Montana historically has a lower heartworm disease incidence than other states—see this incidence map—but the heartworm risk in our state is growing. Increased mobility of people and pets truly means “it’s a small world” with regard to heartworm spread.

  • When a dog moves here from a hot, humid climate, such as the southeastern United States, they may carry heartworms. 
  • New mosquito species and expanded mosquito ranges mean a greater heartworm threat.
  • Rescue groups often transport dogs into our state, sometimes unknowingly bringing heartworm disease. 
  • Pet-friendly travel means that tourists may bring heartworm-positive dogs from out of state.

Highlands Veterinary Hospital is your partner in the fight against heartworm infection in your pet. Heartworms cause serious heart and lung damage in dogs and cats, but, fortunately, the disease is preventable. Read on to learn about heartworm infection, disease, diagnosis, treatment, prognosis, and prevention.

Heartworm infection in pets

Heartworms are not directly contagious from pet to pet—the organisms must go through part of their life cycle in the body of a mosquito, a process called vector transmission. These parasites live in the bloodstream of dogs, cats, wolves, coyotes, and foxes. When mosquitoes bite an infected animal, they can transmit the heartworms in their microscopic (i.e., larval) stage to your pet. The larvae then migrate to your pet’s heart and pulmonary arteries through your pet’s bloodstream, maturing into adults. The journey to your pet’s heart takes six to seven months, and the mature worms can grow up to a foot long and reproduce in the hundreds. In dogs, who are the natural heartworm hosts, the heartworm infection causes inflammation and blocks blood flow in the heart and pulmonary arteries. Untreated, the infection causes heart failure and death.

Heartworm disease in pets

Some heartworm-positive dogs show no outward symptoms, but the longer they are infected, the worse the problem becomes. Heartworm disease signs include:

  • Coughing
  • Decreased exercise tolerance
  • Weight loss
  • Pale gums
  • Labored breathing
  • Fluid buildup in the abdomen
  • Weakness and collapse

The liver can suffer damage, in addition to the heart and lungs. A severe infestation can cause sudden collapse (i.e., caval syndrome). 

Cats can harbor only a few adult heartworms, since they are not the parasite’s natural host. However, those few worms can cause a severe reaction, called heartworm-associated respiratory disease (HARD). This complicated infection causes cats to cough, have labored breathing, vomit, and lose weight. Sometimes, the first sign of heartworm disease in cats is sudden death, making prevention critical.

Heartworm diagnosis in pets

Our hospital performs a heartworm antigen test at every annual wellness exam to determine your dog’s heartworm status. A positive test indicates the presence of adult heartworms. A dog who tests positive may also receive a microfilaria test to check for immature heartworm larvae in their bloodstream, as well as a chest X-ray, chemistry panel, urinalysis, and complete blood count (CBC). We study your dog’s physical exam and test results, and their history, to assess their heartworm disease severity. We then classify your pet as stage one, two, three, or four, with four the most advanced disease stage.

Heartworm treatment in pets

Heartworm treatment is a multi-step process that takes months, and is more complex and risky than prevention. Each pet’s treatment plan is customized to their needs, but here is an outline of heartworm treatment’s key aspects:

  • A tetracycline antibiotic is prescribed to weaken the worms.
  • Medication is administered to kill the larvae in the bloodstream. 
  • Antiinflammatory medication is prescribed, and pets monitored for reactions.
  • Two to three arsenic injections, which can be painful, are administered one month apart  to kill the adult worms.
  • A strict exercise restriction must be enforced throughout the treatment process, to lower the risk of a thromboembolism, which is a blockage caused by dead worms that constitutes a veterinary emergency.

Heartworm prognosis in pets

If the disease is caught early, heartworm treatment carries a good prognosis. For dogs with stage one or two disease, their heart and lung damage will often heal following treatment. If the disease has progressed to stage three, however, the prognosis is more guarded, and dogs who have progressed to the stage of heart failure (i.e., stage four) seldom survive treatment. For cats, no heartworm treatment is available, and their prognosis is always guarded.

Heartworm prevention in pets

Heartworm disease is preventable in dogs and cats. Many forms of preventives are available, including chewable tablets or a topical preventive administered monthly. Injections that are given every 6 or 12 months are also available for pet owners who have a hard time administering a monthly medication. Since mosquitoes are the heartworm disease vector, their habitat should be eliminated by preventing standing water around your home. Mosquito control and repellents also help prevent heartworm transmission. Keep in mind that heartworm prevention is much less costly, both in terms of stress for your pet, and dollars in your budget, than heartworm treatment. Treating heartworm disease is difficult, but prevention is easy.

Highlands Veterinary Hospital wants you to prevent the peril of heartworm disease in your pet. Call our caring team to discuss heartworm prevention and to set up a protocol that best suits you and your pet.