Hydatids cysts are fluid filled sacs in the liver and lungs but also the brain and other tissues of predominantly sheep but also cattle. Pigs, other wildlife such marsupials, and people can be infected.
The cysts are the larval stage of the dog tapeworm Echinococcus granulosus. Adult hydatid tapeworms are very small (about 3-8 mm) and consist of four or five segments that are motile once released into the environment.
Hydatid infections of domestic dogs are now considered to be uncommon, but infection is still widespread in cattle grazed in pastoral areas where wild dogs are also present. Transmission of the disease between wild dogs and dingoes, and marsupials (the ‘sylvatic’ cycle) occurs along the entire eastern side of Australia, particularly in high rainfall areas.
Hydatid disease is zoonotic - it can infect man!
People may be accidentally infected, often after patting domestic dogs which may have the sticky tapeworm eggs on their coats, then touching their mouths or by kissing the dog. Infection can result in multiple cysts in internal organs and cause serious disease.
Hydatid cysts affect weight gain in cattle. After the initial exposure, cattle are generally resistant to new infestations, but this immunity does not affect the already existing cysts. Cysts in the liver and lungs of cattle are usually ‘sterile’ and non-infective, whereas those in sheep are usually fertile and infective.
Figure 1. Immature Echinococcus granulosus larvae from a hydatid cyst. Image courtesy of Constantin Constantinoiu
Figure 2. Larval Echinococcus granulosus protoscoleces from a hydatid cyst. Image courtesy of Constantin Constantinoiu
Adult tapeworms are resident in the small intestine of the dog, dingoes and foxes, with hydatid cyst stages mostly in the liver and lungs of cattle.
This is an indirect predator / prey life cycle where a larval stage occurs in cattle. The definitive (predator) host is the farm or wild dog and the intermediate (prey) hosts are cattle.
Figure 3. Liver of a cow with hydatid cysts. Image courtesy of Constantin Constantinoiu
Dogs become infected by eating offal containing hydatid cysts. Adult tapeworms released into the small intestine grow and reach maturity in six to seven weeks, and shed segments containing microscopic eggs into the environment when the dog defaecates. Eggs passed in the faeces can be spread by insects, wind and water and survive for several weeks but are rapidly killed by sunlight and desiccation.
When eaten by cattle during grazing, the eggs hatch in the small intestine, and the resultant larvae penetrate the gut wall and reach the liver and lungs via the blood stream. Cysts grow to about 20 mm in six months and continue to grow in size sometimes reaching up to 10 cm in diameter, although cattle are usually slaughtered before cysts become very large.
Figure 4. When a hydatid cyst is cut open the granular protoscolices of larval Echinococcus granulosus are visible. Image courtesy of the National Centre for Veterinary Parasitology, ncvetp.org
Hydatid cysts are regularly seen during meat inspection procedures at abattoirs. Cysts protrude mainly from the surface of the liver and lungs. In cattle they are usually degenerative and non-fertile, and show progressive or marked caseation and later calcification. Some cysts may be viable and appear clear and fluid filled.
Figure5. Adult Echinococcus granulosus live in wild dogs. Image courtesy of Leoncastro Wikimedia commons
This disease is still of medium-high prevalence in Australian cattle, despite the fact that farm dogs are usually prevented from eating fresh offal, are treated with praziquantel, and fed dry dog food. Adults and children should be particularly mindful of washing hands after patting dogs whose coats may be soiled with faeces containing tapeworm eggs.
Note: The large tapeworm segments often visible in the dung of cattle, and described as looking like cooked grains of white rice, are not from the hydatid tapeworm, but are segments of the intestinal tapeworm Moniezia benedeni and are not a cause for concern.
Treatment of cattle for the intestinal tapeworm will have no effect on the hydatid cysts in the liver or lungs.
Figure 6. The eggs of Taenia spp and Echinococcus spp look the same. Image courtesy of the National Centre for Veterinary Parasitology, ncvetp.org