Everything You Need to Know about Heartworm

Prevent heartworm in your pets, but heartworm treatment in rescued dogs is almost always successful.


Photo contributed

Pudding was heartworm positive, successfully treated and adopted through Homeward Trails.


Coco was an upbeat, loving ten-year-old beagle when Homeward Trails Animal Rescue – a Northern Virginia-based rescue organization – took her under their wings. 

Sue Bell, founder and owner of Homeward Trails, was eager to whisk Coco away from her previously lonely, uncomfortable (at best) life in a rural southwestern Virginia town. First, the Homeward Trails team would treat Coco for her multiple health issues, and then they would find her a loving owner in Northern Virginia who would finally give her the home and life that she deserved.

“Coco had the sweetest personality,” Bell recalled. “She came to us from rural Virginia, where clearly she had been neglected her entire life. Among so many preventable health problems, she had a heavy load of heartworms. Still, despite all her pain, she was a tail-wagging bit of sunshine. Sadly, she succumbed to the heartworms.  She suddenly started coughing up gobs of blood, the oxygen level in her blood plunged, and despite the veterinarian’s efforts, she didn't make it.”

Coco’s tragic story is a cautionary tale. While she was one of many dogs with heartworm – a mosquito-borne illness – who have come into the care of Northern Virginia rescue organizations over the past decade, she has also been one of the few that Bell has seen die from the disease.

Bell established Homeward Trails in 2001, and through the non-profit organization, her team pulls animals from low-income shelters, owners who cannot provide for them, and from other unfortunate situations. They then place their rescues into loving, permanent homes – primarily in the Mid-Atlantic.

Through her journey with animal rescue, Bell has discovered that heartworm disease is entirely preventable – and usually treatable when caught early (and even when caught not so early).

“With the thousands of dogs we’ve treated over the course of 21 years, we have a 99.99% success rate with heartworm,” Bell said. “Of thousands of dogs, we’ve lost less than ten to heartworm complications. The overwhelming majority of our dogs with the disease go on to live happy and healthy lives.”

This statistic, though, hinges on awareness of heartworm disease and taking necessary actions as early as possible.

What Is Heartworm Disease?

Heartworm disease is a mosquito-borne parasitic infection that impacts dogs, cats, and other animals, and it is every bit as unpleasant as its name indicates. While heartworm disease can infect a host of mammals, canines are the most commonly infected.

For dogs who have not been treated with prevention medication, heartworm disease is a relatively common disease. The infection is “caused by foot-long worms (heartworms) that live in the heart, lungs, and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body,” according to the American Heartworm Society. 

Through just one bite, a mosquito can inject the larvae of parasitic worms directly into a dog’s body. These larvae travel through the bloodstream and settle into the dog’s heart, vessels, and lungs, where they grow and multiply. Over the course of six to seven months, they fully mature into worms that can be up to one foot long. Tangled up and taking up precious real estate in a dog’s vital organs, the parasites cause life-threatening, painful inflammation.

“Dogs are the natural host for heartworms and they can reproduce and survive in a dog,” Dr. Matt Novarr, DVM, veterinarian at Columbia Pike Animal Hospital, said. “Since cats, for example, are an atypical host, the worms do not progress into adulthood within them, and they only tend to have a few worms at a time if they do get an infection.”

On the other hand, dogs can ultimately host up to hundreds of worms (though the average is around 15). These worms can live within a dog between five and seven years.

Testing is required to confirm heartworm because dogs don’t always exhibit symptoms – at least not for some time.

“The cycle of heartworm, once bitten, takes about six months to get it and test positive,” Bell said. “Symptoms may take a while to present in a dog, and include excessive coughing, panting, or having a round belly that can be mistaken for pregnancy. Heartworms clog the arteries of the heart, and oftentimes there are no outward indications of this. Largely, we do not see physical signs. It takes a blood test – usually combined with other tests.”

Heartworm disease is rare in dogs who begin their lives in Northern Virginia, but it is more common to find the parasite among animals who come to local organizations from more rural locations. 

“Because the DC area is such a hub for the importation of animals from other areas, there is a relatively high prevalence of heartworm,” Bell said. “Where pet owners in Northern Virginia are resourced both in terms of education about heartworm and in the ability to procure resources for preventing it, people in other areas are not in the same position.”

Dr. Novarr’s firsthand observations align with Bell’s experience with the disease.

“Heartworm is present in Virginia, but tends to be more common down south,” he said. “It has been present here since I began practicing in 2008, and the majority of cases we get are from dogs that are adopted from more southern states. I am not an expert on the epidemiology of heartworms, but generally anything that would increase the amount of mosquitos would potentially lead to a higher incidence of heartworms.”

Bell attributes the rise of animal rescue as a “growing business” in Northern Virginia to the increased awareness and lower instances of heartworm in the area.

“I would think we know more about it because the business of animal rescue and sheltering has grown so much,” Bell said. “I think that’s thanks in part to some of the very large animal associations. For the last 20 years, those organizations have been doing a lot of communication and outreach to the public about homeless animals. Millions and millions of dollars have gone into animal rescue and welfare. At one time, millions of animals were dying in shelters needlessly, and in the last few years, more and more people have been getting dogs from these places. Decades ago, it was far more common that you’d buy a dog from a breeder.”

Peoples’ ability and unequivocal willingness to invest in their animals’ health and wellbeing – to treat them with the same priority as with human family members – also translate to better heartworm statistics in Northern Virginia.

“Once people have pets, research has proven over and over again that care for animals is inelastic for people in areas like ours,” Bell said. “On the other hand, this – along with animals being a big, big business and the awareness of heartworm – doesn’t translate to rural areas.”

Bell said that the way so many dogs live in rural southern areas contribute to making conditions ripe for the spread of heartworm disease.

“I can tell you behaviorally that dogs are generally more sedentary than cats and other animals,” Bell said. “While the dogs we see with heartworm are varied in their breeds, the largest breed of heartworm-affected breeds we see are hounds and beagles. So many of them are owned by hunters or other people who house them outside year-round. These dogs are sitting ducks, often on chains and living in small pens outdoors, 24/7/365.”

Even though heartworm prevention is legally considered critical care, Bell said that the pets she pulls from under-resourced shelters – namely in southern Virginia and West Virginia – are just not getting the treatment.

“These outdoor dogs are also not being  given preventative medication,” Bell said. “This is oftentimes because of barriers – both financial and otherwise – for pet owners to obtain preventative care. A lot of people in under-resourced areas do not understand what heartworm is. There is that educational deficit. Even then, prevention would be cost prohibitive, where it really isn’t a financial issue in places like Northern Virginia.”

“DON’T BE SCARED” Adopting or fostering a dog with heartworm disease

According to animal rescue experts in Northern Virginia, heartworm disease – which is not transmittable to humans – should not deter potential fosters or adopters from bringing home infected dogs.

“Don’t be scared to adopt a heartworm dog,” Bell said. “It is the exception to rule that there will be long-term complications. I am living proof – one of my own dogs had heartworm. The treatment was manageable for both of us and now she is a completely normal dog.”

Chelsea Jones, Senior Communications Specialist for the Animal Welfare League of Arlington (AWLA), agreed with Bell – based on what she’s seen at AWLA and also from her experience adopting her own beloved previously heartworm-positive dog. 

In her dog’s case, heartworm treatment was a bit more complicated. Still, she said that even though supporting her dog, Obie, through the process could be overwhelming at times, she couldn’t be happier that she didn’t let the diagnosis stop her from adopting him.

"I knew Obie was heartworm positive when I adopted him, and I was also lucky that I knew more about the disease than another dog owner might because of my 11 years in animal sheltering,” Jones said. “Despite some difficult nights and a long period of rest, pills, shots, testing, and some more pills, Obie is heartworm free and living his best life. I don't regret adopting him or going through treatment with him; this guy is my little shadow and I would do anything for him. My only regret is that I can't time travel to make sure he was getting his heartworm preventative so he didn't have to go through it all.”

The key to beating and overcoming heartworm, as emphasized by both Bell and Jones, is early testing and timely treatment – and then the consistent administration of monthly prevention treatment to dogs throughout their lives.

“If a dog tests positive for heartworm, we follow American Heartworm Association protocols,” Bell said. “These have varied, but have always involved some dosage of doxycycline [an antibiotic], a heartworm preventative medication, and injections of Immiticide. There are various ways to do it, but it’s usually some kind of combination of these treatments. And it usually works.”

Jones said the AWLA follows a similar course of action, adding that a dog who is treated for heartworm must lay low while in recovery.

“This can vary from case to case, dependent upon the severity of the infection, but the standard in-shelter treatment for heartworm is a dose of ivermectin (heartworm preventative), then 30 days of doxycycline, then another dose of ivermectin, as well as the first dose of melarsomine,” Jones, the AWLA’s Senior Communications Specialist, said. “Thirty days later they get two more melarsomine shots, 24 hours apart. During this time they get prednisone and other medications as needed. From start to finish, treatment takes about four months. The animal has to be on strict exercise restriction the entire time, and for another eight weeks after their last injection.” 

Acknowledging the limited exercise requirement, Bell insisted that caring for a dog in the midst of heartworm treatment is still absolutely manageable.

“It’s keeping your dog somewhat calm for a relatively short time after treatment,” Bell said. “People think that means a dog has to be in a crate 24/7, but that’s not the case. They can go out on slow, leisurely walks. While it’s harder when you have young, energetic dogs, in the grand scheme of things, the time it takes for them to undergo treatment is nothing to blink at.”

Dr. Novarr noted that there is never an official time that it becomes “too late” to treat a dog with heartworm disease.

“I wouldn't say that it is ever too late to treat, but as the patient develops a more severe infection, there can be secondary changes to the heart and lungs and the treatment would be more difficult,” he said. “There would be a higher risk of side effects secondary to more numerous dying worms as well as changes to the heart and lungs.”

He said the main issue with intensive heartworm treatment is that it can be costly. 

Fortunately, because rescue organizations in Northern Virginia do not often face the same resource challenges as their counterparts in other areas, it is pretty standard for local groups –  including Homeward Trails and the AWLA – to pay for the entire course of treatment for every infected animal.

Prevention: The best heartworm strategy

According to the Federal Drug Administration, heartworm testing should be conducted on all dogs who are seven months and older.

The American Heartworm Society also recommends that dogs be tested for heartworm annually.

Then, once a dog has tested negative for the disease – whether it is post recovery or he or she has never had heartworm – owners should always be diligent about prevention measures – no matter where they live. After all, while it is less common in areas of high elevation and in the northeastern part of the US, heartworm has been detected in all fifty states.

“Thankfully, prevention is very easy,” Jones said. “All dogs (and indoor and outdoor cats) should get a monthly preventative heartworm medication – commonly a tablet – along with their flea and tick preventative treatment. Talk to your vet if you aren't sure if your monthly preventative covers heartworm.”

Monthly prevention is a non-negotiable for pet health, even in Northern Virginia’s coldest months. 

“Heartworm prevention not only prevents heartworm, but some other intestinal parasites – some of which may be contagious to humans,” Dr. Novarr said. “It is also much cheaper to prevent heartworms and intestinal worms than it is to treat infections once they are present. Typically heartworm preventatives aren't very cost prohibitive, but there are always less expensive options that can be discussed if cost is the main concern.”

He added that preventing one dog from getting heartworm can translate to safeguarding many dogs in a community against heartworm. 

“Mosquitoes can pick up the infection from other dogs who are infected, then spread it to different dogs if they aren't on their prevention,” Dr. Novarr said. “So by using the heartworm preventative, you are helping to protect other dogs as well.”

To learn more about heartworm disease, or to discover how to help affected animals, talk to your veterinarian, contact the AWLA (www.awla.org)  or Homeward Trails (www.homewardtrails.org).