AVMA News

Heartworm incidence climbs despite preventive efforts

Veterinary groups emphasize need for more communication with clients

Heartworm infection rates across the U.S. have steadily increased for nearly two decades despite the greater availability and efficacy of preventatives.

Every three years, the American Heartworm Society (AHS) conducts a survey that looks at reporting clinics and the total dog population. And each time, the number of dogs that have tested positive for heartworm goes up by well over 50,000, says Dr. Doug Carithers, a parasitology expert and board member of the AHS for over 18 years.

In 2001, the number of heartworm-positive dogs was estimated at 800,000. Twenty years later, that estimate has reached well over 1.2 million.

Normal pulmonary artery
Normal pulmonary artery (Photos courtesy of Dr. Steve Jones)

“It’s very disheartening, especially since this disease is preventable” said Dr. Carithers, who is a long-time AHS board member and employee of Boehringer Ingelheim.

The disease is found in all 50 states, though its geographic distribution seems to be changing, as it spreads to regions where it was once considered rare. Expansion of the territories of heartworm-positive wild canids, low owner compliance with heartworm preventives, relocation of microfilaremic dogs to other states, and an increasing frequency of weather conditions conducive to mosquito proliferation, driven in part by climate change, are all likely contributing to its spread.

“As a profession, we have to work together to help veterinary practitioners create urgency about this,” Dr. Carithers said. “We have to educate our colleagues that the spread of heartworm is a highly efficient process.”

Understanding the spread

Recent research indicates that what was once thought to be a random process is not random at all. A study published in 2013 by Dr. Tanja McKay, a professor at Arkansas State University, and others, found that mosquitoes are 10 times more likely to find and feed on infected dogs, wolves, or coyotes than noninfected canids.

Similarly, a 2023 study published in the Brazilian Journal of Veterinary Medicine  showed that heartworm-positive dogs are the richest source of infection for mosquitoes, increasing the risk of transmission.

Often, the disease will result in necrosis of pulmonary tissue before an animal tests positive for heartworm. At 153 days, the animal could be exhaling volatile organic compounds (VOCs) from the necrotic tissue, attracting mosquitoes, and thus increasing the likelihood of transmission as mosquitos feed on other animals.

Pulmonary Artery on Day 153 after heartworm infection
Pulmonary Artery on Day 153 after heartworm infection. Antigen test would not yet be positive.

“To make matters worse, wild canids tend to forage in anthropic areas where they can interact with mosquito-vectors that have fed on microfilaremic dogs and already harbor (the third larval stage of heartworm). Once microfilaremic, these wild canids become a source of infection to sylvatic mosquito vectors all over their living area, spreading the parasite in the wild,” the Brazilian authors wrote.

Dr. Carithers recounted a time when he noticed an increase in the prevalence of dogs in Montana testing positive for heartworm. In 2006, area veterinarians saw one case per clinic, which rose to five per clinic in 2009. He asked a local practitioner about it, and he said it was because Yellowstone National Park had reintroduced wolves into the park.

“If one is positive, they will pack up together, mosquitos feed on them, and then come back to regular spots to bed down and other packs get infected,” Dr. Carithers said. “The biggest issues occur when there’s a higher density population of pets, mosquitos, and a high population of infected dogs such as in Southern cities.”

Microenvironments and weather

Climatic conditions can also profoundly affect opportunities for mosquitoes to transmit heartworm disease.

In 2008-09, the Southeast experienced a severe draught. One would expect the number of heartworm cases to go down, but they just became localized around bodies of water, Dr. Carithers said. Maps from 2010 showed one Texas county, which previously had high incidences of heartworm, having zero heartworm-positive dogs among seven clinics. Yet, the next county over had 10 clinics report over 100 positive dogs per clinic. When he followed up with them, they told him the former county was high and dry and the latter had numerous lakes.

Hurricanes or other major storms can also cause heartworm outbreaks because of the increased water in an area. In California, some areas have seen increased heartworm incidence because tree hole mosquitos will lay eggs in underground urns at cemeteries that accumulate water.

Grey wolf (Canis lupus) portrait
Wild canids, such as the grey wolf, tend to forage in areas where they can interact with mosquito-vectors that have fed on microfilaremic dogs. Once infected with heartworm disease, the wolves can be a rich source of infection for mosquitoes, increasing the risk of transmission to other dogs, wolves, and coyotes.

“Mosquitos can lay eggs in a water source the size of a thimble. If it’s in the shade the water will last until the larvae develop. There’s great potential out there,” Dr. Carithers said.

Heartworm transmission does decrease in colder months, but the presence of microenvironments in urban areas suggests that the risk of heartworm transmission never reaches zero.

“Urban sprawl has led to the formation of ‘heat islands’ as buildings and parking lots retain heat during the day, creating microenvironments with potential to support the development of heartworm larvae in mosquito vectors during colder months, thereby lengthening the transmission season,” according to the AHS’s Canine Heartworm Guidelines.

In the Northeast, heartworm incidence is somewhat influenced by the frequent transport of animals from South, where heartworm is endemic. These animals are put up for adoption after being spay or neutered.

“The problem in these cases is when the dogs are taken into any shelter, that’s when they are tested (for heartworm). They might be infected at that point, but if they are prepatent, the results will come back negative. If you don’t follow up test, and the infection matures, then that dog is infecting mosquitos in the new neighborhood,” Dr. Carithers said. “This is how we end up with heartworms in previously nonendemic areas, like we have seen in Massachusetts near a large adoption center. So, veterinarians should always be vigilant, even if they feel they are in a low endemic area.”

Treatment and prevention

The biggest issue, according to Dr. Carithers, is that heartworm disease can’t be treated.

“No, you can’t treat heartworm disease,” he said. “You can get rid of the heartworm, but not the disease. The pathology, even from a single worm that dies in a single lung field, is still present year later. … Worms cause pathology that lasts forever.”

Further, results from a study that published this January in Frontiers showed that 6,138 patients treated for heartworm infection were found to have significantly elevated risks of right and left heart failure or cardiomyopathy compared with over 4 million patients that never had a positive heartworm test.

Yet, one study estimates only a third of dogs in the U.S. received a single dose or more of heartworm preventive in 2023, while 67% didn’t. That’s an increase from 64% in 2009, when there were only five heartworm preventives on the market. Now, at least 20 different heartworm preventive options are available, and all products are nearly 100% effective in preventing heartworms when used year-round and administered as directed.

To help address misconceptions and misinformation about this deadly and preventable disease, professional veterinary and animal health advocacy organizations have teamed up, including the AVMA, AHS, and Boehringer Ingelheim, to launch the #ProtectMoreDogs initiative—a program with digital resources for veterinarians and pet owners.

Pet owners are often told to get heartworm preventive but there’s a lack of follow up, Dr. Carithers says.

“There’s a disconnect between the exam room and the front desk. How much do you remember (as a patient) after stepping out of the exam room with a doctor? They ask you questions you don’t want to answer and give you lots of info and you have to keep track of that. I can understand easily how that would be lost in translation,” he said.

“We need to get vets to prescribe instead of recommend products before the client walks out of the clinic door. Or have products in the exam room and take them to the front desk.”

Clients can visit the AVMA website for information about heartworm disease, including the importance of prevention. Or send them home with AVMA’s Heartworm Disease brochure, also available in Spanish: Enfermedad del Parásito del Corazón.

In addition, the AVMA’s Language of Veterinary Care tools provide successful strategies–and words–that help veterinarians better connect with clients, build trust, improve compliance, and ultimately bring about better patient outcomes. They include a free ebook, available exclusively to AVMA members, as well as an interactive continuing education module for everyone on the veterinary team.