Heartworm Disease – Is This (Still) Really a Big Deal?

By Susan O’Bell, DVM, DACVIM

Angell General Medicine Service

angell.org/generalmedicine

617-522-7282

The simple answer – a resounding “yes”! As a veterinarian and pet owner living in Massachusetts, heartworm disease is always at the forefront of my mind.

For many of us as dog owners we faithfully administer some sort of heartworm preventive to our dogs each month (and hopefully to our cats as well!). This is a lifelong habit, one we probably don’t give much thought at this point.

But what is heartworm disease? Is it really an issue up here in Massachusetts? Do I really need to administer this medication to my dog or cat all year long?

Heartworm disease results in foot-long worms living in the heart, lungs, and vessels of infected animals. Dogs are considered the natural hosts (the species in which heartworms mature into adults and produce offspring). Other species of animals can also be infected including cats, ferrets, wolves, coyotes, foxes, and in very rare instances, humans. Heartworm infections are found in animals worldwide.

Mosquitos play an important role in disease transmission. When a mosquito bites an animal who is already infected with heartworm disease, it can end up ingesting baby heartworms (called microfilaria). Then when the mosquito feeds on a new animal it can transmit these baby heartworms into the new host. In the dog it takes about 6 months for these baby heartworms to mature into adults. This timeline forms the basis for when veterinarians recommend heartworm testing. Heartworms have a relatively long lifespan, so each year an animal remains infected with heartworms, they continue to produce offspring and develop a greater and greater worm burden. This worm burden continues to cause organ damage and eventual symptoms of heartworm disease, progressively worsening until the condition is quite severe and often fatal without treatment.

Many dogs remain asymptomatic for months given this lengthy timeline, and sometimes even years. This is one of the main reasons we recommend an annual blood test even if there are no symptoms of heart worm disease present. Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life-threatening form of cardiovascular collapse. This is called caval syndrome, and is marked by a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the heartworm blockage, few dogs survive.

The good news is that almost all cases of heartworm disease are preventable. There are approximately a dozen FDA approved, safe products for use in dogs to prevent heartworm disease. There are also several options for cats. The majority of these products are flavored, chewable treats or tablets administered once a month. There are also topical “spot-on” treatments useful for dogs with food allergies or other contraindications to the oral products. There is no heartworm vaccine, but there is an injectable version of heart worm preventative that lasts for 6 months. All of these products have a wide margin of safety at the labeled dosage, even in dogs with known genetic mutations that make them more susceptible to the side effects of numerous medications.

Added benefits of monthly heartworm preventatives are that they serve to prevent and/or treat numerous other internal and/or external parasites such as round worms, hookworms, or whipworms. Many also have combinations of ingredients in them to provide efficacy against fleas and mites.

Why do you need to have your dog tested for heartworm disease even if you give him a heartworm preventive faithfully every month? There are several reasons I test every dog at least annually. We routinely use a “combo test,” so we are not just screening for heartworm disease but also common tick borne infections found in Massachusetts. Early detection of any of these infections provides useful information about the health of your dog, as well as helps us understand the risks posed to other animals in the area and the humans who share your dog’s lifestyle. There is also concern about strain(s) of heartworm resistant to the most commonly employed monthly preventives. Fortunately this problem seems to be very rare and currently not endemic to Massachusetts, but we should be proactive about identifying these cases with ongoing annual screening. We also see a significant number of dogs originating from Southern states rehomed to families in Massachusetts. For several reasons including climate, heartworm disease is far more prevalent in these Southern states. Some of these dogs have existing known or occult heartworm infections at the time they move up north. You may notice us being even more vigilant with heartworm testing in the first 1-2 years after adoption.

Don’t let your guard down even if you feel like you don’t see many mosquitos in your area. All it takes is one heartworm positive dog in your vicinity to increase your own pet’s risk by seven to eight fold. The best news of all is that heartworm disease is very preventable.  And if your dogs are anything like mine, they love when they hear me pop the monthly treat out of the foil wrapper!

Some final take home points:

Make the administration of heartworm prevention HABIT. Use technology to your advantage to set reminders. Many pharmaceutical companies even have apps to help you stay on target.

Have your dog tested annually for heartworm disease. Sometimes which tests we run or the frequency of testing will change over the years, but this is simply because we are trying to be as thorough as possible to prevent this epidemic.

Don’t forget about your cat! Even indoor cats are at risk for a stray mosquito bite. Safe monthly preventives are readily available through your veterinarian.

Heartworm preventives are considered prescriptions and should not be available over-the-counter or on-line without a prescription from your licensed veterinarian. You are also reminded that products purchased from your veterinarian’s pharmacy are often the only ones guaranteed by the pharmaceutical company who makes them because they can determine point of origin of the medication.

Lastly, if you are particularly interested in heartworm disease, visit the American Heartworm Society website at www.heartwormsociety.org or contact your veterinarian.

For information about Angell’s General Medicine service, please visit www.angell.org/generalmedicine or call 617-522-7282.