Widespread flooding in the Midwest in June caused an explosion in the mosquito population, leading to the potential spread of heartworm in dogs, cats and ferrets, according to the American Heartworm Society.

And the flooding from hurricanes in the Gulf Coast will put pets at risk of heartworm, the disabling and deadly parasitic disease, said Sheldon Rubin, DVM, president of the Batavia, Ill.-based society

"Flooding causes an exponential increase in the risk of heartworm infections. Mosquitoes are prolific,'' said Rubin, a Chicago veterinarian. "The larvae begin to multiply almost immediately after flooding in stagnant pools of water.''

Sandi Sawchuk, DVM, a veterinarian at the University of Wisconsin School of Veterinary Medicine in Madison, Wis., encountered the flooding first-hand: "I lost several trees. My yard was like a swamp."

"The mosquito population is definitely higher than it has been in previous years," Sawchuk said. She also said she is confident that her patients are protected with heartworm-preventive medication. But she is concerned about pets that live outdoors and do not receive veterinary care.

High-risk animals

Risks can be high in such animals. Sawchuk said the full impact of infections acquired during the flooding won't be known until six to seven months after the mosquito bites occur. "If a dog was bitten in June by an infected mosquito, heartworm disease will be found the following year when it's in for a heartworm test. It takes a long time for these little microscopic larvae to migrate and develop to a point that the test tissue actually picks up proteins secreted by the uterus of an adult female heartworm.''

She said the risks are probably greater in shelters where the animals that have been picked up and brought in haven't been receiving preventive medicine.

Effects of Katrina

An estimated 60 percent of the pets displaced by Hurricane Katrina were infected with heartworm, which is spread by mosquitoes that have bitten other infected dogs, cats, wolves and coyotes.

Rubin said the American Heartworm Society later this year will announce the results of its first heartworm survey that includes shelter animals. The society's last survey in 2004 received reports on more than 250,000 heartworm infections in dogs.

He said about three-quarters of dogs under veterinary care receive regular heartworm protection, which can cost less than $10 a month, far less than treating the disease. "Prevention is nearly 100 percent effective,'' Rubin said.

Rubin said that animals that start on the preventive medication can beat the infection. "If we start them now (in late August), the medication is going to work backward by approximately 30 days, so it's going to prevent infections that took place in July and August,'' he said. "It is very important that dogs be checked for heartworm with a simple blood test before heartworm prevention is started.''

The American Heartworm Society recommends annual testing, even if a pet is on monthly prevention.

Reveiwed by Susan E. Aiello, DVM, ELS, and John A. Bukowski, DVM, MPH, PhD

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