Further Studies on the Effect of Thiabendazole on Trichinosis in Swine, with Notes on the Biology of the Infection

Abstract

Eight experi-ments were conducted, involving 179 pigs, and dealing with the efficacy of thiabendazole in the prophylaxis and therapeusis of Trichinella spiralis infections in swine. A single subcutaneous dose of thiabendazole at 150 or 200 mg/kg effectively suppressed the enteral phase of trichinosis, when given 48 hr. before inoculation. Smaller dosages gave variable protection, and in 2 instances allowed the development of an infertile enteral population. Immature trichinella were more sensitive to thiabendazole therapy than were adult worms. Immature worms were eliminated from the gut by a single dose of 150 mg/kg; lower dosages were not tested. Adult worms were usually eliminated by doses of 25 mg/kg given twice daily (for 2 to 3 weeks), beginning 1 or 2 weeks after inoculation. During the invasive phase of trichinosis, beginning 1, 2, or 3 weeks after inoculation, the number of trichinella larvae developing in the musculature was greatly reduced by twice-daily doses of 25 or 75 mg/kg. Under these conditions, the effect of the drug on the parenteral phase may have been enhanced by its effect on the enteral phase. When treatment was begun at 5 to 8 weeks after inoculation, doses of 25 mg/kg, twice daily, had no detectable effect on larvae in the muscle of pigs, but twice-daily doses of 75 or 150 mg/kg reduced the number and viability of the larvae. The last dosage, amounting to 300 mg/kg/day, was toxic to the pigs. In histopathological sections some of the larvae appeared degenerate and presumably had been killed by treatment. Some of the larvae that survived treatment of the host were apparently incapable of withstanding peptic digestion; other larvae withstood digestion but were noninfective to mice. Whether such larvae are in fact moribund in the host muscle is not known. Data obtained from untreated control pigs indicated that a gross expulsion of enteral trichinella occurred 3 to 5 weeks after inoculation. The expulsion was accompanied by a reversal in the sex ratio of the worms, so that the small residuum of worms consisted mainly of males. Pigs exposed to 10,000 larvae/kg of body weight developed no fever and only moderate eosinophilia.