Correlation between susceptibility to malaria and babesia parasites and to endotoxicity.

Adult (185g) rats are about twice as sensitive to the harmful effects of injected endotoxin as are younger (65g) rats. This relationship correlates with an earlier report on the densities of Plasmodium berghei at which deaths occur in rats of these two age groups. Similarly lizards, which withstand very high parasitaemias of malaria parasites, are refractory to very large doses of endotoxin. This correlation appears to hold for malaria and babesiosis in all host species for which information is available, with man, for instance, very sensitive to these infections and to injected endotoxin. It is now realized that endotoxicity is not caused by the direct effects of endotoxin, but is the consequence of the release of a range of harmful soluble mediators, mainly from macrophages. Since the susceptibility of a host species to endotoxicity and to malaria and babesiosis correlate, and the illness produced in each case is very similar, these harmful mediators which cause endotoxicity are likely candidates for the origins of much of the pathology of malaria and babesiosis. This concept may also explain the relationship between parasite density and illness in these infections in man.