Patti Casey is a hunter, though she’s not looking to bag more typical Vermont game like deer or moose. Instead, Casey tracks down mosquitos.
As the environmental surveillance program manager at the Vermont Agency of Agriculture, Casey has tested thousands of mosquito populations all over the state for the presence of two particular diseases: West Nile virus and Eastern Equine Encephalitis, or EEE. Encephalitis is an inflammation of the brain.
In August, the Vermont Department of Health announced the first positive test of the year for West Nile virus found in a mosquito population in Essex. A second population has recently tested positive in Newport City. West Nile is consistently found in Vermont year after year since the disease was brought to the United States from overseas in the late 1990’s.
“We start collecting and identifying species and then having them tested in June,” Casey said, Tuesday morning before going out into the field again. “We don’t typically see any diseases show up in mosquito populations until later in the summer, but we like to monitor it from early on so that we will see the first occurrences of it.”
Casey also works with the tick surveillance program, where testing is conducted over two six week periods: one during May and June, and the second during October and November. The remaining months during the summer are devoted to mosquito surveillance. The surveillance ends for the year after the first hard frost.
There are four full time technicians throughout the state who constantly work to monitor mosquitoes. More than 100 trap sites are employed used with different kinds of traps.
One kind, light traps, can capture thousands of mosquitoes in one night because they are attracted to the light and carbon dioxide. The mosquitoes are then stored on dry ice until tested.
There are two other types of traps the agency uses. Gravid traps have a small pool of dirty water in the bottom which females are attracted to in hopes of laying eggs. A small fan sucks the mosquitoes into a net for collection. The third type of trap is called a resting box trap. Technicians will place these traps on the ground near wetlands with little overgrowth. After a female mosquito has a blood meal, she will settle to the ground to digest the blood and make eggs. The dark boxes provide a safe and attractive place for the mosquitoes to rest. Every week, a technician will visit the box and use a vacuum and nylon sock to collect mosquitoes inside the box.
“Four of the days [of the week] are devoted to being out in the field all day, and then on the fifth day, they bring the specimens in, and recharge their supplies,” Casey said.
Mosquitoes are tested at the Vermont Department of Health laboratories in Colchester.
“We work around reminding people of the low but actual risk of mosquito borne illnesses in Vermont,” Patsy Kelso said, who is the state epidemiologist at the Vermont Health Department.
After Casey’s team collects the mosquitoes in vials and sends them to the test lab in Colchester, technicians dawn protective gear and place the mosquitoes in the freezer.
“The goal is to break open the mosquitoes and release the viral RNA for EEE and West Nile virus,” said Christine Matusevich, the health department’s microbiology unit coordinator. A solution is added to the vials before they are placed in a machine, a mixer mill, which shakes the vials quickly to break the mosquitoes.
The next step is to take the liquid from the mixer, with crushed up pieces of mosquito in it, and place the vials in a different machine which can extract the RNA. Another instrument will heat and cool the vials for about two hours, which allows the RNA to duplicate rapidly.
“We can actually watch this amplification in real time,” Matusevich said. If the vial does have West Nile or EEE in it, a line on a graph in a computer program will slowly start to rise and then plateau.
The lab will test about 150 “pools” per day. A pool is a sample of anywhere from one to 50 mosquitoes. Agriculture will send pools on Mondays and Wednesdays, and the health department tests on Tuesdays and Thursdays.
When the health department labs receive a positive test, they notify the agency of agricuture and the town health officer where mosquitos came from, as well as the public through a press statement.
The mosquito and tick surveillance program is funded through a grant by the CDC.
There are about 45 species of mosquito in Vermont. Casey’s team targets areas around the state where specific species are more likely to carry EEE or West Nile virus. Mosquitoes are born out of standing water, which is why health officials remind homeowners to change the water in bird baths weekly, and to tip over children’s wading pools when not in use.
“For EEE, we look for hardwood swamps. Typically acidic swamps so they could be cedar swamps, or red maple swamps,” Casey said. A lot of these swamps are located in the western part of the state and the Lake Champlain Islands. “We’re looking more for woodland pools early in the spring.”
So far this year, more than 2,000 mosquito pools have been tested. They are taken back to the lab and sorted by species before being placed in vials and sent to the health department.
Mosquitoes can contract West Nile from biting another animal which is already carrying the virus, or vice versa. Casey said these can be different kinds of birds, primarily during the summer months. Reptiles and amphibians can carry the virus through the winter. However, with a changing climate around the world, migratory bird patterns are changing, which directly affects the behavior of mosquitoes.
“As the migratory bird populations are coming back to the area a little bit earlier every year, and leaving a little bit later. It’s not a huge amount, but even a few days, it can give mosquitoes a little jump on their breeding season,” Casey said. “Warm, early, wet springs allow for earlier hatches and earlier feeding.”
Increasing temperatures also allow for mosquito larvae to hatch quicker and can change the gestation period from two weeks to one week. While climate change is being blamed for increased temperatures and damaging weather events, it is also being blamed for periods of drought.
“It’s really hard to predict,” she said. “This year it seemed like it was sort of a long, cold, wet spring. It slowed the start of the mosquitoes.”
The effects show, as Casey said only two positive tests for West Nile have come back this year.
“That’s really, really light,” Casey said. “We’ve had a year where we had over a hundred West Nile virus positive pools come back.”
“West Nile virus is everywhere,” she said. Every year, tests in every county come back positive. Casey said the West Nile testing program might soon come to an end, because the state knows it shows up every year. Since 2011, only 12 Vermonters have become ill with the disease.
Centers for Disease Control and Prevention data shows a human’s chances of becoming ill with West Nile after a bite are slim — about one out of every 150 cases develop serious symptoms which primarily affect the central nervous system. If serious symptoms develop, about one in ten people will die from the disease. There is no vaccine for West Nile. Minor symptoms can be treated with over-the-counter medicines, while severe cases can require hospitalization. Severe symptoms can last for weeks or months, or even be permanent. Eight out of ten cases show no symptoms at all.
Casey said EEE has become her greatest concern. EEE has more serious symptoms and can lead to a painful death for humans, horses and other animals. While a small number of people are infected each year, about 30 percent of them die, while survivors are left with ongoing neurological issues. This year, EEE killed a Massachusetts woman, and positive tests have been found in New York and New Hampshire.
“This year, it hasn’t shown up [in Vermont], but it’s shown up all around us,” Casey said. In New Hampshire, WMUR reported a horse in Northwood, N.H. has contracted the disease. The last human case of EEE in Vermont was recorded in 2012, when Celotti said two people in Addison County died during the summer. The last group of mosquitoes which tested positive for EEE was recorded in 2015.
“We’re really doubling down,” Casey said. “In fact, I’m actually headed out today to scout some sites in the southern part of the state just to act out of an abundance of caution.”
Fortunately for humans and horse owners, Casey said a test for the species of mosquito which primarily carry EEE found that 95 percent of their blood meals came from birds.
“Worrying that we’re getting it right,” is the hardest part of the job for Casey. “As I drive around the state, I always have my eyes open for these little swamps. I’m always thinking, ‘Am I missing something? Is there a swamp out there we haven’t found?’”