Transmission of Cholera

on 6.2.09 with 0 comments

Cholera is spread by the faeco-oral route, via contaminated water and food. Asymptomatic infections are common, especially in case of El Tor. People excrete bacteria for about 10 days. This is sufficient time to ensure continued contamination of the environment. Chronic carriers are very rare. In third world countries many people have no chlorinated, filtered, treated, pure drinking water. The lack of good toilets and sewers leads to contamination of the surface or ground water. Too often untreated sewage water is still poured into surface water. Sometimes sewage pipes and drinking water pipes are laid in the same trench, which may result in contamination if there are leaks or greatly varying water pressures in the pipes. If drinking water is infected in this way, bacteriological checks of the drinking water when it leaves the pumping station will not show anything amiss. In houses, drinking water containers with a wide opening often become infected, because people are inclined to scoop up water in their (dirty) hands. Containers with a small spout, from which water must be poured, are much safer.

There is also direct transmission from person to person, but it is rare. The number of bacteria on dirty hands is usually lower than the minimum infectious dose necessary for direct transmission (>105 bacteria). A large inoculum is necessary to cause infection. Health workers who respect basic hygiene are at extremely low risk. Filter feeders such as mussels or oysters concentrate the bacteria in their bodies. These molluscs were the source of a recent outbreak in Hong Kong. When the organisms adhere to food particles (e.g. the chitin of crustaceans) and in the case of hypochlorhydria, lack of gastric acid due to gastric surgery, antacids, anti-ulcer drugs or atrophic gastritis, the number of organisms needed to trigger infection is much smaller. Food may be infected by dirty hands during or after preparation. The bacteria can survive and reproduce in food such as cereals, rice or lentils and crustaceans. This intermediate replication step is very important. If someone dies of cholera and a meal is made for the mourners by the persons who have washed the corpse, the risk of further transmission is very real. The bacteria are very sensitive to drying out, sunlight and acid. Meals which contain acid, e.g. tomatoes and/or lemon, are much less dangerous than neutral or alkaline meals. Vegetables and fruit on the market are often sprayed with water to make them appear fresher and more attractive. If this is done with infected water, transmission may occur.

Category: Medical Subject Notes , Microbiology Notes



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