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 spor ozoites, 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, sh owed a marked decline between the mid-1930s and the mid-1970s, they then be gan to rise again. This fall and rise in mosquito infectivity is attributed to the widespread use of antimalarial drugs, which initially tended to red uce the infectivity of patients for mosquitoes, and the subsequent developm ent of resistance to these drugs in the malarial parasites. The rise observ ed in the sporozoite rates in Muheza in the 1980s-1990s may be attributed t o widespread resistance of P. falciparum to chloroquine, until recently the drug of choice for the treatment of malaria in Tanzania. Changes in the su rvival 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.