Hydatid disease in humans is caused by the cystic stage of the complex life cycle of the hydatid tapeworm, Echinococcus granulosus.
The adult tapeworm lives in the small intestine of the dog. It is a very small tapeworm measuring only about 5 millimeters. It consists of a head and 3 to 4 segments. Even when present in very large numbers it causes little disturbance within the intestine so there are no outward signs that a dog may be infected.
Eggs are formed in the large end segment of the tapeworm which is shed every 14 days. Each mature segment may contain up to 1,000 eggs which pass out in the dog’s feces and are then dispersed on to the pasture or surroundings of the dog’s kennel. When one considers that many farmers keep 4 or 5 dogs one can imagine the degree of contamination possible.
The eggs of the tapeworm are eaten by a suitable intermediate host and pass into the small intestine where they hatch to produce a very active form called an oncosphere. These penetrate the wall of the intestine and migrate to the liver and lungs, to the brain, and sometimes, to the bone marrow. After some months they have formed a cystic structure which can gradually increase in size and which may produce pressure effects on the surrounding organ.
Within these cysts thousands of immature tapeworm segments develop, all capable of developing into an adult tapeworm if swallowed by the final host – a dog or dingo.
Occasionally a cyst may rupture within the intermediate host which results in a multitude of daughter cysts developing within the intermediate host’s body. In humans this may occur in the lungs or in the abdominal cavity and create a very serious clinical problem.
By far the most important intermediate host is the sheep, and it is the ease of transmission from dog to sheep that ensures the spread of the disease. Hydatids can be controlled. The key to control is the breaking of the link between the dog and the sheep.
Dogs are only infected by eating the immature eggs within the intermediate host. If dogs are not fed infected offal or do not find freshly dead sheep they will not become reinfected. The adult tapeworm within the dog is effectively controlled by the regular dosing of all dogs with Droncit.
Humans may become infected by the ingestion of the eggs of the tapeworm shed from the dog. Patting a dog carrying the eggs on its coat or working in an area contaminated by the feces of dogs then eating or smoking without washing, are the usual sources of infection. The eggs of the tapeworm can survive some weeks in the environment.
Of course the majority of dogs infected with hydatid tapeworms live in country areas and have been infected by careless feeding of offal or by eating freshly dead carcases of sheep. Dogs in the urban areas fed on commercial dog food have little chance of becoming infected unless taken on to properties or allowed to roam where there are sheep carcases.