April is finally here after another long, cold winter, and that means it’s once again Heartworm Awareness Month.

Heartworm is a potentially fatal disease spread by mosquitoes. Though dogs are the natural hosts, heartworm can infect cats as well, and because the parasites can be spread to any animal a mosquito can reach, even indoor-only animals are susceptible.

2013 Heartworm incidence map

Graphic courtesy of American Heartworm Society

After taking a blood meal from an infected host, the mosquito picks up the microscopic offspring of the heartworm that are circulating through the host’s bloodstream. Once inside the mosquito, these tiny baby worms, called microfilaria, develop in 10 to 14 days into larvae, which are the infective stage of the organism. As the mosquito seeks another blood meal from a dog or cat or other susceptible animal, it deposits the larvae on the new host’s skin; the larvae then enter the animal through the wound caused by the mosquito.

In around six months the larvae grow into thread-like adult heartworms which are about twelve inches long, and may be found in the host’s arteries and blood vessels, lungs, and of course, its heart. The worms will mate, and their offspring will enter the animal’s bloodstream, starting the cycle anew.

As their natural host, dogs are more susceptible to infection from the heartworm and there have been cases which, left untreated, resulted in an infestation of worms numbering in the hundreds, though in most cases there are far fewer. Because cats are not their normal hosts, most of the larvae do not survive to grow into adult worms; cats infected with adult worms usually have only a few, though even immature worms can cause serious problems. Unfortunately, there is no approved safe treatment for heartworm in cats (the medication used for dogs is unsafe for cats), so administering a monthly heartworm preventive is just as important for cats as for dogs.

Symptoms of infection can vary. In dogs, look for loss of appetite and weight loss, a lack of desire to exercise, tiredness after light exertion, or a mild but chronic cough. In cats, take note of loss of appetite or weight loss, vomiting, unstable movement or difficulty walking; as the larvae migrate through the lungs, they can cause inflammation which may cause an asthma-like respiratory condition.

Treating heartworm is risky, traumatic and expensive. Fortunately, prevention is easy, convenient and affordable. Heartworm prevention comes in the form of chewable tablets, pills, topicals and injections. Please ask us how we can protect your pet.

For a wealth of information on heartworm, visit the American Heartworm Society. And remember, prevention is the best medicine!

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Ann Arbor Animal Hospital is a locally-owned animal hospital operating for over 90 years in Ann Arbor, MI.