Malaria: Vector

The development of plasmodia (from gametocytes to sporozoites) in a mosquito takes at least 9 days (sometimes as much as 30 days, depending on the temperature). After an infected blood meal a female mosquito will pass through at least 4 or 5 egg-laying periods (and blood meals) before it becomes infectious. There is thus ample time over several days to destroy the vector before she can transmit. Mosquitoes which are infectious are already “middle-aged". They may sometimes survive a month or more, but often the life span is much shorter. This long period before a mosquito becomes infectious is a weak point in the cycle of plasmodia. For example: at a temperature of 30°C at least 70% of mosquitoes need to survive every day if more than 1% of mosquitoes are to survive the 10-day development period.

A practical example can be seen in "airport malaria". Sometimes (due to lack of decontamination or resistance to insecticide) infected mosquitoes are brought to northern airports in aeroplanes. If such a mosquito can remain active due to high environmental temperatures (in a hot summer), bites someone and injects sporozoites, it will already be at least 10 days old. Adding the incubation period for P. falciparum and the time between the first fever and making the diagnosis (in northern regions one does not think quickly of the possibility of malaria), it can then be assumed that the mosquito which caused the disease will probably already be dead when the diagnosis is finally made. Patient's delay and doctor's delay are important factors in lethal malaria. One mosquito can infect several persons (for example if the blood meal is interrupted the mosquito will bite several times), also possibly outside the airport (carried by the wind, or flying). Also, several mosquitoes may be introduced. Due to these factors, clusters of airport malaria may occur in hot summers. The risk that descendants of an infected (or non-infected) mosquito would survive in Europe and find a gametocyte carrier and cause infections again after the incubation period, is very small.

Category: Medicine Notes