The rise and fall of malarial sporozoite rates in Anopheles gambiae s.l. and An. funestus in north-eastern Tanzania, between 1934 and 1999.

The proportion of Anopheles mosquitoes found to be carrying Plasmodium sporozoites, usually called the 'malarial sporozoite rate', has often been used as a measure of mosquito infectivity. Although the sporozoite rates found in Anopheles gambiae and An. funestus in Muheza, north-eastern Tanzania, showed a marked decline between the mid-1930s and the mid-1970s, they then began to rise again. This fall and rise in mosquito infectivity is attributed to the widespread use of antimalarial drugs, which initially tended to reduce the infectivity of patients for mosquitoes, and the subsequent development of resistance to these drugs in the malarial parasites. The rise observed in the sporozoite rates in Muheza in the 1980s-1990s may be attributed to widespread resistance of P. falciparum to chloroquine, until recently the drug of choice for the treatment of malaria in Tanzania. Changes in the survival rates, abundance, or predominant species of the mosquito vectors are unlikely to have influenced the pattern observed. The role of antimalarial drugs in malaria transmission risk is discussed.