Hydatids – when a dog is not man's best friend
Key text
This topic is sponsored by The Bayer Group and the bequest of J S Anderson, FAA.
Dogs are the main culprits in the spread of hydatids, a common but largely ignored disease in Australia that can kill people, livestock and wildlife.
There are few things a young child likes more than to be licked in the face by a friendly dog. But this lick of affection might also be a kiss of death – it could infect the child with hydatids, a nasty and potentially fatal disease.
What is hydatid disease and where does it occur?
Hydatid disease is a parasite infection of humans and animals. In Australia hydatid disease is caused by a tiny tapeworm, Echinococcus granulosus.
Hydatid disease occurs throughout most of the world. In Australia, the disease is most common in the sheep-farming areas of New South Wales, the Australian Capital Territory, Victoria, southwest Western Australia and eastern Queensland. It is believed to occur in South Australia but since no recent studies have been conducted there, its status is unknown. It has also been found in cattle populations in the Kimberley region of Western Australia, in northern Queensland and near Darwin in the Northern Territory.
Scientists believe that E. granulosus arrived in Australia soon after European settlement, probably in infected sheep or dogs. Its distribution is determined largely by climate. Its eggs, which sometimes must survive in the outside world for long periods, are vulnerable to high temperatures and desiccation, although they can withstand freezing. The disease is therefore most prevalent in areas where the average monthly rainfall is above 25 millimetres for six months of the year.
The life-cycle of E. granulosus
E. granulosus has a peculiar life cycle that requires definitive hosts and intermediate hosts. Definitive hosts are carnivores such as dogs, foxes and dingoes, while intermediate hosts are usually herbivores such as sheep, cattle, kangaroos, wallabies and wombats. Humans can also act as intermediate hosts – but since the life cycle relies on the carnivores (eg, dogs) eating infected herbivores (eg, sheep), humans are usually a ‘dead-end’ for the parasite.
Intermediate hosts such as sheep become infected by ingesting the eggs of the parasite, which are passed into the outside world in the faeces of the definitive host such as a dog (Box 1: The life-cycle of E. granulosus). Inside the sheep (or human), the eggs hatch and release tiny hooked embryos which travel in the bloodstream, eventually lodging in an organ such as the liver, lungs, bone marrow, kidneys or brain. There, they develop into hydatid cysts, which are the cause of medical problems in humans and are the most damaging manifestation of the disease. Disease symptoms arise as the cysts grow bigger and start blocking or putting pressure on blood vessels or organs. Inside the cysts are thousands of tapeworm larvae, the next stage in the life cycle of the parasite.
The cysts are small to begin with but grow steadily – in a year they will expand by 1-1.5 centimetres in diameter – and they may continue to grow during the life of the host. Cysts the size of golf and tennis balls are common, while cysts the size of soccer balls have been observed. The largest ever discovered in a human had a volume of 50 litres – equivalent to about a third of an average-sized bathtub. The sheer physical size of these cysts will eventually cause problems in the organs in which they are lodged. And, if the cyst ever bursts – if the sufferer falls, for example – the resultant toxic shock (anaphylaxis) will probably be fatal.
Sustaining the life-cycle
In Australia, hydatid disease has historically been sustained in a farm-dog to sheep cycle. When dogs are fed the offal of infected sheep, they become infected with E. granulosus and in turn produce eggs in their faeces, which infect new sheep with the disease.
A few basic changes in farm practices can help eradicate or at least greatly reduce the incidence of the disease. The first and most important of these is to prevent the farm dogs from eating the offal of dead livestock. The second step is to ‘worm’ dogs regularly, which usually involves feeding them a drug called praziquantel to kill the tapeworm. By employing such practices, hydatid disease has been virtually eliminated in Tasmania and New Zealand, where it was once very common.
The wildlife ‘reservoir’
However, eradicating the disease from the Australian mainland is a much more difficult task – in fact, scientists consider it to be virtually impossible. Most farmers these days are careful in their management of livestock offal because of the risk of hydatids, but a wild ‘reservoir’ of the disease confounds their efforts. Feral dogs, dingos and foxes are all carriers of the E. granulosus tapeworm; on forays from their bushland refuges, they carry eggs into farming areas where they are inevitably ingested by livestock.
Making matters worse, some Australian native species of wildlife are also susceptible. Kangaroos, wallabies and wombats have all been found with hydatid cysts. Since these animals form a large part of the diet of feral dogs, dingos and foxes, we can see that the E. granulosus life cycle can be sustained even without sheep, cattle and farm dogs playing a role.
The risk to human health
Hydatid disease causes significant problems to the health of wildlife and livestock and may have ecological and economic effects. But perhaps the biggest concern is for human health. It can be a killer. In Victoria, for example, it killed at least 584 people in its ‘heyday’ years between 1862 and 1881. It is less of a killer today but still a concern. For example, 321 people were diagnosed with hydatid disease between 1987 and 1992 in New South Wales and the Australian Capital Territory. The extent of human hydatid disease in Australia is not accurately known. There is no register of infections and it is not a notifiable disease here. However, infected people usually are tested for blood antibody levels and this provides some insight into the extent of the problem. Scientists estimate that there are approximately 500 people currently being treated and monitored for hydatid infection and that about 50 new cases of infection arise each year.
Direct contact with infected dogs is perhaps the most common way by which people become infected by the parasite. As explained earlier, E. granulosus eggs are passed into the environment in dog faeces. We all know that dogs have a habit of sniffing each other’s rear ends, and licking their own. In this way, eggs can be transferred to the dog’s muzzle, tongue and fur. The eggs can then be transferred to people when they pat the dog or are licked by it.
People who live on farms are most commonly infected with hydatids, but not exclusively. The hydatids tapeworm is reported to be prevalent in dogs used by pig and kangaroo shooters in Perth. These shooters feed the offal of their kills to their dogs. Since hydatids is present in the wild population of kangaroos, pigs and other animals, the dogs become infected and may then pass the disease on to their owners.
Flies might be an agent of transfer if they come into contact with infected dog faeces and then alight on your face or on food you are about to eat. And it may be possible to ingest eggs simply by touching grass containing the eggs and then touching your face or mouth. This could happen on farmland or even in bushland when the disease is prevalent in the resident wildlife. It could even happen in suburbia – foxes are known to raid suburban areas in search of food. Since they are potential carriers of the disease, they may serve to distribute E. granulosus eggs into picnic areas and outdoor cafes in the hearts of towns and cities.
The cure
One of the biggest problems with curing hydatids is that it may go undetected in the body for many years, by which time an infected person may have one or more large cysts. (Often a cyst is only detected when it has grown large enough to interfere with the proper functioning of the infected organ.) Another problem is that the cyst wall forms a physical barrier to foreign substances, protecting the tapeworm larvae from the person’s own immune system and from drugs.
The most common and effective treatment is surgery to remove the cysts, followed by a course of drugs to kill any remaining tapeworm larvae that might be in the body. Nevertheless, even surgery will not always cure the disease. In one study, 22 per cent of 34 surgically treated patients developed new cysts within 3 years of the initial surgery. The application of drugs immediately after surgery may reduce this rate.
Prevention is much better than cure. Washing your hands before eating or drinking is important, particularly after contact with animals. Scientists have also developed an effective vaccine for sheep, giving rise to hope that a hydatids vaccine suitable for humans may one day be available (Box 2: Hydatids vaccine).
Boxes
1. The life-cycle of E. granulosus
2. Hydatids vaccine
Posted July 2000.