Email Sign-Up Go to Shopping Cart (0)



Customer Service

Heartworm Disease: Transmission and Prevention

Drs. Foster & Smith Educational Staff
Dewormer Comparison Chart 
Dog Deworming Guidelines 
Heartworm Disease: Transmission and Prevention 
Heartworm Disease: Transmission and Prevention

Veterinary insight about heartworm disease is constantly evolving. This devastating disease was once thought to only affect dogs in the southern United States. Now, veterinarians and pet owners know that this condition is much more widespread and pet owners everywhere need to be vigilant about prevention.

AREAS OF THE U.S. AFFECTED BY HEARTWORM DISEASE HEARTWORM DISEASE, caused by the parasite Dirofilaria immitis, has been diagnosed in all 50 states. Additionally, heartworm is not just an American problem - heartworm disease has been diagnosed worldwide.

HEARTWORM LIFECYCLE HEARTWORM IS TRANSMITTED by many species of mosquitoes. Transmission occurs when a mosquito takes a blood meal from an infected animal and then feeds on another animal. Heartworm infection can involve both dogs and cats, but the infection progresses differently in each species.

IN DOGS, the immature larval stage of the worm is deposited into the dog's skin by an infected mosquito. These larvae enter the dog's skin through the bite wound of the mosquito. Then in about 3 months, the juvenile worms migrate through the bloodstream to the right side of the heart. There they grow into adult worms (some can grow up to 12 inches in length!). The female then begins to produce tiny offspring, called microfilariae. Microfilariae are tiny and require a microscope to be seen. The microfilariae continue to live in the blood of the infected animal as they mature into adult heartworms, a process which takes about 5-7 months. As they mature, the heartworms can lodge in the heart and in the large blood vessels going from the heart to the lungs. Adult heartworms can live up to 7 years in a dog's body. If untreated, dogs usually die of heart failure.

HEARTWORM DISEASE IN CATS is different than in dogs on many levels. For instance, heartworms are smaller in cats than in dogs and it takes about one month longer for the larvae to develop into adults in the heart. Additionally, the lungs are the major organ affected in cats and it only takes a small number of worms to cause disease. According to the American Heartworm Society, the life span of the heartworm is shorter in cats than in dogs. Remember that the vector for heartworm disease, the mosquito, does not bite exclusively outside the house. Indoor-only cats are affected by this disease, as well.

SIGNS OF HEARTWORM DISEASE IN DOGS, the first outward signs of heartworm disease may not be apparent until a year after infection and may begin simply as a soft cough. The severity of this disease depends on the number of heartworms present. As the disease progresses, the infected pet will find it more difficult to breathe. He will be reluctant to exercise, and he may have a decreased appetite and weight loss.

INFECTED CATS have clinical signs that mimic many other diseases. These include difficult or rapid breathing, coughing, vomiting, fainting, seizures, blindness, loss of appetite, and weight loss. Heartworm is more difficult to diagnose in cats than in dogs.

For additional information, view our "How to Manage Heartworm" video.

Dog Heartworm Lifecycle


HEARTWORM DISEASE IS EASY TO PREVENT. A heartworm prevention program is effective and simple, and consists of three parts:

1. REGULAR BLOOD TESTS - This ensures your pet is free from heartworms before he begins or continues on his preventive medication. Your veterinarian will advise you as to the frequency of regular blood tests.

2. PREVENTIVE MEDICATION - The American Heartworm Society recommends administering a heartworm preventive every month to all cats and dogs year round. Prevention for dogs includes monthly oral preparations like Iverhart Max® (Brand), Heartgard® products, Tri-Heart® Plus Chewables by Merck or Sentinel® Tablets for Dogs (which includes flea prevention). These medications are given in treat form that dogs readily accept. Topical combination heartworm/flea preventives include Revolution® Topical and Advantage Multi® Topical For Dogs. Heartgard®, Revolution® and Advantage Multi® also offer monthly heartworm preventives for cats.

3. REDUCING YOUR PET'S EXPOSURE TO MOSQUITOES - This means making your pet's environment less hospitable to mosquitoes, which in turn decreases the risk of your pet being infected with heartworm in the first place. There are topical products available which can kill and/or repel mosquitoes on your pet as well.

Heartworm prevention is safe, easy, and inexpensive compared to treatment. Talk to your veterinarian today about starting a prevention program for your pets.


Heartgard ® Plus Chewables (Brand) by Merial

Heartgard® Plus Chewables (Brand) by Merial

Tri-Heart ® Plus (Generic) by Merck

Tri-Heart® Plus Chewables by Merck

Iverhart Max® (Brand)

Iverhart Max® (Brand)

Revolution® Topical for All Dogs & up to 5lb Cats (Prescription)

Revolution® Topical for All Dogs & up to 5lb Cats (Prescription)


Click here for a more printer-friendly version of this article.  
Click here for a pdf version of this article.  


Contact us

8 am - 7 pm CST
7 days a week

7 am-8 pm, CST
7 days a week