Hydatid disease caused by the tapeworm Echinococcos granulosus is alive and very well, living in large areas of all states in eastern Australia1, but also in some areas of Western Australia. Currently we have no idea what is happening in South Australia, since nobody has looked for about 40 years.
The definitive host of E. granulosus is classically domestic dogs. The tiny (2-3mm) hydatid tapeworm lives in the small intestine (pre-patent period about 42 days in Australia) with eggs released in gravid terminal segments and passed into the environment with faeces from infected dogs. Eggs can be further distributed by agents such as wind, rain and coprophagus flies. The eggs are thought to remain viable, under optimal conditions for about one year. However, this estimate may have to be reassessed following the publication of a study in Argentina where eggs extracted from dog faeces left in the environment for 41 months were still infective for sheep! The intermediate host for E. granulosus is mainly sheep but a range of other domestic livestock hosts are susceptible to infection, namely goats, cattle, camels, alpacas, llamas donkeys and horses. However, infection in Australian-bred donkeys, horses, alpacas and llamas has yet to be reported. Importantly, humans can also act as an intermediate host and infection may lead to debilitation and in extreme cases death. Hydatid cysts in sheep grow slowly and do not become “fertile”, containing protoscoleces (the next generation of hydatid tapeworms) until they have been in the sheep for about two years
The hydatid transmission situation in Australia is further complicated by a widespread wildlife reservoir that is important in perpetuating transmission of the parasite. Wildlife provides a reservoir of infection that periodically “spills over” into domestic dogs, livestock and humans2. Transmission of E. granulosus in Australian wildlife occurs between wild dogs (dingoes and dingo/domestic dog hybrids) and macropod marsupials, mainly wallabies, but infection is also seen commonly in kangaroos and less commonly in wombats. Foxes are a less important wildlife definitive host in transmission because few foxes within a population are infected and only with small tapeworm burdens, usually less than 50 worms. This contrasts with worm burdens of wild dogs that are commonly several tens of thousands to hundreds of thousands of tapeworms. Feral pigs can also act as intermediate hosts, but compared to macropods they are unimportant. This is mainly because a large proportion of pig cysts fail to become fertile and also wild dogs predate almost exclusively on young, small pigs, animals whose cysts are unlikely to have developed sufficiently to have become fertile.
The distribution of E. granulosus in Australia is rainfall dependant with transmission occurring in areas where there is 25mm or more of rain for 6 months of the year3. The parasite occurs most abundantly in all eastern states of Australia in areas associated with the Great Dividing Range.
A study funded by Novartis Animal Health surveyed intestinal helminths in 1,425 rural domestic dogs, focusing particularly on E. granulosus4. Farmers were asked to fill in a questionnaire focusing on domestic dog management, including de-worming frequency and diet and also feral animals on properties, hunting and offal disposal. Faeces were collected from domestic dogs living in all eastern states, from Tasmania to Queensland (1,119 from the mainland and 306 from Tasmania). Faeces were tested using traditional faecal flotation and microscopy to visualize helminth eggs and an E. granulosus coproantigen-ELISA as a screen for substances released into faeces by E. granulosus tapeworms. Since all species of taeniid eggs are morphologically identical, DNA was extracted from taeniid eggs present in faeces and subjected to PCR and sequencing to identify the species of taeniid eggs present. Also, some of the E. granulosus coproantigen-ELISA positive faeces were also subjected to an E. granulosus copro-PCR, as confirmation, to identify free DNA of E. granulosus in faeces.
There was a good response by rural dog owners with 179 questionnaires being returned but not everyone answered all questions. The dogs included in the study came from a variety of farm types but mostly (67%) were from properties where sheep were raised as the sole enterprise or part of a mixed enterprise. The average number of dogs per property was 4, comprising three working dogs and one house dog. Less than one percent of owners also kept hunting dogs. Almost all dogs (98.3%) were fed dry dog food either exclusively or as part of their daily diet. Of those fed a mixed diet, 29.6% were also fed fresh raw meat of which 5.6% were fed sheep meat. Home slaughter was undertaken by almost half the owners (49.2%) and between 7.8% and 8.9% of owners fed offal of lambs or mutton sheep to their dogs, mainly hearts. In addition, 74.3% of owners thought their dogs could have access to carcasses of dead animals and just over one third (35.2%) of owners also fed whole rabbits to their dogs
There were two preferred dog de-worming regimes, two monthly or four monthly and five percent of owners reported never de-wormed their dogs. More than half owners reported weighing their dogs before treatment.
The faeces of 11 dogs were found to contain eggs of taeniid tapeworms. None was found to be E. granulosus. Six were identified as Taenia pisiformis, one T. serialis, and four as T. hydatigena. Taenia ovis was not found.
The 1119 mainland faecal samples were obtained on a voluntary basis whilst the 306 samples from Tasmania were targeted, being collected by the Tasmanian DPI hydatid control officer from owners living in areas around where hydatid-infected cattle had been reported. Positive tests were obtained in 21/1,119 of the mainland samples and in 24/306 of the Tasmanian samples. Fifteen faecal extracts from Tasmania, positive in the coproantigen ELISA were also subjected to an E. granulosus copro PCR. Three of the samples returned a positive result for E. granulosus. Nevertheless, a major issue in this study was the lack of E. granulosus eggs or segments in the faeces from the coproantigen-positive dogs suggesting the results could have been false positives. However, we do not think this is the case, we are confident these dogs were positive for E. granulosus.
This study demonstrated E. granulosus still occurs in Australian rural domestic dogs, but at low levels. The two most important influences on reducing the prevalence of E. granulosus in Australian dogs has been the development of palatable dry dog food and the highly efficient cestocidal drug praziquantel. Since coming off patent, praziquantel has become readily available being incorporated in a wide range of relatively cheap multi de-wormers for dogs available at supermarket and stock and station agents. On mainland Australia, the risk of spill-over from an extensive wildlife reservoir remains in many rural areas, therefore, we should not allow hydatids to “drop off the radar”.
Funding for this study was provided by Novartis Animal Health