Epidemiology

Cases of trichinellosis in humans have been reported from most regions of the world. Although most documented cases have been associated with ingestion of pork products, the number of potential alternative meat sources is great and recognition of cases caused by other sources of infection continues to increase.

The USA is one example of a country in which trichinellosis has long been recognized as an endemic public health problem. Historically, the infection has been mainly associated with ingestion of pork from domestic pigs; however, this meat source has declined in importance and, in recent years, cases caused by ingestion of meat from a variety of wild animals roughly equal the number of those associated with pork. The presence of the parasite in the USA was first described by Leidy in 1846 and, subsequently, numerous outbreaks involving hundreds of cases were reported in the literature. A National

Fig. 19b.4 Annual incidence of reported cases of trichinellosis, USA 1940-1996

Institutes of Health report published in 1943 and based on data collected at autopsies found that one of every six people (16.7%) in the USA were infected (Wright et al., 1943). National reporting of trichinosis did not begin until 1947, at which time an average of 400-500 cases and 15-20 deaths were reported each year (Schantz, 1983) (Figure 19b.3, courtesy of Dr D. Despommier, see Plate VII). The incidence of the disease declined subsequently as a result of legislation prohibiting the feeding of raw garbage to swine (Federal Swine Health Protection Act, 1980), widespread commercial and home freezing of pork, and increased public awareness of the dangers of eating inadequately cooked pork products. In 1982-1986, an annual average of only 57 cases was reported, with three associated fatalities (Bailey and Schantz, 1990) and during 1991-1996, the most recent period for which summarized data are available (Figure 19b.4), an average of 38 cases per year were reported, with three deaths (Moorhead et al., 1999). Pork was implicated in 60% of the cases, bear meat in 23%, walrus meat in 10% and cougar meat in 7%. The proportion of cases attributable to consumption of commercial pork in the USA continues to decline due to a combination of factors, including the continued reduction in the prevalence of Trichinella spiralis in domestic swine through improved production systems (Gamble et al., 1999), widespread use of home freezers and the common practice of thoroughly cooking pork.

In most years the majority of cases reported in the USA occur in multiple-case outbreaks associated with a common meat source. Outbreaks have often occurred in persons belonging to ethnic groups that prefer pork raw, partially cooked or lightly processed. The usual higher incidence of human trichinellosis in the northeastern USA probably resulted, in part, from the greater concentration of ethnic groups (e.g. German, Italian, Eastern European) that have a fondness for lightly cooked sausage dishes (Schantz et al., 1977). Immigrants from southeast Asia have been identified as the most recently identified group at risk because of their preference for raw spiced pork (Stehr-Green and Schantz, 1986). Persons at high risk for bear and other wild animal meat-associated cases have included native Alaskans, who traditionally eat the meat of bears, walrus and other species that may harbor T. nativa; however, cases also occur sporadically in hunters of such game throughout the country.

In parts of Europe, T. spiralis is enzootic in domestic pigs and T. britovi occurs widely in foxes and other sylvatic carnivores (Pozio, 1997).

In most Western European countries, rigorous standards of meat inspection involving examination of a piece of muscle from the carcass of each slaughtered pig has effectively prevented swine-associated trichinellosis. However, occasional outbreaks, sometimes involving hundreds of cases, have been reported as a result of evasion of established swine inspection procedures. Since 1975, horse meat has become the most important cause of trichinellosis in humans in Western Europe. Cases associated with ingestion of horse meat have emerged as an important problem in France and Italy, where this meat source is popular and usually ingested raw or lightly cooked. Although no horsemeat-associated cases were reported before 1975, since then at least 13 horsemeat-associated outbreaks involving more than 3000 cases have been reported (Dupouy-Camet, 2000). Although horses are generally considered to be herbivores and would not seem vulnerable to meat-borne infections, it is believed that horses may occasionally be fed ground meat for 'fattening' before slaughter, or accidentally ingest rodents or other small mammals that are inadvertently ground up in horse rations.

Trichinellosis has re-emerged in Russia, a number of the former Soviet Socialist Republics (Latvia, Lithuania, Ukraine) and Eastern European countries (Bulgaria, Croatia, Romania), in apparent association with lapses in governmental services, including careful meat inspection. Hundreds and even thousands of cases are now reported every year from these countries (International Commission on Trichinellosis, 1999).

In populations in which pork ingestion is proscribed for religious or other reasons, trichi-nellosis associated with pork ingestion has been rare or non-existent. Nevertheless, outbreaks have been described, in which such populations unknowingly ingested pork. Examples are outbreaks involving hundreds of cases in southern Lebanon, associated with ingestion of ground meat dishes traditionally prepared with lamb but for which pork was substituted (Haim etal., 1997).

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