Interactions between parasites and their hosts: Metabolic aspects

Anaerobic mechanisms are the predominant energy-yielding pathways of endoparasites. However, all parasitic helminths use oxygen when it is present, and appear to be capable of oxidative phosphorylation. Bryant (1982) has argued that anaerobic pathways are primitive and that aerobic pathways are a more recent adaptation. The question as to why oxygen is not used more efficiently by parasitic helminths has still not been answered satisfactorily.The phosphoenolpyruvate branchpoint is critical in helminth metabolism, and the controlling factors influencing PK and PEPCK are still not clearly understood. This is further compounded by differences between strains (Mettrick & Rahman, 1984).The effect of size of an intestinal parasitic infection may, at low levels of infection, stimulate net glucose and ion flexes, while high levels of infection produce malabsorption. It is of particular interest that the washings from a low level intestinal infection may stimulate glucose and ion fluxes in an uninfected animal; this appears to be a new component in the compensatory mechanisms of the host response.5-hydroxytryptamine both influences helminth glycolysis and has a role in helminth neurophysiology. Current evidence indicates that 5-HT is synthesised in very small amounts by H. diminuta and is also taken up from the intestinal luminal pool. There is evidence that 5-HT and acetylcholine are antagonistic; 5-HT appears to be a modulator of longitudinal muscle contraction with glutamate acting as the neurostimulator.