Combined cross-sectional and case-control study on Echinococcus multilocularis infection in pigs in Switzerland

Abstract

The canid tapeworm Echinococcus multilocularis causes alveolar echinococcosis (AE) in humans and other intermediate hosts. Depending on the permissiveness of the intermediate host, the larval form of E. multilocularis (metacestode) may be either fertile, e.g. in rodents, and thus supporting the life cycle of the parasite, or infertile, e.g. in pigs, and thus interrupting the life cycle. Pigs have been shown to act as aberrant hosts for the metacestode and consequently develop liver lesions but represent a dead-end for the parasite. Routine liver inspection at slaughter provided the basis for a large-scale surveillance study on E. multilocularis infection in pigs. The aim of this combined cross-sectional and case-control study was to estimate the minimal prevalence of E. multilocularis in pigs in Switzerland, to find factors associated with infection, and to assess potential regional clusters of infection. During the 12-month-study period, approximately 85% of all pigs slaughtered in Switzerland were assessed. In total, 450 pig livers with macroscopic lesions suggestive of E. multilocularis infection were analysed. Of those, 200 samples were positive by E. multilocularis-PCR. Thus, the overall minimal prevalence detected by molecular means was 0.009% in all slaughter pigs (200 of 2'143'996), 0.008% in finishing pigs (177 of 2'123'542), and 0.11% in breeding pigs (22 of 20'454). Histology revealed the unique presence of a laminated layer in 105 cases, and an additional germinal layer detected in a single case. Protoscoleces could not be observed in any of the lesions. Factors positively associated with infection were "foxes seen in the pig shed", "foxes on premises", "presence of other animals in the shed", "absence of a hygiene barrier", "outdoor feeding", "feeding grass", "lack of rodent control", "not having own dogs on the farm" and "infrequent deworming of sows". Infection was present in all regions sampled and was representative of the important pig rearing areas of Switzerland, without evidence of any obvious geographical cluster. Conclusively, our study provided further evidence of widespread environmental contamination with E. multilocularis eggs in Switzerland. Furthermore, the absence of protoscoleces in any of the lesions supported the concept that pigs act only as a dead-end host and thus do not contribute to the life cycle of the parasite. Factors associated with E. multilocularis infection were in-line with parasite biology, and many can be addressed by increasing hygiene and management standards.

Keywords: Echinococcus multilocularis; alveolar echinococcosis; case-control study; liver; risk factors; slaughter pigs; swine.