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 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.