Cholera is considered as a gastro-intestinal disease. An acute, secretory diarrhea caused by infection with Vibrio cholerae of the O1 and O139 serogroups. This bacterium is transmitted via contaminated food or water that has come in contact with fecal matter of the infected person. In some severe form, cholera can be a very terrifying illness in which profuse painless watery diarrhea and copious effortless vomiting may lead to hypovolemic shock and death in less than 24 hours, if untreated. Management of patient with cholera include aggressive fluid replacement, antibiotics. Prevention include safe water and good sanitary conditions. Two oral vaccines are available. Researchers have estimated that each year there are approximately 1.3 million to 4.0 million cases of cholera, and 21 000 to 143 000 deaths occurring in world due to cholera. Total of seven cholera pandemics have occurred in the past 200 years. The first pandemic originated in India.
Morphology and Identification
A. Typical Organisms V.cholerae is a gram negative, comma-shaped, curved rod 2–4 μm long. It is actively motile shows presence of polar flagellum.
(Vibrio cholerae, the bacterium that causes cholera)
B. Cultural characteristics and Plating media.
V.cholera are strongly aerobic. They grow well at 37°C on many kinds of media, including defined media containing mineral salts and asparagine as sources of carbon and nitrogen. On Mac Conkeys agar the colonies are colorless at first then become pink on prolonged incubation due to slow fermentation of lactose. V.cholerae grows on thiosulfate-citrate-bile-sucrose (TCBS) agar, a media selective for vibrio’s, on which it gives yellow-colored colonies that are readily visible against the dark-green background of the agar. Monsur’s gelatin taurocholate trypticase tellurite agar (GTTA) medium is also used. They produce small, translucent colonies with a greyish black Centre and a turbid halo. Most Vibrio species are halotolerant, and NaCl often enhances their growth. Some vibrios are halophilic, requiring the presence of high concentration of NaCl to grow. Vibrio species are susceptible to the compound O/129 (2,4-diamino-6,7di-isopropylpteridine phosphate)
C. Holding or Transport Media. Cary-Blair medium is used as a transport medium, it is a buffered solution of sodium chloride, calcium chloride, sodium thioglycolate, disodium phosphate at pH 8.4. Venkatraman-Ramakrishnan (VR) medium, in this the organisms do not multiply but remain viable for few weeks. It is dispended in screw capped bottles in 10-15 ml amounts. About 1-3 ml of stool is added to each bottle. Autoclaved sea water can also be used as a holding medium.
D. Biochemical Reactions. V.cholerae shows following features: It is catalase positive and oxidase positive. V.cholerae ferments sugars with production of acid only no gas formation. It ferments glucose, sucrose, maltose, mannitol, and mannose. It is a late lactose fermenter ferments lactose on incubation for several days. It does not ferment arabinose, inositol, and dulcitol. It forms indole and reduces nitrates to nitrites. It gives methyl red positive and urease test negative. It liquefies gelatin and decarboxylates lysine and ornithine, but not arginine. A positive oxidase test is a basic step in the identification of V.cholerae and other vibrios.
E. Antigenic Structure and Biologic Classification. Many vibrio’s possess a single heat-labile flagellar H antigen. They are classified as Group A vibrio’s, and the rest as Group B. Based on major somatic O antigen, Group A vibrio were further classified into subgroups or serovars also called as serogroups. Antibodies to the H antigen are not involved in the protection of susceptible hosts. V.cholerae contain an O lipopolysaccharide that confer serologic specificity. There is a minimum of 206 O antigen groups. V.cholerae strains of O group 1 and O group 139 that cause classic cholera; non-O1/non-O139 V.cholerae causes cholera-like disease. The V.cholerae serogroup O1 antigen has determinants that make further typing possible; the serotypes are Ogawa, Inaba, and Hikojima. V. cholerae O139 is similar to V.cholerae O1 El Tor biotype. V.cholerae O139 does not produce the O1 lipopolysaccharide and is incapable of making this antigen. V.cholerae O139 produce a polysaccharide capsule, but V.cholerae O1 does not produce a capsule.
Virulence factor and Resistance. Virulence factor of V.cholerae include cholera toxin, adhesin factor, toxin regulated pilus, siderophores, hemagglutination-protease, neurotransmidase and some others also. They produce a heat labile enterotoxin. Which consists of subunits A and B. Ganglioside GM1 act as the mucosal receptor for subunit B, which promotes entry of subunit A inside the cell. Activation of subunit A1 yields increased levels of intracellular cyclic adenosine monophosphate (cAMP) and results in hypersecretion of water and electrolytes. Electrolyte-rich diarrhea occurs—as much as 20–30 L/day—which results in dehydration, shock, acidosis, and death. The genes for V.cholerae enterotoxin are present on the bacterial chromosome. Cholera enterotoxin can stimulate the production of neutralizing antibodies. Toxin regulated pilus, helps in adherence to mucosal cells of intestine. Hemagglutination- protease, splits mucus and fibronectin and cholera toxin. Thereby inducing intestinal inflammation and helps in releasing free vibrios from bound mucosa to the intestinal lumen. Neuraminidase, destroys muramic acid and increases toxin receptors for V. cholerae. Siderophores is responsible for sequestration of iron. These organisms are susceptible to heat, drying and acids, but resist high alkalinity. Survival in water is influenced by pH, temperature, salinity and organic pollutants.
Immunity and Pathogenesis. After ingestion of V.cholerae, the majority are killed by gastric acid. Specific IgA antibodies are found in the lumen of the intestine. Similar antibodies in serum develop after infection but last only for few months. Vibriocidal antibodies in serum are associated with protection against colonization.
The pathogenesis of cholera and of diarrhea caused by enterotoxigenic bacteria other than V.cholerae 01 comprises three main stages: (1) bacterial colonization; (2) production and delivery of enterotoxins; and (3) toxin action and intestinal fluid secretion. (Ananthanarayan and Paniker, 1948;)
The structure and function of cholera toxin (CT) and its effects on fluid transport processes have been particularly well elucidated. It is believed that colonization may involve, sequentially: (1) chemotactic attraction of the bacteria to the surface of the mucus gel; (2) penetration of the mucus gel;'(3) adhesion to the epithelial cell surface; and (4) multiplication of mucus gel- and mucosa-associated bacteria. The bacterial cell surface receptor for CTXφ is the toxin-co-regulated pilus, which is itself encoded within a genomic island, vibrio pathogenicity island (VPI-1). Evolution of virulence in V.cholerae involves sequential acquisition of VPI-1 followed by CTXφ. Under normal conditions, V.cholerae is pathogenic only for humans. A person with normal gastric acid secretion may have to ingest as many as 1010 or more V.cholerae to become infected. When the medium is food, as few as 102–104 organisms are necessary because of the buffering capacity of food. Any medication that decreases stomach acidity makes a person more susceptible to infection with V cholerae. The organisms do not invade the bloodstream but remain within the intestinal tract. Pathogenic V cholerae organisms attach to the microvilli of epithelial cells. They multiply and secrete cholera toxin and also mucinases and endotoxin.
Fresh stool specimen collected before administration of antibiotics is the specimen of choice.
Dark field microscopy and phase contrast microscopy is preferred to check out motility and inhibition by antisera. Direct immunofluorescence is another rapid method used for detection of vibrios in the stool sample.
The specimen collected in holding media is inoculated in enrichment media for 6-8 hrs., before inoculating on selective and general-purpose media. The specimen collected in transport media are incubated for 6-8 hrs. The inoculated plates are incubated at 37oC for a period of 24 hrs.
4. Specific Tests
V.cholerae organisms are also identified by slide agglutination tests using anti-O group 1 or group 139 antisera and also by biochemical reaction patterns. The diagnosis of cholera under field conditions has been reported to be facilitated by a sensitive and specific immunochromatographic dipstick test.
(Antisera to the O1 serogroup of V. cholerae will agglutinate homologous organisms (left). A normal serum or saline control (right) does not show agglutination)
Both contaminated water and contaminated food can serve as medium for the transmission of cholera. In Bangladesh and India, water appears to play a major role. In other areas, such as the South Pacific islands, foodborne outbreaks have occurred. In situations where water is the medium, it need not only be drinking-water that is responsible, since contaminated water may be consumed in other forms. In addition, contaminated water may inoculate food, leading to foodborne cholera. The role of fomites, fingers, bed linen, or other soiled objects in the transmission of cholera remains unclear. Type of transmission more often when there is overcrowding and hygiene is very poor. Children who acquire nosocomial cholera may be more susceptible than normal children because of their underlying illness.
Diagnosis and Treatment.
Physicians in endemic areas diagnose cholera based on its manifestations, particularly so-called “rice-water stool,” which is watery, colorless, odorless, and flecked with mucus, which looks like bits of rice. The necessary and immediate part of therapy consists of water and electrolyte replacement to correct the severe dehydration and salt depletion. Oral tetracycline and doxycycline tend to decrease stool output in cholera and shorten the period of excretion of vibrios. In some areas, tetracycline resistance of V.cholerae has emerged; the genes are carried by plasmids. In children and pregnant women, alternatives to the tetracyclines are erythromycin and furazolidone.
Epidemiology, Prevention and control.
In endemic regions, the major cases occur among children below 5 years of age and in reproductive-age women. In some countries like Bangladesh and India, cholera infections occur every year. It is found that environmental factors such as climate, temperature, and salinity play a major role in cholera transmission. Reoccurrence of epidemic cholera has also been related to population density, urbanization, and overcrowding. For the prevention and control of cholera, it is necessary to understand the factors that are responsible for initiation and transmission of cholera in a community. Measures for the preventing cholera include provision of clean water, hygienic food and proper sanitary conditions to the cholera-endemic communities. Health education regarding personal hygiene and food safety should be provided. Media, community leaders, and religious leaders should participate in health education and social mobilization campaigns. Today, there are two oral cholera vaccines, namely Dukoral and Shanchol. Dukoral is made up of killed whole cell vaccine including V. cholerae O1 serogroup and recombinant B subunit of cholera toxin. This vaccine can be given to children above 2 years and to adults. Shanchol is a killed bivalent whole‐cell vaccine suspension. It can be dosed to 1 year of age and above. he primary methodologies for cholera control are suitable administration of cholera cases; fortifying research centers; preparing and limit working of medical care laborers; and accessibility of sufficient clinical supplies for the executives. Likewise, admittance to safe water, legitimate disinfection, suitable waste administration; individual cleanliness and food cleanliness rehearses; improved correspondence and public data are required for the control of cholera episodes.
Despite the fact that cholera has been around for a long time, the illness came to conspicuousness in the nineteenth century, when a deadly flare-up happened in India. There have since been various flare-ups and seven worldwide pandemics of cholera. Every year, cholera taints 1.3 to 4 million individuals around the globe, slaughtering 21,000 to 143,000 individuals. The primary cholera pandemic rose out of the Ganges Delta with a flare-up in Jessore, India, in 1817, coming from polluted rice. The infection immediately spread all through the majority of India. The pandemic ceased to exist 6 years after it started. The second cholera pandemic started around 1829. The pandemic would vanish and reappear all through various nations for almost twenty years until it died down around 1851. Six resulting pandemics executed huge number of individuals over all mainland. The seventh pandemic began in South Asia in 1961, and arrived at Africa in 1971 and the Americas in 1991. Cholera is presently endemic in numerous nations.