The Pandemic State of Cholera
Cholera is an acute diarrheal infection caused by the bacteria Vibrio cholerae, a flagellated gram-negative bacterium (Fig. 1). The pathogen primarily exists as a free-living water-based organism, usually in brackish coastal waterways, and is often associated with chitin-shelled organisms and zooplankton. However, recent outbreaks in Africa and Haiti have demonstrated the bacterium’s ability to survive in fresh water. Transfection primarily occurs through the consumption of contaminated food or water, and symptoms include profuse vomiting and diarrhea, which depending on the degree of illness may result in severe dehydration and electrolyte imbalance (Ryan 2013).
To date, there have been seven world-wide pandemics of cholera and six of them alone occurred between 1817 and 1923. Of approximately 200 serogroups, the epidemic state is associated with strains O1 and O139. Descriptions of a disease in Sanskrit having symptoms similar to those of cholera date back to the 5th century BC, and the disease is believed to have existed on the Indian subcontinent for several centuries. In 1817, the first epidemic was reported among British troops in Calcutta, and abruptly spread beyond the Indian subcontinent. The second pandemic began in 1827, also in the Ganges delta area, and spread across Asia, Europe, the United States, and even Australia until it receded in 1837. The third pandemic began in 1839 among British troops in Afghanistan and spread in a manner similar to that of the second pandemic, and receded by 1855. During the third epidemic, London physician John Snow proposed that cholera was a communicable disease and that stool contained infectious material. In addition, Italian physician Fillipo Pacini isolated the curved baccilus from the stool of cholera victims in 1854 and named it Vibrio cholera because it appeared to vibrate under a microscope. The fourth pandemic occurred in 1863 in Hajj and spread through Africa, Europe, Latin America, and the United States before receding in 1870. This pandemic holds particular historical significance because it marked the ascendancy of the contagiousness theory of disease as well as the water-borne nature of cholera. The fifth pandemic began in 1881 and is referred to as the Suez and Steam epidemic, and spread from India through Egypt through Africa and Europe, as well as China and Japan. During this time, Robert Koch isolated the cholera organism in Calcutta in 1884 and it was his work that gave rise to laboratory confirmatory tests for cholera. The sixth pandemic occurred between 1899 and 1923 and spread through Asia, and Africa. A large outbreak occurred in 1905 at the El Tor Quarantine Camp in the Sinai and gave rise to El Tor V. cholerae, a new biotype of the disease. The El Tor V. cholerae reemerged in Indonesia in 1961 and manifests itself as the ongoing seventh cholera pandemic in and has spread to Asia, Africa, Europe, and South America (Ryan 2013) (Harris 2012).
The onset of symptoms occurs due to increased chloride secretion from intestinal cells that resulting upon toxin entry. Once inside the cell, the toxin leads to the adenylate cyclase and therefore an elevated level of cAMP. The elevated levels of cAMP results in the activation of chloride channels and cause an efflux of negative as well as positive ions, thus leading to water efflux and establishing an osmotic gradient that differs from the equilibrium state of the cell. This mechanism is briefly outlined in Fig. 2 and will be expanded upon in the Molecular Basis of Disease section.
The WHO reports an estimated 3-5 million cases a year, predominantly occurring in Asia and Africa with recent outbreaks in Haiti. Diarrheal diseases including cholera are the second leading cause of mortality in children under the age of 5, with approximately 760,000 deaths occurring each year (Harris 2012). Diagnostic delays contribute greatly to these rates and therefore early detection is of utter importance. Although isolation and identification of V. cholerae serogroup O1 or O139 by culture of stool specimen remains the gold standard for laboratory diagnosis, rapid diagnostic testing is of great use in outbreak situations and account for delays due to stool transport. Rapid diagnostics that are currently available are based on the detection of the lipopolysaccharide of V. cholerae O1 and O139 by monocloning antibodies using vertical-flow immunochromatography. However, due to limited capabilities of rapid testing, laboratory testing is still needed in order to molecularly characterize isolates. Stool transport often requires the use of the Cary-Blair medium which due to its high pH of 8.4 is highly effective in the preservation of V. cholerae. In addition to microscopy, secondary detection using PCR techniques is also often required for the molecular characterization of cholera toxin. For PCR detection, the ctxA gene encoding the A subunit of cholera toxin, which is the primary catalytic component, is often the preferred target. Multiplex PCR also offers the advantage of determining the V. cholerae O1 biotype by exploiting the sequence difference between classic biotype and El Tor biotype tcpA genes to distinguishing the two (Keddy 2013).