MALARIA INFECTION IN HUMANS
The presence of malaria parasites in a blood sample is an indication of infection, but not necessarily of disease. People who live or have resided for several years in areas where malaria is endemic may have rare (extremely rare) parasites in their blood; in such cases, the symptoms they report to their physician, i.e. fever, may be due to a variety of causes, not necessarily to the malarial infection. Only the asexual cycle in the peripheral bloodstream (blood schizogony) is capable of triggering disease. Exo-erythrocytic stages (liver schizogony) do not give rise to any pathological manifestation. Human carriers of mature gametocytes represent the only “reservoir” of human plasmodia responsible for disease transmission: no animal “reservoirs” exist, though monkeys are susceptible to infection by P. malariae. In tropical regions of Africa chimpanzees may in some cases constitute a natural “reservoir” of infection. Individuals who live in malaria endemic areas and are subject to repeated plasmodium infection develop a special type of immunity called “premunition”, which is capable of hindering the parasite’s multiplication. As a result, clinical paroxysms become increasingly infrequent and less serious and an asymptomatic parasitemia eventually sets in. However, this kind of immunity tends to wane if the individual spends a lengthy period in a non-endemic zone (as in the case of immigrants to Western countries), which means they may once again be susceptible to a severe form of malaria infection.