The Heartworm Story

The first published description of heartworm in dogs appeared more than 100 years ago in an issue of “The Western Journal of Medicine and Surgery.” Heartworm in cats was first described in the early 1920’s. Since then, naturally acquired heartworm infection in cats and dogs is identified as a worldwide clinical problem. Despite improved diagnostic methods, effective preventives and increasing awareness among veterinary professionals and pet owners, cases of heartworm infection continue to appear in pet dogs around the world. The diagnosis of the disease is still complex and elusive in cats.

Heartworm in dogs and cats

What do we know about it?

Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are but one of many species of roundworms. The specific roundworm causing heartworm in dogs and cats is known as Dirofilaria immitis. Until recently canine heartworm disease was considered to be a problem only in warm climates, but in the past few years it has been found in almost all areas of the United States and Canada. Since dogs travel widely with their owners, and infected dogs can carry heartworms for several years, heartworm disease may be a problem anywhere in the nation. Heartworm infection is transmitted by mosquitoes. When a mosquito bites an infected dog, it takes up blood which may contain microfilarae. These incubate in the mosquito for about two weeks, during which they become infective larva. Then, when the mosquito bites another dog, the infective larvae are passed into the second dog, infecting it. The infective larvae migrate through the tissues of the body for 2-3 months. They develop into several stages called L1, L2, and L3 stages. The L1 stage only lasts for 1-2 days. The L2 and L3 stages last for approximately two months. They then enter the heart where they reach adult size approximately 3 months after infecting your pet.

The mosquito is the only natural vector of transmission for canine heartworms, and about 70 species are capable of carrying the disease. As you might expect, heartworm infection is more common in areas where mosquitoes are numerous, and outdoor dogs constantly exposed to mosquitoes are the most frequent victims.

How can I find out if my dog has heart disease?

Your veterinarian is your dog’s healthcare expert. Regular veterinary visits are important for early detection of health problems.

Your veterinarian may ask you for specific information about your dog before performing a thorough physical examination. If indicated, blood and urine tests, X-rays, an EKG or other tests may be ordered. Regular testing is important for early detection of heart disease in dogs.

“Too often, dog owners do not take their dogs to visit the veterinarian until they are displaying severe signs of heart failure, and by then it may be too late,” says Dr. Bicknese. “When heart disease is detected in your dog, your veterinarian can recommend a schedule of regular visits and discuss a treatment plan that can help.”

Can dogs with heart disease be treated?

Yes. Although there is no cure for most heart disease in dogs, new treatments are available. Success of treatment depends on various factors, but early detection is always best. By following your veterinarian’s recommendations, you can help your dog live a longer, more comfortable life.

Heartworm 2001 map