Marm Kilpatrick Returns from Five-Month Study of West Nile Virus

by Linda Shockley

Marm Kilpatrick, disease ecologist with the Consortium for Conservation Medicine at Wildlife Trust, recently returned from a five-month field study that involved trapping and testing birds and mosquitoes for West Nile virus at nine sites in Maryland and Washington D.C.

This summer's work marks the completion of the third year of a six-year study of West Nile virus transmission. The study is funded by the National Institute of Allergy and Infectious Disease.

Dr. Peter Daszak, executive director of the Consortium for Conservation Medicine at Wildlife Trust, says, "West Nile virus is an introduced virus that has moved rapidly across the USA and is now our most significant vector-borne disease. Our work at the CCM aims to understand how it moves through wild bird populations and what makes it a major risk for people. Our work on how this virus spreads have allowed us to predict and help prevent its movement to new high risk regions such as Hawaii and Galápagos."

Dr. Kilpatrick also recently authored a paper "Predicting pathogen introduction: West Nile virus spread to Galápagos," slated for publication in the journal Conservation Biology.

In this WT online interview, Dr. Kilpatrick articulates the threat of this emerging infectious disease, its global ramifications and the key questions of his research.

What is West Nile virus and why is it such a global threat?

West Nile virus is a pathogen that was introduced from the old world (Africa, the Middle East and Europe) into North America via New York in 1999. The virus is a global threat to human and wildlife health because it causes substantial mortality in a number of different species of birds and other animals, and can cause serious illness in humans, especially older individuals. Since its introduction in 1999, it is estimated that more than a million people have been infected with West Nile virus, with over 215,000 becoming ill, and 7600 suffering from West Nile virus encephalitis. Over 720 people have died from this infection, which has now spread across North America into the Caribbean, and into South America. Data from the past seven years suggests that it will cause 2,000-10,000 cases each year in the US. Its impacts on wildlife and human health in the tropics are unknown, but it poses a serious threat to endemic wildlife in Hawaii, Galapagos and other sensitive regions.

What are the important hosts?

Nearly all birds are susceptible to infection, as well as most mammals, and even crocodiles and alligators! However, the most important hosts for amplifying the virus and causing epidemics are American robins and house sparrows. In contrast, European starlings and rock doves may reduce transmission of West Nile virus by diverting mosquitoes from feeding on more competent species.

What makes them such effective hosts?

American robins are effective hosts because they are highly preferred by mosquitoes. In fact, mosquitoes feed on robins 10-30 times more often than would be expected from their abundance. In addition robins are moderately competent hosts, in that approximately 15% of mosquitoes feeding on them in the seven days after infection will become infectious for the virus.

House sparrows are effective hosts for other reasons. First, they are amazingly abundant, and although they are actually partially avoided by mosquitoes, they still represent a significant fraction of mosquito feedings. In addition they are more competent hosts than robins. About 23% of mosquitoes feeding on house sparrows in the week after infection will become infectious. Although crows and jays are often the victims of West Nile virus infection, they are actually much less abundant than other species, and are relatively unimportant in West Nile virus transmission. (Their loud raucous and gregarious behavior makes them seem more abundant than they actually are.)

What are the key questions for your investigation?

The focus of our research is to determine what drives spatial and temporal variability in West Nile virus transmission. In short, why are hot spots hot, and what can we do to decrease the impacts on wildlife and human health?

What is your process for completing the research?

For the bird crew, our days start early - usually at 3:30am-4am. We drive from our farm house in Harwood, MD to one of our nine field sites, and try to set up 10-20 mist nets before dawn so that the birds will be captured during their first flights when lighting is still poor and the nets are least visible.

We also perform censuses or counts of the bird community at each site at dawn by recording the number, species identity and distance of birds around four-six points at our sites during six-minute "point counts." We continue catching birds with mist nets until early afternoon and then return home to store the samples in the -80C freezer before sending them to our collaborator, Laura Kramer, at the New York State Department of Health, for testing.

The days are often long (4am-4pm), but it helps that we change sites every two-three days so each day brings something new. The most exciting part of the work for me is going on net runs and seeing what we've caught. A highlight this year was catching a pileated woodpecker (see photo).

For the mosquito crew, the days start later. We set up two kinds of mosquito traps, called CDC light traps and CDC gravid traps, which are used by people to catch mosquitoes and other biting insects all around the world, including for malaria in Africa. The light traps are actually a misnomer – they are baited with dry ice, which is solidified CO₂. Mosquitoes are attracted to CO₂ when we breathe out, which is how they can find birds, mammals, and other animals.

The gravid traps are baited with a nasty drink we call stinky water. We make the water by putting grass, rabbit food and yeast in water and putting it in a sealed jug in the sun for a week. The most important mosquitoes for transmitting West Nile virus prefer to lay their eggs in organically rich water and so we make a special treat for them! The gravid traps are basically fans that capture mosquitoes that have traveled here to lay their eggs in the stinky water.

After setting up the traps, the mosquito crew also hunts for mosquitoes using a giant backpack mounted aspirator, which looks a bit like a "Ghostbuster" contraption. We search for mosquitoes this way to try to catch mosquitoes that have recently taken a blood meal. Our collaborators actually sequence the DNA inside the blood in their stomach to determine which species of bird or other animal they have fed on.

A day after setting up the traps, we collect the mosquitoes we have caught, bring them back to the lab and then identify them to species. It's a bit tedious, as we sometimes catch over 1000 mosquitoes in one trap in one night. However, it's important to determine which of the 20+ mosquito species we have caught, as each mosquito species has different feeding habits and biology. Some prefer to bite birds, some mammals, and some frogs and lizards. Some also are much more efficient at transmitting West Nile virus.

Why was D.C. selected as the testing site?

We chose to work in Maryland and the Washington, D.C.-area because a collaborator in the project, Peter Marra from the Smithsonian, had completed some preliminary research on West Nile virus in 2001 and 2002. When we initiated our work in 2003, he already had a large set of field sites established. We were able to choose the best subset of these sites where we now perform our work.

We chose our sites to fall along a gradient from intact forest to downtown urban jungle (see aerial maps). Along this gradient, bird and mosquito communities change substantially which allows us to determine the relative influence of different factors on West Nile virus transmission.

What's next in the research?

Two of the long-term goals of our research are to determine how climate affects disease and whether West Nile virus transmission will increase or decrease in the future as birds develop immunity and the virus mutates. Both questions have important ramifications for the long-term impacts of this pathogen on human and wildlife health.