[Public Health] West Nile Virus: Worst Year Ever

So far this year, business has been good for Ron Dawson, owner of DFW Pest Control, Dallas, Texas. While much of the Midwest suffered through drought conditions this summer, Dawson said much of Texas did not experience as extreme a problem. Heavy spring rains combined with a mild winter have helped Dawson grow his company’s general pest services by 45 percent, so far this year. But one part of that bump — mosquitoes — has come with a cost.

“It’s one of those situations where, even though I do mosquito control, I don’t even want to go outside,” Dawson said, speaking of the West Nile virus outbreak that has taken a toll throughout the U.S. this year. The virus has been especially prevalent in the Dallas area, where Dawson lives and does business.

“We’ve had so many people come down with it, we had a lady on our block actually contract West Nile,” said Dawson, who lives in Garland, a city just outside Dallas. “There’s a lot of older people in our neighborhood, a lot of people out walking their dogs, and [that’s] stopped.”

Why so many cases? As of press time, the Centers for Disease Control and Prevention had reported 1,590 cases of West Nile virus in 48 states, with a death count of 66. Texas accounts for nearly half of that national case count, with 733 cases on the books, according to the CDC. It is the worst outbreak of the disease since West Nile first appeared in the United States in 1999.

Joseph Conlon, technical adviser for the American Mosquito Control Association, said the outbreak is more prevalent in Texas for several reasons. “A lot of people say it is due to an abnormally mild winter and abnormally high spring rains. I don’t think that a good argument can be made against that.” Conlon said with high rains come more places for mosquitoes to breed. “You have an immunological cohort of immigrant birds (a carrier of West Nile) that are flying through there that have not been exposed to West Nile virus before. And you have a largely immunologically naïve human population being subjected to it for the first time,” he said. “So you have lots of mosquitoes, new birds, lots of virus floating between the birds and the mosquitoes, and the immunologically naïve humans that are prepared to receive the virus.”

The good news is, as of the end of August, the CDC believed the worst of the outbreak was nearing an end. “I’m not sure how much more widespread it can get. Here again we have a relatively recent epidemiological phenomenon, so it is difficult to make predictions,” Conlon added.

“A majority of cases generally occur in the last couple weeks of August and the first couple weeks of September,” Conlon said. “I imagine it will go through September and the first couple weeks of October. The majority of these are happening in the lower United States.”

Cyclical Nature. West Nile virus is cyclical in nature, and can ultimately be traced back to bird populations within the areas where the virus has been prevalent, according to Dr. Jerome Goddard, an entomologist at Mississippi State University. PCT recently interviewed Goddard for a podcast appearing on PCT Online (http://bit.ly/SrcHQJ), in which he discussed the outbreak. As of press time, Mississippi was tied with South Dakota as the state with the second highest number of reported West Nile cases, at 98.

“There is definitely a cyclical nature to West Nile virus,” Goddard said. “It’s all about the birds. Bird to bird, it’s transmitted by different mosquitoes. When mosquitoes bite the birds and the birds get West Nile, and the birds become exposed, there are a lot of them. It’s like herd immunity. When (the birds) become immune to it, then it can’t get going. After this year, most of the birds are going to be immune to West Nile virus, so maybe next year it will go down.”

Humans who come down with WNV are accidental or secondary hosts. As bird populations become immune to the virus, less is found, and when new birds are born, an outbreak is more likely. “We’ve seen this cycle before,” he said. “Maybe every five to six years.”

Goddard said weather plays an important role in the cycle of West Nile, though it is not as simple as more rain/standing water = more mosquitoes = more WNV. “More than people realize, West Nile is related to lack of rain. So there is an inverse relationship between rainfall and West Nile cases,” he said. “The reason is this particular mosquito that carries West Nile virus (Culex pipiens) loves thick and soupy, organic water. So it is counterintuitive, but when it doesn’t rain much is when you have a lot of West Nile.”

Role of the PMP. DFW Pest Control offers mosquito control options including the installation of misting systems, and periodic mosquito treatments. But Dawson said that few customers have purchased services, citing the West Nile outbreak.

“That’s not really the reason we sell [mosquito control]” he said. “It’s just people don’t like being bitten by mosquitoes when they’re enjoying their pools, sitting outside with their families.”

While the public may not realize it, the pest management professional can play an important role in this sort of public health crisis. Goddard said PMPs can serve as educators. “Speak at community events, Kiwanis meetings, schools, those types of things,” he said. “They also can offer mosquito control as an add-on service.”

Goddard noted that in Jackson, Miss. — the state’s capital and largest city — mosquito control is done by a private pest control company. “Companies can do these types of things,” he said. “My opinion is that if a pest control company is going to do mosquito control, do it right. Dedicate some technicians to do it alone. Send those technicians to mosquito control meetings, because [mosquito work] has its own language and jargon and technology and they need to learn that stuff. I think those pest control companies that want to do it can be involved and serve in this public health effort.”

The authors are associate editor and managing editor of PCT. Contact them at bdelaney@giemedia.com and bharbison@giemedia.com.