About Heartworm Disease in Dogs

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Protecting your dog from heartworm disease is easy with one Trifexis® (spinosad + milbemycin oxime) chewable tablet, once a month, year-round. Trifexis works by killing heartworm larvae after an infected mosquito bites your dog and before the larvae mature and become adult heartworms (D. immitis).

What is heartworm disease in dogs?

You may wonder, “How does a dog get heartworm disease?” Heartworm infection can be deadly, and it starts with the bite of an infected mosquito. From there, the worms penetrate the dog’s tissue, migrate to the bloodstream, and enter his heart and lungs.

Risks of heartworm disease

Heartworm disease symptoms in dogs can include1,2:

  • Coughing
  • Exercise intolerance
  • Difficulty breathing
  • Enlarged liver
  • Death

However, a dog with a heartworm infection may have no visible symptoms. Regardless, if left untreated, heartworm disease can be deadly.2 Your veterinarian should perform a yearly blood test, which is an important part of your dog’s routine health check, in order to detect a heartworm infection.

Heartworms can live in an infected dog for 5 to 7 years.1,2 In time, damage to your dog's heart and lungs can be severe and can include1,2:

  • Clogging and damage to the lining of the artery leading from the heart to the lungs 
  • Heart valve malfunction
  • Liver and kidney dysfunction
  • Heart failure

Heartworm incidence nationwide as of 2016

Heartworm incidence nationwide map

Map © American Heartworm Society: The severity of heartworm incidence as shown in this map is based on the average number of cases per reporting clinic. Some remote regions of the United States lack veterinary clinics, therefore we have no reported cases from these areas.

Year-round protection is essential

Your love for your dog knows no season, and neither should his heartworm protection. That’s why the Companion Animal Parasite Council and the American Heartworm Society recommend uninterrupted, year-round treatment with a broad-spectrum parasite control product without interruption, regardless of where you live.1,2

Why administer heartworm prevention year-round?

Heartworm larvae generally develop in temperatures above 57°F. However, colder temperatures can just pause the process and result in larval growth resuming when the air warms up.2 It’s just one more reason why year-round heartworm protection is important.

If your veterinarian prescribes less than year-round protection, it’s important to administer Trifexis for at least three months after your dog’s last exposure to mosquitoes. It can take up to six months or longer for the symptoms of heartworm disease to manifest in your dog, so it’s advisable to administer Trifexis monthly—even in winter or other times you don’t see mosquitoes. 

Life cycle for heartworm in dogs

Illustrated stages of heartworm infection
A dog with an active heartworm infection. Microscopic larvae (microfilariae) are produced by adult worms and circulate in the blood. Larvae appear in the dog’s blood about 6-9 months after infection.

A mosquito bites an infected dog and ingests blood containing microscopic heartworm larvae.

Microscopic larvae develop within the mosquito and enter the infective stage in about 2 weeks, under ideal conditions.

Infective larvae are contained in the saliva of the mosquito.

The infective larvae enter the bite wound from a mosquito and migrate into the tissue of the dog.

Larvae migrate through tissue and mature to enter the bloodstream, making their way to the dog’s heart and pulmonary arteries, about 70 days after infection.

Larvae develop into adult heartworms and reside in the pulmonary arteries. In heavy infections, adult worms may invade the chambers of the heart and cause symptoms including coughing, sluggishness and difficulty breathing, though some dogs may show no signs at all.

Adult heartworms produce microscopic larvae that are released into the bloodstream.


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When it comes to protecting your dog from heartworms, prevention is your best weapon. That’s because treating an infected dog can be painful, expensive and may not be 100% effective.2 Plus, your dog could have serious health complications.

Two key steps to keep your dog from developing heartworm disease:

1) Annual testing

Consult your veterinarian about testing your dog annually for heartworm infection. If a test is positive, your veterinarian will discuss your treatment options.

2) Year-round protection

Provide your dog with year-round heartworm prevention with Trifexis. It makes heartworm prevention as easy as one chewable tablet, once a month, year round. If your veterinarian prescribes less than year-round prevention, it’s important to administer Trifexis each month for at least three months after your dog's last exposure to mosquitoes.

See how Trifexis can stop this deadly heartworm cycle.

Learn more about Trifexis

Trifexis is a prescription product that should only be purchased from a veterinarian.
Learn how to buy