By Harvey Black
Researchers have found another invasive mosquito species that carries the virus responsible for La Crosse encephalitis in the Appalachian region. In the April 2015 edition of Emerging Infectious Diseases, Dr. Camille Harris, a wildlife disease ecologist, and her colleagues report finding Aedes japonicus mosquitoes carrying the virus in southwestern Virginia. This is the first time that field-collected Ae. japonicus mosquitoes have been found with La Crosse virus.
Prior to 2001, the only mosquito known to transmit the disease was the Eastern treehole mosquito, Aedes triseriatus. Then it was discovered that an invasive species known as the Asian tiger mosquito (Aedes albopictus) could also transmit La Crosse virus.
“With three mosquito species being able to carry the virus, there is the potential that it could persist in areas that it might not have been found before,” said Harris, who conducted the research as a graduate student at Virginia Tech.
La Crosse encephalitis is the leading cause of arboviral encephalitis in children.
While many infected people remain symptom-free, the virus can cause fever, headache, nausea, vomiting, and lethargy. Children under 16 are particularly vulnerable, according to the U.S. Centers for Disease Control, and may suffer swelling of the brain, seizures, paralysis and coma. Most cases occur from early spring through the fall.
According to the CDC, most cases have been reported in the upper Midwest. But increasingly there have been more cases found in the Southeast. Between 2004 and 2013, for instance, 182 cases were reported in North Carolina and 102 cases in Tennessee.
Typically, 80 to 100 cases are reported annually.
“The findings certainly are very interesting in the context of states where these mosquitoes might be more abundant and more likely to feed on humans in comparison with Ae. triseriatus,” said Dr. Susan Paskewitz, an entomologist at the University of Wisconsin-Madison who was not involved in this research.
In order to get a better handle on the situation, Dr. Harris says that researchers need to see if the live virus can be found in Ae. japonicus in states like West Virginia and North Carolina.
Harris and her colleagues obtained samples of Ae. japonicus as far back as 2005, demonstrating that the invasive species had indeed found a home in the region. She obtained the samples using mosquito traps in a forested area in southwestern Virginia.
Ae. albopictus was first detected in Houston in 1985 and is thought to have arrived in tire casings from Asia. It has since become widespread in the U.S. and is now found as far north as southern Ohio, Indiana, Illinois, and Pennsylvania.
Like Ae. albopictus, Ae. japonicus is also from East Asia and is native to Japan and Korea. It is commonly known as the Asian bush mosquito or the rock pool mosquito. It was initially found in the U.S. in 1998, and, like Ae. albopictus, it has become widespread around the world.
Ae. japonicus breeds in standing water, and is commonly found in forests and high elevations.
While much is known about these two species, questions still remain. For instance, researchers would like to find out more about how Ae. japonicus transmits the La Crosse virus. Mosquitoes are infected by the virus when females take blood meals from infected rodents, like squirrels or chipmunks. But scientists do not yet know whether Ae. japonicus can spread the virus vertically — that is, whether an adult mosquito can pass it on to its eggs and larvae. If so, it would clearly increase the likelihood that the virus is transmitted to humans.
Another question concerns how the virus affects the feeding behavior of Ae. japonicus mosquitoes. Dr. Harris points to research by Dr. Bryan Jackson and colleagues showing that it alters the way Ae. triseriatus feeds. In a laboratory study, infected mosquitoes took smaller blood meals from rodents and fed more frequently than uninfected mosquitoes. The same study reported that infected Ae. albopictus mosquitoes also took smaller blood meals from rodents, but did not feed more frequently.
In addition to La Crosse virus, there is evidence that Ae. japonicus may also be a competent vector of dengue fever, chikungunya, and other diseases.
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Harvey Black is a freelance science writer. A long-time resident of Madison, Wisconsin, he has written for numerous publications including Environmental Health Perspectives, Scientific American Mind, New Scientist, The Scientist, and the Milwaukee Journal Sentinel.