The prevalence of heartworm disease is increasing in New York state, and ranges from 6 to 25 cases per veterinary clinic each year. The Homestead Animal Hospital team encourages you to learn more about this mosquito-borne blood parasite, and the importance of heartworm prevention to keep your pet safe from a serious, potentially fatal disease. 

How can my pet get heartworm disease?

Heartworm disease occurs after a mosquito that is infected with immature heartworms (i.e., larvae) bites a dog or cat. The larvae are deposited with the mosquito’s saliva, and enter your pet’s bloodstream to wreak havoc on their body. 

In dogs, the larvae spend five to seven months swimming toward the pulmonary artery and heart, where they become adult worms. When adult male and female worms mate, they give live birth to “microfilariae,” which are released back into the bloodstream, and transmitted by a mosquito to another animal host. Microfilariae may also be passed from mother to puppies while in the womb. 

Cats are not natural heartworm hosts, so the cat’s immune system vigorously attacks the larvae, resulting in an inflammatory response that can progress to lung disease. Pets do not need direct contact with other animals to become infected with heartworm, and approximately 25% of infected cats live indoors. 

How does heartworm disease affect pets?

In dogs, adult heartworms can grow up to 14 inches long and can obstruct pulmonary artery blood flow. Also, pieces of dead worms can block adjacent vessels, reducing oxygen availability and creating blood clots. A dog’s immune system will try to attack heartworms as a foreign invader, and the combination of tissue damage and reduced blood flow can result in heart failure. In addition, heartworms often carry a bacterium that is released during reproductive stages, which causes more problems. 

In cats, a relentless immune system assault prevents 75% of the larvae from maturing into adult worms. Therefore, only one to three adult worms may develop in a cat, but they can still cause significant disease signs. If an adult worm manages to reproduce in a cat, the microfilariae are quickly killed by the immune system. However, a cat’s inflammatory response to a heartworm infection can cause lung damage, and clots from dead larvae or adult worms can cause sudden death. 

What are heartworm disease signs in pets?

Your pet has a greater chance of recovery if a heartworm infection is caught before signs develop. Signs that may appear in an infected animal can include: 

  • Decreased appetite
  • Weight loss
  • Coughing
  • Shortness of breath
  • Exercise intolerance
  • Rapid or difficulty breathing
  • Wheezing
  • Vomiting
  • Nosebleeds
  • Fluid accumulation in the lungs or abdomen
  • Blood clots
  • Sudden death

How is heartworm disease diagnosed in pets?

In dogs, a blood sample can be tested for an antigen indicating adult female heartworms are present, and annual testing is recommended. If results are positive, a confirmatory test is performed, plus a check for microfilariae. A complete blood panel, and chest X-rays or an echocardiogram, can help assess the stage of a heartworm infection to guide treatment.

In cats, the diagnosis is more complicated because an adult female heartworm, or microfilariae, may not be present. An antibody test can be helpful, but may be positive because of a previous heartworm infection that has been cleared by the cat’s immune system. If a cat shows heartworm disease signs, we recommend both an antigen and antibody test, a complete blood panel, and chest X-rays or an echocardiogram for a full assessment.

How is heartworm disease treated in pets?

If your dog tests positive for heartworm, the Homestead Animal Hospital team will evaluate the disease severity, and whether your dog needs stabilization prior to treatment. The American Heartworm Society guidelines include the use of a heartworm preventive to initially kill microfilariae, and antibiotics to eliminate heartworm-associated bacteria. After completion of these steps, your dog will receive intramuscular injections of an arsenic-based derivative to kill adult heartworms. During all treatment stages, your dog requires strict exercise restrictions, because physical exertion increases the amount of heartworm damage, and can cause sudden blockages from dead and dying worms. 

Cats do not have a licensed product to kill adult heartworms, so prevention is essential. Heartworm disease symptoms in cats are treated with antibiotics and long-term corticosteroid medications, with the goal of preventing body-tissue damage from a cat’s immune response, and from heartworms dying in different developmental stages.

How can heartworm disease be prevented in my pet?  

Heartworm disease prevention is much safer, and less costly, than treatment. Heartworm preventives, which kill the larvae to halt their development, are available as monthly oral or topical products, or as a biannual or annual injection in dogs. Year-round prevention is recommended, because of the mosquito’s increasing adaptation to cold and their ability to overwinter indoors, and the variations in mosquito season. Many heartworm preventives also act as a dewormer for intestinal parasites, and may include flea and tick prevention.

The heartworm is a microscopic parasite that causes a major disease in pets. Contact the Homestead Animal Hospital team to keep your pet safe with regular heartworm testing and prevention.