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Hydatid disease (echinococcosis) is widespread and endemic in western parts of China, particularly among Tibetan pastoral herders inhabiting grazing lands above 3,800 meters. In Tibet, animal infection rates of 54% in yaks (Bos grunniens) and 81% in sheep have been recorded. Human hydatid infection rates in exclusively stock-raising areas were 8–27%, of which 50% were due to Echinococcus granulosus and 50% to E. multilocularis.1
To initiate a demonstration of hydatid control for the area, 100 four-year-old yaks from the four townships of the Datangba Flatlands in Ganzi County, Sichuan, People’s Republic of China were necropsied to provide baseline data (Table 1
). An additional 25 older yaks slaughtered for meat and identified as originating from Datangba were examined at slaughter. Visually, cysts found in any of the 125 yaks investigated were classified as multilocular (probably E. multilocularis) or unilocular (certainly E. granulosus). Often, both types of cysts were found in the same animal (Table 1). Qiu and others2 had previously described atypical E. multilocularis infections in the livers of yaks and sheep. These cysts did not contain brood capsules or protoscoleces, but had a strong resemblance to E. multilocularis infections in humans. If the multilocular cysts contained infective protoscoleces, this would be the first record of large grazing animals acting as an intermediate host for E. multilocularis. This might be expected because dogs in this area have been shown to be heavily infected with both Echinococcus species.1 An echinococcal lesion in the liver of a yak from a neighboring region (Shiqu County) in the Qinghai-Tibet plateau region was shown by mitochondrial DNA typing to be the result of infection by the G1 genotype of E. granulosus.3 As emphasized by Xiao and others,3 the identification of echinococcal lesions in yaks as E. granulosus rather than E. multilocularis should be confirmed by molecular typing of a larger number of samples, which is what we now report.
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Cysts were cut open and then dissected from liver or lung tissue. They were then fixed immediately in 95% (v/v) ethanol. Portions of unilocular and multilocular cysts (Figure 1) were sent to the Australian laboratory for genotyping. Methods for genomic DNA isolation and purification, and polymerase chain reaction amplification, automatic sequencing, and alignment analysis of fragments of the mitochondrial cox1 gene have been described.4,5 The cox1 sequences obtained from cysts were aligned with published sequences for various E. granulosus genotypes, E. multilocularis, and other Echinococcus spp.4,5
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The analysis of unilocular and multilocular cysts thus showed that both types were caused by the sheep strain of E. granulosus and not E. multilocularis. This was supported by examination of hematoxylin and eosin–stained histologic sections. The convoluted laminated membranes in the multilocular cysts were lined on the inside by germinal membrane, but showed no budding to the exterior, and there were no protoscoleces present. These multilocular cysts were probably a manifestation of an immune response to E. granulosus that walls off the developing cyst so that the laminated and germinal membranes continue to proliferate within a confined space. The unilocular cysts also had no evidence of protoscoleces or developing brood capsules.
Although the liver and lung cysts from the 25 necropsied older yaks were often 2–3 times the diameter of those from 4-year-old yaks, there was no increase in prevalence or incidence of cysts. It is tempting to conclude that the continued exposure to E. granulosus eggs in this environment, in which no hydatid control has been practiced until now, results in the stimulation and maintenance of immunity to reinfection.
A Ganzi Hydatid Control and Community Health Project in Sichuan is intended to provide guideline information for the development of future hydatid control programs in China, and is focused on interrupting the lifecycle of the hydatid parasite by dosing dogs with praziquantel and vaccinating6 the animals that host the cystic stage of the tapeworm. An understanding of the tapeworm life cycle and how people can avoid becoming infected with hydatid disease is included as part of the community and health education activities.
At the beginning of the project, it was thought that Datangba Flatlands yaks, sheep, and goats could all produce cysts that would be able to reinfect dogs with the parasite. Vaccination of all these animals would prevent infections from becoming established and reduce the chances of dogs becoming reinfected by eating animal organs containing hydatid cysts.
Emerging technology has shown that not all types of grazing animals are involved in the transmission of cystic hydatid disease caused by E. granulosus to dogs.7,8 Zhang and others5 reviewed previous work in China showing the predominance of the sheep strain (G1 genotype) and a report of this genotype in a sample of hydatid material from yaks.
We have now shown that this local Datangba sheep strain of E. granulosus usually only produces protoscoleces in sheep and goats, and not in yaks. There are reports of yaks contributing to human hydatid disease in a population of yaks around Qinghai Lake in Qinghai and in cattle in Xinjiang. The Qinghai yaks and Xinjiang cattle are now thought to actively host a different (G5) genotype,4 which does produce cysts that are infective for dogs, but this cycle may not occur elsewhere in western China. The G5 (cattle-dog strain) has been shown by genotyping human cyst material to be infective to humans.7
We are now proposing to collect human hydatid cyst material from people in Datangba undergoing hydatid surgery at the Ganzi Hospital to determine the infective hydatid genotype. If it is solely or predominantly G1, as we predict, the control of hydatid disease caused by E. granulosus will concentrate there on sheep and goats, while putting less emphasis on yaks.
Received March 11, 2004. Accepted for publication July 7, 2004.
Financial support: Work in the Brisbane Laboratory is supported by grants from the National Health and Medical Research Council of Australia and the Wellcome Trust.
Authors’ addresses: David D. Heath, Wallaceville Animal Research Centre, AgResearch Limited, PO Box 40063, Upper Hutt, New Zealand. Li Hua Zhang and Donald P. McManus, Molecular Parasitology Laboratory, Australian Centre for International and Tropical Health & Nutrition, The Queensland Institute of Medical Research and University of Queensland, 300 Herston Road, Brisbane, Queensland 4029, Australia, Telephone: 61-7-3362 0401, Fax: 61-7-3362 0104, E-mail: donM{at}qimr.edu.au.
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