The incidence of the spaghettilike parasites is up by more than 20 percent since 2013
By Kim Campbell Thornton
Andrews McMeel Syndication
Climate change, failure to give preventive products, and the beginnings of resistance to preventives are among the reasons why veterinarians are seeing more cases of heartworm disease in dogs — and cats. When the American Heartworm Society performed its triennial incidence survey last year, it found that while the highest incidence remains in the southern United States, no state is free of the harmful internal parasites, spread by the bite of an infected mosquito or, in the case of states such as Alaska, arriving by way of already-infected dogs brought from out of state.
Dogs are natural hosts for heartworms. Once an infected mosquito injects microfilaria — microscopic baby heartworms — into a dog’s bloodstream, the worms begin to mature and reproduce. As they get larger — heartworms can achieve a length of 1 foot during their 5- to 7-year lifespan — and increase in numbers, they clog the heart, lungs and associated blood vessels, causing heart failure, lung disease and other organ damage.
Cats are more resistant to the parasites, but they can acquire them. Clinical signs include weight loss, exercise intolerance, vomiting, diarrhea, coughing, gagging, difficulty breathing and wheezing. Even indoor cats are at risk. Approximately 25 percent of indoor cats are heartworm positive, according to the American Heartworm Society.
Heartworm disease is easy to prevent with a monthly pill or topical treatment, and it’s comparatively less expensive than treating a pet with heartworms. But people forget to give preventives, or they don’t give them year-round, giving infective mosquitoes a shot at spreading the parasites. Cool or dry weather slows transmission, but it doesn’t eliminate it.
“Most people think they don’t need to give it in the winter,” says Craig Prior, DVM, owner of Murphy Road Animal Hospital in Nashville, Tennessee. For instance, he says, dogs should stay on preventives for two months after the last exposure to mosquitoes and go on them one month before mosquitoes become active again. With climate change, some species are staying active longer throughout the year and venturing into new areas.
For those reasons, parasitologists recommend treating pets with parasite preventives year-round.
An associated concern is the beginning of resistance to preventive products. Some populations of heartworms, primarily in the Mississippi Delta area so far, are becoming resistant. “By keeping pets on year-round preventives, we decrease the risk of developing more resistant populations and increase the effectiveness of the preventives,” says Leni K. Kaplan, DVM, community practice service lecturer at Cornell University College of Veterinary Medicine in Ithaca, New York.
Adding a dog-safe mosquito repellent (avoid anything containing DEET) to your dog’s arsenal against mosquitoes can beef up his protection. Research published in 2016 found that the combination of a heartworm preventive with the mosquito repellent in the study, Vectra 3D, was 100 percent effective in blocking transmission of immature heartworms from dogs to mosquitoes — one of the stages of the heartworm lifecycle — and more than 95 percent effective in repelling and killing mosquitoes for 28 days after treatment.
“The addition of a topical product that prevents mosquito feeding adds a second element of protection to the pet,” says Byron Blagburn, Ph.D., a parasitologist at Auburn University College of Veterinary Medicine. “So not only do you prevent heartworm infection if the pet is on prevention, but you prevent the likelihood that the pet will see a mosquito.”
While Vectra 3D isn’t safe for cats, the good news is that if the repellent is used on a dog in the same household, the cat will share in protection because fewer mosquitoes will be present.