Trichinella spiralis in Mammals at Mountain Lake, Virginia

Abstract

The occurrence, epizootiology, and serology of Trichinella spiralis infection in a sylvatic environment were studied. Eleven of 85 medium-sized mammals were infected, including 4 opossums, 6 spotted skunks, and 1 longtail weasel. None of 997 rodents and insectivores examined were positive. The Card Flocculation Test was used as a simple, rapid serological test for surveying the prevalence of trichinosis in wildlife populations. The epizootiology of Trichinella spiralis (Owens, 1835) in naturally infected hosts in Iowa was reviewed by Zimmermann et al. (1962, 1963). Recently, Martin et al. (1968) studied T. spiralis infections in wild animals on two Illinois swine farms. This investigation is concerned with the occurrence, epizootiology, and serology of T. spiralis infections in mammals trapped in a sylvatic environment. MATERIALS AND METHODS During the summers 1966-68, a total of 1,094 mammals, representing 25 species, were examined for T. spiralis. Animals were trapped in the vicinity of the Mountain Lake Biological Station, Giles County, Virginia. Medium-sized mammals were live-trapped along Secondary Highways 700 and 613 from Sinking Creek to the entrance of Jefferson National Forest (1,800 to 3,800 ft above sea level). Most of the small mammals (examined in 1967 and 1968 only) were trapped by the Mammalogy Class at 24 trap sites in a 5-mile radius of the Station and from 4 major habitats: rocks, forest, field, and stream bank. Each animal was screened for infection by tissue press examination of diaphragm and intercostal muscle. Three 25to 30-g samples of ground tissue (diaphragm, intercostal, tongue, leg, and back muscle) from positive animals were digested in pepsin-hydrochloric acid, and the average number of larvae per gram of tissue determined by total counts and dilution methods. Medium-sized mammals were chloroformed and blood samples were obtained by heart puncture. The serum was separated by centrifugation, placed in sterile vials, coded, frozen, and subsequently serologically tested with the Trichinosis Card (Flocculation) Test, described by Anderson et al. (1963) and Schultz et al. (1967). The Trichinosis Card Test antigen suspension is a Melcher's acidReceived for publication 21 February 1969. * Supported in part by NSF (GB-3439) summer postdoctoral fellowship awards (1966-68) from the University of Virginia and by a Grant-in-Aid of Research from the Society of the Sigma Xi. soluble fraction of T. spiralis larvae adsorbed onto cholesterol-lecithin crystals in a suspension of charcoal particles. RESULTS AND DISCUSSION The present findings (Table I) indicate a definite focus of T. spiralis infection in Southwest Virginia. During a 3-year period, 4 of 26 opossums, 6 of 16 spotted skunks, and 1 longtail weasel were infected with T. spiralis. The prevalence rates are considerably higher than those reported by Zimmermann et al. (1962) for an 8-year period in Iowa, and undoubtedly more extensive sampling throughout Virginia would reduce these rates. Although the primary suggested method of transmission of T. spiralis in wildlife populations is scavenging of carrion, other methods such as predation, fecal, and mechanical transmission by carrion beetles and fly larvae have been recognized (Zimmermann and Hubbard, 1963). T. spiralis has been reported from the opossum, spotted skunk, and several species of Mustela (Zimmermann et al., 1962, 1963); however, the recovery of T. spiralis from the longtail weasel (Mustela frenata) is apparently the first such finding in North America. None of the 25 striped skunks examined by the tissue press technique were infected. The longtail weasel, opossum, and spotted skunk are known to feed on carrion. Garbage-fed swine, local slaughtering practices of discarding viscera in the woods, and feral dogs and cats in the vicinity of the trapping area are possible sources of transmission. A total of 997 small mammals representing 18 species were examined by the tissue press technique and were not infected with T. spiralis (Table II). Predation was thought to be of secondary importance in transmission of