What is Heartworm?

Adopting dogs from the USA and Central America has become common practice over the last ten years. While this is wonderful that dogs are finding homes when otherwise they may not have, many of these dogs have heartworm and being adopted without their new owners being fully informed of what this means for their new pets health.

What is heartworm?
Heartworm disease is a serious and potentially fatal disease in pets in the United States and many other parts of the world. It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe (sometimes fatal) lung disease, heart failure and damage to other organs in the body. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect the dog’s health and quality of life long after the parasites are gone.

How do dogs get heartworm?
While we in eastern Canada are able to live blissfully ignorant of this life threatening parasite due to our climate (for now), it is recommended that all dogs in the USA be on monthly preventative for this disease. Shelter dogs are unlikely to be protected and are at a high risk of contracting it, especially in the southern states where many of these dogs are brought up from, due to its transmission being via mosquito.

How is Heartworm Treated?
The recommended treatment for heartworm is a series of 3 intra-muscular injections to kill the adult worms. The biggest risk associated with this is that the dying worms can cause blood clots which can be life threatening. The lower the worm burden, the lower the risk of this happening. It requires strict exercise restriction of the dog for 2-3 months (no off leash time, short walks only) to further decrease the chances of this happening (the slower and softer a heart beats the less likely chance of throwing a clot). This treatment is expensive, but highly successful especially in asymptomatic patients.

The other option, and one that is only recommended if financials do not allow for the injection protocol, is to keep the dog on heartworm prevention until the adult worms die off on their own.  This simply prevents the dog from re-infecting itself through the adult worms already established in their heart by killing the babies they produce. It is not in fact killing any adults despite the common colloquial name for this process (“slow kill”). This treatment protocol is not recommended for several reasons. First, it does not address the established adults and allows them to live, and cause physical damage to the vessels and organs they are living in, which can cause issues long after they are dead. The second reason this is not recommended is because these animals are still at risk of throwing clots from dying worms but there is no way to predict when this will happen. If, for example, a worm or worms die and the dog goes to daycare like they always do, the risk of suffering from a clot is much higher because their heart rate could be elevated when the worm(s) die. With the injection protocol there is a clearly defined timeline of worm death that we know we need to keep them as calm as possible for.  Finally, this treatment course can take years and the dog is at risk for developing potentially life threatening illness secondary to the infection at any time.

The issues we are seeing as veterinarians is that new owners are not aware of the potential for this disease to be life threatening, cost of treatment options, and risks associated with those options. Please, if you are considering adopting a dog from the USA, learn the risks of heartworm and what is involved regarding treatment and side effects: https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics.

Myth Busting:

Myth: Dogs with no symptoms don’t need injections.
Fact: The severity of heartworm disease does not always correlate with the severity of symptoms, and dogs with many worms may have few or no symptoms early in the course of the disease.

Myth: Dogs with only a “light” positive result don’t need injections.
Fact: There are only positive and negative heartworm test results.

Myth: If a dog tests negative when they are transported, they will stay negative since they are no longer exposed to mosquito vectors.
Fact: A positive test can take up to 6 months post infection (for example, a dog can be infected in March, test negative in June then positive in September). It is recommended that all dogs coming from heartworm endemic regions be re-tested 6 months later because of this.