Pathogenic mechanisms not operating in Eimeria necatrix infections.

Having investigated certain aspects of Eimeria necatrix coccidiosis in chickens, workers of the Hannover Veterinary School postulated "that death following a single inoculation of a large number of oocysts is due to an alarm reaction and not a specific pathogenic action of the parasites". Because this hypothesis is somewhat revolutionary in its consept, several pieces of evidence on which it is based and several logical deductions which can be made from it have been examined. It has been confirmed that injection of chichens with cysteamine or 5-hydroxytryptamine 30 min before inoculation of the birds with a lethal dose of E. necatrix oocysts reduces subsequent mortality; the reason for this, however, appears not to be the neutralisation of the proposed shock reaction, but rather an inhibition of the excystation process, brought about indirectly through the host. Inoculation of chickens with a non-lethal dose of E. necatrix oocysts 30 min before inoculation with a lethal dose of oocysts was followed by increased mortality rather than the decreased mortality which the hypothesis would predict. Treatment of chickens with sulphadimidine starting 48 h after inoculation resulted in survival of the birds rather than death which would ensue if in fact mortality was due to a shock reaction irreversibly initiated at the time of inoculation. A direct effect of sulphadimidine on the parasite has been shown both in vivo and in vitro.