National Report — Experts say the summer of 2010 has been one for the record books. And it could signal trouble when it comes to the transmission of parasitic diseases this fall.
Sweltering heat, high humidity and above-average rainfall in much of the United States have combined to make a perfect storm for parasite proliferation. And most veterinarians clearly understand the risk from a host of vectors: West Nile virus, eastern equine encephalitis (EEE), heartworm infection, Lyme disease and even plague in the West.
"The mosquitoes are thick, aggressive and healthy this year," says Dr. Steven Halstead, state veterinarian for the Michigan Department of Agriculture. In fact, some Midwestern states, including Wisconsin, Illinois and Iowa, have had the wettest Julys on record, according to the National Climactic Data Center.
EEE has been particularly prevalent this year with large outbreaks in Michigan and Indiana. In mid August, nearly 70 horses died from complications of EEE, mostly in the southwest area of the state near Lake Michigan.
Specific mosquitoes are vectors for EEE and West Nile virus, Halstead explains. "One mosquito tends to circulate the virus among birds, and another mosquito bridges it out to humans and horses. It's a complex virus cycle, and all of the factors have to work together to get the case numbers (we've gotten)," he adds.
In fact, for mosquitoes that spread EEE, "pretty categorically, rainfall is the primary determinant because there are more potential habitats for mosquitoes to breed and less of an opportunity for mosquito control because there are so many there," says Joe Conlon, an entomologist and technical adviser to the American Mosquito Control Association. "Once you have that high rainfall, you'll have mosquitoes three, four or five weeks afterward. Some mosquitoes like to lay eggs in dry areas, and then once they get moist, they hatch."
EEE was identified in mosquitoes in northern Indiana as well. The disease causes neurological symptoms and can be deadly to horses. There is, however, a vaccine, and Halstead says a once per year vaccination usually will protect horses. In higher-risk areas, some DVMs recommend vaccinating twice per year. "The bottom line is, (vaccinate) at least once per year in the spring when it is warming up so you have the best coverage through mosquito season," he adds. While usually this would be in April or May, this year Michigan began warming in March, he says.
In Florida, dengue fever made a splash in the headlines. A Key West man was diagnosed with the virus in April, and that case comes on the heels of three others in Florida last year. Though dengue is the most common vector-borne illness worldwide, it has had a relatively low profile in this country compared with other viruses, such as West Nile. Two mosquitoes, Aedes aegypti and the Asian Tiger mosquito are the key vectors. Aedes aegypti is the most efficient transmitter and requires humans for blood meals. The Tiger mosquito is less efficient, but it is hardy and extremely difficult to control because it is considered a container breeder. "I've seen it breed in discarded Coke bottle caps," says Conlon.
Thankfully, veterinarians can assure pet owners that, despite this year's uptick in mosquito activity, their animals won't be infected with dengue. "Dengue has a human-to-mosquito, mosquito-to-human transmission cycle," explains Dr. Harold Margolis, chief of the Atlanta-based Centers for Disease Control and Prevention's dengue branch. "No animals are affected other than macaques, where the virus circulates in forest canopies in Malaysia and Africa. It just doesn't go in anything else."
But, according to Edward Breitschwerdt, DVM, Dipl. ACVIM, veterinary professor of internal medicine at North Carolina State University, it doesn't negate the concern about its spread. "We're concerned about it being transmitted from an endemic place to a non-endemic place," Breitschwerdt says. He likens the threat to that of heartworm. "When we transported dogs from Louisiana after Katrina to California and other parts of the world, veterinarians were frequently seeing heartworm (cases)."
In fact, mosquito-borne heartworm is the greatest threat to dogs, and Americans' increased mobility and the change in weather patterns led to the year-round medication recommendation. "Both Dengue (in humans) and heartworm in dogs are a function of the rapid national, and, at times, international movement of people and animals...We're moving infectious diseases at a faster rate than ever," Breitschwerdt says.
In regard to the weather, "it's a function of an environmental change that is occurring. There does seem to be a trend in Europe and the United States for various vectors that were more limited to a southern climate now being found in the central or even northern United States."
Control elements are key to reducing risk, experts add. Vaccinating animals and reducing populations of parasites remain critical prevention strategies. While mosquito spraying works to control some species, removing the breeding grounds is the best bet for all species. For the Asian Tiger mosquito, for example, he says that the flight range is only 300 to 500 feet, so removing planters, paint cans and trash will significantly help. "Even in a tarp, if it isn't flat and has creases on it, mosquitoes can breed there."
Some factors, however, can't be controlled, "particularly in the Midwest," says Breitschwerdt. "If you have a week of warm weather in a winter month, it's not unusual for mosquitoes to appear."
Ms. Karapetian is a freelance journalist in Chicago.