Chuck Haga, Forum News Service, Published August 15 2013
Mosquito control officer tracks West Nile virus - by liquefying the insects
Todd Hanson seems almost to admire the intricacy of the mosquito’s predatory form as he examines her at the city’s mosquito control lab, where he is supervisor.
“It’s just part of nature that’s out there,” he says, smiling.
True, it is just one of maybe 40 species of mosquitoes that inhabit North Dakota, but it is one of seven monitored by public health officials because of their capacity to transmit disease to humans.
And culex tarsalis is the transmitter of the West Nile virus, which was first detected in the Grand Forks area in 2002, causing officials here to ramp up the city’s mosquito control program, now a more than $800,000-a-year campaign of trapping, testing and spraying.
And preaching, with Hanson as a senior preacher: Don’t deny yourself the joys of outdoor activities, he urges, but take precautions, including the liberal use of repellant with DEET.
“We run the program as if West Nile is always present,” Hanson said, and citizens should make the same assumption, especially in parks and on golf courses and other areas likely to have higher concentrations of mosquitoes.
“This year, we recognize there is an elevated risk,” he said, due probably to climate conditions. On Monday, mosquito control crews identified two pools that had tested positive for the virus, and city crews sprayed along the Red River and the English Coulee on Wednesday.
Spraying is done after sunset at least once a week now because of the presence of West Nile and is not triggered by high counts from the 20 traps scattered about and beyond the city, 15 using lights and five using carbon dioxide to attract mosquitoes.
This mosquito season started out wet, conducive to a bumper crop, but has been relatively dry more recently – though one trap Monday produced 680 mosquitoes.
Hanson is concerned, in fact, that low trap numbers and the apparent absence of mosquitoes in backyards, while a widely praised result, may lull residents into a false sense of security.
Wednesday’s trap count, or “skeeter meter,” was just 2, but the city sprayed Wednesday evening. Two pools had been identified Monday as positive for West Nile, and of 85 mosquitoes collected from one trap about 10 days ago, about half were culex tarsalis.
“There are very few ‘nuisance’ mosquitoes out there, and people think there’s not a problem,” Hanson said. “People see a low trap count and ask us, ‘Why are you spraying when there’s no mosquitoes?’
“That’s why it’s important to have a good surveillance program, so you’re not spraying willy-nilly.”
Of eight human cases of West Nile in North Dakota reported by the state Health Department this month, two were in Grand Forks County. The first West Nile-related death in the state was confirmed early this week, a woman older than 60 living in the southeastern part of the state. She had no underlying health conditions.
Minnesota officials also confirmed the state’s first death of the year early this week but did not release any information on the person who died. The elderly, especially those with compromised immune systems or other health issues, are most vulnerable.
Most people who become infected with the West Nile virus don’t develop any symptoms, according to health authorities, but about one in five may develop flu-like symptoms, such as fever, headache, body aches, joint pains, vomiting or a rash.
Finding the virus
Hanson works out of a city building in the industrial park, in labs above a motor pool that houses the city’s fleet of sprayer-armed pickups and all-terrain utility vehicles.
Lab walls are appointed with colorful photographs and fine drawings ... of mosquitoes.
To test a batch Thursday, Hanson began by placing 20 of the critters each into two small glass tubes with a buffer solution and three metal BBs, which help to liquefy the material in a vortex mixer.
That “mosquito smoothie” then goes into a centrifuge to spin all solids to the bottom. Hanson extracts a portion of the mosquito slurry, mixes it with fluorescent dye and deposits a tiny amount, about 70 microliters, in a small cassette.
He lets the slurry dry for 90 minutes before inserting the cassette into a machine that reads the dye-enhanced material and assigns a number, from 0 to 640, to indicate the presence of West Nile virus.
“A reading of 100 confirms West Nile,” Hanson said. “If it’s up at 640, we know that was a very hot pool” that produced the mosquitoes.
The verdict on Thursday’s tests?
“They were both negative,” he said. “They were both less than 10.”
Results of all tests are logged and shared with the state Health Department, which in turn shares them with the Centers for Disease Control and Prevention in Atlanta.