Researchers at the University of Texas Medical Branch at Galveston (UTMB) have discovered a quick new way that mosquitoes can pass West Nile virus to each other. The new study challenges fundamental assumptions about the virus' transmission cycle and may help explain why it spread so rapidly across North America despite experts' predictions that it would progress more slowly or even die out. In the conventional understanding of West Nile transmission, mosquitoes acquire the virus when they bite birds with high levels of virus (or "high viremia.") in their blood. Those levels are reached several days after the birds are initially infected by other mosquitoes. But experiments at UTMB show that when infected and uninfected mosquitoes feed simultaneously on previously uninfected laboratory mice, the virus can pass from mosquito to mosquito within an hour.
"We were amazed to see that it could happen," said UTMB associate professor Stephen Higgs, lead author of a paper on the discovery that will be published online in the Proceedings of the National Academy of Sciences the week of June 6. "It is basically a brand-new component of the virus' life cycle."
In the paper, Higgs and his co-authors--UTMB graduate student Bradley S. Schneider, senior research associate Dana Vanlandingham, research assistant Kimberly A. Klingler and Ernest A. Gould of the United Kingdom's Centre for Ecology and Hydrology--note that although such "non-viremic transmission" (that is, transmission before virus can be detected in the blood) has been observed in cases involving viruses transmitted by ticks, it has never before been documented in a virus carried by mosquitoes.
To determine whether West Nile virus could be transmitted non-viremically, the researchers placed an anesthetized, uninfected lab mouse on a mesh-topped container holding infected "donor" mosquitoes, which fed on the mouse through the mesh. Five minutes later, they moved a second mesh-topped container in position so that its uninfected "recipient" mosquitoes could feed on the same mouse, allowing the simultaneous feeding by infected and uninfected mosquitoes to continue for an hour. In repeated experiments, tests revealed that between 2 and 6 percent of the recipient mosquitoes acquired the virus. In one trial, a single bite from a donor mosquito was sufficient to infect two out of 87 recipient mosquitoes.
The discovery calls into question the current conception of mosquito transmission of West Nile virus and possibly other viruses such as the one that causes dengue fever. According to that theory, many vertebrates were considered to be "dead-end hosts" that did not pass along the virus. Only birds, which develop much higher levels of the West Nile virus in their blood, were thought capable of passing it on to uninfected mosquitoes.
"None of the models that have attempted to predict the spread of West Nile virus take into account the possibility that mammals such as horses may be involved in the proliferation of this virus. Direct transfer of virus from the infected mosquitoes that initially feed on them to others that feed on them afterwards, could significantly accelerate the spread of the disease," Higgs said. "Instead of only birds infecting mosquitoes, all sorts of animals may be involved, and transmission could be happening much faster because you don't have to wait for a high viremia."
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