Do mosquitoes get malaria? Do rats catch bubonic plague? If not, why not?
鈥� Congratulations to the children of the Christopher Hatton School in London, UK, for asking such a penetrating question.
Rats can get quite sick from plague fleas and some will die, but usually not too quickly. Plague-carrying rats are at their most dangerous when they are about to die, because their fleas leave them as soon as they are dead to find new hosts.
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The malaria parasite Plasmodium does not usually kill its host mosquitoes, though it may take a high enough toll that it is better for the mosquitoes not to get infected.
If we could breed mosquitoes that were resistant to the parasite we might find that they outcompete ordinary mosquitoes, and this might ultimately help get rid of malaria. This kind of strategy would not work with yellow fever, as the mosquitoes that carry the virus responsible for the illness in humans hardly seem to be affected.
Jon Richfield, Somerset West, South Africa
鈥� , the parasite that causes malaria in humans, infects mosquitoes. The mosquitoes then transmit it to people when feeding on their blood. As for the plague microbe, Yersinia, it blocks the gut of the flea that transmits it. As a result, when an infected flea feeds on the blood of a human or rat, it will regurgitate some blood containing the microbe and so spread the germ to a new host.
To address the question directly, the important thing to note is that being infected with a microbe or other parasite does not necessarily cause disease, because it is often in the interest of the microbe to cause no harm to its hosts.
鈥淏eing infected with a microbe or other parasite does not necessarily lead to disease in the host鈥�
However, the mosquitoes that transmit Plasmodium are affected by it, as the parasite grows in their salivary glands. Such infection can reduce the ability of the salivary glands to function and thus the viability of the mosquitoes.
A related parasite called Theileria, transmitted between cattle by ticks, can damage the gut and salivary glands of the ticks, and can even kill them in the laboratory. Epidemiologists make a point of studying the extent of such effects under natural conditions.
Alan R. Walker, Edinburgh, UK