May 3, 2004
School Researchers Work to Improve West Nile Virus Surveillance
Dr. Douglas Norris
"West Nile virus is the first new mosquito-borne illness to emerge in the United States in recent times,” explains Douglas Norris, PhD, an assistant professor at the Johns Hopkins Bloomberg School of Public Health. “The virus was first isolated in Africa in the late 1930s and by that time had probably already spread to Europe.” Surprisingly, it only reached U.S. shores in the late 1990s.
Now that West Nile is here, Norris and others at the School have been working to streamline the methods used to keep track of it.
Dr. Norris began working in the School's Department of Molecular Microbiology and Immunology in 1999, right at the beginning of Maryland's West Nile outbreak. After the Center for Disease Control and Prevention released the first data on the disease, he and Gregory Glass, PhD, professor, Molecular Microbiology and Immunology, noticed that the State's surveillance methods were not as efficient as they could be.
Dr. Gregory Glass
“At that time, the standard method of West Nile virus surveillance,” says Norris, “was to set up traps, collect all the mosquitoes you could, and test each species batch for West Nile.” Norris and Glass realized that if every species of mosquito that could normally be caught in Maryland (20 to 30 different species) were included in surveillance studies, only one positive mosquito would be found for every 2,000 caught. “That's a lot of work for very little return,” says Norris.
Building a Better Mosquito Trap
“We decided,” he continues, “to see what would happen if we limited ourselves to just one genus of mosquitoes, Culex, which is the primary West Nile vector in Africa. There are just five or six species of Culex that occur with any frequency in Maryland.” The researchers figured that by modifying standard traps so they would preferentially snare Culex mosquitoes, their samples would be greatly enriched with virus-carrying insects right off the bat.
“We wanted to be able to say, 'If you used traps baited with such-and-such, this high off the ground, during this part of the year, in this ecosystem, then you would preferentially catch this particular species of mosquito,” explains Norris. “People doing mosquito surveillance could then use such a system to enrich their samples or catch a series of particular species.”
The elevation of the traps was therefore one variable the two researchers wished to study. They asked Scott Shone, a fifth-year PhD student with Dr. Glass, to devise a method such that trap height could easily be adjusted from close to the ground all the way up into the tree canopy.
The bait used in the trap was another important variable. To draw mosquitoes into the traps, they used carbon dioxide (from dry ice), which is the standard bait, either alone or in combination with octenol, a compound originally derived from ox breath, and so attractive to many mosquito species.
The ecosystem in which the traps would be set was the third variable studied. In 2002 and 2003 they tested the traps in three typical Maryland ecosystems—a salt marsh, a freshwater marsh and in flooded woodland—to find out which species of mosquitoes preferred which combinations of trap height, baits and habitat.
Testing the Trapping Scheme
There was a serious drought in 2002, so they managed to collect “only” 40,000 mosquitoes that year. The trapped insects are frozen and each is sorted by species; all insects of the same species are pooled and ground up and each species batch is tested for West Nile
Last year, 2003, was wet and over 500,000 mosquitoes were caught. Although the data are still being analyzed (three undergraduate students are doing much of the sorting and counting), so far their trapping methods have been able to target specific species of mosquitoes.
The Future of the Virus
Norris notes that over the past few years West Nile has swept across much of the United States, therefore the virus won't cause too many problems in most parts of this country in the future. “In the virus's wake, we'll still see some sickness, but not as much as we've seen in the past few years.” But this year West Nile will spread through California, a completely naive population, probably resulting in an epidemic on the West Coast. After this year, the virus will remain endemic in the U.S., as it has been in Africa and Europe, always present with occasional outbreaks.
The Norris-Glass method of mosquito surveillance may become even more important in the future because—although cases of West Nile in humans and horses are still increasing—Maryland is no longer gathering data on infected birds and funding for surveillance operations has dwindled. In addition, news operations are not covering the story as they did at first, reducing awareness in the public sector.
Norris and Glass are now also working to determine whether they can predict where and when particular mosquito populations will turn up in Maryland. “If, based on climate data, place, and time, we can be 80 percent sure that in a couple of weeks a potent and dangerous mosquito species will appear at this time and at a certain site, then mosquito control workers can get a real advantage,” explains Glass. --Rod Graham