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Heartworms: The Silent Killer

A cute lover valentine havanese puppy dog with a red heart is looking upward, isolated on white background

Heartworm disease is serious and potentially fatal. It affects dogs, cats, and up to 30 other species of mammals. Heartworm disease has been reported in all 50 states. It is caused by parasitic worms (heartworms) living in the major vessels of the lungs and, occasionally, in the heart. Heartworms are transmitted (as microscopic larvae) through the bite of an infected mosquito. The scientific name for the heartworm parasite is Dirofilaria immitis.

Heartworms can cause a variety of medical problems affecting the lungs, heart, liver, and kidneys. Any of these problems, alone or in combination, can lead to death. While treatment is available for dogs, it can sometimes be costly and complicated. In cats, heartworms can cause a respiratory disorder that mimics feline asthma. However, there is no approved medical treatment for heartworm disease in cats.

Although heartworm disease is virtually 100% preventable, many pets are still diagnosed with it each year. The American Heartworm Society (AHS) estimates that 1 million dogs in the United States are infected with the disease and that its incidence may be rising. Cats are susceptible to heartworms, too, and even indoor cats are at risk. Studies have shown that more than 25% of heartworm-infected cats live indoors.

Treating Heartworm Disease

In dogs, if heartworm disease is detected early enough, it can be treated before permanent damage is done to the heart, lungs, and blood vessels. However, if the infection has been present for a long time or consists of a large number of worms, the risk of complications can increase. In these cases, treatment can be more expensive and complicated, and dogs may need many months to recover from the infection. Hospitalization may be required.

For cats, there is no approved medical treatment for heartworm disease. Your veterinarian can discuss with you how to monitor your cat and manage the signs of disease. Antibiotics, steroids, and other medications are sometimes recommended. For cats with severe breathing problems or other complications, hospitalization may be needed. In some cases, surgical removal of adult worms may be attempted. However, this surgery is costly and has some risks. 


Fleas and heartworms can be easily prevented by using safe, effective, and easy- to-administer monthly medications. Some of these products are given orally, whereas others are applied topically to the pet’s skin (these are called spot-on medications). There is also an injectable heartworm preventive for dogs that can be administered every 6 months by your veterinarian.  

Some heartworm and flea preventive products have the added benefit of also controlling other internal parasites of concern, such as roundworms and hookworms (in dogs and cats) and whipworms (in dogs). Some products also target other external parasites, such as ticks and mites. 

In some cases, the best protection for your pet may not be the use of a single product, but rather the simultaneous administration of more than one product to effectively control parasites. Your veterinary team can help you decide which strategy may be best for your pet.

Preventing heartworms and fleas before they can become a problem is the safest, smartest, and most effective way to combat these parasites and keep your beloved canine and feline friends healthy! Ask your veterinarian which product(s) he or she recommends for your pet’s situation. 

Caution: Some parasite control products cannot be used on cats. Consult your veterinarian regarding which specific products can be used for cats to safely prevent fleas and heartworms.

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