Conjoint Heligmosomoides polygyrus and Trypanosoma congolense infection in mice

The economic importance of gastro-intestinal nematodes and African trypanosome infections of farm animals in the tropics has been presented as the rationale for research into the aspects of these infections with the hope of devising control measures which will increase animal productivity. The scientific literature on the development, immunology and pathology of gastro-intestinal nematode and protozoan infections with particular reference to Heligmosomoides polygyrus and African trypanosomes and the effects of concurrent infections, with these classes of parasites, on the host has been reviewed.Experiments were conducted to develop a suitable laboratory model involving H. polygyrus and Trypanosoma congolense in female TO mice in order to study the effects of some factors such as the timing of conjoint infections, the role of immunity to the helminth infection in the face of conjoint challenge infection and the responses of the host as well as that of the parasites to conjoint infections.Infections with stabilated blood stream forms of T. congolense (TREU 1881) but not the infective metacyclics grown in vitro gave reproducible parasitological results which together with infections of 500 L3 of H. polygyrus in mice produced a suitable host-parasite system in which aspects of chronic gastrointestinal nematode and blood protozoan infections were studied.Except when T. congolense was superimposed on a 10-day old H. polygyrus infection, mice conjointly infected with T. congolense during a primary H. polygyrus infection were severely compromised, resulting in enhanced mortality. The synergistic pathogenic effects of dual infections in mice were particularly marked when T. congolense infection preceded infection with the nematode. T. congolense infection depressed the immune (cellular and humoral) responses which normally occur in mice after primary H. polygyrus infection. The protozoan infection either reduced or totally inhibited immunity against a challenge H. polygyrus infection. Possible mechanisms of this immunosuppression which include among others, the generation of suppressor macrophages and the inhibition of eosinophils, are discussed. These observations suggest that, conjoint infections with these parasites produce deleterious synergistic interactions which affect productivity and even cause further deaths.Although the protective responses against homologous challenge in mice immunized by abbreviation of H. polygyrus adult infection were completely lost as a result of concurrent infection with T. congolense, the stronger protection in those immunized by an abbreviated larval infection was merely reduced. These observations suggest that animals with a strong immunity to gastro-intestinal nematodes may largely overcome the suppressive influence of the trypanosome. It is suggested that enhanced resistance through abbreviation of primary nematode infection within the first one week of grazing an infected pasture at the beginning of the grazing season might combat problems of synergistic interactions during natural infections of nematodes and trypanosomes in ruminants