Chapter 260. Cholera

Cholera is an acute life-threatening disease characterized by enormous loss of fluid and electrolytes due to profuse diarrhea and vomiting. This waterborne disease has been responsible for global scourges for centuries; it is often seen in the wake of disaster situations, both man-made and natural.

Descriptions resembling cholera exist in ancient Sanskrit literature (500 BC to 400 BC) from India.1 Until the early 19th century, this disease was primarily confined to the Indian subcontinent, especially the Ganges delta region. The first pandemic of cholera in 1817–1823 began with spread of disease outside the Indian subcontinent along trade routes to the west as far as southern Russia. The second pandemic started in 1826 and it reached the major European cities by the early 1830s and the eastern United States in 1832. This pandemic continued for 19 years. John Snow, a prominent London physician, observed a correlation between the disease and the source of public water, leading to the control of the epidemic by removing the handle of the Broad Street water pump. During the third pandemic (1852–1859), Filippo Pacini described the curved bacilli from the intestinal contents of cholera patients and named them Vibrio cholerae.1,2 Later, Robert Koch proved that the curved bacillus was the cause of cholera. The seventh pandemic, which began in 1961, is the most well studied.3 Unlike the previous pandemics, which were caused by classical V cholerae O1 biotype; the V cholerae O1El Tor biotype, which first appeared in Indonesia in 1937, caused the seventh pandemic. This strain began to spread to India in 1964, Middle East in 1965, Africa and southern Europe in 1970, and South America in 1991. This pandemic led to the use of oral rehydration solution (ORS) to treat cholera. Early during this pandemic, mortality rates were approximately 50% in Africa. Later, with rapid response time and effective treatment, less than 1% mortality rate was observed in Peru.4 Until 1992, only V cholerae O1 serogroup caused epidemic cholera. Some other serogroups could cause sporadic cases, but not epidemic cholera. However, the most recent outbreak of cholera in 1992 in Chennai (formerly Madras), India, was caused by a new V cholerae serogroup, designated O139 or Bengal strain. This strain rapidly spread from India to Bangladesh and later to other 11 countries in Asia. If this strain continues to spread throughout the world, then it would be the eighth cholera pandemic.1,5,6 The outbreaks caused by V cholerae O139 have spread rapidly even among the elderly who were previously exposed to cholera caused by V cholerae O1, suggesting that the immunity to V cholerae O1, whether from natural infection or vaccination, is not protective against V cholerae O139. Serogroups O139 and O1 now coexist and continue to cause large outbreaks of cholera in the Indian subcontinent.

Endemic cholera caused by V cholerae O1 primarily affects children. For instance, in Bangladesh the infection was most prevalent among children in the 2- to 4-year age group; however, V cholerae O139 has been reported in infants as young as a 4-day-old newborn with diarrhea. Endemic cholera cases mainly occur during the summer and monsoon months in the Indian subcontinent.7,8 In the United States, the average number of cholera cases during 2001 to 2005 was 4.6 cases per year, all of which were caused by V cholerae O1.9 Approximately one third of those infected acquired the infection overseas, one third acquired the infection by consumption of raw or undercooked domestic seafood, and one-third were residents of Guam with no identified sources. Shellfish harvested from the US gulf coast after Hurricane Katrina was the source of illness for certain cases of V cholerae O1 infection reported in 2005.10

Microbiology and Pathophysiology

V cholerae is a gram-negative, highly mobile, curved bacillus with a single polar flagellum. There are at least 200 somatic serogroups based on somatic (O) antigens. Until 1992 only strains of serogroup O1 were associated with epidemic cholera. Other non-O1-V cholerae serogroups result in sporadic diarrhea and occasionally cause a variety of severe extraintestinal infections including wound infections and sepsis, especially among patients with liver disease or immunosuppression. The V cholerae O1 are divided into three serotypes, Ogawa, Inaba, and Hikojima based on specific antigenic determinants. Ogawa strains produce the A and B antigens and a small amount of C antigen, whereas Inaba strains produce only the A and C antigens, and Hikojima strains produce all 3 antigens. The V cholerae O1 are further divided into 2 biotypes, classical and El Tor. V cholerae O139, first identified in 1993, represents a mutation of O1 antigen in V cholerae O1 El Tor strain, and, therefore, resembles V cholerae O1 El Tor strain.1

V cholerae is oxidase positive. It is a slow lactose fermenter, and also ferments glucose and sucrose. It grows easily in alkaline media in the presence of bile salts and is exquisitely sensitive to acid and to drying. Cholera is caused only by cholera toxin-producing strains of O1 or O139 serogroups. Nontoxigenic strains of V cholerae are occasionally isolated from patients with diarrhea or sepsis. The organism can be isolated on a variety of culture media, but a selective media, such as thiosulfate-citrate-bile salt sucrose (TCBS) agar, is recommended. On TCBS agar, V cholerae appear as 2- to 4-mm large, smooth, round, yellow colonies with opaque centers and translucent edges that contrast with the blue-green agar. Suspicious colonies can be rapidly identified by agglutination tests utilizing specific V cholerae O1 and O139 antisera.

The current characterization of the ecology of cholera includes global weather patterns, aquatic reservoirs, bacteriophages, zooplankton, the collective behavior of surface-attached cells, an adaptable genome, and the deep sea, together with the bacterium and its host.11 Strains of V cholerae have been isolated from aquatic environments in areas where the disease is endemic as well as where no cholera cases occur. The organism is well adapted to warm stagnant environments with increased salinity. The ecological relationship of cholera and planktonic copepods was first described in 1983; V cholerae attach to live copepods in the Chesapeake Bay and around Bangladesh. This suggests that cholera is probably not an eradicable disease, because its causative organism lives naturally in riverine, brackish, and estuarine ecosystems. Outside of the host and in the aquatic phase, V cholerae can be found as free-swimming cells attached to plants, filamentous green algae, copepods, crustaceans, insects, and egg masses of chironomids.1,11 Biofilm formation and entry into a viable but nonculturable state in response to nutrient deprivation are thought to be important in facilitating environmental persistence during periods between epidemics. Although V cholerae is part of the normal estuarine flora, toxigenic strains are mostly isolated from the environment in areas where infected individuals are present. There are two crucial steps in the evolution of a pathogenic V cholerae. First, the nontoxigenic strains have to acquire the vibrio pathogenicity island (VPI) that carries genes for the pilus colonization factor (Tcp). Second, having acquired a specific receptor, the Tcp-positive strains are infected with and lysogenized by a bacteriophage designated Ctx that carries the genes encoding cholera toxins. Experiments in animal models have shown that the intestinal milieu is the site where strains acquire these elements effectively.

Bacteria easily grow on chitinous shellfish which explains the reports of cholera following ingestion of seafood. Crabs harvested from the US gulf coast are a common source of cholera, especially during warmer months when environmental conditions favor the growth and survival of V cholerae in blackish and coastal water.1,12-14 The classic fecal–oral paradigm is a well-known means of transmission; however, contamination of a variety of foods by food handlers or contamination of fresh fruit and vegetables by cholera-contaminated water has also been attributed to the spread. Healthy individuals shedding the organism also contribute by contaminating food items as well as the environment. For more developed countries, contaminated food, especially undercooked seafood, is the usual vehicle for transmission, and contaminated water is more common in less-developed countries.

Cholera has pronounced seasonality. In Bangladesh, 2 peaks occur each year corresponding to the warm seasons before and after the monsoon season. In Peru, epidemics are strictly confined to the warm season. The seasonality seems to be related to the ability of Vibrios to grow rapidly in a warm environment. Other than shellfish and plankton, there are no animal reservoirs.

Several genes are responsible for the virulence of V cholerae. Only epidemic strains of V cholerae O1 and V cholerae O139 possess the ctxA and ctxB genes that encode for enterotoxin subunits A and B.1,15 Persons with blood type O are more susceptible to contract cholera as well as are likely to have a severe illness.16 Similarly, persons with low gastric acidity are more prone to disease. Because V cholerae are sensitive to acid, a large inoculum (> 108 bacteria) is required to cause the disease. Considerably lower inoculum size (105 bacteria) may produce disease if given with antacids or in individuals with less gastric acidity. Once in the small intestine, the rapid penetration of the intestinal mucous coat is achieved through the organism’s motility, chemotaxis, and elaboration of mucinase, which enables the organism to penetrate the mucus barrier that covers the intestinal epithelium. The toxin coregulated pilus A gene, TcpA, governs effective colonization. V cholerae adheres to enterocytes without invading the mucosal barrier, and thus avoids peristaltic expulsion. Adherent bacteria multiply and elaborate an enterotoxin, also known as cholera toxin or choleragen. The toxin is composed of one A subunit, which is further divided into A1 and A2 and 5B subunits. A regulatory gene called ToxR coregulates ctxA, ctxB, and TcpA genes. The zonula occludens toxin gene zot also plays a critical role in pathogenesis as it makes the tight cell junctions leaky.15

The B subunits of cholera toxin bind the toxin to GM1 ganglioside receptors on enterocytes, after which A1 subunit enters the cell. The A1 subunit irreversibly activates the adenylate cyclase system in the mucosa, leaving it in the “on” position, leading to increases in the intracellular concentration of cyclic AMP, which results in sodium and water loss. Thus, the active secretion of sodium and chloride into the gut lumen with water following it passively results in secretion of isotonic fluid into the small intestine surpassing the absorptive capacity of the colon. This results in volume depletion and shock. Loss of bicarbonate and potassium also occurs, and this leads to metabolic acidosis and hypokalemia. Influencing the sodium output of the organism by cholera toxin also contributes to the survival of V cholerae in environments with a low level of salinity.1,15

Clinical Features

The incubation period for cholera is short, ranging from 6 hours to 5 days, with most cases occurring between 1 and 3 days. Toxigenic strains of V cholerae O1 and O139 produce infections ranging from asymptomatic infection to a severe fatal illness. Most infected individuals have no symptoms, and approximately 25% develop mild to moderate symptoms, while less than 5% of infected individuals develop the classic symptoms of cholera. Symptoms are generally abrupt and include watery diarrhea and vomiting. Anorexia and mild abdominal pain may precede the onset of diarrhea. Initially there is brownish fecal matter in the liquid stools. Once the diarrhea becomes copious, the stools are pale gray in color with a faint fishy smell and contain mucous flecks giving them a classic “rice water” look.1,2,6 Vomiting may occur after the onset of diarrhea. The patient may be normothermic, hypothermic, or may have a low-grade fever. Because of the massive amount of fluid and electrolyte loss, occasionally exceeding 1 liter per hour, severe dehydration and shock may develop within a few hours. Signs of severe dehydration include absent or low-volume peripheral pulse, undetectable blood pressure, poor skin turgor, sunken eyes, and reduced urine output. A listless detached mental status is common. Wrinkling of the skin over hands and fingers described as “washer woman’s hands” indicates severe depletion of extracellular fluid. Patients at first are restless and extremely thirsty, but as shock progresses, they become apathetic and may lose consciousness. Many also show respiratory signs of metabolic acidosis with Kussmaul gasping breathing. If not treated vigorously and promptly, fluid loss may be so rapid that the patient is at risk of death within hours of the onset of symptoms. If rehydration fluids are provided in insufficient quantities, the patient may survive temporarily, only to die a few days later.

Children may also develop seizures, hypoglycemia, and may become unconscious. Hyponatremia and hypokalemia are more pronounced in children because of the greater loss of sodium and potassium in stools.7,8 Hypokalemia may cause severe muscle cramps, marked weakness, and hypotonia as well as ileus and cardiac arrhythmias. Acute renal failure from protracted hypotension may develop if insufficient fluids are given. Many patients may have hypoglycemia and a few may have severe hypoglycemia. In pregnant women, cholera can be severe, resulting in very high fetal mortality and premature labor as a complication of shock and placental hypoperfusion.

Diagnosis

The clinical diagnosis of cholera is suggested by the acute onset of profuse watery diarrhea in the absence of significant abdominal pain and systemic manifestations. A monoclonal antibody coagglutination test for direct detection of V cholerae O1 in fecal specimens is available. The test is sensitive and specific, does not require culturing of fecal specimens, and can be completed in less than 5 minutes. Rapid etiologic diagnosis can also be made by dark field or phase microscopy. Actively motile Vibrio organisms are seen in large numbers in stool. Adding specific antisera for V cholerae O1 and O139 to the sample will immobilize and extinguish the characteristic “shooting star” movement of the Vibrio. Rectal swab or stool culture in Carry-Blair transport medium should be used for transportation of the specimen to the laboratory. The sample should then be placed onto TCBS agar. In addition, the specimen should be inoculated into alkaline peptone water, a high pH enrichment broth, which preferentially supports the growth of Vibrio. After 6 to 12 hours of incubation in the enrichment broth, a second TCBS plate is inoculated. The subsequent identification of the organism is established by agglutination test with O1 or O139 specific antisera. Molecular methods including polymerase chain reaction assay and DNA probes are also available but are not widely used and are not practicable in many areas where cholera is common.

Treatment

Without appropriate treatment, the case-fatality rate for severe cholera can be as high as 50%. The primary therapy is replacement of fluid and electrolyte losses as outlined in Chapter 385.1,6,12 Antibiotic therapy is of secondary importance.

Altered sensorium and convulsions should prompt investigation and management of hypoglycemia, hypernatremia, or cerebral edema. Replacement of potassium may be indicated even with anuria to prevent hypokalemia-induced cardiac arrhythmias. Conservative treatment of renal failure is desired.

Antibiotic treatment for 2 to 3 days will shorten the clinical course from 4 to 5 days to 2 to 3 days, reduce the volume requirement for rehydration, and decrease the period of bacterial excretion. Tetracycline, 50 mg/kg per day given every 6 hours, or a single-dose doxycycline is the antibiotic of choice. V cholerae is also generally sensitive to trimethoprim-sulfamethoxazole, erythromycin, azithromycin, ciprofloxacin, furazolidone, chloramphenicol, and the aminoglycosides. The rapid emergence of multidrug-resistant isolates all over the world is of grave concern.12 In an antibiotic susceptibility of El Tor biotype from India, only 46% of isolates were susceptible to tetracycline, 54% to chloramphenicol, 24% to furazolidone, 30% to trimethoprim-sulfamethoxazole, and 80% to cefotaxime. The recently identified V cholerae O139 is susceptible to tetracycline but has been reported to be resistant to trimethoprim sulfamethoxazole and furazolidone; the mechanism of resistance is due to the novel conjugative, self-transmissible, chromosomally integrating SXT element (a colinstin) that confers resistance to sulfamethoxazole, trimethoprim, chloramphenicol, and low levels of streptomycin. Quinolones generally have excellent activity against V cholerae, but fluoroquinolone-resistant strains have been reported from India. Therefore, during an outbreak, samples from representative patients should be tested for antibiotic susceptibility in order to select the most appropriate antibiotic on the basis of current sensitivity patterns. Antibiotics should not be given to asymptomatic contacts because that could increase the risk of development of resistance and is not cost-effective.

Prevention

The stool of cholera patients contains high concentrations of the organism, and they are highly infectious. When passed into the environment, they can contaminate water sources and food which are the main vehicles of transmission. Sanitation, a safe water supply, and good personal hygiene are of primary importance in controlling cholera, but these measures are often difficult to implement in poor, developing countries.

Clinical cholera confers effective and long-lasting immunity. The protection stems from the development of local vibriocidal antibodies directed against the V cholerae cell wall lipopolysaccharide and antitoxin antibodies directed against the cholera toxin B subunits. Either class of antibody is protective; together they exert synergistic protective effects. Hence, the ideal vaccine should stimulate production of both types of antibody in the intestinal mucosa.

Two oral cholera vaccines have been licensed for commercial use in many countries except the United States: the killed whole cell V cholerae O1 plus recombinant B subunit of cholera toxin vaccine (rCTB-WC; Dukoral; SBL Vaccin AB, Stockholm, Sweden) and the live attenuated V cholerae O1 strain vaccine (Orochol; Berna Biotech, Berne, Switzerland; known as Mutachol in Canada) (Table 260-1).

The rCTB-WC vaccine does not contain the A subunit toxin and, therefore, no pathogenic toxin is present. A large randomized, double-blinded, placebo controlled trial in Peru evaluated the rCTB-WC vaccine given as 2 doses following by a booster dose at 10 months.17 Protection starts 10 days after the second dose. The overall protective efficacy after 3 doses was 61%. The efficacy against severe cholera requiring hospitalization was 82%. The vaccine efficacy was 52% for children 2 to 5 years old, 46% for those 6 to 15 years old, and 72% for those older than 15 years. In a report of mass vaccination using a 2-dose regimen of rCTB-WC vaccine before cholera outbreak in Mozambique, receipt of 1 or more doses of the vaccine was associated with 78% to 84% protection.19 The vaccine was found to be equally effective in children younger than 5 years of age and in older persons. The rCTB-WC vaccine has been shown to confer moderate immunity up to 3 years in adults. Young children, however, develop immunity that lasts less than 1 year. Therefore, the booster interval is shorter for children. This vaccine is currently recommended for use in refugee settings at risk of cholera.

The live attenuated V cholerae O1 strain vaccine (Orochol) is derived from wild type classical V cholerae O1 with 94% of the gene encoding the A subunit deleted (strain CVD 103 HgR). It has been shown to be immunogenic and safe; it is minimally excreted in the feces of the vaccinated children.20 Several research trials of adults, children, and infants have demonstrated the safety and immunogenicity of a single dose of CVD 103-HgR vaccine. A single dose provides complete protection against moderate or severe cholera caused by either El Tor or classical V cholerae O1 within 8 days of vaccination. Although protection was observed in a challenge study,21 a large field trial undertaken in Indonesia failed to demonstrate convincing protection.22 The production of this vaccine, although licensed, was stopped in 2004. Cholera immunization is not required for travelers entering the United States from cholera-affected areas, and the World Health Organization no longer recommends immunization for travel to or from areas with cholera infection.23 The recent identification of V cholerae O139 as another cause of epidemic cholera poses an additional hindrance in the development of the optimal cholera vaccine, because protective antibodies against V cholerae O1 do not protect against cholera caused by V cholerae O139. Furthermore, oral vaccines will not prevent all cases of cholera because local intestinal immunity can be overcome with a high bacterial inoculum.20,24

The vaccine program could work synergistically with sanitation programs; the inoculum needed to cause disease would be raised, and the numbers of pathogenic organisms entering the environment would be decreased.