The bacterium V. cholerae is the cause of cholera. The epidemiology of cholera closely parallels the recognition of V. cholerae transmission in water and the development of sanitary water systems. Cholera is associated with poor sanitation, as well as direct contact with or consumption of contaminated water and/or food (eg, water used for drinking, cooking, bathing, and crop irrigation).

Morphology and Identification

A. Typical Organisms

Upon first isolation, V. cholerae is a comma-shaped, curved rod 2–4 µm long (Figure 1). It is actively motile by means of a polar flagellum. On prolonged cultivation, organisms may become straight rods that can resemble other Gram negative enteric bacteria.

Fig1. Gram-stain of V. cholerae. Often they are comma shaped or slightly curved (arrows) and 1 × 2 to 4 µm. Original magnification ×1000.

 B. Culture

V. cholerae produces convex, smooth, round colonies that are opaque and granular in transmitted light. V. cholerae and most other vibrios grow well at 37°C on routine agar media to recover enteric bacteria (eg, blood agar and MacConkey agar); however, selective agars for Vibrio species, such as thiosulfate-citrate-bile salts-sucrose (TCBS) agar and enrichment broth (eg, alkaline peptone broth), can also be used to recover vibrios, especially from specimens (eg, stool) when a mixture of organisms is expected. All vibrios, including V. cholerae, grow well on TCBS agar; V. cholerae produces yellow colonies (sucrose fermented) on TCBS agar that are readily visible against the dark-green background of the agar (Figure 2). Non-sucrose-fermenting vibrios (eg, most strains of V. parahaemolyticus and V. vulnificus) produce green colonies on TCBS agar. Characteristically, vibrios grow at a very high pH (8.5–9.5) and are rapidly killed by acid. To ensure optimal recovery of vibrios, stool specimens should be collected early in the course of the diarrheal illness; prompt inoculation onto appropriate agar media is necessary. If processing of specimens may be delayed, the stool specimen should be mixed in a Cary-Blair transport medium and refrigerated.

Fig2. Colonies of V. cholerae growing on thiosulfate, citrate, bile salts, and sucrose agar. The glistening yellow colonies are 2–3 mm in diameter and are surrounded by a diffuse yellowing of the indicator in the agar up to 1 cm in diameter. The plate is 10 cm in diameter.

In areas where cholera is endemic, direct cultures of stool on selective media, such as TCBS, and enrichment broth cultures (eg, alkaline peptone water with 1% NaCl, pH 8.5) are appropriate. In the United States and other countries where cholera is rare, routine use of TCBS agar for stool cultures in clinical laboratories is generally not necessary or cost effective; exceptions may be made if recovery of other vibrios (eg, V. parahaemolyticus) is a frequent and/or seasonal occurrence (eg, coastal U.S. regions with regular and frequent consumption of bivalve mollusks and crustaceans).

C. Growth Characteristics

 V. cholerae regularly ferments sucrose and mannose but not arabinose. A positive oxidase test result is a key step in the pre liminary identification of V. cholerae and other vibrios. While most Vibrio species are halophilic, requiring the presence of NaCl (range from < 0.5–4.5%) to grow, V. cholerae can grow on most agar media without additional salt.

Antigenic Structure and Biologic Classification

Many vibrios share a single heat-labile flagellar H antigen. Antibodies to the H antigen are probably not involved in the protection of susceptible hosts.

V. cholerae has O lipopolysaccharides that confer serologic specificity. Based on the O antigen, there are over 200 serogroups; however, only V. cholerae strains of serogroup O1 and O139 cause epidemic and pandemic cholera. Occasionally, non-O1/non-O139 V. cholerae strains have been described as causes of cholera-like diarrheal disease. Anti bodies to the O antigens tend to protect laboratory animals against infections with V. cholerae.

The V. cholerae serogroup O1 antigen has determinants that make possible further subtyping; these serotypes are Ogawa, Inaba, and Hikojima. Furthermore, two biotypes of epidemic V. cholerae have been defined, classic and El Tor. The El Tor biotype produces a hemolysin, gives positive results on the Voges-Proskauer test, and is resistant to polymyxin B. Molecular techniques can also be used to type V. cholerae. Typing is used for epidemiologic studies, and tests generally are done only in reference laboratories.

V. cholerae O139 is very similar to V. cholerae O1 El Tor biotype. V. cholerae O139 does not produce the O1 lipopolysaccharide and does not have all the genes necessary to make this antigen. V. cholerae O139 and other non-O1 V. cholerae strains, as well as V. vulnificus produce acidic polysaccharide capsules; however, V. cholerae O1 does not make a capsule.

Vibrio cholerae Enterotoxin

V. cholerae produce a heat-labile enterotoxin with a molecular weight (MW) of about 84,000, consisting of subunits A (MW, 28,000) and B. Ganglioside GM1 serves as the mucosal receptor for subunit B, which promotes entry of subunit A into the cell. Activation of subunit A1 yields increased levels of intracellular cyclic adenosine monophosphate (cAMP) and results in prolonged hypersecretion of water and electrolytes. There is increased sodium-dependent chloride secretion, and absorption of sodium and chloride by the microvilli is inhibited. Electrolyte-rich diarrhea occurs with as much as 20–30 L/day, resulting in dehydration, shock, acidosis, and death. The genes for V. cholerae enterotoxin are located on the bacterial chromosome. Cholera enterotoxin is antigenically related to LT of Escherichia coli and can stimulate the production of neutralizing antibodies. However, the precise role of antitoxic and antibacterial antibodies in protection against cholera is not clear.

Pathogenesis and Pathology

 While V. cholerae is pathogenic only for humans, the organ ism is not solely dependent on the human host for propagation. V. cholerae also grows in brackish and marine waters in close association with copepods and zooplankton; the organism can also survive in water of low salinity when it is warm and sufficient organic substrates are available to support growth. A person with normal gastric acidity may have to ingest as many as 1010 or more V. cholerae to become infected; therefore, contaminated food and water are the all more likely source of infections, rather than person-to-per son contact. The infectious dose, however, is significantly lower (10²–10⁴) in a person with achlorhydria or hypochlorhydria. Any medication (eg, proton-pump inhibitors) or condition that decreases stomach acidity makes a person more susceptible to infection with V. cholerae.

V. cholerae is a noninvasive mucosal pathogen. The organisms do not reach the bloodstream but remain within the intestinal tract. Virulent V. cholerae organisms attach to the microvilli of the brush border of epithelial cells. There they multiply and liberate cholera toxin and perhaps mucinases and endotoxin.

Clinical Findings

The spectrum of disease due to V. cholerae ranges from asymptomatic intestinal colonization to mild, moderate, or severe diarrhea. About 50% of infections with classic V. cholerae are asymptomatic, as are about 75% of infections with the El Tor biotype. The incubation period after ingestion of a sufficiently high infectious dose of V. cholerae is 12 hours to 3 days for persons who develop symptoms, depending largely on the size of the inoculum ingested. There is a sudden onset of nausea and vomiting, followed by profuse diarrhea with abdominal cramps. Stools, which resemble “rice water,” contain mucus, epithelial cells, and large numbers of vibrios. In cases of severe cholera the volume of diarrheal fluid loss can exceed 1 L/h. The rapid loss of fluid and electrolytes can lead to profound dehydration, painful muscle spasms, metabolic acidosis, hypokalemia, and hypovolemic shock with circulatory collapse, and anuria with associated renal failure. The mortality rate without treatment is between 25% and 50%, and can be as high as 70%. On the contrary, mortality in patients promptly treated with fluid replacement has been reported as low as 1% or less. The diagnosis of a full-blown case of cholera presents no problem in the presence of an epidemic. However, sporadic or mild cases are not readily differentiated from other diarrheal diseases. The V. cholerae O1 El Tor biotype tends to cause milder disease than the classic biotype.

Diagnostic Laboratory Tests

 A. Specimens

As stated above, stool specimens should be collected early in the course of the diarrheal illness and inoculated within 2–4 hours of collection onto appropriate agar media, to ensure optimal recovery of vibrios. If processing of specimens may be delayed, the stool specimen should be mixed in a Cary-Blair transport medium and refrigerated.

B. Smears

Direct detection of V. cholerae on smears made from stool samples is not distinctive of the organism, and therefore not routinely recommended. Dark-field or phase-contrast microscopy can be used to detect V. cholerae O1 directly from stool samples or the enrichment broth. Observation of “shooting star” motility is suggestive of V. cholerae O1; if the motility is extinguished after mixing the sample with a polyvalent O1 antiserum, the organism is confirmed as V. cholerae O1. However, if there is no motility or the type of motility does not change after applying the antiserum, the organism is not V. cholerae O1.

C. Culture

Vibrios, including V. cholerae, grow well on most agar media (including MacConkey and blood agar) used in clinical laboratories. Some strains of V. cholerae may however be inhibited on MacConkey agar. Growth is rapid in alkaline peptone broth or water, containing 1% NaCl with a pH of 8.5, or on TCBS agar; typical colonies can be picked in 18 hours of growth. For enrichment, a few drops of stool can be incubated for 6–8 hours in taurocholate peptone broth (pH, 8.0–9.0); organisms from this culture can then be stained or subcultured onto other appropriate agar media. Accurate identification of vibrios, including V. cholerae, using commercial systems and kit assays is quite variable. MALDI-TOF MS is a promising newer methodology for identification of vibrios, and studies have shown rapid and reproducibly accurate identification for V. parahaemolyticus.

D. Specific Tests

Other rapid detection methods for V. cholerae include immunofluorescence, latex agglutination, and coagulation assays. V. cholerae organisms are further identified by slide agglutination tests using anti-O group 1 or group 139 antisera and 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.

Immunity

 Gastric acid provides some protection against vibrios, including V. cholerae.

An attack of cholera is followed by immunity to reinfection, but the duration and degree of immunity are not known. In experimental animals, specific IgA antibodies occur in the lumen of the intestine. Similar antibodies in serum develop after infection but last only a few months. Vibriocidal antibodies in serum (titer ≥1:20) have been associated with protection against colonization and disease. The presence of antitoxin antibodies has not been associated with protection.

Treatment

The most important part of treating cholera patients consists of water and electrolyte replacement to correct the severe dehydration and salt depletion. Many antimicrobial agents are effective against V. cholerae, but these play a secondary role in patient management. Appropriate antimicrobial therapy can also reduce the duration and amount of shedding of Vibrio organisms in the stool. The antibiotic choice should be based on the local antimicrobial resistance profiles. Tetracycline has shown to be very effective treatment for cholera, and generally has better efficacy than furazolidone and chloramphenicol. Erythromycin and/or azithromycin are an appropriate choice of antimicrobial therapy in children and in pregnant women; other antimicrobial agents that are effective include trimethoprim–sulfamethoxazole, fluoroquinolones, and doxycycline. Increasing antimicrobial resistance in V. cholerae has been noticed, globally. Specifically, in areas where cholera is endemic or epidemic, tetracycline resistance has been reported with increasing frequency; the resistance genes are carried by transmissible plasmids. In addition, another likely risk factor for the emerging antimicrobial resistance is the widespread use of antibiotics, including the mass distribution for prophylaxis in asymptomatic individuals. During previous epidemics, antibiotic resistance emerged in the context of antibiotic prophylaxis for household contacts of cholera patients.

Epidemiology, Prevention, and Control

 Cholera and cholera-like illnesses have been mentioned in various writings since antiquity; since 1817, seven cholera pandemics (worldwide epidemics) have been recorded. Six pandemics of cholera occurred between 1817 and 1923 caused most likely by V. cholerae O1 of the classic biotype and largely originating in Asia, particularly the Indian subcontinent. The seventh pandemic began in 1961 in the Celebes Islands, Indonesia, with spread throughout Asia, and the Middle East Europe, and Africa. This pandemic was caused by V. cholerae O1 bio type El Tor. Starting in 1991, the seventh pandemic spread to Peru and then to other countries of South America and Central America. Subsequently, variant atypical or hybrid V. cholerae O1 El Tor strains have emerged in Africa and in Asia; these strains appear to be more virulent than the original El Tor or the classical strains. In 2011, the WHO reported an estimated occurrence of 600,000 cases of cholera with approximately 8000 deaths, annually among 58 countries. However, it is rea sonable to state that these numbers for morbidity and mortality underestimate the global burden of this disease. More recent estimates suggest that approximately 3 million cases with an associated mortality of 95,000 cases occur globally and on an annual basis. In 1992, a new serotype of V. cholerae, the strain V. cholerae O139 Bengal, emerged in India and in Bangladesh. It is believed that horizontal gene transfer of a novel somatic antigen and capsule from an unknown bacterium onto the El Tor strain caused this new strain to emerge. The clinical disease of this new strain is very similar to cholera caused by the O1 strain; however, adults are more often affected by the O139 strain, since prior infection with the O1 strains does not confer immunity. Some consider the cholera epidemic caused by the serotype O139 strain to be the eighth pandemic that began in the Indian subcontinent in 1992–1993, considering the subsequent spread throughout Southeast Asia. However, to date, no cases of V. cholerae O139 have been reported outside of Asia; in 2011, China was the only country reporting cases of cholera due to the O139 strain.

Cholera is endemic in India and in Southeast Asia. From these epicenters, it is carried along shipping lanes, trade routes, and pilgrim migration routes. Between 1996 and 2009, most cases of cholera were reported from countries in Africa; in the Americas, cases were infrequently reported. However, in 2010, Haiti experienced a magnitude 7.0 earth quake that devastated the country’s infrastructure, and subsequently a severe cholera epidemic began in Haiti on the Caribbean island of Hispaniola. By 2012, more than 600,000 cases of cholera and 7400 deaths associated to the illness were recorded. Cholera subsequently spread to the Dominican Republic, also located on the island of Hispaniola, and then to Cuba; since then imported cases have been reported in many other countries in the Americas, including the United States. Various epidemiologic investigations provided evidence that United Nations peacekeepers from countries in Southeast Asia, who were invited to Haiti for providing support, may have introduced V. cholerae O1, serotype Ogawa, biotype El Tor. These people were likely asymptomatic carriers, and V. cholerae O1 was introduced into local waterways that are used by the local populace as a source of water for drinking, cooking, and bathing. This cholera epidemic has been the worst in recent history; more than 665,000 cases with more than 8000 associated deaths have been reported so far and cholera is now endemic in this Caribbean nation.

Cholera is a disease that is spread by contact involving individuals with mild or early illness and by water, food, and flies. In many instances, only 1–5% of exposed susceptible persons develop disease. The carrier state seldom exceeds 3–4 weeks, and the importance of carriers in transmission is unclear.

V. cholerae has no known animal host aside from humans; however, the organisms is capable of surviving in various aquatic environments for some time. Such aquatic environments are considered the vibrios’ natural reservoir, where V. cholerae lives in close association with algae, copepods, and crustaceans.

People infected with cholera shed the organisms only during the first few days of the illness; however, there is no long-term carriage in humans. Infection control rests on education and on improvement of sanitation; such measures involve adequate sewage management, water purification systems, and methods to prevent food contamination. Additional measures to prevent spread of cholera during outbreaks include isolation of patients and disinfection and appropriate disposal of their excreta. Antimicrobial therapy may be beneficial as it reduces clinical symptoms and the transmission of vibrios from patients to healthy contacts; similarly chemoprophylaxis with antibiotics given to household contacts of cholera patients can help limit the spread of the organisms. In addition, oral and parenteral vaccines are available; in June 2016, the FDA approved a single-dose, live oral cholera vaccine for use in adults (18–64 years old) who are traveling to areas of cholera endemicity and transmission. Three other, killed oral cholera vaccines are prequalified by the WHO and are currently available outside the United States; the whole cell V. cholerae O1 with recombinant cholera toxin B subunit vaccine is used primarily as a vaccine for traveler’s to cholera endemic areas; one whole-cell V. cholerae O1 and O139 vac cine is available for use in Vietnam, only, whereas another whole-cell V. cholerae O1 and O139 vaccine is available for global market use. While the injectable cholera vaccine made from phenol-inactivated strains of V. cholerae is still manufactured in a few countries, this vaccine is no longer recommended by the WHO due to the limited efficacy of this vaccine and its short duration of protection. Because cholera vaccines offer only incomplete protection from disease, vaccination should not replace the other standard prevention and control measures described earlier in this paragraph.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم شؤون المعارف ينظم دورة متقدمة في تحقيق المخطوطات لملاكاته

قسم الشؤون الفكرية يصدر العدد العشرين من مجلة دراسات إفريقية

المجمَع العلمي يكرم الأساتذة المشاركين في مشروع الدورات القرآنية الصيفية في كربلاء

العتبة العباسية المقدسة تختتم فعاليات البرنامج المركزي الأسبوعي لمنتسبيها

المزيد

الآخبار الصحية

تحذير من "عدوى بكتيرية" تسبب نوبات قلبية

ماذا يحصل لجسمك عند التوقف عن استخدام الزيت في الطهي؟

الأمل في كبسولة.. "فيتامين شهير" يبطئ التقدم في العمر

اشتهاء أطعمة بشكل مفاجئ.. ناقوس خطر للسرطان ؟!

المزيد

الآخبار التكنلوجية

زلزال أفغانستان يحصد أكثر من ألف قتيل.. ويدمر آلاف المنازل !

ناجٍٍ وحيد من انزلاق أرضي.. دمّر قرية سودانية بأكملها !

نهر الفرات.. مورد مائي بين الندرة والتقاسم

ما السر وراء مشية إيلون ماسك الغريبة والسريعة ؟

المزيد

مواضيع ذات صلة

Vibrio Parahaemolyticus and Vibrio vulnificus

Burkholderia cepacia Complex

Burkholderia pseudomallei and Burkholderia mallei

Rapidly Growing Nontuberculous mycobacteria (RGM)

Slow-growing Nontuberculous Mycobacteria

Mycobacterium Tuberculosis Complex

Anaerobic Gram-Positive and Gram-Negative Cocci

The Salmonellae

The shigellae

Diseases Caused by Enterobacteriaceae Other than Salmonella and Shigella

Antigenic Structure of Enterobacteriaceae

Gram-Negative Rods