Comparative studies on geographical strains of Trichinella spiralis

Abstract

A comparison was made of the infectivity of strains of T. spiralis isolated from a serval cat in Kenya, a bear in Alaska, a domestic cat in England and a domestic pig in Poland. With an infective dose of 100-200 larvae, the English strain produced good infections in all the normal laboratory animals, but the Kenya and Alaska strains were much less infective, especially in albino and hooded rats. Even when the dose was increased progressively from 100 to 12,800 there were no deaths; almost all the rats exposed to the Alaska strain remained uninfected and very few larvae were produced with the Kenya strain. However, light infections were produced in a highly inbred strain of Wistar rats. In contrast, the English strain produced larval densities over 10,000 per gramme at a dose of 6,400 in all 3 types of rats and at higher doses all the rats died. With a dose of 5,000 larvae in hooded rats, the Kenya and Alaska strains produced mean densities in the blood averaging 0.6 and 12 per ml. as compared with 165 and 237 per ml. with the English and Polish strains. This was partly due to fewer worms of the Kenya and Alaska strains reaching maturity, but mainly the result of larvae failing to enter the circulation owing to a suppressive activity in the bowel. It was suspected that this reaction might provoke an unusually strong immunity. This was confirmed by exposing rats to 100 larvae of the Kenya strain and challenging them 6 weeks later with 1,000 larvae of the English strain. When the animals were examined 6 weeks later, they had larval densities averaging only 75 per gramme as compared with 465 per gramme in the group immunized with the homologous English strain. The epidemiology and epizootiology of trichinosis must depend as much on the strain of parasite as on the habits of the host, and that the reservoir of infection in wild animals in the Arctic andtropics may not be such a menace to man as is generally believed.