Heartworms in Cats

Laurie Anne Walden, DVM

Heartworms are parasites transmitted by mosquitoes. The worms grow to be several inches long and live in the heart, lungs, and blood vessels of the lungs. Cats are infected less often than dogs but can die of heartworm disease. There are currently no good options for removing heartworms in cats. Giving cats heartworm preventive medication all year round is the best way to protect them from heartworm disease.
 
Infection in Cats
Cats can be infected with heartworms wherever dogs are infected with heartworms—and heartworms have been diagnosed in dogs in all 50 states.[1] Studies have shown that 12% to 16% of cats have been exposed to heartworms.[2] In a 2017 study, 15% of cats with heartworm infection were indoor cats.[3]
 
Animals become infected with heartworms through mosquito bites. Mosquitoes take in heartworm microfilariae (baby heartworms) when they bite an infected dog, coyote, or other canid. When the mosquito bites another animal, heartworm larvae are deposited in the other animal’s body. Over the next few months, the larvae travel to the blood vessels and eventually to the heart.
 
Dogs are heartworms’ natural hosts. Cats are atypical hosts, which means that heartworms aren’t as well adapted to living in cats. In dogs, heartworm larvae grow up to be adults that can make baby heartworms. Heartworms in cats are less likely to produce offspring. Heartworms that reach adulthood in cats are usually smaller than in dogs and don’t live as long (heartworms can live for 2-4 years in cats and 5-7 years in dogs).[2] Cats typically have only a few heartworms, whereas dogs can have many. However, because cats and their hearts are so small, even 1 or 2 worms are a significant burden for them.
 
Symptoms
In cats, heartworm disease is mainly a lung disease. Most of the symptoms are caused by an inflammatory reaction to worms in the lungs and the blood vessels of the lungs. Although some cats never show any signs, others have a severe inflammatory response that leads to a condition called heartworm-associated respiratory disease. The symptoms of this disease can be mistaken for asthma.
 
Symptoms of heartworm infection in cats can include the following:

• No symptoms
• Coughing
• Wheezing
• Difficulty breathing
• Vomiting
• Decreased activity
• Weight loss
• Seizures
• Collapse
• Sudden death

 
Diagnosis
Diagnosis of heartworm infection is not always straightforward in cats. Antigen tests are used routinely in dogs, and a cat with a positive antigen test is definitely infected. But antigen tests detect only adult female heartworms. These tests are more likely to miss infections in cats than in dogs because cats are sometimes infected with only immature worms (no adults) or with only 1 or 2 worms (which might not be female). Antibody tests show whether cats have been exposed to heartworms but not whether they are still infected.
 
Heartworm diagnosis in cats usually involves a combination of antigen testing, antibody testing, and possibly ultrasound to look for worms in the heart and the blood vessels of the lungs. Chest radiographs can show changes compatible with heartworm disease. Cats also typically receive general bloodwork including a complete blood count and chemistry profile.
 
Treatment
In cats, the goals of treatment are to reduce the inflammatory response and provide support until symptoms improve or the infection clears on its own. The level of treatment depends on the individual cat’s condition and symptoms. Cats with severe disease may need intensive care.
 
The drug used to eliminate heartworms in dogs is not recommended for cats because it is more toxic to cats. No medications so far have been shown to be effective and safe for removing heartworms in cats. Surgical removal of heartworms may be attempted in some cats but has risks. Studies of other treatment options are ongoing.[1]
 
Prevention
Safe and effective monthly heartworm preventives are available for cats and kittens. Some products are flavored chews; others are drops applied to the skin. All heartworm preventives in the United States require a prescription from a veterinarian. No over-the-counter products are effective at preventing heartworms in dogs or cats.
 
For More Information
Heartworm in cats (American Heartworm Society): https://www.heartwormsociety.org/heartworms-in-cats
Heartworm in cats (Cornell Feline Health Center): https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/heartworm-cats
 
References
1. American Heartworm Society. Prevention, diagnosis, and management of heartworm (Dirofilaria immitis) infection in cats. Published 2020. Accessed April 16, 2020. https://d3ft8sckhnqim2.cloudfront.net/images/pdf/2020_AHS_Feline_Guidelines.pdf?1580934824
2. Heartworm: cat. Companion Animal Parasite Council. Updated July 1, 2015. Accessed April 16, 2020. https://capcvet.org/guidelines/heartworm/
3. Levy JK, Burling AN, Crandall MM, Tucker SJ, Wood EG, Foster JD. Seroprevalence of heartworm infection, risk factors for seropositivity, and frequency of prescribing heartworm preventives for cats in the United States and Canada. J Am Vet Med Assoc. 2017;250(8):873-880.
 
Photo by Rana Sawalha

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