Mosquito season is here: More than a third of states have detected West Nile virus

NBC News

There have been nine confirmed cases of West Nile virus so far this year. Disease experts have warned that the illness appears to be circulating more than usual during this time of summer.

As of June 25, the Centers for Disease Control and Prevention (CDC) had confirmed human cases in at least seven states: Arizona, Arkansas, Kansas, Maryland, Michigan, Mississippi and Tennessee. At least 18 states have detected the virus this year in humans, mosquitoes, birds or other animals.

Five of the reported human cases were neuroinvasive, meaning people developed severe illness such as inflammation of the brain or the membranes surrounding the brain and spinal cord. This can lead to disorientation, vision loss, coma or paralysis. In rare cases, it can be fatal.

There are no vaccines or treatments for West Nile virus, so people with severe infections usually receive only supportive care, such as fluids or pain relievers.

About 8 in 10 people infected with West Nile virus have no symptoms. A smaller number may develop fever accompanied by headache, body aches, joint pain, vomiting, diarrhea, or rash. These symptoms can often be confused with those of other viruses, so most cases of West Nile are never diagnosed.

The United States records hundreds to thousands of cases each year, with most reported in August and September.

“We’re seeing West Nile virus (WNV) activity earlier this year, so it’s really important for everyone to take steps to protect themselves and their families from mosquito bites,” CDC press officer Kate Fowlie said in a statement. “WNV tends to be unpredictable and varies from year to year, so we don’t know specifically how this year will compare to others.”

The impact of climate change

Rising global temperatures due to climate change have increased human exposure to West Nile virus, as mosquitoes can reproduce faster, bite more, and survive for longer periods. Climate change has also expanded mosquito habitats.

“For a number of species, there are pretty strong trends that populations are growing earlier in the year than they were decades ago,” said Scott Weaver, director of the Human Infection and Immunity Institute at the University of Texas Medical Branch.

“If it’s a warm winter, more will survive the winter,” he said. “And secondly, if the spring is warm, they will start multiplying earlier in the year.”

These factors have also fueled the rise of other mosquito-borne diseases, such as dengue, in the United States. The CDC issued a warning late last month that the country was seeing an unexpectedly high number of dengue cases: The agency had recorded nearly 2,400 cases as of Tuesday, compared with about 3,000 cases for all of last year.

Weaver said mosquitoes carrying West Nile virus tend to be more active in central states like Texas or Colorado, as well as in rural areas with lots of farmland. Areas with cooler temperatures, like the Northwest, don’t have as much West Nile virus activity, she added.

“If the temperature is not warm enough, it takes too long for the virus to replicate in mosquitoes and transmit efficiently,” Weaver said.

The CDC’s count for West Nile cases this year is likely an undercount, as several states have reported additional human cases in recent weeks.

The Southern Nevada Health District has recorded seven cases since June 26, five of which were announced Wednesday. Four of the total cases were neuroinvasive. According to the health district, more than 8,000 mosquitoes in Southern Nevada had tested positive for West Nile as of June 27.

“We had our first positive mosquitoes in May, which is pretty early because we typically see positive West Nile mosquitoes in early July,” said Vivek Raman, environmental health supervisor for the Southern Nevada Health District.

Raman said he is concerned about increased transmission of West Nile during the monsoon season in the Las Vegas area, which is expected to begin soon. Certain mosquitoes thrive in areas with lots of rain or standing water from storm drains or neglected swimming pools.

Douglas County, Nebraska, officials said in a news release that their mosquito population also appears to be higher than usual for this time of year. The county announced in late June that one person, a blood donor, had tested positive for West Nile virus.

“This report is concerning because it may indicate an early start to the West Nile season,” said Dr. Lindsay Huse, county health director.

The Texas Department of State Health Services also confirmed one human case of West Nile virus in the Houston area. Weaver said she expects more cases in Houston this summer.

“They’re finding a lot of viruses in mosquitoes very early in Houston, so I think it’s a bad sign of things to come,” he said.

In Springtown, Texas, a woman told NBC 5 Dallas-Fort Worth that her husband died from West Nile in late June. The woman, Dranda Hopps, said she was informed by hospital staff that her husband had tested positive for the virus shortly before his death.

The city of Springtown said Friday it had not yet received official notification from the state health department about a confirmed case. The Texas Department of State Health Services said additional suspected cases may be under investigation.

Weaver said West Nile surveillance tends to be limited to major metropolitan areas, so it’s often difficult to know how widespread the virus is across the country.

“It’s actually quite frustrating that we haven’t made more progress in being able to predict where we’ll see West Nile virus epidemics so we can focus more resources in the right places,” he said.

However, some counties have taken innovative approaches to locating infected mosquito populations. Clark County, Nevada, uses drones to find mosquito breeding sites, and the Scott County Health Department in Illinois has asked the public to report sightings of dead birds: Birds are the natural hosts of the virus and mosquitoes contract it by feeding on infected birds. Illinois tests birds for West Nile to help predict when and where humans may be at risk.

To reduce exposure, disease experts recommend getting rid of any sources of standing water on your property, such as flower pot saucers or dirty pools. Installing screens on doors and windows can also prevent insects from entering the home.

When outdoors, experts recommend using insect repellent and wearing long-sleeved shirts and pants, especially during times when mosquitoes are most active, at dawn and dusk.