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Heartworm in Cats

Heartworm (Dirofilaria immitis) is rare in cats but can be life threatening when it occurs. The adult worms live in the right side of the heart and pulmonary artery. Outdoor cats are most at risk.

Heartworm is spread by mosquito bites carrying infective larvae in their saliva. The larvae burrow into the cats tissues and undergo several changes to become small adults worms which make their way into a vein and move towards the heart and pulmonary artery. One or two worms may be enough to cause serious trouble or even sudden death.

Signs of infection include:

Coughing (made worse by exercise)
Lethargy
Loss of weight
Poor coat condition
Vomiting
Difficulty breathing and congestive heart failure appear late in the disease

Worms may be discovered at post mortem (necropsy) following sudden unexplained death.

Treatment is complex and potentially dangerous. Drugs are available to kill adult worms (adulticide treatment) but the mortality rate is 30%. Pulmonary thromboembolism following adulticide treatment is fatal. Treatment of the larval stage is not usually necessary. Spontaneous cure is much more common in cats than dogs. Cats without clinical signs should not be treated. Cats showing clinical signs should be stabilized (given oxygen to help breathing).

Warm humid climates are ideal for heartworm.

If your outdoor cat lives in a hotspot for canine heartworm disease, consider giving a heartworm preventative medication. Heartgard for Cats (containing ivermectin) is given by mouth every 30 days for prevention. Revolution (containing selamectin) is given as a top spot application on the back of the cats neck and can also be used as a heartworm preventative.

References

Carlson DG & Giffin JM, Cat Owners Home Veterinary Handbook, New York: Howell Book House, 1995

Fogle B, Natural Cat Care, London: Dorling Kindersley, 1999

Tilley LP & Smith FWK, The Five Minute Veterinary Consult - Canine and Feline (2nd Edition), Philadelphia: Lippincott Williams and Wilkins, 2000

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