Trichinella in wildlife and pork production: evaluation of risk-based monitoring

Abstract

Nematodes of the genus Trichinella are parasites of mammals, birds and reptiles. Twelve taxa are recognised in this genus. Humans may develop trichinellosis through consumption of undercooked meat from Trichinella-infected wildlife and domestic animals. The severity of the disease depends on the dose and species ingested. Meat originating from wild boar and pigs that were kept under non-controlled housing is the most important source for trichinellosis outbreaks in humans in Europe. The overall aim of this thesis was to study prevalence of Trichinella spp. in wildlife host species in the Netherlands, and to assess the risks for humans by using a quantitative microbial risk analysis (QMRA) approach. A QMRA approach allows quantification of hazards in the meat production chain, including attribution of factors as carcass control, pig husbandry system, types of meat and meat products to support risk-based methods for Trichinella control. We studied three wildlife host species that play a role in the sylvatic lifecycle of Trichinella. We show that the Trichinella prevalence in Dutch red fox (0.27%) decreased significantly in the study area, compared to fifteen years ago (3.9%, p = 0.0006). In contrast, other helminth fauna in Dutch foxes increased in biodiversity over the last three decades. Trichinella seroprevalence in Dutch wild boar during the period 2007 – 2015 (0.16%) was shown to be significantly lower than in 2004 - 2005 (0.61%, p = 0.0311), using a validated in-house ELISA. However, we were not able to demonstrate Trichinella in wild rats using artificial digestion. Using molecular techniques, we demonstrate hybridisation between T. spiralis and T. britovi from wild boar and red fox in a highly endemic area in Europe (Poland), where both Trichinella species co-exist. In view of proven absence of Trichinella from slaughter pigs reared under controlled housing during the last decades, and the large sums that are spent on individual carcass control, a risk-based control approach has been approved for Trichinella control in Europe, but the scientific background is lacking. We developed a QMRA model for Trichinella, which quantifies distribution of Trichinella ML in and among swine, test sensitivity at carcass control, partitioning of edible pork parts, Trichinella ML distribution in edible muscle types, heat inactivation by cooking and portion size. This model was combined with a dose response model for Trichinella species that was previously developed in our lab, to estimate the incidence of human illness after consumption of infected meat. The QMRA model was validated with Polish data and we estimated an average incidence of 1.56 trichinellosis cases per million persons per year (Mpy) from consumption of pigs that were reared under non-controlled housing, and 1.97 cases per Mpy from consumption of wild boar. The total estimated incidence is similar to the observed incidence of human trichinellosis (1.15 Mpy) in Poland. Overall, in the European Union, we estimated less than one predicted case of trichinellosis every 39.4 years from pork produced under controlled housing. Therefore, Trichinella testing of pigs reared under controlled housing is not adding any value to protect human health.