Chipmunks In Medical Research

During epidemic viral encephalitis, only a minority of those infected develop encephalitis. Others develop viral meningitis, fever and headache only, or no symptoms at all. These diseases are all zoonoses maintained in nature in birds or rodents, humans being an accidental host. In the United States, LaCrosse encephalitis virus usually causes most of the reported encephalitis cases. In its natural cycle (figure 26.13), the LaCrosse virus infects Aedes mosquitoes, which pass it directly from one mosquito to another in semen, and it can survive the winter in the mosquito's eggs. These mosquitoes feed on and infect squirrels and chipmunks, which markedly amplify the amount of virus and spread it to uninfected female mosquitoes feeding on the rodents' blood. These mosquitoes are forest dwellers that normally breed in water-containing cavities in hardwood trees or in discarded tires.

Figure 26.13 LaCrosse Encephalitis Virus, Natural Cycles The virus is maintained both by mosquito-to-mosquito transmission through semen and eggs, and by mosquito-to-squirrel (or chipmunk)-to-mosquito transmission. Infection of the rodent markedly increases the quantity of virus and the potential to infect many mosquitoes.

Figure 26.13 LaCrosse Encephalitis Virus, Natural Cycles The virus is maintained both by mosquito-to-mosquito transmission through semen and eggs, and by mosquito-to-squirrel (or chipmunk)-to-mosquito transmission. Infection of the rodent markedly increases the quantity of virus and the potential to infect many mosquitoes.

The natural cycle of other encephalitis-causing viruses involves wild birds rather than rodents, and involves different mosquitoes with different habitats and feeding habits. Figure 26.14 shows the results of testing for these viruses in mosquitoes, wild birds, and sentinels in states doing surveillance.

There is still much to be learned about the ecological factors responsible for the spread of these viruses to humans, but the proximity of humans to their natural cycles, rainfall, winter and summer temperatures, numbers and range of amplifying hosts, and predators could all be important.

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