Experts: Mosquito scourge to persist; more disease likely

North Carolina had three recorded cases of West Nile virus this month, and state experts said the numbers will likely increase in the coming weeks as the post-Debby mosquito population continues to boom.

Several suspected cases of West Nile virus are under investigation by the N.C. Division of Public Health’s Communicable Diseases Branch, Dr. Carl Williams, public health veterinarian with the N.C. Department of Health and Human Services, said this week.

Those results should be known in the next week or two.

“We know the end of summer and beginning of fall is typically and historically when we receive the greatest report of human cases of West Nile virus and La Crosse encephalitis virus,” Williams said in a video interview on Thursday. “It’s late August, so the cases are coming in.”

While the numbers that have been reported so far are typical, Williams said they might not stay that way.

That’s because of heavy bands of rain from Tropical Storm Debby earlier this month, said Dr. Mike Doyle, public health entomologist with DHHS. Mosquito control programs are seeing up to 10 times as many mosquitoes in rural North Carolina since the storm.

“What we’ve seen this year is that it was really dry (when summer began) and the numbers were really low for the early part of the season,” Doyle said. “Then there was (Tropical Storm Debby), a large storm for a week that filled up a lot of containers, and now it’s becoming dry again, so we are starting to see the cycle.”

That’s because mosquitos that carry the West Nile virus, and species that carry Eastern equine encephalitis and La Crosse encephalitis, breed in stagnant water.

West Nile virus, along with eastern equine encephalitis and La Crosse encephalitis, are “arboviruses,” viruses that are transmitted by the bite of an infected arthropod, usually a mosquito. They are the three most usually transmitted arboviruses transmitted to humans and animals in North Carolina. Because they have the potential of causing serious neurological problems, diagnosed cases must be reported to local health departments, the state and Centers for Disease Control and Prevention.

However, for every one diagnosed case, experts estimate between 100-150 people are infected. Because the symptoms are mild, patients don’t seek medical attention, said Doyle.

Because tracking infections relies on people going to the doctor for testing, state and national officials also depend on local mosquito testing and vector control work to determine if species that carry the virus are present in the environment and how many are there.

Twenty-five North Carolina counties, including Pitt County, undertake mosquito testing and vector control work, which is voluntary, Doyle said.

“Fortunately we have a partnership between those counties and the state lab of public health and several of those counties send in mosquitos on a regular basis to be tested for West Nile virus, eastern equine encephalitis and La Crosse encephalitis,” he said. “It acts as sort of a canary in the coal mine. The mosquitos are going to be infected first, before people become infected, theoretically. You can tell from the mosquitos if a virus is being transmitted out there and if there is a risk to humans.”

Ongoing testing in situations where West Nile virus has been identified also helps officials determine if the mosquito spraying has reduced the population enough to break that cycle and to stop seeing infected mosquitos, Doyle said.

Pitt County’s vector control manager was unavailable this week so local data wasn’t available.

The City of Greenville also operates a mosquito trapping program to measure the local population, said Delbert Bryant, the city’s sanitation manager.

Greenville received 4.46 inches of rain from Tropical Storm Debby as of Aug. 9, according to the N.C. State Climate Office, bringing the city’s total rainfall to 19.4 inches in the previous 30 days.

This significant rain, and the subsequent dry weather, produced stagnant standing water that increased the local adult mosquito population.

“Water is the issue,” Bryant said. “Many of the pools are still active breeding sites for mosquitoes. The eggs can remain dormant in a dry pond or dry area and when the water rises, the eggs will hatch.”

The city and county are coordinating efforts to monitor and respond to complaints about mosquitos. Bryant said public works has been spraying across the city.

“Residents can help by removing standing water from their property and encouraging their neighbors to do the same,” Delbert said. “Many mosquitoes do not fly very long distances thus indicating a breeding site may be near. Residents can protect themselves with long sleeves, clothing and the use of insect repellents according to label direction. Please use caution when applying insect repellents to children.”

Residents can also protect themselves by avoiding outdoor activities at dusk and dawn, which is when mosquitoes are most active, and check gutters and rain barrels to make sure they are not harboring water and/or mosquitos, he said.