Epidemiology of Trichinella infection in the horse: the risk from animal product feeding practices

A discovery in 2002 of a Trichinella spiralis-infected horse in Serbia offered an opportunity to conduct needed epidemiological studies on how horses, considered herbivores, acquire a meat-borne parasite. This enigma has persisted since the first human outbreaks from infected horse meat occurred in then 1970s. The trace back of the infected horse to a farm owner was carried out. Interviews and investigations on the farm led to the conclusion that the owner had fed the horse food waste in order to condition the horse prior to sale. Further investigations were then carried out to determine the frequency of such practices among horse owners. Based on interviews of horse producers at local horse markets, it was revealed that the feeding of animal products to horses was a common practice. Further, it was alleged that many horses, particularly those in poor nutritional condition would readily consume meat. A subsequent series of trials involving the experimental feeding of 219 horses demonstrated that 32% would consume meat patties. To confirm that horses would eat infected meat under normal farm conditions, three horses were offered infected ground pork balls containing 1100 larvae. All three became infected, and at necropsy at 32 weeks later, were still positive by indirect IFA testing, but not by ELISA using an excretory-secretory (ES) antigen. This result indicates that further study is needed on the nature of the antigen(s) used for potential serological monitoring and surveillance of horse trichinellosis, especially the importance of antigenic diversity. The experimentally-infected horses also had very low infection levels (larvae per gram of muscle) at 32 weeks of infection, and although the public health consequences are unknown, the question of whether current recommended inspection procedures based on pepsin digestion of selected muscle samples require sufficient quantities of muscle should be addressed. It is concluded that horses are more willing to consume meat than realized and that the intentional feeding of animal products and kitchen waste is a common occurrence among horse owners in Serbia (and elsewhere?). This is a high risk practice which demands closer scrutiny by veterinary and food safety authorities, including the implementation of rules and procedures to ensure that such feeds are rendered safe for horses, as is now required for feeding to swine.