PHOENIX — A new study suggests that a strain of the West Nile virus is going to remain in Arizona's most populous county for the foreseeable future.
Arizona researchers say that the same mild winters that bring snowbirds to Maricopa County also let mosquitoes and certain virus-reservoir birds survive winter to spread West Nile anew when the weather warms up.
Phoenix radio station KJZZ reports the study concludes that potentially deadly virus seems to be endemic to the county which includes the Phoenix metro area.
Experts say the West Nile is the foremost source of mosquito-borne disease in the U.S. The virus reportedly first entered the country in 1999 in New York City and was detected in Maricopa County four years later.
West Nile is transmitted to humans by the bite of an infected mosquito.
Authorities say about 20 percent of people infected with virus will feel flu-like symptoms within 3 to 15 days after the mosquito bite. Symptoms may include fever, headache, nausea, vomiting, swollen lymph glands and skin rash.
A small percentage of those infected with West Nile could die or suffer severe symptoms such as meningitis, encephalitis or paralysis. Persons over age 50 generally are at a higher risk for severe symptoms.
There were 110 confirmed or probable cases of West Nile in Arizona in 2017 and the Vector Control District of Maricopa County Environmental Services found the virus in 221 mosquito pools in metro Phoenix.
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 Maricopa 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 David Engelthaler, director of TGen North in Flagstaff and co-author of the study with Crystal Hepp of Northern Arizona University.
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," Hepp said.
Engelthaler said the "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."
The study was backed primarily by the Arizona Biomedical Research Committee and the Arizona Board of Regents' Technology Research Infrastructure Fund.