Epidemiology

Malaria is transmitted by the bite of female anopheline mosquitoes. The parasite undergoes a temperature-dependent cycle of development in the gut of the insect, and its geographical range therefore depends on the presence of the appropriate mosquito species and on adequate temperature. The disease occurs in endemic or epidemic form throughout the tropics and subtropics except for areas above 2000 m: Australia, the USA, and most of the Mediterranean littoral are also malaria-free. In hyperendemic areas (51-75% rate of parasitaemia, or palpable spleen in children 2-9 years of age) and holoendemic areas (> 75% rate) where transmission of infection occurs year round, the bulk of the mortality is seen in infants. Those who survive to adulthood acquire significant immunity; low-grade parasitaemia is still present, but causes few symptoms. In mesoendemic areas (11-50%) there is regular seasonal transmission of malaria. Mortality is still mainly seen in infants, but older children and adults may develop chronic ill health due to repeated infections. In hypoendemic areas (0-10%), where infection occurs in occasional epidemics, little immunity is acquired and the whole population is susceptible to severe and fatal disease.

Malaria can also be transmitted in contaminated blood transfusions. It has occasionally been seen in injecting drug users sharing needles and as a hospital-acquired infection related to contaminated equipment. Rare cases are acquired outside the tropics when mosquitoes are transported from endemic areas ('airport malaria'), or when the local mosquito population becomes infected by a returning traveller.