Cholera is an infection of the small intestine by the bacterium Vibrio cholerae. The disease causes profuse watery diarrhoea, which can lead to dehydration and death in severe untreated cases.
Cause and incidence
Infection is acquired by ingesting contaminated food or water. Outbreaks of the disease occur regularly in northeast India, but worldwide cholera is controlled by sanitation.
Cholera starts suddenly, between one and five days after infection, with diarrhoea that is often accompanied by vomiting. More than 500 ml of fluid may be lost each hour and, if this fluid is not replaced, severe dehydration and then death may occur within hours. The fluid loss is caused by the action of a toxin produced by the cholera bacterium that greatly increases the passage of fluid from the bloodstream into the large and small intestines.
Treatment is with water containing salts and sugar (see oral rehydration therapy) and, in severe cases, by intravenous infusion. Antibiotic drugs can shorten the period of diarrhoea and infectiousness. After adequate rehydration, affected people usually make a full recovery from the infection.
Cholera is controlled worldwide by the improvement of sanitation, and in particular by ensuring that sewage is not permitted to contaminate water supplies used for drinking. Vaccination against cholera is not recommended. Travellers planning to visit cholerainfected areas are advised to consume only water that has been boiled, or bottled drinks from reliable sources.
Cholera in detail - technical
Vibrio cholerae is a Gram-negative organism that can be subdivided into over 200 serogroups based on the somatic O antigen, with only serogroups O1 and O139 causing epidemic and pandemic disease. Historically it has killed millions from dehydrating diarrhoea, encouraged the birth of modern epidemiology, the sanitary revolution, and oral rehydration therapy; it persists today as a glaring reminder of poverty and inadequate water/sanitation. Contaminated food (especially undercooked seafood) is the usual route of transmission in developed countries; contaminated water and street food vendors are more common vehicles in less developed countries.
Clinical features and diagnosis—typical presentation is with sudden onset of voluminous, painless, watery diarrhoea, which can exceed 500 to 1000 ml/h, leading to severe dehydration in a couple hours and risk of death. Definitive diagnosis is by isolating V. cholerae from stool or rectal swab samples.
Treatment—oral rehydration therapy with sugar or starch, water, and salts must be provided in the community and at field stations, clinics, and hospitals where most patients present: this reduces the case fatality of untreated severe cholera from about 50% to 1% or less. Antibiotics can shorten the illness and decrease diarrhoeal purging: tetracycline, cotrimoxazole, ciprofloxacin, or azithromycin have been effective, but there is increasing resistance.
Prevention—effective preventive measures include (1) ensuring a safe water supply; (2) improving sanitation; (3) making food safe for consumption by thorough cooking of high-risk foods, especially seafood; and (4) health education through mass media. Two oral cholera vaccines can provide significant (but not complete) protection.
Introduction and historical perspective
Cholera, the dreaded scourge causing death from dehydrating diarrhoea, existed for centuries in South Asia until, in 1817, it broke out along trade routes; since then there have been seven pandemics across all six inhabited continents. Cholera was largely responsible for encouraging the birth of modern epidemiology and for driving the sanitary revolution in Western Europe and North America in the 19th century. In the last one-third of the 20th century, it helped drive scientific discoveries of cell signalling, intestinal ion transport, and oral rehydration therapy (ORT), which have brought global diarrhoea mortality down from over 5 million/year to below 2 million/year. Yet cholera persists today as a disease of poverty, along with other faecally transmitted pathogens, a sign of inadequate water and sanitation among the desperately poor and displaced around the world.
Aetiology, gentics, and pathophysiology
Thirty years before the causative agent Vibrio cholerae was discovered during the fifth pandemic in 1884 in Kolkata, India, by Robert Koch, John Snow’s classic epidemiological study of cholera in London in 1854 suggested that it was transmitted by contaminated drinking water. Snow even postulated that a toxin might cause the dramatic fluid loss. V. cholerae is a halophilic flagellated curved Gram-negative organism classified by biochemical tests and further subdivided into serogroups based on the somatic O antigen. Among over 200 serogroups, only O1 and O139 cause epidemic and pandemic disease. The other strains are classified as non-O1 and non-O139 V. cholerae. Serogroup O1 is further subdivided into three serotypes, Inaba, Ogawa, and Hikojima, and into two phenotypically different biogroups, Classical and El Tor (named for the Egyptian village quarantine station where it was first isolated in 1905 from Indonesian pilgrims travelling to Mecca). This strain then became the cause of the seventh pandemic that continues around the world today. The O139 serogroup, first seen in 1992, appears to have emerged from horizontal gene transfer of a fragment of DNA that encodes O-antigen biosynthesis from another serogroup (perhaps O22) into the seventh pandemic V. cholerae O1 El Tor strain. O139 and O1 (both Classical and El Tor biotypes) now coexist and continue to cause large outbreaks in India and Bangladesh. V. cholerae O1 (biotype El Tor) has two circular chromosomes and the entire genome sequence was recently described. The large chromosome has most of the genes required for growth and pathogenicity and the small chromosome encodes components of several essential metabolic and regulatory pathways.
Critical to the pathogenicity of V. cholerae (and distinct from environmental isolates) is the acquisition of two distinct phages. The first contains a ‘pathogenicity island’ (VPI) encoding the ‘toxin coregulated pilus’ (TCP). Remarkably, TCP serves as both a major intestinal colonization factor and as the receptor for the second phage, CTXϕ, that encodes for cholera toxin and accessory proteins (including ACE and Zot) as well as containing genes required for phage replication, integration, and regulation in the RS2 region. Genes encoding colonization factors or toxin are regulated in response to environmental conditions. The 32-kDa transmembrane protein ToxR binds upstream of ctxAB to increase transcription and synthesis of cholera toxin. ToxR also regulates the expression of other genes in the ToxR regulon; hence, the expression of ToxR is controlled by environmental factors.
Vibrios are acquired from contaminated water or food and they must pass though the acidic stomach before they are able to colonize the upper small intestine. Colonization occurs with filamentous protein fimbriae, called toxin coregulated pili, which extend from the vibrio wall and attach to receptors on the mucosa. V. cholerae adhere to the M cells without causing tissue damage and rapidly multiply to 107 to 108 cells/g of tissue. Attached vibrios efficiently deliver cholera toxin directly to the epithelial cells. The A subunit consists of two peptides linked by a disulphide bond. The larger, A1, containing the toxic activity, is endocytosed following toxin binding via its B subunit to GM1 ganglioside. A1 subunit catalyses the covalent bonding of adenosine diphosphoribose from nicotinamide adenosine dinucleotide to the α-subunit of Gs, the heterotrimeric adenylyl cyclase-stimulating G protein, thus activating adenylate cyclase to form cAMP. cAMP then acts to open the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel causing increased chloride secretion by the intestinal crypt cells and a blockade of neutral sodium and chloride absorption by villous cells. This leads to voluminous fluid efflux into the small intestinal lumen which exceeds the absorptive capacity of the bowel and results in watery diarrhoea. The diarrhoeal fluid contains large amounts of sodium, chloride, bicarbonate, and potassium, but little protein or blood cells. The loss of electrolyte-rich isotonic fluid leads to blood volume depletion with attendant low blood pressure and shock. Loss of bicarbonate and potassium leads to metabolic acidosis and potassium deficiency.
Ever since Snow’s seminal epidemiological treatise, cholera has been described as the classic water-borne disease. However, it is also transmitted by contaminated food, especially undercooked seafood or food mixed with contaminated water. Contaminated food (especially undercooked seafood) is the usual vehicle for transmission in developed countries, and contaminated water and street food vendors are more common vehicles in less developed countries. V. cholerae is found in brackish surface water and in shellfish, and survives and multiplies in association with zooplankton and phytoplankton independently of infected human beings. There is no known other animal reservoir for V. cholerae.
V. cholerae is endemic in the Indian subcontinent and the re-emergence of cholera in other continents is highly dependent on environmental factors. The association of the bacteria with plankton has led to the suggestion that ship ballast is a cause of its global spread. V. cholerae has evolved to survive in the aquatic environment and then in the host. In water, V. cholerae vibrios are free swimming or attached to plants, green algae, copepods, crustaceans, or insects. In humans, the intestinal milieu fosters the acquisition of genetic elements from the TCP bacteriophage, lacking in most environmental strains. TCP phage encodes type IV fimbria which serves as colonization factor and receptor for the CTX phage that carries genes encoding cholera toxin. Thus both bacteriophages integrate into the bacterial genome and form episomal replication intermediates. The production of cholera toxin and the biogenesis of CTX phage both depend on a type II secretion apparatus, encoded within the bacterial genome. In Bangladesh and Peru, where the disease has been endemic and epidemic, cholera tends to occur in the warm seasons albeit before and after the monsoon rains in Bangladesh.
Most V. cholerae infections are asymptomatic (case:infection = 1:3 to 1:100) or associated with mild nonspecific diarrhoea. Since a high inoculum dose is required for infection, person-to-person infection is rare without intervening water or food contamination. Infection and its severity also depend on the gastric acid barrier, local intestinal immunity, and blood group. Those with blood group O are at higher risk of severe El Tor cholera than are those with other blood groups. This susceptibility may explain the lower prevalence of blood group O in the Ganges delta area. In cholera-endemic areas, the highest attack rates are in children aged 2 to 4 years. In newly invaded areas, attack rates are similar for all ages. First illnesses are often seen in adult men, presumably because of greater exposure to contaminated food and water.
The current seventh pandemic began in 1961, in Sulawesi (Celebes), Indonesia. By 1966 the disease had spread to other countries in eastern Asia including Bangladesh, India, the former Union of Soviet Socialist Republics, Iran, and Iraq. Cholera reached West Africa in 1970, and in 1991 it appeared in Latin America for the first time in more than a century. Until 1992 only serogroup O1 had been implicated in epidemics while other serogroups had caused only sporadic cases of diarrhoea. However, in late 1992 cholera broke out in India and Bangladesh caused by a previously unrecognized serogroup of V. cholerae, designated O139. It is unclear whether this new serogroup from Southeast Asia will spread to other regions of the world. It is estimated that 120 000 people die from cholera worldwide each year.
All cases of suspected cholera should be reported to local and national health authorities, since cholera outbreaks can become massive epidemics. These cases should be confirmed by laboratory investigation. If a patient older than 5 years develops severe dehydration or dies from acute watery diarrhoea, or if there is a sudden increase in the daily number of patients with acute watery diarrhoea, a cholera outbreak should be suspected.
Prevention and vaccines
Since contaminated water and food are the main vehicles of transmission, effective preventive measures include ensuring a safe water supply (especially for municipal water systems), improving sanitation, making food safe for consumption by thorough cooking of high risk foods (especially seafood), and providing health education through mass media (Bullet list 1).
Two safe and well-tolerated oral cholera vaccines that provide significant protection have been licensed for commercial use:
- 1 Killed whole cell V. cholerae plus recombinant B subunit of cholera toxin vaccine given as two doses 1 to 6 weeks apart (rCTB-WC; Dukoral). In Dukoral the A subunit of cholera toxin is deleted.
- 2 Live attenuated V. cholerae O1 strain, CVD 103-HgR vaccine given as a single dose (Orochol; known as Mutachol in Canada). In Orochol, the gene for encoding the A subunit of cholera toxin has been largely deleted.
Neither vaccine is licensed in the United States of America.
Twenty-five trials of oral cholera vaccines were reviewed to assess the effect of cholera vaccines in preventing cases of cholera and preventing deaths. Eighteen efficacy trials of relatively good quality, testing parenteral and oral killed whole cell vaccines and involving over 2.6 million adults, children, and infants were included. Eleven safety trials were conducted using killed whole cell vaccines and involving 9342 people. The efficacy of the killed whole cell vaccines compared to placebo to prevent cholera at 12 months was 49%. Both parenteral and oral administrations were effective, although killed whole cell vaccines had a significant protection extended in older children and adults. Parenteral killed whole cell vaccines were associated with increased systemic and local adverse effects compared to placebo. Oral killed whole cell vaccines were not associated with adverse events compared to placebo. In conclusion, killed whole cell cholera vaccines are relatively effective and safe. Because vaccine efficacy is overcome by larger infectious doses, vaccine should be seen as synergistic with improvements in water and sanitation that reduce the numbers of vibrios ingested.
Bullet list 1 Prevention of cholera
- Ensure a safe water supply.
- Wash hands after defecation and before food preparation.
- Improve sanitation, making water and food safe for consumption.
- Provide health education through mass media.
- Vaccination and improvements in sanitation work synergistically.
Recently an oral killed whole cell low cost 2-dose vaccine has been modified to comply with WHO standards, and it was safe and provided 67% protection in 2 years at all ages above 1 year old.
The incubation period of cholera usually ranges from 18 h to 5 days. There is a sudden onset of voluminous watery diarrhoea with occasional vomiting. Diarrhoea is severe in 5 to 10% of those infected. Its most distinctive feature is the painless purging of voluminous stools resembling rice-water with a fishy odour. The vomitus is generally a watery and alkaline fluid. Severe diarrhoea can exceed 500 to 1000 ml/h, leading to severe dehydration in 2 h and risk of death. Dehydration can be classified based on the presence and severity of clinical findings (Table 1). Signs of severe dehydration include absent or low-volume peripheral pulse, undetectable blood pressure, poor skin turgor, sunken eyes, and wrinkled hands and feet. Metabolic acidosis can develop and lead to gasping (Kussmaul) breathing. Urine output is diminished or absent until dehydration is corrected.
|Table 1 Assessment of patients with diarrhoea for dehydration|
|Feature||No dehydration||Some dehydrationa||Severe dehydrationa,b|
|General appearance||Well, alert||Restless, irritable||Lethargy or unconscious; floppy|
|Eyes||Normal||Sunken*||Very sunken and dry*|
|Mouth and tongue||Moist||Dry*||Very dry*|
|Thirst||Drinks normally, not thirsty||Thirsty, drinks eagerly||Drinks poorly or not able to drink|
|Skin pinchc||Goes back quickly||Goes back slowly||Goes back very slowly|
a Two or more of these signs including one indicated by *.
b Absence of radial pulse and low blood pressure are also signs of severe dehydration in adults and children older than 5 years.
c The skin pinch is less useful in patients with marasmus (severe wasting), kwashiorkor (severe malnutrition with oedema), or in obese patients.
(From Azurin JC, et al. (1967). A long-term carrier of cholera: cholera Dolores. Bull World Health Organ, 37, 745–9.)
Complications generally result from inadequate fluid replacement, acute renal failure due to protracted hypotension, hypoglycaemia, hypokalaemia, and cramps due to electrolyte imbalance.
Most cases are indistinguishable from other cases of diarrhoeal diseases, but since the treatment of any dehydrating diarrhoea is the same—fluid replacement—identification of the pathogen is not essential for patient management. However, if an adult patient becomes severely dehydrated and is in the right epidemiological setting or with a history of travelling, the clinician and public health authorities should be alert to the possibility of cholera.
Criteria for diagnosis
Definitive diagnosis is by isolating V. cholerae from stool or rectal swab samples on selective media. V. cholerae survives in faecal specimens if kept moist. Cary–Blair transport medium should be used for transport to the laboratory for plating onto thiosulphate citrate bile salts sucrose (TCBS) agar that inhibits most other normal faecal flora but supports the growth of the vibrios. Specimens should also be inoculated into alkaline peptone water, an enrichment broth that preferentially supports the growth of vibrios. After 6 to 12 h of incubation, a second TCBS plate is inoculated. These plates are incubated for 18 to 24 h, and V. cholerae colonies appear as smooth yellow colonies with slightly raised centres. V. cholerae is a Gram-negative polar monotrichous oxidase-positive asporogenous curved rod that ferments glucose, sucrose, and mannitol and is positive in the lysine and ornithine decarboxylase tests. The organism is classified by biochemical tests and is further subdivided into serogroups based on the somatic O antigen. Presumptive identification of V. cholerae O1 or O139 can be made on the basis of typical colonies, which are oxidase-positive and agglutinate with O1 or O139 antiserum.
Rapid tests include dark-field microscopy and rapid immunoassays which can be useful for monitoring epidemiological patterns in remote areas where cultures are not readily available. New outbreaks must be confirmed by cultures. Polymerase chain reaction (PCR) and DNA probes are available but are not practicable in many areas where cholera is common.
Treatment must be provided at the community and field stations, clinics, and hospitals where most of the patients present. ORT was a major therapeutic breakthrough that has drastically decreased mortality from cholera and other dehydrating diarrhoeal diseases. The case fatality rate of untreated severe cholera approaches 50%, but with ORT it is decreased to 1% or less. The physiological basis for ORT is the Na+-coupled transport with glucose; transport from the enterocyte to the lateral intercellular space creates a local osmotic gradient that initiates water flow. The oral rehydration salts (ORS) formulation approved by the World Health Organization (WHO) is based on the electrolyte composition lost in stool in patients with cholera. Table 2 ;summarizes the electrolyte concentrations from cholera stool and several oral rehydration formulations, including that approved and recommended by the WHO.
Table 1 summarizes the clinical assessment and management of patients with mild, moderate, or severe dehydration. In all cases the key is to rapidly replace fluid deficits, correct metabolic acidosis and potassium losses, and to continue replacing ongoing fluid losses. Because cholera toxin has prolonged effects, it is imperative to continue replacing fluid losses, for which a ‘cholera cot’ with a central hole, plastic sheet, and bucket to monitor purging can be tremendously helpful to both the patient and medical attendants.
Five to 7.5% of the bodyweight should be given as ORS with additional ORS to compensate for other losses. In patients who are severely dehydrated, having lost at least 10% (5 litres for a 50-kg patient) of their bodyweight, volume replacement must be rapid. Lactated Ringer’s solution is an excellent commercially available intravenous fluid. Other polyelectrolyte solutions with added potassium can also be used. Since ORS is the best polyelectrolyte solution to compensate for the acidosis and potassium deficiency, they should be given as soon as possible after initial intravenous fluid resuscitation.
A formulation of ORS that uses rice rather than glucose is better for cholera patients because it reduces the purging rate by providing polymeric glucose with lower osmolarity. ORS have been modified to prevent hypernatraemia (more common with other diarrhoeas) by having a reduced concentration of sodium (75 mmol/litre). This hypo-osmolar solution is also acceptable for cholera. ORS are easily prepared by adding the following simple ingredients to 1 litre water: 2.6 g sodium chloride, 2.9 g trisodium citrate, 1.5 g potassium chloride, and 13.5 g glucose (or 50 g boiled and cooled rice powder).
Adults and children are encouraged to eat, and breastfeeding can continue as there is no scientific basis for resting the gut.
Antibiotics can shorten the illness and decrease diarrhoeal purging. One- to 3-day courses of tetracycline, co-trimoxazole, or ciprofloxacin have been effective but there is increasing resistance. Azithromycin has been used more recently, but growing macrolide resistance may limit its use as well. Antibiotic sensitivity testing is therefore recommended during outbreaks. Antibiotics are not indicated for asymptomatic contacts. Prophylactic use of antibiotics increases the risk of the development of resistance and it is not indicated to prevent cholera.
Case fatality should be 1% or less if adequate ORT is used early in the illness, even at the community level. Adequate fluid and electrolyte replacement reverses or prevents complications such as acute renal failure or hypoglycaemia even in moderate or severe cholera. Cholera may well persist in its brackish marine reservoir, but improved water and sanitation and increasingly available vaccines promise to control this dreaded disease.
|Table 2 Composition of cholera stools and electrolyte rehydration solutions used to replace stool losses|
|Fluid||Sodium (mmol/litre)||Chloride (mmol/litre)||Potassium (mmol/litre)||Bicarbonate (mmol/litre)||Carbohydrate (g/litre)||Osmolality (mmol/litre)|
|Oral rehydration salts|
a Trisodium citrate (10 mmol/litre) is generally used, rather than bicarbonate.
b Glucose 13.5 g/litre (75 mmol/litre).
c Depending on degree of hydrolysis, 30–50 g rice contains about 30 mmol/litre glucose.
d Base is lactate.
e Base is acetate.
(From Sack DA, et al. (2004). Cholera. Lancet, 363, 223–33.)
Other issues (health economics, areas of uncertainty or controversy, and likely developments ahead)
Areas of uncertainty or controversy include the mechanisms and importance of natural reservoirs of cultivable and even noncultivable vibrios and marine organisms from plankton to shellfish in the ecology of cholera. Despite the remarkable advances in understanding the pharmacological mechanisms of cholera toxin action, reliable, effective, and inexpensive means of blocking the effects of the toxin remain elusive.
With molecular genetic understanding of virulence and protective immunity, likely developments in the near future include the promise of new and better vaccines, toxin-blocking or absorption-enhancing drugs, and continued improvements in ORT, perhaps with nutrients, micronutrients, or probiotics that compete with vibrio colonization or deliver proabsorptive drugs or nutrients.