Enhanced Parasite Transmission in Mosquitoes

Sporozoites of the malaria parasites (Plasmodium spp.) invade the salivary glands of the female mosquito. Her salivary glands are very well developed and histologically regionalized. The sporozoites preferentially invade the distal-lateral and medial (4 of 6) lobes where the anti-platelet aggregation apyrases are made. The sporozoites damage the cells that make apyrase while leaving those that make saliva proper, alone. The result is that apyrase decreases by 70-90% while saliva volume is unaffected (Beaty and Marquardt, 1996). As a result of this change, Ribeiro et al (1985) noted that probing activity of mosquitoes increased in a directly inverse manner to the decrease in apyrase content in the saliva. They hypothesize that the lack of apyrase prevents feeding haematomas to form, resulting in increased probing and saliva injection as the mosquito strives to feed. Of course the saliva is laden with sporozoites which are injected each time the mosquito injects more saliva (A mosquito usually starts feeding with an initial significant saliva injection). Furthermore, the probing mosquito is more likely to disturb its hosts and leave without a full blood meal and, consequently relocate to another host and try to feed again, further spreading the malaria parasites (Ribeiro, 1989).

Another interesting salivary component that probably won't be properly investigated for years to come, are components used by parasites as chemotaxins. There is some mystery surrounding how microfilaria infect new mosquitoes when they are in such low numbers in the dermis to maintain their infectious cycle. It may be that they use mosquito saliva as a signal to congregate at the feeding site and so infect the feeder (Beaty and Marquardt, 1997). How far and fast they move to get to the site is unknown.