Heartworm Disease – The Basics: What Do You Need to Know?

Heartworm disease is caused by the parasite Dirofilaria immitis.  This is a worm that lives in the heart, lungs, and surrounding vasculature.  It is a serious disease that primarily affects the heart and lungs but can also affect the liver, kidneys, central nervous system, and if left untreated, can cause death.

Heartworms are transmitted by mosquitoes: they take a blood meal from an infected animal and transmit the microfilariae (larval stage/baby worms) into another animal with subsequent blood meals. These microfilariae will then make their way to the heart where they grow into adult worms, causing heartworm disease. Mosquitoes are required for the parasite’s life cycle which means that a dog cannot re-infect itself.

Both dogs and cats can get heartworm disease from mosquitoes!  A cat is an atypical host, and unfortunately many times goes undiagnosed. In some cats, 1-3 adult worms can be devastating and create respiratory issues, and one of the main risk factors for cats developing feline asthma is heartworm! The treatment that we use for dogs cannot be used in cats, so prevention is key for kitties.

What are the symptoms?

Some dogs are asymptomatic, meaning that they act normally. There are no changes in their breathing, exercise levels, or appetites.  With chronic infections or heavy worm-burdens, owners can notice coughing, exercise intolerance (unable to go on a walk without stopping and/or coughing), decreased appetite, sleeping more, and even weight loss.

Clinical signs in cats can be very subtle to very dramatic.  These symptoms can include coughing, asthma-type symptoms, vomiting, weight loss, lack of appetite, and fluid build up in the abdomen.

Grading Scale

There are 4 grades to heartworm disease:

1. Grade I: Asymptomatic dog, tests positive on the annual test that is recommended by veterinarians.  Chest x-rays, blood work and urine testing is normal.

2. Grade II: Asymptomatic or mild symptoms in dogs.  Chest x-rays will show some abnormalities or the pet may have mild changes on blood work and urine testing.

3. Grade III: Symptomatic dogs, chest x-rays show obvious changes and blood work and urine testing is very consistent with chronic inflammation and parasitic infection.

4. Grade IV: Severely symptomatic dogs, chest x-rays show enlarged and abnormal vessels; they may have fluid build-up in the abdomen and are in right-sided congestive heart failure.  These pets have a guarded prognosis (and in some cases treatment may need to involve surgical extraction of the worms from the heart, through the jugular vein)

Why is annual testing recommended if my pet is on regular prevention?

Heartworm disease can be devastating.  The earlier the detection, the better chances for survival.  Since many dogs are asymptomatic at time of diagnosis, the only way it is found is through an annual test, which requires only a small amount of blood

All pets over the age of 7 months old should be tested for heartworm disease on an annual basis, but we start giving the heartworm preventative medication as young as 8 weeks of age.

How is heartworm disease treated?

If your dog has been found to have heartworm disease and all the testing indicates that it is safe to then go ahead with treatment, it is done with a medication called Immiticide (an arsenic derivative!).  The American Heartworm Society recommends giving three injections: one injection on day one and the other two injections one month later, 24 hours apart.  Post-injection care includes strict exercise restriction for 30 days (so, for a traditional treatment – that means TWO MONTHS of STRICT restrictions), keep them on all prescribed medications (often steroids to reduce inflammation in the lungs, sedatives as needed and pain medications for injection-site discomfort) for the heartworm disease, and monthly heartworm prevention.

There is no approved treatment for cats.

What is the best way to prevent this disease?

Keeping dogs and cats on monthly prescription preventatives, year round (even in the cold months), is the best way to prevent this disease.  The two main ways to administer this are topical or oral medications.  Both are only available as prescriptions through a veterinarian.

This is definitely a disease where prevention is a lot better (and cheaper) than treatment!

The life cycle and intricacies of treatment are a lot more complicated that the basic information we’ve provided here. If you’re interested in learning more – ask your veterinarian! At Clarendon Animal Care(goes to new website) we work with a number of local rescue groups and manage heartworm positive dogs frequently – we’re always happy to answer any questions you may have about this disease – detection, prevention, management, and general biology/life cycle.

The American Heartworm Society is also a great point of reference for pet owners.  Please visit www.heartwormsociety.org(goes to new website) for additional information.