Madonna, a three-year-old pit bull mix with a sweet disposition, arrived at the ASPCA last May, part of a group of eight dogs rescued by the New York City Police Department (NYPD). All of the dogs were in extreme stages of neglect and were suffering from skin and ear infections, intestinal parasites, dental disease and other illnesses, but only Madonna tested positive for heartworm.

Heartworm is a serious disease: The spaghetti-like worms, which can grow up to a foot in length, live in the hearts, lungs and associated blood vessels of infected animals. They are carried in a microscopic form (known as microfilaria) by mosquitoes that transmit the worms when they bite other animals. They can circulate in the bloodstream, mature, multiply and can eventually obstruct the flow of blood to the heart and lungs. If not treated, heartworm can be fatal.

While dogs are the most common hosts for this parasite, they can also be found in other species, including cats, ferrets, foxes, even wolves and horses. Dogs can live for years without symptoms after infection, but the heartworms’ long-term effects in an untreated dog may cause lasting damage to the heart, lungs, and arteries. Cats may develop chronic respiratory disease and, unfortunately, the first signs in infected cats can be sudden collapse or death.

Fortunately, the ASPCA caught Madonna’s case early. She was successfully treated for her infection and subsequently tested for both adult heartworms and microfilaria. Today she is in a happy home and takes a monthly preventive medication.

Heartworm treatment in dogs is a multiple-step, three-to-four month process that involves injections and oral medication to kill the heartworms, as well as prolonged periods of exercise restriction. Since it is very challenging to treat cats for heartworm disease, it is essential to prevent the disease in the first place.

“The best way to avoid heartworm disease is to give your dog heartworm preventive, a once-a-month oral or topical prescription medication,” says Dr. Louise Murray, Vice President of the ASPCA Animal Hospital in New York City. Prevention comes in several formulations and your veterinarian can advise you as to the best choice for your pet. Heartworm preventives commonly also treat a variety of other internal and external parasites.

Puppies should start on preventives no later than eight weeks of age without a test, but should be tested in six month intervals after the first dose and then yearly after that.

Heartworm infection is harder to detect in cats, because they are less likely to host adult heartworms. Cats should be tested before being put on medication and re-tested as vets deems appropriate to monitor exposure and risk.

Heartworm symptoms in dogs include persistent coughing, fatigue after exercise, decreased appetite, decreased desire to exercise, and weight loss. Heartworm in cats can cause wheezing and respiratory symptoms, as well as vomiting, loss of appetite and weight loss.

April is National Heartworm Awareness Month. Visit our Pet Care section to learn more about heartworm in dogs and in cats.