Fecally contaminated water is the most common source of cholera infection, although foods such as crab and vegetables fertilized with human feces have also been implicated in outbreaks.
Chapter 24 Alimentary System Infections
The ecology of cholera is fascinating but still incompletely understood.The O139 Vibrio cholerae strain arose in India in the fall of 1992 from a serotype O1 strain, the same serotype as the pandemic and American Gulf Coast strains. Interestingly, the change in O antigen was accompanied by acquisition of a capsule, these changes probably resulting from genes acquired by transduction or conjugation.Various strains of V. cholerae, most of which are not pathogenic for humans, live in the coastal seas around the world largely in association with zooplankton.These zooplankton can sometimes also harbor the V. cholerae strains responsible for pandemic cholera. Moreover, the zooplankton feed on phytoplankton and therefore increase markedly in number when warm seas and abundant nutrients cause explosive growth of phytoplankton.These findings could explain (1) how new pandemic strains could arise through genetic interchange, (2) an association between cholera and climatic changes such as EI Niño, and (3) the onset and rapid spread of epidemic cholera by ocean currents along coastal areas. ■ zooplankton, p. 301
A person with cholera may discharge a million or more V. cholerae organisms in each milliliter of feces. Although cholera is relatively common worldwide, few cases due to the pandemic strain have been acquired in the United States since the early 1900s. Since 1973, however, sporadic cases have occurred along the Gulf of Mexico. Most of these infections have been traced to coastal marsh crabs that had been eaten. These cases were due to a strain of V. cholerae different from the pandemic strain. Ominously, in September 1992, a new strain of V. cholerae, belonging to a different O group (O139), appeared in India and spread rapidly across South Asia, attacking even those people with immunity to the seventh pandemic strain. This new strain and its rapid spread suggested it could initiate another pandemic, but after about a year it declined in incidence and remained endemic, causing resurgent localized epidemics. ■ O antigen, pp. 256,282
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