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Heartworm Disease in Dogs and Cats
Heartworm disease is aptly named - it truly is a disease that results in worms living in the heart. The good news is it can be easily prevented and managed for your pets. Keep reading to learn the symptoms and steps towards prevention.
What is heartworm disease?
Heartworm disease, also known as dirofilariasis, is a mosquito-transmitted disease caused by a nematode called Dirofilaria immitis. Nematodes are better known as roundworms, which resemble pieces of cooked spaghetti. In the US, heartworm is most common in the southeast, but has been diagnosed in every state in addition to across the globe. Any dog or cat not on heartworm prevention can become infected. We’ll focus on dogs since they are most commonly infected with heartworm disease.
How is heartworm disease transmitted?
When a female mosquito has a blood meal from an infected host, she picks up the first larval stage of the heartworm called a microfilaria. Within the mosquito, the microfilariae mature until they can be transmitted to another host, like a dog or cat, during another blood meal. The more mature larvae migrate through the tissues and blood vessels until they find the heart, which is where they generally stay, though they can find themselves in unwanted locations like blood vessels, eyes, the abdomen, and other places. It takes about 5 months for the larvae to mature into adults, and about 6 months for the adults to start producing microfilariae to be picked up by other mosquitoes (and the tests we use, more on that later).
How do I know if my dog has heartworm disease?
At each annual exam after 7 months of age, a small blood sample is taken to run a heartworm screening test which checks for heartworm antigen, which is a small piece of the adult heartworm. Because of the life cycle, there is a long prepatent period - it takes at least 6 months for a dog to test positive after infection. If you recently adopted a dog from someplace with a lot of heartworm, your vet may recommend rechecking a test 6 months after the initial test.
Depending on where you live and the clinic you go to, that test may also check for exposure to some tick-borne illnesses like Lyme disease. If your dog tests positive for heartworm then it is generally recommended to confirm the positive result with some more testing.
Some dogs diagnosed with heartworm disease need a couple of additional tests prior to treatment, especially if they are sick. The most common additional test is a set of chest x-rays, which can show the extent of the damage done to the lungs and blood vessels within the chest.
What symptoms should I expect to see if my dog has heartworm disease?
Fortunately, most dogs diagnosed with heartworm disease show no signs. However, even if they look healthy on the outside, it can be fatal especially if many worms are present. Symptoms like weight loss, coughing and breathing abnormalities, fatigue, fainting, or a distended abdomen can occur. In some of the more severe cases, a condition called caval syndrome can occur, which is when a large number of worms in the heart block the flow of blood. Caval syndrome causes death within days if not addressed.
How is heartworm disease treated?
Heartworm disease is treated by killing the adult heartworms with a series of injections of a drug called melarsomine into the back muscles. In addition there are some oral medications recommended, including a steroid to reduce inflammation and an antibiotic called doxycycline to kill a bacteria released from dead heartworms. It is extremely important to keep your dog rested and quiet during treatment. After the adult heartworms die they are broken down and absorbed, but if they leave the heart in too big of a piece they will get stuck in blood vessels and cause serious complications.
It is no longer recommended to use a “slow kill” method to treat heartworm disease. This was a treatment regimen in which heartworm prevention was given and eventually sometimes succeeded in killing adult worms.
How is heartworm disease prevented?
Fortunately, heartworm disease is easily preventable! There are many options easily available including monthly topical and chewable medications in addition to long-acting injections. No matter what you choose it is recommended to keep your dog protected all year round. It can be helpful to also use a product that repels mosquitoes to further lower your dog’s chances of becoming infected. It’s best to consult with your veterinarian to determine the best course of action for you and your dog.
What about cats?
Cats are often left out when discussing heartworm disease. They seem to be more resistant to heartworm infections when compared to dogs, but can certainly still contract heartworm disease and suffer significant complications. Testing for heartworm disease in cats is tricky for several reasons, mostly because they tend to have small numbers of worms present. If your cat has clinical signs consistent with heartworm disease, which can be vague, diagnosis may require a series of tests including blood tests and imaging like chest x-rays and a heart ultrasound (echocardiogram). Unfortunately, due to how cats handle heartworm disease, it can result in sudden death.
There is no approved medication for the treatment of heartworm in cats like there is in dogs. Because of this, prevention is especially important, even for those who live inside since mosquitos can find their way indoors. Heartworm prevention for cats comes as a monthly topical or chewable medication.
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Additional Resources
Occurrence Maps - can filter by disease to see worldwide distribution
Sources
“Highlights of the Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs.” 2020.
“Summary of the Current Feline Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Cats.” 2020.
Palermo, Valentina and Rishniw, Mark. “Dirofilariasis (Heartworm Disease).”
vin.com, https://www.vin.com/Members/Associate/Associate.plx?from=GetDzInfo&DiseaseId=1176&pid=607. Accessed 24 May 2020.
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