West Nile takes vacation as coronavirus rages in Southern Nevada

It was a summer in Southern Nevada with a record number of illnesses caused by a virus.

But the year we speak of was 2019, not 2020, and the culprit was the West Nile virus, not the new coronavirus. Flash forward a year, and Southern Nevada is experiencing a very mild season for the mosquito-transmitted disease.

“Thankfully,” said Devin Raman, senior disease investigator for the Southern Nevada Health District.

“I don’t think we could handle another season like last year,” she continued. “It would be very difficult to manage that type of West Nile outbreak and doing those extensive case investigations while trying to make any headway with our coronavirus cases.”

Last season, there were 43 reported cases of West Nile virus in Clark County — including 34 of the more serious form of the illness that attacks the nervous system — and one death. In August 2019, the health district declared an outbreak after reaching the highest number of cases in a season since the virus was first detected in the state in 2004.

In contrast, there have been no reported cases at all of the virus in Clark County this season, which extends from spring into the fall.

It has been a mild season for the virus across the country, with just 22 cases and no deaths reported, compared with 958 cases last year and 54 deaths, according to data from the Centers for Disease Control and Prevention.

Most people with West Nile virus have no symptoms or very mild ones. Some develop a fever along with headache, body aches, vomiting, diarrhea or rash. About one in 150 who are infected by West Nile will develop a severe illness affecting the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord).

There are theories as to why this season has been so mild, including some that suggest the coronavirus has played a role.

West Nile, coronavirus convergence?

Humans contract West Nile virus when they are bitten by an infected mosquito and do not pass the virus on to other people.

Mosquitoes become infected when they bite an infected bird. So the severity of the West Nile season depends in part on bird migratory patterns and whether weather conditions are conducive to mosquitoes breeding.

Virus researcher Amy Stone theorizes that changes in human behavior stemming from the coronavirus could be a factor contributing to the mild season.

“Normally during the summer, a lot of people are … going to lakes and rivers and going camping and doing all of these other things that expose them to mosquitoes,” said Stone, an assistant professor at Touro University Nevada in Henderson. “But because of the pandemic, more people are staying home, they’re not traveling as far. They’re not doing some of those activities that they would do normally.”

Or take the Fourth of July, she said, when fewer people this year were at barbecues socializing with friends or gathering outdoors to watch fireworks, making it less likely that they’d be bitten by mosquitoes.

The bird and the bugs

While there could a human behavior factor, there is a definite bug factor.

Last season, mosquitoes with the virus were found in 43 Clark County ZIP codes. Of more than 43,000 mosquitoes tested, nearly 8,000 were in samples in which the virus was detected. (There are multiple mosquitoes in a single sample.)

This season, infected mosquitoes have been found in just five ZIP codes: 89012, 89014, 89074, 89120 and 89122. Nearly 54,000 mosquitoes have been tested, with fewer than 400 in virus-infected samples.

Last year, Southern Nevada, Arizona and California made up the nation’s hot zone for illness from West Nile virus. This year, however, surveillance by public health agencies is showing far fewer infected mosquitoes in the region.

Arizona researcher Crystal Hepp believes there are fewer infected mosquitoes because there are fewer infected birds. She expects that research underway will confirm that many birds are now immune to the virus after becoming infected during last year’s severe season.

The birds developed an antibody response to the virus, which has resulted in some level of immunity. “And if they can’t be infected, then of course they can’t participate in the bird-mosquito cycle,” the main route of transmission for the virus, said Hepp, an assistant professor at Northern Arizona University.

The reprieve, however, may only last as long as this season.

“I don’t think the verdict is in on how long birds are immune” to the virus Hepp said, adding that immunity typically varies by bird species.

What is known is that each year, baby birds are hatched that will be susceptible to the virus.

“Those new birds don’t have immunity to West Nile virus,” she said, meaning that the cycle of spreading the illness will continue in seasons to come.

This article was updated to add a missing ZIP code.

Contact Mary Hynes at mhynes@reviewjournal.com or 702-383-0336. Follow @MaryHynes1 on Twitter.