Hydatids - A Disease of Dogs that Affects People
The hydatid tapeworm (Echinococcus granulosis) is one of seven tapeworms known to infect dogs in Australia. The lifecycle of this parasite can involve a number of animals, including humans, but the most important species involved are sheep, kangaroos and dogs. Hydatid disease is diagnosed in tens of humans every year.
The hydatid tapeworm
The hydatid tapeworm lifecycle requires two host animals for its survival.
- Hydatids occur as a small tapeworm in the intestine of dogs or dingoes and to a lesser extent in foxes. These are known as definitive hosts.
- Hydatids also occur as watery cysts in the soft tissues of sheep, cattle, pigs, kangaroos, wallabies and occasionally man. These are known as intermediate hosts. Rabbits do not act as intermediate hosts for true hydatids (Echinococcus granulosis). In humans, hydatid cysts can cause serious illness and even death.
The hydatid tapeworm in dogs is only about 6mm long, and consists of very small segments. The last segment carries the mature tapeworm eggs. Heavily infested dogs or dingoes may pass many millions of eggs in their faeces each year. One dingo caught near Mt Buller had 108,000 hydatid tape worms throughout its intestines but was otherwise healthy.
Hydatids in dogs/dingoes/foxes (definitive hosts)
Hydatid tapeworms develop when the dog consumes hydatid cysts in the offal (internal organs) of sheep, cattle, kangaroos and pigs. Dogs (particularly wild dogs) have been found to carry thousands of hydatid tapeworms in their intestines, yet there is rarely any sign of disease. Large, white tapeworm segments, often seen in the faeces of dogs, indicates the presence of other species of tapeworm, not the hydatid worm. The tiny hydatid tapeworm can live for up to two years in a dog. It is considered the most important tapeworm of dogs, not because of disease they may cause in the definitive host, but because of the disease that may be caused in the intermediate hosts, particularly humans.
Hydatids in sheep, cattle and kangaroos (intermediate hosts)
Grazing animals become infected when they swallow eggs from contaminated pasture. When hydatid eggs are swallowed by an intermediate host (sheep, cattle, kangaroos or humans), they migrate through the stomach wall into the bloodstream. They are then carried to various internal organs, usually the liver and lungs, but sometimes the brain. A watery hydatid cyst then forms in these soft tissues.
In cattle, approximately 10% of hydatid cysts are viable, and these cysts usually die within a few months. In sheep and kangaroos however, most cysts are viable and continue to grow throughout the life of the animal. Older sheep in particular may have very large cysts. Cysts contain many capsules, with each capsule containing up to 40 hydatid heads. Each head is capable of developing into an adult tapeworm in the intestine of a dog.
Sheep and kangaroos therefore act as the most important source of infestation for dogs, however the effect on the sheep or kangaroo themselves is minimal; few noticeable signs of infection will be seen in sheep or kangaroos with hydatid cysts.
Hydatids in humans
People become infected with hydatids by swallowing the microscopic hydatid eggs, passed by adult tapeworms in the intestine of the dog. Humans do not become infected with hydatids by eating sheep or kangaroo offal contaminated with the hydatid cysts. Human infection most commonly occurs when infested dogs are handled, because the sticky hydatid eggs are present on the dogs coat. Infection is also possible from eating home-grown raw vegetables, contaminated with the faeces of an infected dog.
The effect of hydatid infection in humans depends on the organ in which the cysts develop, and the size the cyst eventually reaches. Obviously, a small cyst in the brain will be potentially more serious than a larger cyst in the liver or lung. Extensive cyst development in any organ will affect the organs function.
While it is easy to treat the tapeworm in the intestine of the dog with anthelmintics (deworming drugs), in man the removal of cysts may require major surgery. In some instances, multiple major surgeries may be required to remove cysts from affected organs.
Hydatid eggs in the environment
Hydatid tapeworm eggs, when passed by dogs, dingoes and foxes into the environment, are quite resistant to harsh environmental conditions. Under favourable conditions (cool, moist, overcast), eggs may remain infective to susceptible intermediate hosts for several months.
High risk areas
Sheep and cattle that graze pastures adjacent to forested land where wild dogs are present have a higher incidence of hydatid cysts. Wild dogs, and to a lesser extent foxes, have become a reservoir of hydatid infection, as they are not regularly treated for worms.
Domestic dogs become infested when they are fed or scavenge the offal of infected sheep or kangaroos. Hunters dogs fed offal from kangaroos, wallabies or occasionally wild pigs, can bring hydatid disease into urban areas.
Holiday-makers or rural retreat owners and their friends who bring domestic dogs into bush areas are often unaware of the hydatid risk, and may not regularly treat their dogs for the hydatid tapeworm. They should not allow their dogs to roam, or scavenge from sheep or kangaroo carcasses.
Wild (sylvatic) and domestic cycles
Wild dogs carry thousands of hydatid tapeworms, and they usually contract this disease by eating infected kangaroos and wallabies. This is known as the wild, or sylvatic, cycle. When these feral dogs or dingoes pass faeces on farmland, hydatids enter the domestic cycle, where farm livestock and rural dogs perpetuate the disease. Similarly, domestic dogs that gain access to the offal of infected kangaroos can bring the hydatid tapeworm into the domestic cycle.
Controlling hydatid disease
To control hydatid disease in both man and animals, the life cycle of this parasite must be broken. This is achieved in two ways.
Ensuring dogs are regularly treated with effective anthelmintics to remove the hydatid tapeworm
Preventing dogs from accessing offal from infected intermediate hosts (particularly sheep and kangaroos) by feeding dogs commercially cooked dog foods
Dogs should be regularly treated with praziquantel tablets (5 mg/kg) to remove the hydatid tapeworm. Dogs that could have accidental access to the offal of infected animals should be treated every 6 to 8 weeks to keep them free of hydatids; hydatid tapeworms mature and lay eggs 6 weeks after the cysts are eaten by the dog.
Treatment need not be so frequent for dogs that are fed only commercially cooked food, and that have no access to animal carcasses. Owners of dogs in low risk areas are recommended to treat their dogs with praziquantel every 3 months.
Home killing of livestock
The large, encapsulated, fluid-filled hydatid cysts are easily detected in lung and liver tissue, but early infections can be too small to be detected. If a cyst is cut with a knife, the clear, watery fluid containing the hydatid heads may splash out over the muscle tissue or other surfaces. This material is not infective to man, but is highly infective to dogs.
Feeding of offal to dogs, even if boiled, carries a significant risk of transmitting hydatids to the dog, and perpetuating the hydatid life cycle.
Offal of sheep, cattle, kangaroos or pigs should never be fed to dogs.
Basic rules for hydatid prevention
- Wash hands after handling dogs, and before eating, smoking, etc.
- Dont allow dogs to lick your own, or your childrens, faces.
- Never feed offal (internal organs of sheep, cattle, kangaroo or pigs) to dogs boiling is not always sufficient to kill hydatid cysts in offal.
- Dont allow dogs to defecate near vegetable gardens or childrens play areas.
- Dont allow dogs to roam, or gain access to carcasses of wild or farm animals.
- Treat dogs at high risk with praziquantel (5 mg/kg) every 6 to 8 weeks, and all dogs at least every 12 weeks.