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

Nine cases of West Nile virus have been confirmed so far this year, and disease experts say the virus appears to be circulating more than usual for this time of summer.

As of June 25, the Centers for Disease Control and Prevention 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 in people, mosquitoes, birds or other animals this year.

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

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

About 8 out of 10 people infected with West Nile virus do not show any symptoms. A smaller number may develop fever accompanied by headache, body aches, joint pain, vomiting, diarrhea, or a rash. These symptoms can often be confused with those of other viruses, so the majority of West Nile cases are never diagnosed.

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

“We’re seeing West Nile virus (WNV) activity a little earlier this year, so it’s really important that everyone takes steps to protect themselves and their families from mosquito bites,” Kate Fowlie, a CDC press secretary, said in a statement. “WNV is typically unpredictable and varies from year to year, so we don’t know exactly how this year will compare to other years.”

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

“There are pretty strong trends for several species that populations are growing earlier in the year than they were decades ago,” said Scott Weaver, director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch.

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

These factors have also fueled the rise of other mosquito-borne diseases, such as dengue fever, in the U.S. Late last month, the CDC warned that the country was facing an unexpectedly high number of dengue fever cases: the agency had recorded nearly 2,400 cases on Tuesday, compared with about 3,000 cases for all of last year.

Weaver said mosquitoes that transmit West Nile are most active in central states like Texas or Colorado, and in rural areas with lots of farmland. Areas with cooler temperatures, like the Northwest, don’t have as much West Nile activity, he added.

“If the temperature is not warm enough, the virus may multiply too slowly in the mosquitoes and not spread efficiently,” Weaver said.

The number of West Nile cases the CDC has counted 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 normally see mosquitoes that are positive for West Nile in early July,” said Vivek Raman, the environmental health supervisor for the Southern Nevada Health District.

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

Officials in Douglas County, Nebraska, said in a news release that the mosquito population also appears to be higher than normal 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 as it may indicate an early start to the West Nile season,” said Dr. Lindsay Huse, the region’s health director.

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

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

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

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

Weaver said surveillance for West Nile virus is typically limited to large urban areas, making it difficult to tell how widespread the virus has become in the country.

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

Some counties, however, have adopted innovative approaches to locate infected mosquito populations. Clark County, Nevada, is using drones to find mosquito breeding grounds, and the Scott County Health Department in Illinois has asked the public to report dead birds — birds are natural hosts for the virus, and mosquitoes get the virus by feeding on infected birds. Illinois is testing birds for West Nile to help predict when and where people might be at risk.

To reduce exposure, disease experts advise removing standing water sources on your property, such as dirty flower pot saucers or swimming pools. Installing screens on doors and windows can also prevent insects from entering your home.

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

Leave a Comment