TGen, NAU Study Finds West Nile Strain Endemic To Maricopa County

Published: Monday, January 14, 2019 - 6:25pm
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A study by Arizona researchers suggests that a strain of West Nile virus has taken up permanent residence in Maricopa County.

The same mild winters that bring snowbirds flocking to the Valley of the Sun also let Culex mosquitoes — and certain virus-reservoir birds — survive winter to spread West Nile anew when the weather warms.

"This confluence of events has allowed Maricopa County in specific areas to really be not just a hot bed of West Nile, but a long-term source of it, now that it really seems to be endemic in this region," said co-author David Engelthaler, director of TGen North in Flagstaff.

The study, which appears in the journal PLOS One, was backed primarily by the Arizona Biomedical Research Committee and the Arizona Board of Regents Technology Research Infrastructure Fund.

West Nile, which is a flavivirus like yellow fever, dengue and Zika, is the foremost source of mosquito-borne disease in the U.S.

Roughly 1 in 5 infected people develop fever and flu-like symptoms such as body aches and vomiting, and 1 in 150 develop a severe nervous system illness like encephalitis or meningitis.

2017 saw 110 confirmed or probable cases of West Nile in Arizona, and the Vector Control District of Maricopa County Environmental Services found the virus in 221 mosquito pools in the Phoenix Metro Area.

The virus first entered the U.S. in New York City in 1999.

Experts have detected the virus in Maricopa County since 2003. To find out if it is endemic or repeatedly imported, the researchers developed a new technique for sequencing 14 West Nile genomes in mosquitoes across the county.

"It's kind of an Ancestry.com look at it. We can really understand the relationships between the viruses that are showing up in mosquitoes throughout the West," said Engelthaler.

Their results turned up two family lines, one of which has been circulating in Arizona for at least seven years.

By calculating when different viral strains broke off from their common ancestors, researchers also could track their spread.

"In 2017, it looks like the strain that was endemic to Maricopa County was exported to southern California and to southwestern Utah," lead author Crystal Hepp of Northern Arizona University.

Humans and other mammals are dead-end hosts for the virus. West Nile mainly is spread by Culex mosquitoes and survives best in certain birds.

Although the virus tends to spread during warm-weather months, scientists believe it could overwinter within mosquitoes and birds that reside in the area, such as communal roosting house sparrows (Passer domesticus), house finches (Haemorhous mexicanus), great-tailed grackles (Quiscalus mexicanus) and mourning doves (Zenaida macroura).

The group is now working on a National Institutes of Health grant to develop a model incorporating bird data, mosquito data and human data to make better predictions.

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