What is Heartworm Disease and Why is Prevention Best

Heartworm (Dirofilaria immitus) is an infection that can affect dogs and sometimes cats that has the potential to be life-threatening.

Heartworm is transmitted to a pet by mosquitos when they take a blood meal and happen to carry the larval stages of the parasite.  These larvae can then molt to a next larval stage of their life, before becoming mature heartworms after roughly 50-70 days in their canine host.  During this maturation the worms migrate in the body until they reach their desired location of the pulmonary arteries.

Heartworm infection can be detected at minimum 7 to 9 months after infection as it requires sexually mature worms to detect microfilaria or antigens.  Microfilaria are the stage in the heartworm lifecycle that can also be consumed by the next mosquito to then mature and carry on to another host animal.

Heartworm becomes concerning when a burden of mature worms reside within the pulmonary arteries and sometimes into the heart itself.  This puts the heart, vessels and lungs at risk as it becomes more difficult for them to work.  Worms that die can also cause emboli if they travel in the vasculature and lodge themselves in a blood vessel where they cut of blood and oxygen supply to that area of the body.  Emboli risk must also be considered when trying to rid your pet of a heartworm infection.

The safest way of dealing with heartworm is by prevention.  Your veterinarian may recommend regular heartworm testing, generally taking a small blood sample from you dog (or cat) to check if they remain heartworm negative.  A drop of blood may also be examined under the microscope as the microfilaria are large enough to be seen and can be used to verify a positive heartworm case.

Based on a negative test result, prevention is then safe to give.  Here in southwestern Ontario, prevention is at minimum given from June 1st to November 1st (6 doses when it is a monthly treatment).  Areas that have a mosquito population year-round, such as in the United States or further south, year-round prevention is recommended.  Most medications are monthly and work retroactively to kill any heartworm larvae that may have been transmitted to your pet over that previous month.  They are generally effective, but not every drug is 100%, doses can be missed or given late, or possible drug resistance can occur.  Products you may be familiar with are Heartgard, Simparica Trio, Interceptor, Revolution, etc.

If your pet has a positive heartworm result, then more careful treatment is needed to eliminate that worm burden.  Firstly, a second blood draw for re-testing will be done to ensure it is a true positive result.  Next, imaging of your pet’s chest may be done as x-ray and ultrasound can help confirm heartworm disease and possible severity.  With very high burdens of worms a vascular catheter may need to physically grab the worms and extract them a few at a time.  This is a dangerous procedure and is only used when killing the adult worms with medications is too dangerous.  Most heartworm positive cases require a regimen of adulticide medications in combinations with antibiotics given on very specific days and over many months.  This is a costly protocol, and can still be high risk to the patient.  Part of treatment includes strict cage rest during and is vital to their successful recovery from a heartworm infection.  This restriction in activity helps minimize cardiopulmonary complications.

Knowing all this, prevention is key along with regular testing to ensure your pet remains as healthy as possible and at the lowest risk of severe and costly complications.

A note about heartworm in cats:

Cats can contract heartworm disease, but overall tend to be more resistant to infection than their dog counterparts.  Though cats may have fewer worms, their smaller body size means a low worm burden can still impact them negatively.

 Overall cats can be more difficult to diagnose with heartworm, and even when positive, if they seem clinically normal they may, with time, eliminate the infection themselves via their immune system.  Normal antigen tests used for canine patients may not work for feline heartworm patients.  Chest x-rays may be quite beneficial in raising clinical suspicion of heartworm in a cat, and ultrasound of the heart and vessels can be quite revealing of a positive heartworm case. 

Medical treatment may not be required for most cats, and the drug used for dogs is not recommended for cats.  Often support of sick patients with steroids can be considered.  Surgery is often the treatment of choice if the burden is large and causing poor health to the patient, but as with dogs comes with its own set of risks.

Cats can also be given heartworm prevention during the same interval as above for dogs and is considered safe and effective.  Even indoor cats should be considered as mosquitos often make it into our homes and living spaces.

Let’s keep those pesky mosquitos away from our pets this Summer and Fall!

Dr. Hillary