Hydatid disease is a serious condition for humans. It is caused by a relatively small tapeworm, but the cysts that develop from the juvenile parasite may be quite large.
Hydatid disease is recorded world-wide and cycles naturally in wild animals (this is called a sylvatic cycle). In Australia wildlife populations provide the major reservoir of the disease as it cycles through wild dogs or dingoes and their prey, for example wallabies or other marsupials. It is more prevalent in the southeast and northwest of the continent.
The opportunity for human infection occurs due to the ability of the parasite to also cycle through livestock and domestic animals, for example sheep and dogs. In some sheep growing areas the farm dogs are allowed to feed on sheep offal. If the sheep were infected with the tapeworm then the parasite will be passed to the dog. Adult tapeworms living in the intestine of dogs release eggs which pass out with their faeces. The eggs can lie dormant for long periods in the environment but are susceptible to high temperatures and dehydration.
Humans contract hydatid disease by accidentally swallowing the tapeworm eggs, usually as a result of patting an infected dog. Infections like this, that can be transmitted to humans from other animals, are called zoonotic infections.
In humans the eggs hatch and the infective stages, called onchospheres, migrate through the bloodstream to the liver, lungs, kidneys, spleen and occasionally the brain. In these organs they slowly change into a cyst-like structure, called a hydatid, which contains fluid and many developing baby tapeworms, called protoscolices.
The location and size of the hydatid will determine the symptoms experienced and infected people may not experience any symptoms for many years. However liver infections can be very painful and if the cysts rupture it may cause a potentially fatal anaphylactic shock.
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