heartworm prevention

Heartworm Disease in Dogs and Cats

Have you ever wondered if your pet is at risk for heartworm disease or why your vet recommends monthly heartworm prevention? Continue reading to learn how heartworm disease can affect your beloved pet.

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What is heartworm?

Heartworm, Dirofilaria immitis, is a worm that can cause significant disease of the pulmonary arteries, lungs, and ultimately the heart of dogs and cats. 70+ species of mosquitos can transmit the microfilaria (larval stage) of heartworm by biting dogs and cats. Heartworm also affects wolves, foxes, coyotes, raccoons, ferrets, some seals, and sea lions.

Where is heartworm disease most common?

Heartworm disease has been reported in the U.S., Canada, Australia, Latin America, and Southern Europe. Environments with warmth and moisture are preferable for mosquito reproduction. All animals who live in sub-tropical and tropical climates are at risk, including those that are indoors only because mosquitos can fly right into your home. In colder climates, animals that spend time outdoors are at a greater risk.

What happens when a pet is infected with heartworm?

Dogs are more frequently and severely affected by heartworm than cats.

The microfilaria develops through several larval stages in the mosquito before it bites your dog. The larvae first stay in the dog’s subcutaneous tissues, chest, and abdomen for about 2 months after the initial infection. They then move into the pulmonary arteries and heart. Heartworms can grow up to 25cm long, clogging up the pulmonary arteries, and eventually invading the right ventricle and even atrium of the heart.

Clinical Signs of Heartworm in Dogs and Cats

  • Asymptomatic (most dogs) - This is why yearly testing and monthly prevention is important!
  • Weight loss
  • Exercise intolerance
  • Coughing
  • Labored breathing
  • Additionally, aberrant larval migration can cause lesions in the central nervous system, eye, scrotum, peritoneal cavity, systemic arterial system, and in visceral and subcutaneous sites. This is less common but can happen in any infected animal.

The severity of heartworm disease is based on:

  • Worm numbers and health
  • Host immune response
  • Duration of infection
  • Host activity level
  • More active animals tend to develop pulmonary hypertension
  • More sedentary dogs may show no clinical signs even if carrying a high worm burden

Diagnosis and Testing

Heartworm testing should be performed yearly on dogs that are on monthly heartworm prevention. The 8-minute snap test your vet typically performs on your dog at an annual exam will only be positive if there are enough adult female worm antigens detected. This typically shows up after at least 5 months of infection.

A separate test for microfilaria (the larvae) may not show to be positive until 6.5 months after infection. Due to the lifecycle of the heartworm, it is advised to retest a dog with an unknown history 6 months later just to be sure you didn’t get an initial false negative.

You can view the heartworm life cycle here:

Heartworm Life Cycle

If a dog is heartworm positive, full bloodwork (complete blood count, chemistry, electrolytes) should be run to evaluate whether other organ systems are affected and how healthy or compromised the dog is. Chest x-rays to evaluate the lungs and heart are important to assist in determining the severity of the disease.

Treatment of Heartworm Disease in Dogs

Treatment is directly dependent upon the health status of the dog.

The American Heartworm Society has a standard protocol that lays out the specific steps, tests, and timing of the very deliberate treatment of heartworm disease in dogs.

You can view the treatment protocol here:

Heartworm Treatment Guidelines

Initially, after a diagnosis is made, 30 days of doxycycline is prescribed to kill Wolbachia pipiens, a bacteria that lives inside the cells of heartworms. Without it, the worms become sterile and eventually die. However, it only kills adult worms. So, there is also a series of three melarsomine injections (administered intramuscularly by your veterinarian) which kills the microfilaria as well.

During the entire treatment process, dogs continue to receive monthly heartworm prevention. This also helps kill the adult worms. It is vitally important to keep animals quiet (exercise restriction) during the entire process of treatment, which lasts several months.

Steroids may also be prescribed to assist in decreasing inflammation. The concern is that dying worms moving through the vessels can cause clotting in the heart and lungs leading to strokes and/or death.

Can cats be affected by heartworm disease?

Cats aren’t as severely affected by heartworm disease, but they can get it, and they can succumb to it.

Feline Heartworm Facts:

  • 1-3 adult worms at most
  • Worms typically survive within the cat for 2-3 years
  • Many worms die after entering the vasculature when they’re still immature
  • Heartworm-Associated Respiratory Disease (HARD) - Due to the body’s response to the dead/dying immature heartworms in the pulmonary vasculature

How is heartworm disease prevented?

The most important message about heartworm is to always give your pets monthly heartworm prevention, as it is far safer, easier, and cheaper than treating existing heartworm disease. Some pets have such a large worm burden that has already affected or compromised their heart and lungs that they aren’t healthy enough to receive treatment. Unfortunately, these pets tend to eventually be euthanized due to extreme respiratory distress.

Read more:

Heartworm: The Parasite

American Heartworm Society

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