Rhabdoviruses: rabies and rabies-related lyssaviruses.
- Rabies in animals
- Clinical features in humans
- Clinical and differential diagnosis
- Laboratory diagnosis
- Control of rabies in animals
- Prevention of human infection
- Rabies-related virus infections of humans
- Further reading
The Rhabdoviridae are a large family of RNA viruses, two genera of which infect animals: the genus Lyssavirus contains rabies and rabies-related viruses that cause at least 55 000 deaths annually in Asia and Africa.
Transmission and epidemiology
The risks and problems posed by rabies and other lyssaviruses vary across the world. Virus can penetrate broken skin and intact mucosae. Humans are usually infected when virus-laden saliva is inoculated through the skin by the bite of a rabid animal, usually a dog. Although the greatest threat to man is the persistent cycle of infection in stray dogs, several other terrestrial mammal species are reservoirs of infection. In the Americas, bat viruses are also classic genotype 1 rabies and insectivorous bats have become the principal vectors of infection to humans in the United States of America. Elsewhere in the world, there is increasing evidence of widespread rabies-related lyssavirus infection of bats. Unrecognized infection of organ donors has proved fatal to transplant recipients.
After a highly variable incubation period (usually 20 to 90 days), prodromal symptoms include itching at the site of the healed bite wound. These are followed by symptoms of either furious or paralytic rabies, reflecting whether infection of the brain or spinal cord predominates.
Furious rabies—the diagnostic symptom is hydrophobia, a combination of inspiratory muscle spasms, with or without painful laryngopharyngeal spasms, associated with terror, initially provoked by attempts to drink water. Patients may suffer generalized arousal, during which they become wild, hallucinated, fugitive, and rarely aggressive.
Paralytic rabies—flaccid ascending paralysis develops, starting in the bitten limb.
The diagnosis can be made during life using rapid laboratory methods such as immunofluorescence of brain or punch biopsy specimens of skin taken from a hairy area. The polymerase chain reaction is used increasingly to detect rabies in saliva and skin biopsy material. However, lack of facilities hampers the confirmation of disease in developing countries where the diagnosis usually relies on recognition of hydrophobic spasms and other clinical features of furious rabies. Paralytic disease is rarely identified. Rabies has been misdiagnosed as cerebral malaria, or even drug abuse.
Management and prognosis
The few human survivors of rabies encephalomyelitis had received vaccine and, with one exception were left with severe neurological sequelae. Recently an unvaccinated patient bitten by a bat in North America made a good recovery. However, dog rabies virus infection remains universally fatal in man. Patients with furious rabies rarely live more than one week without intensive care but survival can be up to one month with paralytic disease. The mechanism of neuronal dysfunction remains elusive, and no treatment has proved effective experimentally.
Management—intensive care treatment may be appropriate for patients infected by a bat in the Americas if they present early and are already seropositive. Other patients with rabies should be sedated heavily and given adequate analgesia to relieve their pain and terror.
Highly effective methods for control and prevention of rabies are available.
Control of rabies in domestic dogs—95% of human rabies deaths could be prevented by controlling the transmission of dog rabies, but education and resources are lacking.
Pre-exposure prophylaxis—a three-dose course of rabies vaccine is recommended for travellers and indigenous people in dog-rabies endemic areas, but the cost is often prohibitive.
Postexposure prophylaxis—at the time of a bite, correct cleaning of the wound and optimum postexposure immunization virtually eliminate the risk of rabies (compared to c. 45% in untreated cases). Effective prophylaxis demands urgent wound cleaning with copious amounts of soap and water, followed by vaccine and rabies immunoglobulin. A new improved economical 4-site intradermal postexposure vaccine regimen could increase the availability of affordable treatment in developing countries.
Rabies is a zoonosis of mammals that remains endemic in most parts of the world. A cycle of infection is maintained in several reservoir species, of which the domestic dog is by far the most important. Many wild mammals including bats are also independent rabies reservoirs (sylvatic infection) with identifiable strains of virus. Any mammalian species is potentially susceptible to rabies and may be a vector, e.g. a cat infected by a dog may then bite and infect a person. However, there is no persistent virus transmission between cats. The vector origin of human disease depends on the likelihood of contact with an infected animal. Hence domestic dogs and cats are the source of more than 95% of human cases worldwide, mainly in Africa, Asia, and parts of South America. Rabies control programmes can reduce the risk of rabies in domestic animals to such an extent that wild animals, e.g. insectivorous bats in the United States of America, become the principal vectors of infection to humans. Rabies in wild mammals is usually spread by bites or by ingestion of infected prey.
Rabies and rabies-related viruses
The Lyssavirus genus currently includes classic rabies virus, genotype 1, and six rabies-related genotypes that are continent-specific in Europe, Australasia, and Africa, and are, with one exception, zoonoses of bats (see also ‘Rabies-related viruses known to infect humans’). All but Lagos bat virus have caused fatal human disease. New unclassified lyssaviruses are now emerging. No rabies-related viruses have been found in the Americas. All terrestrial rabies reservoir mammal species (dogs and wildlife) carry genotype 1 rabies, except for the rare Mokola virus in Africa.
Countries currently reported as rabies free include Iceland, Norway, Sweden, Finland, Portugal, Greece, Cyprus and most other Mediterranean islands, Singapore, Sabah, Sarawak, Antarctica, Oceania (including New Guinea and New Zealand), Hong Kong islands (but not the New Territories), Japan, South Korea, Taiwan, and Caribbean islands with the notable exceptions of Cuba, the Dominican Republic, Grenada, Haiti, and Trinidad and Tobago. The British Isles, together with other Western European countries, and Australia have no rabies in terrestrial species, but do harbour rabies-related lyssaviruses in bats. Inadvertent importation of infected animals is a global risk.
Cyclical epizootics of rabies may result from an uncontrolled increase in the population of the key reservoir species, such as the fox epizootic in Europe in the late 20th century. This started in Poland and spread across France, but it has now been eliminated from Western Europe. The AIDS pandemic has increased indirectly the risk of rabies infection in South Africa because many dogs abandoned by people who are sick or dying from AIDS have become feral and rove in packs. Outbreaks in dogs have also followed the movement of refugees.
Although the fox is one of the species most susceptible to rabies, about 3% of animals survive the infection and become immune. Seropositive bats are not uncommon, and rabies antibody has been found in several other species, exceptionally even in dogs. There is no evidence that animals can become chronically infected or be infectious carriers, although an apparently healthy animal may be infectious during the prodromal stage of infection.
Wild mammal reservoir species
Wild mammal reservoir species vary in different areas.
Reservoir species in the central United States of America and California are striped skunks Mephitis mephitis and, to a lesser extent, spotted skunks Spilogale putorius; in Arizona and Texas grey foxes Urocyon cinereoargenteus and red foxes Vulpes vulpes; and in Alaska arctic foxes Alopex lagopus. However, in the east, rabies is most commonly found in raccoons Procyon lotor that transmit it to skunks and foxes. In North America many insectivorous bats are reservoirs, including big brown bats Eptesicus fuscus, Mexican free-tailed bats Tadarida brasiliensis mexicana, little brown bats Myotis lucifugus, and silver-haired bats Lasionycteris noctivagans whose virus is the main cause of human rabies infections in the United States of America (see below) where bat infection has been found in every state except Hawaii. All bat rabies in the Americas is due to genotype 1 virus.
Latin America and the Caribbean
Dog rabies persists in some urban areas of South America despite successful control programmes. The three species of true vampire bats Desmodus rotundus, Diaemus youngi, and Diphylla ecaudata (Desmodontinae) occur from sea level to over 3500 m but usually below 1500 m only in Mexico, Central and South America, and some Caribbean Islands. The common vampire bat D. rotundus is the main reservoir of vampire bat rabies in Trinidad, Mexico, and Central and South America, where humans are occasionally bitten. Carnivorous bats of the family Megadermatidae, such as the Indian ‘vampire’ Megaderma lyra, have given rise to the myth that vampires occur elsewhere. In Latin America, thousands of head of cattle are lost each year from vampire bat-transmitted paralytic rabies (derriengue) with locally serious economic consequences. Mongooses Herpestes auropunctatus are reservoirs of sylvatic rabies in Central America, Grenada, Puerto Rico, Cuba, Haiti, and the Dominican Republic.
Africa and Asia
Dog rabies predominates but there is sylvatic rabies in Africa in foxes, wolves, jackals, and small carnivores of the families Mustelidae and Viverridae (e.g. the yellow mongoose Cynictis penicillata in South Africa), and in Asia in wolves, jackals, ferret-badgers Melogale moschata in China, and palm civets Paradoxurus hermaphroditus in Indonesia.
Foxes, wolves, raccoon dogs Nyctereutes procyonoides, and insectivorous bats are infected (see also ‘Rabies-related viruses known to infect humans’).
There are reports of rabies virus being isolated from wild rodents in many countries but there is no evidence that they are a reservoir species or that rodent bites, which are very common in some places, pose a threat of rabies.
Incidence of human rabies
The true incidence of human rabies throughout the world is not reflected in official figures; 55 000 deaths annually have been estimated to occur in Asia and Africa, including about 20 000 in India alone. However, it is suggested that only 3% of cases are recorded. High mortalities also occur in Bangladesh and Pakistan, and recently the incidence has been rising in China. There are very few data from Africa. In Latin America, mortality from canine rabies persists in Brazil, El Salvador, Mexico, Bolivia, Colombia, Venezuela, and Haiti. There have been recent outbreaks due to vampire bat rabies in Peru, Ecuador, and Brazil. In the United States of America there are on average two human deaths annually. Among 37 indigenous infections occurring in the last 40 years, 92% were caused by insectivorous bats. Europe reported 45 deaths in the last 5 years, mainly from the Russian Federation and the Ukraine. Rabies was apparently eliminated from the United Kingdom by 1903, but, since 1980, there have been nine imported cases and one indigenous European bat lyssavirus infection.
The Rhabdoviridae are a family of more than 100 rod-shaped or bullet-shaped RNA viruses found in vertebrates, insects, and plants. Two genera infect animals, Vesiculovirus and Lyssavirus. Vesicular stomatitis virus is a vesiculovirus of cattle and horses, which occasionally causes an influenza-like illness in farmers or laboratory workers. The genus Lyssavirus contains rabies and rabies-related viruses.
The rabies virion is approximately 180 × 75 nm. Its core is a single spiral strand of negative nonsegmented RNA associated with a nucleoprotein, a phosphoprotein, and an RNA polymerase to form a helical ribonucleoprotein (RNP) complex. This is enveloped in a matrix protein, host cell-derived lipid, and a coat of protruding glycoprotein (G) molecules bearing spikes or knobs 10 nm long. The composition of the glycoprotein determines viral virulence.
The virus is readily inactivated by ultraviolet light, drying, boiling, most organic lipid solvents including at least 45% ethanol, soap solution, detergents, hypochlorite, and glutaraldehyde solutions.
Typing by means of monoclonal antibodies or genetic sequencing techniques allows the identification of diverse strains of rabies and rabies-related viruses from different geographical areas and vector species.
Virus can penetrate broken skin and intact mucosae. Humans are usually infected when virus-laden saliva is inoculated through the skin by the bite of a rabid dog or other mammal. Saliva from a rabid animal can infect if the skin is already broken, e.g. by the animal’s claws. In North America, contact with bats leading to rabies has passed unnoticed; only 39% of patients reported a bat bite and 34% had no history of exposure to bats. Animals can be infected through the gastrointestinal tract, but there is no evidence that this happens in humans.
Inhalation of aerosolized virus created by infected nasal secretions of bats may be an important method of transmission among cave-dwelling bats. In Texas, two men died of rabies after visiting caves inhabited by millions of Mexican free-tailed bats Tadarida brasiliensis mexicana, some of which were rabid, however fleeting bat contact may have caused the infection. Two laboratory workers in the United States of America developed rabies after inhaling aerosolized fixed strains of rabies virus during the preparation of vaccines. The accidental use of vaccine in which the virus was not inactivated has led to fixed virus rabies (rage de laboratoire), e.g. in Fortaleza, Brazil in 1960.
Transmission of rabies between people has been proved in 13 cases of tissue transplantation from donors who had died of undiagnosed neurological diseases. Six recipients of infected corneal grafts developed retro-orbital headache on the side of the graft 22 to 39 days after transplantation and died soon afterwards (other infections spread by corneal grafts include Creutzfeldt–Jakob disease and cryptococcosis). In Texas and Germany, seven recipients of kidney, liver, lung, pancreas, or even just a segment of iliac artery developed rabies encephalitis. Rabies was not suspected in the two young donors despite a history of recent rough travel in India in one and later discovery of a bat bite in the other. Recreational drug abuse was detected in both. One surviving liver transplant patient had had rabies vaccine previously. Postexposure prophylaxis following corneal transplants from infected donors has been successful.
Considering that the saliva, respiratory secretions, and tears of rabies patients contain virus, it is surprising that the disease has not been spread to intimate relatives and nurses.
Transplacental infection has been observed in animals but has only been reported once in humans. Several women with rabies encephalitis have given birth to healthy babies. The transmission of rabies from mother to suckling infant via the breast milk has been suspected in at least one human case and is well known in animals.
The mechanism by which the highly neurotropic rabies virus enters the nervous system and travels into the brain and out again to many organs is intriguing. The virus may replicate locally in muscle cells or attach directly to nerve endings. It can bind to many types of receptors including the neural cell adhesion molecule and the nicotinic acetylcholine receptors at motor endplates, which are blocked by α-bungarotoxin. Several other neuronal binding mechanisms may be involved. Once inside peripheral nerves, virus travels in a strictly retrograde direction within the axoplasm. This progression can be blocked experimentally by local anaesthetics, metabolic inhibitors, and nerve section. The axonal dynein molecular motor is assumed to be the vehicle of transport but the attachment mechanism is elusive. Viral binding might be directly via the naked ribonucleoprotein complex or indirectly as a vesicle containing a whole virion. Rabies virus is experimentally inaccessible to antibodies while concealed in the peripheral nerves.
On reaching the central nervous system, the virus replicates massively within neurons and is transmitted directly from cell to cell across synaptic junctions. Dramatic symptoms can appear before histopathological changes are apparent. Viral virulence is inversely related to neuronal apoptosis. Rabies alters host cell gene expression, but the mechanisms of gross neuronal dysfunction are speculative. Centrifugal spread of virus from the central nervous system, apparently in the axoplasm of somatic and autonomic efferent nerves, deposits virus in many tissues including skeletal and cardiac muscle, adrenal medulla where infection may be clinically significant, and also in kidney, retina, cornea, pancreas, taste buds, respiratory tract, and the skin in nerve twiglets around hair follicles (see below ‘Laboratory diagnosis’). At this stage, productive viral replication occurs, with budding from outer cell membranes in the salivary and lacrimal glands. This is how rabies is transmitted by bites to other mammals. Viraemia has been detected very rarely, only in animals, and is not thought to be involved in pathogenesis or spread.
Immunological response to rabies infection in humans
Some patients die without any detectable immune response, suggesting that rabies virus evades or suppresses the immune system. Rabies antibody might become detectable in serum 7 days or more after the onset of illness and in cerebrospinal fluid a little later. It may rise to high levels in patients whose lives are prolonged by intensive care. A small amount of rabies-specific IgM is sometimes detectable, but is not useful as a means of diagnosis.
There is little evidence of a lymphocyte-mediated immune response to rabies encephalitis. A pleocytosis appears in only 60% of patients, with a mean leucocyte count of 75 × 103/mm. Peripheral blood lymphocyte transformation has been shown in a few patients with furious rabies, but not in those with paralytic disease. Experimentally, in fatal rabies there is suppression of the cytotoxic T-lymphocyte response to unrelated viral antigens and a T-cell response is associated with survival in mice.
Interferon is induced by rabies infection, but appears to be at a very low level in human patients. In animals, latent infections can be reactivated by corticosteroids and stress. This provides a possible explanation for occasional reports of long incubation periods.
Immunological response to rabies vaccination
The viral glycoprotein induces neutralizing antibody, which is detectable by 2 weeks after the start of primary immunization. In animal studies, the neutralizing antibody titre is the best available measure of protection against death. The nucleoprotein antigens also stimulate antibody that is more cross-reactive between lyssaviruses than the more strain-specific glycoprotein. Although peripheral blood lymphocyte transformation occurs following human vaccination, the role of T lymphocytes in protection remains to be demonstrated.
Although neutralizing antibody is undoubtedly protective in the early stages after inoculation of virus, it may be deleterious once central nervous system infection is established. In animals, acceleration of the terminal phase of the encephalitis (‘early death phenomenon’) is associated with the presence of low titres of rabies antibody.
Transient low levels of interferon may be induced after the first dose of tissue culture rabies vaccines. Interferon is effective postexposure prophylaxis against experimental rabies.
Rabies in animals
All warm-blooded animals can be infected with rabies but their susceptibility varies. However, only mammals are infected naturally.
In dogs, the incubation period ranges from 5 days to 14 months, but is usually between 3 and 12 weeks. The first symptom, as in many humans, is intense irritation at the site of the infection. Despite the popular idea of the ‘mad’ rabid dog, probably only a minority develop furious rabies. There is an early and striking change in the dog’s behaviour with dysphagia, ptosis, altered bark, paralysis of the jaw, neck, and hind limbs, hypersalivation, congested conjunctivae, pruritus, shivering, trembling, snapping at imaginary objects, pica, and extreme restlessness causing the animal to wander miles from home. Dogs with furious rabies attack inanimate objects, often seriously injuring their mouths in the process. Virus has been found in the saliva 3 days before symptoms appear, and the animal usually dies within the next 7 days.
This is the basis for the traditional 10-day observation period for dogs that have bitten humans. Very rare old reports from India, Ethiopia, and Nigeria of persistent or intermittent excretion of virus in the saliva of apparently healthy dogs have not been confirmed by subsequent thorough searches. ‘Oulou fato’, a clinical variant of canine rabies with reduced virulence, was seen in West Africa 50 years ago. In Tanzania, a rabies virus of apparently low virulence has been identified in hyenas.
Rabid foxes lose their fear of humans and the majority develop paralytic rabies. An extreme degree of furious rabies is seen in 75% of infected cats. Cattle usually develop paralytic symptoms with dysphagia, hypersalivation, groaning, trembling, colic, diarrhoea, tenesmus, and rectal prolapse. Most other domestic ungulates develop paralytic symptoms. Horses often show furious features with sexual excitement. Most wild animals, like foxes, lose their fear of humans and may appear tame. Rabid skunks, raccoons, badgers, martens, and mongooses may become very aggressive. Dysphagia and inability to drink is common in rabid animals, but they do not exhibit hydrophobia.
Clinical features in humans
The incubation period ranges from 4 days to many years, but it is between 20 and 90 days in three-quarters of cases. It tends to be shorter after bites on the face (average 35 days) than after those on the limbs (average 52 days).
Often, the first symptom is itching, pain, or paraesthesia at the site of the healed bite wound. Nonspecific prodromal symptoms include fever, chills, malaise, weakness, tiredness, headache, photophobia, myalgia, anxiety, depression, irritability, and symptoms of upper respiratory tract and gastrointestinal infections. Subsequently, symptoms of either furious or paralytic rabies will develop, depending on whether the spinal cord or brain are predominantly infected.
Furious rabies is the more common presentation. Most patients have the diagnostic symptom of hydrophobia, which is a combination of inspiratory muscle spasm, with or without painful laryngopharyngeal spasm, associated with terror. Initially provoked by attempts to drink water, this reflex can be excited by a variety of stimuli including a draught of air (‘aerophobia’), water splashed on the skin, irritation of the respiratory tract or, ultimately, by the sight, sound, or even mention of water. The inspiratory spasm is violent and jerky. The neck and back are extended, the arms thrown up, and the episode may end with a generalized convulsion complicated by cardiac or respiratory arrest.
Patients experience hyperaesthesia and, at times, generalized arousal during which they become wild, hallucinated, fugitive, and sometimes aggressive. This behaviour alternates with periods of mental lucidity during which patients may become distressingly aware of their predicament. Despite these dramatic symptoms, attributable to brainstem encephalitis, conventional neurological examination may prove surprisingly normal, leading to the false assumption of hysteria. Reported abnormalities include meningism, cranial nerve lesions (especially III, VI, VII, IX–XII), upper motor neuron lesions, fasciculation, and involuntary movements. Disturbances of the hypothalamus or autonomic nervous system are reflected by hypersalivation, sweating, lacrimation, hypertension or hypotension, hyperthermia or hypothermia, inappropriate secretion of antidiuretic hormone or diabetes insipidus, and, rarely, priapism with spontaneous orgasms, satyriasis, or nymphomania. Hypersexuality suggests similar aetiology to the Klüver–Bucy syndrome created in rhesus monkeys by bilateral ablation of the hippocampus.
Without supportive treatment, about one-third of the patients will die during a hydrophobic spasm during the first few days. The rest lapse into coma and generalized flaccid paralysis, and rarely survive for more than a week without intensive care.
Paralytic or dumb rabies
This is the clinical pattern in less than one-fifth of human cases except in the case of bat-transmitted rabies, especially vampire bat infection, which is usually paralytic. Patients may become literally dumb (‘rage muette’) because their laryngeal muscles are paralysed, but symptoms are quieter (‘rage tranquille’) than in furious rabies. The largest reported outbreak was in Trinidad between 1925 and 1935 when there were 89 human cases, initially misattributed to poliomyelitis or botulism; others have been described from Mexico, Guyana, Brazil, Peru, Ecuador, Bolivia, and Argentina.
The paralytic form of rabies was also seen in patients with postvaccinal rabies, in the two patients who inhaled fixed virus, and is said to be more likely to develop in patients who have received antirabies vaccine. After the usual prodromal symptoms, especially fever, headache, and local paraesthesias, flaccid paralysis develops, usually in the bitten limb, and ascends symmetrically or asymmetrically with pain and fasciculation in the affected muscles and mild sensory disturbances. Paraplegia and sphincter involvement then develop, and finally fatal paralysis of deglutitive and respiratory muscles. Hydrophobia is unusual, but may be represented by a few pharyngeal spasms in the terminal phase of the illness. Even without intensive care, patients with paralytic rabies have survived for up to 30 days.
Other manifestations and complications
Asphyxiation and respiratory arrest may complicate the hydrophobic spasms or generalized convulsions of furious rabies and the bulbar and respiratory paralysis of dumb rabies. Bronchopneumonia is a predictable complication if life is prolonged by intensive care, but a primary rabies pneumonitis may occur. Various abnormal patterns of respiration have been described, including cluster and apneustic breathing. There are some similarities to respiratory myoclonus. Pneumothorax may complicate inspiratory spasms.
A variety of dangerous cardiac arrhythmias have been reported, including supraventricular tachycardias, sinus bradycardia, atrioventricular block, and sinus arrest, together with T wave and ST segment changes. Hypotension, pulmonary oedema, and congestive cardiac failure are attributable to myocarditis.
Raised intracranial pressure resulting from cerebral oedema or internal hydrocephalus has been reported in a few cases, but spinal fluid opening pressure is usually normal and papilloedema is rarely seen. There is clinical and electrophysiological evidence of diffuse axonal neuropathy, consistent with histological appearances of degeneration of peripheral nerve ganglia and axons. Gastrointestinal system ‘Stress’ ulcers and the Mallory–Weiss syndrome are possible explanations for the haematemesis often reported in rabies.
Clinical and differential diagnosis
Rabies should be suspected in any patient who develops neurological symptoms after being bitten by a mammal in a rabies endemic area. However, some patients fail to remember that they have been bitten and others may be infected while they are asleep possibly by contact with lip mucosae (North American insectivorous bats) or near-painless bites by vampire bats in parts of Latin America.
Pathognomonic inspiratory spasms with associated emotional response are provoked by asking the patient to swallow accumulated saliva or by directing a draught of air on to the face.
- Psychiatric conditions: Rabies encephalitis has been misdiagnosed as a variety of psychiatric conditions, including hysteria and behavioural disturbances attributed to recreational drugs. Conversely, patients with a morbid fear of rabies (rabies phobia, lyssaphobia, pseudohydrophobia) may simulate the more melodramatic features of the disease but hydrophobia is unlikely to be mimicked accurately, the incubation period after the bite (hours or a few days) is usually much too short for rabies encephalitis, and the prognosis is, of course, excellent.
- Otolaryngological conditions: Pharyngeal and upper airway symptoms of hydrophobia may be misinterpreted as pharyngitis or laryngitis so that the patient is referred to an otolaryngologist.
- Tetanus: This can also follow an animal bite and is similar to rabies in some respects, especially the pharyngeal form of cephalic tetanus (‘hydrophobic tetanus’). It is distinguished by its shorter incubation period (usually less than 15 days in severe tetanus), the presence of trismus, the persistence of muscle rigidity between spasms, the absence of meningoencephalitis (cerebrospinal fluid is universally normal), and the better prognosis.
- Other encephalopathies/encephalitides: The typical encephalitic progression from severe headache to continuous coma is unusual in furious rabies. Hydrophobia with intermittent excitation and lucid intervals of full consciousness does not occur in other encephalitides. Among children with suspected cerebral malaria in Malawi, some were proved at biopsy to have died of rabies.
- Toxic encephalopathies: Delirium tremens, some drugs (phenothiazines, amphetamines, modafinil, cocaine, and other recreational drugs), and plant poisonings (e.g. Datura fastuosa) can cause excitable and aggressive behaviour that might be confused with rabies.
Other causes of ascending (Landry-type) paralysis may enter the differential diagnosis.
- Postvaccinal encephalomyelitis (see below): This usually develops within 2 weeks of the first dose of the now rarely used nervous tissue rabies vaccines.
- Poliomyelitis: Objective sensory disturbances are absent and fever rarely persists after paralysis has developed.
- Acute inflammatory polyneuropathy (Guillain–Barré syndrome): Cerebrospinal fluid examination will help to distinguish this condition.
- Cercopithecine herpesvirus (B virus) encephalomyelitis: Bites and other types of contact with Asian macaque monkeys (genus Macaca), especially rhesus (M. mulatta) and cynomolgus (M. fascicularis) transmit this dangerous infection. The incubation period (3–4 days) is usually shorter than in rabies and symptoms develop within 1 month of contact. Vesicles may be found in the monkey’s mouth and at the site of the bite, and the diagnosis can be confirmed virologically.
The brain, spinal cord, and peripheral nerves show ganglion cell degeneration, perineural and perivascular mononuclear cell infiltration, neuronophagia, and glial nodules. Inflammatory changes are most marked in the midbrain and medulla in furious rabies and in the spinal cord in paralytic rabies.
Negri bodies are eosinophilic intracytoplasmic inclusions predominantly consisting of masses of viral ribonucleoprotein, with a basophilic inner body containing fragments of cellular organelles including ribosomes and occasional virions. They can be demonstrated by haematoxylin and eosin stains in histological sections of grey matter in up to 75% of human cases, especially in hippocampal pyramidal cells and cerebellar Purkinje cells.
In view of the appalling prognosis of rabies encephalitis, neuronolysis is often surprisingly mild and patchy, and death can occur without any inflammatory response. Vascular lesions such as thrombosis and haemorrhage have also been described. The brainstem, limbic system, and hypothalamus appear to be most severely affected and, in paralytic disease, the spinal cord and medulla. Outside the nervous system, there is focal degeneration of salivary and lacrimal glands, pancreas, adrenal medulla, and lymph nodes. An interstitial myocarditis with round cell infiltration is found in about 25% of cases.
If a mammal suspected of being rabid has bitten, scratched, or otherwise risked infecting a person, it should be killed and its brain examined without delay. The best way to detect rabies antigen in acetone-fixed brain impression smears is by the direct immunofluorescent antibody (IFA) test. Alternatively, if no fluorescent microscope is available, rapid enzyme immunodiagnosis can be used. Sellers’ stain is insensitive and rarely used. Virus isolation takes up to 3 weeks by intracerebral inoculation of mice, or about 4 days in murine neuroblastoma cell culture.
In humans, rabies can be confirmed early in the illness by demonstration of viral antigen by the direct IFA test in frozen sections of full-thickness skin biopsies taken from a hairy area, usually the nape of the neck. Specific diagnostic staining is seen in nerve twiglets around the base of hair follicles. This rapid method is positive in 60 to 100% of cases, and no false-positive results have been reported. Antigen can also be found in brain biopsies, but tests on corneal impression smears are usually falsely negative. The polymerase chain reaction is being used increasingly to detect rabies in saliva, and occasionally cerebrospinal fluid, and also skin biopsy material.
During the first week of illness, virus may be detected in saliva, brain, cerebrospinal fluid, and very rarely urine. Rabies antibodies are not usually detectable in serum or cerebrospinal fluid before the eighth day of illness in unvaccinated patients. Serum antibody may leak into the cerebrospinal fluid in patients with postvaccinal encephalomyelitis, but a very high titre suggests a diagnosis of rabies. A specific IgM test has not proved useful diagnostically.
Patients with rabies must be sedated heavily and given adequate analgesia to relieve their pain and terror. If intensive care is undertaken, the aim is to prevent complications such as cardiac arrhythmias, cardiac and respiratory failure, raised intracranial pressure, convulsions, fluid and electrolyte disturbances including diabetes insipidus and inappropriate secretion of antidiuretic hormone, and hyperpyrexia. Antiserum, antiviral agents, interferon-α, corticosteroid, and other immunosuppressants have proved useless.
Rabies was formerly regarded as a universally fatal disease, but there are reports of seven cases of recovery or prolonged survival following intensive care. All the diagnoses were made serologically and no virus or antigen was identified. Two patients had been given postexposure prophylaxis with nervous tissue vaccines and then intensive care. Four further patients, a microbiologist who inhaled fixed rabies virus, two boys in Mexico, and a girl in India, were given pre-exposure or postexposure tissue culture vaccines, and survived months or years with profound neurological impairment.
The first unvaccinated patient to survive rabies has returned to near normal life following intensive care and antiviral therapy. She was bitten by a bat in Wisconsin in 2004, had no rabies prophylaxis, and developed typical encephalitis without hydrophobia. Rabies neutralizing antibody was detected on the sixth day of illness. Treatment comprised coma induction and antiviral drugs. She made a slow recovery over 5 years and has returned to normal life, although with minor neurological deficits. The antiviral treatments have not proved effective against rabies experimentally; however, she developed antibody at an early stage of the disease. Her treatment possibly maintained her vital functions until the spontaneous specific immunity immune response eliminated the virus, probably with loss of infected neurons.
In animal experiments, American bat rabies virus infection differs from that of canine virus in that it is slower to evolve and progress, virus replication is not restricted to neurons, and histopathological changes are milder with less apoptosis. This suggests that the virus maybe less pathogenic and may also explain the complete recovery of a boy infected by a similar virus in 1970 who had delayed treatment with a nervous tissue vaccine. It is likely that he too had rabies antibody present at an early stage of illness.
The treatment protocol used in Wisconsin has since been used unsuccessfully in several other patients with rabies encephalitis who were infected by bats or dogs. Recently, however, a vaccinated Brazilian boy bitten by a bat survived symptomatic rabies, but the residual neurological deficits are unknown.
No treatment has proved effective in animal models. Human rabies of canine origin remains 100% fatal. Until a new treatment is proved effective experimentally, palliation of the patient and immunization of contacts is recommended. Intensive treatment may be appropriate for patients infected by an American bat, who present early, and are already seropositive. Intensive care treatment is inappropriate for canine virus infection, especially in developing countries, and the cost is prohibitive.
Control of rabies in animals
The elimination of dog rabies would reduce the human mortality by over 95% and drastically reduce the need for human vaccination. Rabies control has been achieved most effectively where the principal reservoir is the domestic dog, as in 19th-century United Kingdom, Malaysia, and Japan, and since then in other areas including Western Europe, Taiwan, North America, and parts of urban Latin America.
In countries where rabies is endemic
The control strategy depends on the local pattern of rabies occurrence in wild and domestic animals. Education and publicity about rabies is always needed. Domestic animals can be protected by regular vaccination. Owned dogs can be muzzled or kept off the streets. People should be discouraged from keeping wild carnivores such as skunks, raccoons, coatis, and mongooses as pets. Unnecessary contact with mammals should be avoided (e.g. stroking stray dogs or apparently friendly wild animals, exploring bat-infested caves). Culling reservoir species has proved an unpopular and ineffective method of long-term control. Impressive reduction of urban rabies in stray dogs has proved possible in India by vaccination, population control, and reducing available food and shelter by removing refuse. Effective oral vaccination of dogs is not yet practicable.
Control of sylvatic rabies has been achieved by vaccination of key wild animal reservoir populations with live oral vaccines distributed in bait. Repeated campaigns distributing attenuated rabies or vaccinia-recombinant rabies glycoprotein vaccines have eliminated fox rabies in Western Europe, and the latter has been used in North American coyotes, foxes, and raccoons. New vaccines are being developed for other species. Vaccination of bats is unlikely to be feasible. Vampire bat rabies is controlled by destroying roosts and poisoning the bats with anticoagulants.
In countries where rabies is not endemic
The inadvertent importation of a mammal incubating rabies is a universal risk. The movement of potential vectors, especially domestic dogs and cats, wild carnivores, and bats, should be strictly controlled. Serological evidence of successful vaccination should be provided for imported mammals, or they should be vaccinated on arrival and quarantined.
Prevention of human infection
Pre-exposure vaccination is the most effective form of rabies prevention. No rabies deaths have been reported in anyone who had pre-exposure vaccine followed by postexposure booster doses. It is recommended for people who handle imported animals, workers in zoos and rabies laboratories, and those who are resident in or intend to travel to dog rabies-endemic areas, especially children. Others particularly at risk in certain areas include veterinarians, dog catchers, farm workers, cave explorers, naturalists, and animal collectors. In dog rabies-endemic areas, pre-exposure prophylaxis is advisable but is rarely used. Travellers should be educated to seek immediate local medical help if they are bitten, scratched, or licked by animals. However, recommendations vary in different areas and local advice may be unreliable. Tissue culture vaccine and especially rabies immune globulin may not be readily available.
Primary pre-exposure vaccine course
A course of three doses of tissue culture rabies vaccine (see below) is given intramuscularly into the deltoid, or the anterolateral thigh in children, on days 0, 7, and 28. The last dose may be advanced towards day 21 if time is short. An effective economical alternative is intradermal injections of 0.1 ml at the same intervals. If the injection is too deep to produce a papule, withdraw the needle and repeat the procedure. The whole vaccine ampoule should be used within a day or discarded. If chloroquine is being taken for malaria prophylaxis (unlikely today), or in other cases of suspected immunosuppression, the intramuscular route must be used. Many travellers cannot afford three doses of an expensive vaccine, so the economical intradermal route is ideal for family, student, or other groups who can be vaccinated on the same day.
A booster dose 1 to 2 years after the primary course enhances and prolongs the presence of antibody. Although the titre falls more rapidly after intradermal than intramuscular inoculation, the response to a booster dose is equally prompt. Confirmation of seroconversion is recommended only if immunosuppression is suspected. Further booster doses may be given intradermally or intramuscularly at intervals of 2 to 10 years depending on the risk of exposure. If the rabies neutralizing antibody level is at least 0.5 IU/ml, boosters are not necessary. Laboratory staff at high risk should have more frequent serology tests. Travellers who will have rapid access to vaccine if exposed need not have further immunization, but, if medical resources will be unreliable, a booster vaccination should be given before departure if 3 to 5 years have elapsed since the previous dose. A personal record of immunization must be kept, and urgent treatment is essential after possible exposure. Lyophilized rabies vaccine is relatively stable even at tropical ambient temperatures. It is sensible to take a dose on expeditions to remote rabies endemic areas. An extra emergency injection can then be given immediately after a risky encounter with an animal. If more than one person is exposed, the ampoule can be shared by giving multiple intradermal doses to each, using the whole dose (see postexposure regimens). This does not replace the normal postexposure treatment, which must still be given as soon as possible.
Despite intensive care, rabies encephalomyelitis of canine origin remains 100% fatal. At the time of the bite, however, correct cleaning of the wound (see below) and optimum postexposure immunization reduce the risk of rabies to nearly zero compared to about 35 to 57% in untreated cases. The risk varies with the biting species and the site and severity of the bites. It is highest following bites to the head by proved rabid wolves, which carries a case fatality exceeding 80% in unvaccinated people. The decision to give postexposure treatment depends on an assessment of the risk of infection by asking about the precise geographical location of the exposure; its severity, whether it was a bite or lick on broken skin; the site of the lesion; and the nature, appearance, behaviour, and fate of the biting animal, and, whether it had been recently vaccinated against rabies. The animal’s brain must be tested for rabies if possible. If there is any doubt, the patient should be given full postexposure prophylaxis, even if the bite is several months old.
The aim of prophylaxis is to neutralize inoculated virus before it can enter the nervous system. Wound cleaning and active and passive immunization must be implemented as soon as possible.
his is effective in killing virus in superficial wounds, but is often neglected. First aid includes vigorous cleaning of the wound with soap or detergent and water under a running tap for at least 5 min. Foreign material should be removed and a viricidal agent such as povidone iodine, or 40 to 70% alcohol, should be applied liberally. Quaternary ammonium compounds such as benzalkonium chloride are inactivated by soap and so are not recommended. Hospital treatment of wounds involves thorough exploration, debridement, and irrigation of deep wounds, if necessary under local or general anaesthetic. Suturing should be avoided or delayed and the wound left without occlusive dressings. Attention should be given to tetanus prophylaxis (Chapter 7.6.22) and the large range of viral, bacterial, and fungal pathogens particularly associated with mammal bites. These include Cercopithecine herpesvirus (B virus) from Asian macaques (Chapter 7.5.2); Pasteurella multocida (Chapter 7.6.18), Francisella tularensis (Chapter 7.6.19), Streptobacillus moniliformis, and Spirillum minus (Chapter 7.6.13) from rodents; and Pasteurella multocida, Capnocytophaga canimorsus, and Bartonella henselae (Chapter 7.6.42) from dogs and/or cats. Most of the bacteria are sensitive to amoxicillin/clavulanic acid, cefoxitin, or tetracycline.
Three highly immunogenic tissue culture vaccines that meet the World Health Organization (WHO) recommended standards are human diploid cell vaccine (HDCV), purified chick embryo cell (PCEC) vaccine, and purified VeRO cell rabies vaccine (PVRV).
Several tissue culture vaccines are produced, mainly for national use, in China, India, Japan, Russia, and other Asian and South American countries.
Obsolete nervous tissue rabies vaccines, no longer sanctioned by the WHO, are still produced in a few countries. Semple vaccine, a sheep or goat brain suspension, or suckling mouse brain (Fuenzalida) vaccine is used in a few countries in Asia, Africa, and South America. Daily subcutaneous doses for 7 to 21 days, followed by booster doses, are usually given over the abdomen. Neurological reactions including postvaccinal encephalomyelitis still occur.
Postexposure tissue culture vaccine regimens
The standard intramuscular five-dose (Essen) regimen is 5 × 1-ml (PVRV 0.5 ml) doses injected into the deltoid (or anterolateral thigh in children) on days 0, 3, 7, 14, and 28.
The alternative 2-1-1 intramuscular regimen is two full doses (1.0 ml or for PVRV 0.5 ml), injected into the deltoids on day 0, and one dose on days 7 and 21. A total of four full doses are given, but the antibody level may fall more rapidly.
The intramuscular regimens are unaffordable in many countries. However, two economical multisite intradermal methods are available, each requiring only 40% of the vaccine used in the standard intramuscular method. Each of the intradermal injection sites drains to a different group of lymph nodes, intended to stimulate more lymphoid tissue to produce antibody.
The new simplified four-site intradermal regimen consists of a whole ampoule of vaccine divided between four intradermal injections over the deltoid and the thigh or suprascapular areas. The volume per site is about 0.1 ml for PVRV and the equivalent dose for vaccines containing 1 ml per ampoule is 0.2 ml. On day 7, two intradermal injections of 0.1/0.2 ml in the deltoid and thigh areas are followed by a single intradermal dose on day 28. If PCECV (1 ml/ampoule) is used, a reduced ID dose of 0.1 ml/ID site was found to be immunogenic. If resources are limited and more than one patient is treated on the same day, ampoules of vaccine can be shared, and an alternative dose is 4 x 0.1 ml ID on day 0, and thereafter 0.1 ml per ID site x 2 on day 7 and one on day 28. The 4-site regimen has several advantages as it requires only three clinic visits on days 0, 7, and 28 and is economical even without sharing any ampoules, using a maximum of 3 doses instead of 5 for the IM regimen. However this involves some vaccine wastage.
The two-site intradermal regimen was designed for use with PVRV. A dose of 0.1 ml for PVRV, or 0.2 ml for vaccines formulated in ampoules containing 1 ml, is given intradermally at two sites in the deltoid area on days 0, 3, and 7 and at two sites on day 28. An intradermal dose of 0.1 ml per site has also been used with PCEC 1 ml vaccine.
For all other vaccines, the manufacturer’s instructions should be followed.
Postexposure vaccine regimen for people who have already received vaccination
If a complete pre-exposure or postexposure course of a potent tissue culture vaccine has been given in the past, or if the neutralizing antibody level has been over 0.5 IU/ml, only two doses of tissue culture vaccine should be given on days 0 and 3. Alternatively, for a one day regimen, a single dose is divided between four intradermal sites. Rabies immune globulin is not required, but otherwise full postexposure treatment must be given.
Side effects of tissue culture vaccines
Mild and transient local redness, itching (especially after intradermal injection), or pain at the site of injection are not uncommon. Influenza-like symptoms and rashes are infrequent. Type I immediate hypersensitivity occurs rarely during primary courses. Type III immune-complex hypersensitivity was reported in 6% of those receiving booster doses of HDCV in the United States of America. This consisted of urticaria, rash, angio-oedema, and arthralgia 3 to 13 days after injection. No fatal reactions have been reported. Very rarely polyneuritis, Guillain–Barré syndrome, or local limb weakness have been reported in patients receiving tissue culture vaccines but no more frequently than for other commonly used virus vaccines.
Neurological reactions to nervous tissue vaccines
These occur in up to 1 in 220 courses of Semple vaccine, with a 3% mortality, and are an allergic response to myelin and related neural proteins in the vaccine. Reactions to suckling mouse brain vaccine are rare. The incubation period ranges from 3 to 35 days after the first vaccine injection. Clinical forms include localized neuropathy, transverse myelitis, paralysis with sensory loss or pain (a Landry-type ascending paralysis), meningoencephalitis, and meningoencephalomyelitis. These can be clinically indistinguishable from paralytic rabies, but recovery is usually complete. Permanent neurological sequelae are rare. Corticosteroids are thought to be helpful, and cyclophosphamide therapy has been suggested. Vaccination should be stopped as soon as symptoms appear and the course continued with a tissue culture vaccine.
Passive immunization: rabies immune globulin
Rabies immune globulin (RIG) has proved valuable in providing protection before neutralizing antibody has been actively generated, presumably by neutralizing rabies virus during the first week after initial vaccination. It is recommended as part of primary postexposure treatment, but it is vital following severe bites (on the head, neck, hands, and multiple or deep bites) (see Box 126.96.36.199).
The dose of human RIG is 20 IU/kg body weight and for equine RIG is 40 IU/kg. Reactions to equine and human RIG have been observed in 1.8% and 0.09% of recipients, respectively, and serum sickness in 0.72% and 0.007%, respectively. These are not predicted by a previous intradermal hypersensitivity test and, since RIG must be given even if the test is positive, skin tests are time-wasting and unnecessary. Adrenaline (epinephrine) should always be available in case of reactions.
All the RIG is infiltrated into and around the bite wound if anatomically possible, but any remaining is injected intramuscularly preferably into the thigh, not the buttock, at a site distant from the vaccine. If RIG is given hours or days before the first dose of vaccine, the active immune response will be impaired. RIG is prohibitively expensive and is neither available nor affordable for 99% of people in developing countries for whom postexposure treatment is indicated.
Failures of postexposure prophylaxis
Deaths from rabies have occurred despite prophylaxis. Failures are attributable to delay in starting vaccination, incomplete vaccine course, use of a substandard (nervous tissue) vaccine, and omission of RIG. Failure to infiltrate RIG around the wound, injection of vaccine into the buttock, or impaired immune responsiveness of the patient may also contribute. Low vaccine potency has been held responsible only with nervous tissue vaccines. Vaccine protection against rabies-related lyssaviruses may be less efficient than against genotype 1 rabies viruses (see below), but no case of vaccine failure has been attributed to this phenomenon.
A reduced or delayed immune response to vaccine can sometimes be predicted. If treatment is started late (e.g. more than 2 days after exposure), no RIG is available for severe bites, the patient is immunocompromised, or a rabies-related virus infection is suspected, the immune stimulus might be enhanced by dividing the first dose of tissue culture vaccine between four sites intradermally, as for the economical four-site regimen (see above).
Rabies-related virus infections of humans
The genus Lyssavirus contains seven genotypes: genotype 1, classic rabies, and six rabies-related genotypes (Fig. 188.8.131.52). Continent-specific rabies-related viruses occur in Africa, Europe, and Australia, and there is serological evidence of lyssavirus infection across Asia. With the exception of Mokola virus, all are viruses of bats. All are known to be capable of infecting humans except Lagos bat virus. They are occasionally detected in other species, but diagnostic tests are available only in highly specialized laboratories, infection is rarely suspected, and the routine tests for genotype 1 rabies virus may be weakly positive or negative. Their true prevalence is, therefore, unknown. Only 13 human cases of rabies-related virus infections have been reported, and disease is likely to remain unrecognized and misdiagnosed.
Bullet list 1 Specific postexposure prophylaxis for use in a rabies endemic areaa following contact with a domestic or wild rabies vector species, whether or not the animal is available for observation or diagnostic tests
Minor exposure (including licks of broken skin, scratches, or abrasions without bleeding)
- Start vaccine immediately
- Stop treatment if animal remains healthy for 10 days
- Stop treatment if animal’s brain proves negative for rabies by appropriate laboratory tests
Major exposure (including licks of mucosa, minor bites on arms, trunk or legs, or major bites i.e. multiple or on face, head, fingers, or neck)
- Immediate rabies immune globulin and vaccine
- Stop treatment if domestic cat or dog remains healthy for 10 days
- Stop treatment if animal’s brain proves negative for rabies by appropriate laboratory tests
a This scheme is a simplification of the recommendations of the World Health Organization Expert Committee on Rabies (1997).
- Lagos bat virus (genotype 2) has not been implicated in any human case.
- Mokola virus (genotype 3) has been isolated from shrews (Crocidura spp.) and rodents, as well as cats and dogs which are presumably vectors. It was isolated from a child with meningitis who recovered, and from another with fatal encephalitis. Mokola virus also caused mild disease in a rabies-vaccinated laboratory worker.
- Duvenhage virus (genotype 4) has been identified in three people, all of whom had had skin lesions inflicted by bats and had developed a fatal illness with clinical features identical to rabies encephalitis.
European bat lyssaviruses
Infected insectivorous bats have been found in Europe since 1954. The European bat lyssavirus (EBLV) group comprises genotype 5 (also known as EBLV 1) and genotype 6 (EBLV 2), both of which have subgroups a and b. EBLV type 1a is found across Northern and Eastern Europe from the Netherlands to Russia; EBLV type 1b in the Netherlands, France, and Spain; EBLV type 2a in the Netherlands and the United Kingdom; EBLV type 2b very rarely in Switzerland and an untyped EBLV 2 in Finland. Five unvaccinated people with bat bites died of encephalitis indistinguishable from rabies: two in Russia, one in the Ukraine, one in Scotland, and a Swiss zoologist visiting Finland was infected with EBLV 2b. Four new, so far unclassified, lyssaviruses have been found in bats in Eastern Europe.
Australian bat lyssavirus
Australian bat lyssavirus (ABL) (genotype 7) has been found in fruit bats (genus Pteropus) (Fig. 184.108.40.206) and insectivorous bats in Eastern Australia since 1996. It caused a fatal rabies-like encephalitis in two women who had handled bats.
The lyssavirus genotypes have been classified into two phylogroups. Mokola and Lagos bat viruses form phylogroup II and the other lyssaviruses are in phylogroup I. All phylogroup I genotypes have caused fatal rabies-like encephalitis in humans, but experimentally phylogroup II viruses are less pathogenic. This is in keeping with the clinical cases reported. The genetic relationships between the whole genome of the genotypes correlates with the degree of serological cross-protection. Since all rabies vaccines are prepared from genotype 1 rabies virus, protection against ABL, which is closely related to genotype 1, should be undiminished. Protection is less efficient against phylogenetically more distant EBLVs and there is little if any protection against Mokola virus. However, there have been no failures of prophylaxis after exposures to bats, and no other treatment is available. Pre-exposure and postexposure immunization is, therefore, more urgent if exposure to a rabies-related virus infection is suspected.
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