The Transmission of Cholera
March 7, 2012
Written by Alisha Kramer
Vibrio cholerae is the causative agent responsible for cholera. It is a bean-shaped bacterium with a long tail that it uses for self-propulsion. The bacteria are transmitted between humans through the fecal-oral route; a bite of contaminated food or a sip of contaminated water can cause infection. More specifically, a toxin secreted by the bacteria, which targets receptors in the human intestine, is responsible for the pathology characteristic of the disease. Symptoms include excessive, watery diarrhea; vomiting; dehydration; and, without proper treatment, rapid deterioration and death.
Before infecting a human host, a key component of the vibrios’s survival strategy in its aquatic environment is the close interaction between the bacteria and a type of shrimp-like crustacean called a copepod. The cholera bacteria acquire strength in numbers through the formation of biofilms on the copepod surface. These biofilms act as a protective barracks for the bacteria—within the biofilm structure, the bacteria can survive unfavorable conditions during inter-epidemic periods.
The aggregation of the vibrios on the copepod also produces a very effective vehicle for transmission to human hosts. Studies suggest that anywhere from 103 to 105 vibrios are required to produce clinical cholera. However, as a result of biofilm formation, a mere 1-10 copepods can harbor enough V. cholera to lead to disease. In areas with poor sanitation and a lack of water infrastructure, copepods are commonly found in drinking water – dramatically increasing the risk of cholera infection.