Amoebiasis

By amoebiasis we mean here infection with Entamoeba histolytica. This is a unicellular cosmopolitan parasite. The first description of the parasite was in 1875 by Fedor Lösch in St Petersburg. This concerned an infection in a young Russian farmer in Arkhangelsk, 150 km from the Arctic circle. This illustrates the fact that the infection is not restricted to the tropics. Transmission depends on the level of sanitation and faecal hygiene in a country or region. In the past, analysis of zymodemes (enzyme analysis) was used to distinguish various morphological identical strains. These days, PCR will be used. One species which is very similar to E. histolytica is Entamoeba chattoni. This species mainly infects monkeys, but may be found in humans.

Life Cycle and transmission

Infection is caused by ingestion of E. histolytica cysts. These cysts develop in the small intestine into motile trophozoites which then find their way into the colon. The trophozoites multiply by asexual reproduction and in turn produce cysts, which are then excreted with the faeces. The cyst is quite resistant and can survive for a long time in the outside world. Excreted trophozoites die quickly and therefore are not responsible for transmission. Cysts of E. histolytica are never found in tissues. The parasite is transmitted faeco-orally as a cyst, usually from person to person. Transmission via water also occurs. Dogs, cats, rats, pigs and monkeys may become infected but do not form a significant animal reservoir (Note: kittens were used by E. Brumpt as a very susceptible animal model to test the pathogenicity of amoebae). Flies and cockroaches may carry cysts. Their role in transmission has not been properly investigated but is probably of minor importance. The main source of infection is humans. Amoebiasis is thus not a zoonosis. Infection via sexual intercourse is rare (via anal contact). The latter method of transmission may result in severe and mutilating lesions of the genitals.

Prevention

Amoebic cysts are resistant to normal chlorination of drinking water. Boiling and filtering drinking water eliminates the parasite. Large scale prevention depends mainly on improved sanitation and hygiene. No vaccine is available. Amoebiasis is not an opportunistic infection in HIV patients.

Category: Medicine Notes