Pathogenesis & Epidemiology
V. cholerae is transmitted by fecal contamination of water and food, primarily from human sources. Human carriers are frequently asymptomatic and include individuals who are either in the incubation period or convalescing. The main animal reservoirs are marine shellfish, such as shrimp and oysters. Ingestion of these without adequate cooking can transmit the disease.
A major epidemic of cholera, spanning the 1960s and 1970s, began in Southeast Asia and had spread over three continental areas of Africa, Europe, and Asia. In 1991, another wave of cholera began in Peru and had spread to many countries in Central and South America. The organism isolated most frequently was the El Tor biotype of O1 V. cholerae, usually of the Ogawa serotype.
The factors that predispose to epidemics are poor sanitation, malnutrition, overcrowding, and inadequate medical services. Quarantine measures failed to prevent the spread of the disease because there were many asymptomatic carriers. In 1992, V. cholerae serogroup O139 emerged and caused a widespread epidemic of cholera in India and Bangladesh.
The pathogenesis of cholera is dependent on colonization of the small intestine by the organism and secretion of cholera toxin. For colonization to occur, large numbers of bacteria must be ingested because the organism is particularly sensitive to stomach acid. Persons with little or no stomach acid, such as those taking antacids or those who have had gastrectomy, are much more susceptible. Adherence to the cells of the brush border of the gut, which is a requirement for colonization, is related to secretion of the bacterial enzyme mucinase, which dissolves the protective glycoprotein coating over the intestinal cells.
After adhering, the organism multiplies and secretes an enterotoxin called choleragen (cholera toxin). This exotoxin can reproduce the symptoms of cholera even in the absence of the Vibrio organisms. The mode of action of cholera toxin is described in the next paragraph and in Figure 1.
Fig1. Mode of action of endotoxin. Endotoxin is the most important cause of septic shock, which is characterized primarily by fever, hypotension, and disseminated intravascular coagulation (DIC). Endotoxin causes these effects by activating three critical processes: (1) activating macrophages to produce interleukin-1 (IL-1), tumor necrosis factor (TNF), and nitric oxide; (2) activating the alternative pathway of complement; and (3) activating tissue factor, an early component of the coagulation cascade.
Choleragen consists of an A (active) subunit and a B (binding) subunit. The B subunit, which is a pentamer composed of five identical proteins, binds to a ganglioside receptor on the surface of the enterocyte. The A subunit is inserted into the cytosol, where it catalyzes the addition of ADP-ribose to the Gs protein (Gs is the stimulatory G protein). This locks the Gs protein in the “on” position, which causes the persistent stimulation of adenylate cyclase. The resulting overproduction of cyclic AMP activates cyclic AMP-dependent protein kinase, an enzyme that phosphorylates an ion transporter (namely, the cystic fibrosis conductance transporter) in the cell membrane. This results in the loss of water and ions, such as Na+, Cl−, K+, and HCO3 from the cell. The watery efflux enters the lumen of the gut, resulting in a massive watery diarrhea that contains neither neutrophils nor red blood cells. Morbidity and death are due to dehydration and electrolyte imbalance. However, if treatment is instituted promptly, the disease runs a self-limited course for up to 7 days.
The genes for cholera toxin and other virulence factors are carried on a single-stranded DNA bacteriophage called CTX. Lysogenic conversion of non–toxin-producing strains to toxin producing ones can occur when the CTX phage transduces these genes. The pili that attach the organism to the gut mucosa are the receptors for the phage.
Non-O1 V. cholerae is an occasional cause of diarrhea associated with eating shellfish obtained from the coastal waters of the United States.
Clinical Findings
Watery diarrhea in large volumes is the hallmark of cholera. There are no red blood cells or white blood cells in the stool. Rice-water stool is the term often applied to the nonbloody effluent. There is no abdominal pain, and subsequent symptoms are referable to the marked dehydration. The loss of fluid and electrolytes leads to cardiac and renal failure. Acidosis and hypokalemia also occur as a result of loss of bicarbonate and potassium in the stool. The mortality rate without treatment is 40%.
Laboratory Diagnosis
The approach to laboratory diagnosis depends on the situation. During an epidemic, a clinical judgment is made and there is little need for the laboratory. In an area where the disease is endemic or for the detection of carriers, a variety of selective media[1] that are not in common use in the United States are used in the laboratory.
For diagnosis of sporadic cases in this country, a culture of the diarrhea stool containing V. cholerae will show colorless colonies on MacConkey’s agar because lactose is fermented slowly. The organism is oxidase-positive, which distinguishes it from members of the Enterobacteriaceae. On TSI agar, an acid slant and an acid butt without gas or H2S are seen because the organism ferments sucrose. A presumptive diagnosis of V. cholerae can be confirmed by agglutination of the organism by polyvalent O1 or non-O1 antiserum. A retrospective diagnosis can be made serologically by detecting a rise in antibody titer in acute- and convalescent-phase sera.
Treatment
Treatment consists of prompt, adequate replacement of water and electrolytes, either orally or intravenously. Glucose is added to the solution to enhance the uptake of water and electrolytes. Antibiotics such as azithromycin are not necessary, but they do shorten the duration of symptoms and reduce the time of excretion of the organisms.
Prevention
Prevention is achieved mainly by public health measures that ensure a clean water and food supply. An oral, live attenuated vaccine called Vaxchora is available in the United States for travelers to areas where cholera caused by serogroup O1 is endemic. Other oral vaccines containing killed organisms or the B subunit of cholera toxin are available in countries where cholera epidemics occur.
The use of tetracycline for prevention is effective in close contacts but does not prevent the spread of a major epidemic. Prompt detection of carriers is important in limiting outbreaks.
[1] Media such as thiosulfate-citrate-bile salts agar or tellurite-taurocholate-gelatin are used.
