A genetic comparison of human and wildlife isolates of Echinococcus granulosus in Queensland: public health implications

Abstract

Objective: To test the hypothesis that the hydatid parasite infecting macropods and dingoes in Queensland is a sylvatic strain of Echinococcus granulosus, distinct from the domestic strain which produces cysts in sheep and humans.

Design: Molecular biological techniques were used to compare DNA isolated from hydatid cysts from humans, local macropods and sheep from New South Wales and the United Kingdom, as well as from adult tapeworms in dingoes.

Setting: The human cysts were surgically resected from two patients seen with hydatidosis in Brisbane teaching hospitals over a one-year period. Neither patient had had previous contact with sheep farms. Macropods and dingoes were shot randomly in the localities where the patients presumably acquired their infections. Sheep liver cysts were obtained from abattoirs.

Methodology: Studies comprised extraction of DNA from cysts, digestion by a series of restriction endonucleases, slab gel electrophoresis. Southern blotting and then hybridisation with defined DNA probes. Polymerase chain reaction, in combination with direct DNA sequencing, was used to compare DNA from cysts and adult worms from dingoes.

Results: The restriction fragment length polymorphism (RFLP) patterns of DNA from all cysts and a defined mitochondrial DNA sequence from all sources were indistinguishable. This finding is significant as both techniques can clearly distinguish between genetically distinct, well characterised strains of E. granulosus.

Conclusions: Hydatid cysts are prevalent in some macropod populations and adult worms are common in dingoes. Since there are relatively few sheep-rearing areas in Queensland, contact with wild animals may be the main source of human hydatid infection in this State. The strain of E. granulosus in both patients was genetically indistinguishable from that found in macropods, dingoes and sheep from New South Wales and the United Kingdom. This strongly suggests that the domestic strain of E. granulosus, or a form very close genetically, freely infects Australian wildlife, and argues against the existence of a distinct sylvatic strain. The implications for public health are considerable.