Nine cases of West Nile virus have been confirmed so far this year, as disease experts say the virus appears to be circulating more than usual at 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 human cases were neuroinvasive, meaning people developed a serious condition 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 and, in rare cases, can be fatal.
There is no vaccine or treatment for West Nile virus, so people with severe illness usually receive only supportive care such as fluids or pain medication.
About 8 out of 10 people infected with West Nile virus have no symptoms. A smaller number of people may develop a fever with headache, body aches, joint pain, vomiting, diarrhea, or rash. These symptoms can often be confused with those of other viruses, so the majority of West Nile virus cases are never diagnosed.
In the United States, there are several hundred to several thousand cases each year. Most cases are reported in August and September.
“We are seeing West Nile virus (WNV) activity a little earlier this year, so it is very important that everyone 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 precisely how this year will compare to others.”
Global warming has increased human exposure to West Nile virus because mosquitoes breed faster, bite more often, and survive longer. Climate change has also expanded mosquito habitats.
“For several species, there are pretty strong trends where populations are increasing earlier in the year than they would have decades ago,” said Scott Weaver, director of the Institute for Human Infection and Immunity at the University of Texas Medical Branch.
“If the winter is mild, more of them will survive the winter,” he said. “And secondly, if the spring is warm, they will start multiplying earlier in the year.”
Those factors have also fueled the rise of other mosquito-borne diseases like dengue fever in the United States. The CDC issued a warning late last month that the country was seeing a surprisingly 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.
Mosquitoes carrying West Nile virus tend to be more active in central states like Texas and Colorado, as well as rural areas with lots of farmland, Weaver said. Areas with cooler temperatures, like the Northwest, don’t see as much West Nile virus activity, he added.
“If the temperature is not high enough, the virus is too slow to replicate in mosquitoes and transmit efficiently,” Weaver said.
The CDC’s tally of West Nile virus cases this year is likely an underestimate, 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 virus as of June 27.
“We had our first positive mosquitoes in May, which is pretty early because we typically see West Nile virus positive 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 virus during the Las Vegas area’s monsoon season, which is expected to begin soon. Some mosquitoes thrive in areas where there is heavy rain or where there is standing water from storm drains or neglected swimming pools.
Douglas County, Nebraska, officials said in a news release that the 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, the county’s health director.
The Texas State Department of Health also confirmed a 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 in Houston very early on, so I think that’s a bad sign for the future,” he said.
In Springtown, Texas, a woman told NBC 5 Dallas-Fort Worth that her husband died of West Nile virus in late June. The woman, Dranda Hopps, said hospital staff informed her that her husband had tested positive for the virus shortly before he died.
The city of Springtown said Friday it had not yet received official notification from the state health department of a confirmed case. The Texas Department of State Health Services said additional suspected cases could be investigated.
Weaver said West Nile virus surveillance tends to be limited to large metropolitan areas, so it’s often difficult to determine the extent of the virus across the country.
“It’s actually quite frustrating that we haven’t made more progress in being able to predict where we’re going to see West Nile virus outbreaks so we can focus more resources on the right places,” he said.
Some counties, however, have taken innovative approaches to locating infected mosquito populations. Clark County, Nevada, is using drones to find mosquito breeding sites, and the Scott County, Illinois, health department has asked the public to report sightings of dead birds — birds are the virus’s natural hosts, and mosquitoes contract the virus by feeding on infected birds. Illinois is testing birds for West Nile virus to predict when and where humans may be at risk.
To reduce exposure, disease experts advise getting rid of any sources of standing water on your property, such as dirty flowerpot saucers or swimming pools. Installing screens on doors and windows can also keep insects out of the home.
When outdoors, experts recommend using insect repellent and wearing long-sleeved shirts and pants, especially during times when mosquitoes are most active, such as dawn and dusk.
