Heartworm: Symptoms and Treatment

The thought of your beloved dog suffering from Heartworm Disease can certainly make you cringe. Those horrid spaghetti-like worms can cause serious health issues and may even be fatal for your dog.

The Basics on Heartworms

As the name suggests, heartworms are worms that live in the heart and lungs of their hosts. Dogs are the most common hosts for heartworms, but other mammals (such as felines, ferrets, and wild canids) have been known to contract these parasites. Humans are not a natural host for heartworms, so you don’t have to worry about contracting them. However, household cats are at risk, which we’ll cover more later in this article.

Heartworms are not directly contagious to other dogs since the parasites transmit through mosquito bites. Here’s how it works:

A mosquito bites an infected animal and picks up microscopic heartworm larvae, called microfilariae. When this mosquito bites another animal, it passes the microfilariae into the bloodstream of the new host.

The microfilariae then migrate through the bloodstream toward the host’s heart and lungs. It can take up to six months for the microfilariae to reach these organs, so detecting heartworms early in the course of the infestation is challenging.

Once in the heart and lungs, the microfilariae mature into adult heartworms and can grow to over a foot in length.

Heartworm risk factors for dogs

Heartworm Risk Factors

Dogs of any age or breed from teacup-sized Terriers to large Labrador Retrievers can contract heartworms from the bite of a mosquito carrying microfilariae.

Breeds with very heavy hair coats are also susceptible to Heartworm Disease. Dogs who are older or who have existing medical conditions, such as heart or respiratory problems, may be more severely damaged by heartworms.

Cases of heartworms have been found in every state across the country. Areas with hot and humid climates, such as the Ohio and Mississippi river basins and the Atlantic and Gulf Coasts, are especially endemic. That’s because they are good breeding grounds for the mosquitos that transmit the disease.

While any and all dogs are susceptible to Heartworm Disease, the greatest risk for transmission in dogs is during mosquito season, even for dogs that live primarily indoors.

Heartworm Symptoms

In its early stages when the microfilariae are migrating through the body, Heartworm Disease may be difficult to detect. However, later in the disease, when the adult worms are invading the heart and lungs, dog parents may notice symptoms such as:

If a large number of heartworms develop in the same blood vessel, they can block the blood flow and cause Caval Syndrome (also called Vena Cava Syndrome). This is a life-threatening situation that can be marked by severe difficulty in breathing and collapse. Caval Syndrome requires immediate emergency medical and surgical intervention to try and remove the blockage.

Heartworm Disease is a complex and life-threatening health issue, especially as it progresses. Remember that we are talking about parasites that are up to a foot long causing severe damage to the heart, lungs, liver, and kidneys.

Heartworm treatment for dogs

Heartworm Treatment

If your dog tests positive for Heartworm Disease, your veterinarian will determine the best course of treatment.

The American Heartworm Society (AHS) recommends initially treating infected dogs with an antibiotic and heartworm preventative medication. This helps combat any secondary infections and begins to eradicate the heartworm larvae. You can download the guidelines at the AHS’s website.

Dogs who have significant clinical signs of Heartworm Disease should also be “normalized” as much as possible before moving onto the next step, which is therapy for the adult worms. During this portion of the therapy, the AHS calls for the use of a three-dose regimen of an adulticide drug which is given by injection to kill off the adult heartworms.

Your veterinarian will administer these three injections over a period of time. The second injection is usually administered approximately one month after the first. The second and third injections are given about 24 hours apart. Please keep in mind that these are guidelines only. Your dog’s treatment is at the discretion of your veterinarian and based on your dog’s specific condition.

Post Treatment

The most important responsibility of dog parents with dogs undergoing treatment for Heartworm Disease is to severely limit their exercise. A dog’s activity must be drastically restricted after treatment since exercise will increase blood flow and cause fragments of the dead heartworms to break off and migrate through the bloodstream. This can cause potentially dangerous blockages.

Keeping an active dog calm and resting can be tricky. If your dog is crate trained, the crate can come in handy as a safe resting space.

Approximately six months after treatment, the dog will need to be tested to make sure the heartworms have been successfully eliminated. If not, the treatment may need to be repeated to take care of the remaining heartworms.

Heartworm prevention for dogs

Heartworm Prevention

Because treating Heartworm Disease is a long, costly, and risky process the best approach is ALWAYS prevention. Talk to your veterinarian about which of the many available heartworm preventative medications is right for your dog.

Heartworm preventative medications come in three forms:

Monthly tablets may be the most convenient method for many dog parents, but that means you have to get your dog to swallow that pill! Wrapping it up in a bit of soft cheese or another treat can help the medicine go down. Get tips on giving your dog a pill.

Since heartworm larvae are passed to your dog by mosquitoes, look out for any standing water around your home and eliminate it as best as possible. Mosquitoes lay their eggs (often 300 at a time) in still or stagnate water. Keeping the mosquito population down will help keep your dog safer.

Heartworm Testing

Annual heartworm testing is just as important as using heartworm prevention for two main reasons:

1. Symptoms may be missed.

It can take up to six months after contracting the microfilariae before adult heartworms develop and cause noticeable symptoms.

2. Early detection is key.

It can help improve the odds for successful treatment of an infected dog if the disease is caught early. As the disease progresses, more adult heartworms develop and increase the risk of treatment.

Heartworm testing is usually performed during your dog’s annual exam. It consists of a blood test that will either check for antigens left behind in the bloodstream by female heartworms or look for the presence of microfilariae. Your veterinarian will determine the correct test for your dog depending on local risk factors.

Heartworms and cats

Heartworms and Cats

It’s a myth that cats can’t contract Heartworm Disease. They’re not a typical host for heartworms because the microfilariae (larvae) have difficulty maturing to adulthood in the heart and lungs. However, a large number of microfilariae can cause health problems, and even a very few adult worms can be fatal to your cat.

Like dogs, cats contract Heartworm Disease from mosquito bites and may benefit from preventative medication. If you have a cat, you should discuss Heartworm Disease with your veterinarian and find out if preventative medication would be appropriate for your feline friend.

Heartworms and Dog Insurance

ASPCA Pet Health Insurance has options that can help you manage the costs of testing, treatment, and preventative medications. See plan options for your dog now.


Find Articles