Climate change is increasing risk for heartworm, researchers say. How to protect your pet

Claire Rafford, Indianapolis Star
·6 min read

Climate change could be contributing to the spread of heartworm in the U.S., creating conditions ripe for the transmitters of the parasite, mosquitoes, to breed.

A 2019 USA TODAY article cited data showing that heartworm infections in the number of dogs per veterinary clinic testing positive rose by 21.7% from 2013 to 2016.

In addition, a study in Hungary published in 2020 found both an expanded geographical area and increased incidence of heartworm infection since 2012, which researchers concluded was due to climate change and ecological changes.

While the study was based in Europe, researchers believe the same heartworm emergence and spread could also be observed in North America.

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Moreover, the National Oceanic Atmospheric Administration is predicting increas.ed odds of above-normal temperatures for most of the U.S. for June, July and August of 2022.

Luckily, preventive treatments are almost 100% effective at preventing heartworm, according to the American Heartworm Society. Here's what pet owners should know heading into summer.

What is heartworm, and what causes it?

Heartworm is a parasite that causes a pulmonary disease in pets like dogs, cats and ferrets.

It's also present in other wildlife, like wolves, coyotes, and sea lions, Dr. Sriveny Dangoudoubiyam, an assistant professor of parasitology at Purdue's College of Veterinary Medicine, said.

An animal can contract heartworm when it's bitten by a mosquito that has heartworm larvae. The larvae are deposited onto the skin and enter the body through the wound, then migrate through the blood and can develop into full-grown worms.

"This process results in a serious disease that involves the lungs, heart and also many other organs, and it can end up resulting in death of the animal," Dangoudoubiyam told IndyStar.

Heartworm and climate change

While heartworm is most common in places where there are high temperatures as well as high incidences of standing water, it is present in all 50 states.

Incidence maps from the American Heartworm Society from 2001 to 2019 show a general trend of increased heartworm prevalence in the United States, especially in the southeastern region.

"When we talk about climate, it's important that we understand that anything that is related to major disasters like a hurricane, or major floodings, anything that can directly contribute to mosquito development ... can all be factors for increased heartworm," Dangoudoubiyam said.

There are also microclimates, small pockets of temperature created where mosquitoes can flourish.

"Urban sprawl has led to the formation of 'heat islands,' as buildings and parking lots retain heat during the day," Dr. Tom Nelson, American Heartworm Society board member, wrote in a Q&A. "Temperatures in these microenvironments can be much higher and more conducive to the persistence of mosquito viability."

According to an infographic from the organization, female mosquitoes lay their eggs in or near water and can produce anywhere from 100 to 300 eggs at once, which will hatch in approximately 48 hours.

According to the Companion Animal Parasite Council (CAPC), which tracks parasite prevalence, Indiana has a 0.96% heartworm canine prevalence rate. Heartworm infection prevalence varies by county, which people can view here: bit.ly/3xqKJhp.

What are the signs of heartworm?

Dr. Lorraine Corriveau, a clinician at Purdue's veterinary hospital, said that heartworm actually affects the entire pulmonary artery vasculature, including the heart, but more so the lungs.

Heartworm signs that emerge in dogs can include cough, decreased appetite or weight loss, as well as fatigue and reluctance to exercise, according the American Heartworm Society.

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However, in the beginning of the infection, the dogs may have few to no symptoms, as it develops slowly. As disease progresses and the dog may develop more worms, more serious symptoms may emerge, like heart failure and excess fluid in the abdomen causing the appearance of a swollen belly.

For cats, the signs can be both more subtle and more dramatic and serious, according to the American Heartworm Society, though cats are not a typical host for heartworms.

Cat owners should be on the lookout for asthma-like attacks, coughing, vomiting, lack of appetite or weight loss, as well as fainting, seizures and difficulty walking as well as fluid accumulation in the stomach.

Ferrets can show many of the same signs of disease as dogs, and are "extremely susceptible" to heartworm infections, the organization's website said.

But because the ferrets have smaller hearts, the symptoms will develop more rapidly. Just one worm can cause extreme illness, leading to possible respiratory distress for a ferret.

How to prevent heartworm

Focus on preventing heartworm should be strong year-round, because by the time signs and symptoms of heartworm emerge, a dog could already have been infected for months, Dangoudoubiyam said.

Dangoudoubiyam and the American Heartworm Society recommend the "Think 12" method, "which means you protect your animal from heartworm by giving it the preventive drugs throughout the year, and you test the animals every 12 months," Dangoudoubiyam said.

The heartworm prevention medication is highly effective because it kills the larvae before they can advance to the adult stage, Corriveau said. In addition, many treatments protect against other types of parasites as well.

The heartworm prevention treatment can take a variety of forms, including a pill, injection or topical medication. Heartworm medications require a prescription.

Perhaps most well known is Heartgard, which contains ivermectin. Heartgard medications vary based on the weight of the pet.

While the medication is very effective, annual testing is still extremely important, Dangoudoubiyam said.

The American Heartworm Society urges against thinking about heartworm in terms of having a "season" of only being a risk during the summer months, or peak mosquito season.

"Giving heartworm preventives only during so-called peak mosquito season can leave pets unprotected/vulnerable," the AHS infographic said. "No one knows when we'll see the first mosquito in the spring or the last one in the fall."

How is heartworm treated?

For dogs infected with heartworm, there are several different steps to treatment, including several types of drugs, steroids and in acute cases, surgery, Corriveau said. Any dog infected with heartworm will also be prescribed a heartworm preventive.

However, for other animals, there are less treatment options, as the drugs used to treat dogs are not approved or recommended for cats or ferrets.

While some cats who test positive for heartworm may be fine with treatment for symptoms alone, others may be affected severely.

A common clinical finding in cats with heartworm? Sudden death, Corriveau said. That's why prevention is key.

"Just prevent it in your ferret and your cat, because it's not good if they get it," she said.

In general, heartworm prevention is highly encouraged for all pets.

"Treatment is very expensive, compared to prevention," Dangoudoubiyam said.

Resources

Contact IndyStar trending reporter Claire Rafford at crafford@gannett.com or on Twitter @clairerafford.

This article originally appeared on Indianapolis Star: Indiana environment: How to protect your pet from heartworm