Hydatid disease is a serious parasitic zoonotic condition associated with sheep farming in the UK and many other parts of the world. The causal agent, Echinococcus granulosus granulosus, is a small (typically only 3 – 5 mm long) tapeworm that lives in the small intestine of canidae (domestic and wild dog species).
The parasite causes no disease in the host but can be present in very large numbers and consequently very large number of onchospheres (“eggs”) can be deposited into the environment. These onchospheres are enclosed in a tough chitin membrane and are very resistant both to environmental degradation and disinfection, and can survive for years in the right condition.
Grazing animals ingest these dormant stages and after being stimulated by digestive processes the onchosphere hatches in the intestine, the embryo then penetrates the gut wall and finds its way into the hepatic portal vein which carries blood from the intestine to the liver. Many lodge in the liver but some pass through and on to the lungs. A few pass through the lungs and can establish anywhere else in the body. These embryos develop into hydatid cysts which in sheep typically grow to a size of a golf or tennis ball. The cysts consist of a wall containing a germinal membrane from which bud off brood capsules containing thousand of scolices (tapeworm “heads”). The final canid host ingests these whilst scavenging carcasses such as sheep and the tapeworms develop in the dog’s small intestine completing the life cycle.
Tapeworms are self fertilising hermaphrodites and as such have evolved to have a very specific final host preference, however, the hydatid cysts can develop in just about any mammalian species. This means that the parasite becomes closely adapted to its host and its normal prey. In Africa this means jackals and hunting dogs and their game prey. The Turkana tribe in Kenya, who leave human bodies out to be consumed by wild dogs, have a very high incidence of hydatidosis with a specific dog/human life cycle.
In the UK there are two recognised strains. The most serious in respect of risk to man is the sheepdog/sheep cycle. Farmers and other people living in or visiting sheep farming areas can become infected with hydatid cysts from their infected dogs. There is also a hound/horse cycle, knackered horses being fed to hound packs by the hunt, but this cycle offers less risk of hu...
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