Heartworm Disease in Dogs

Heartworm disease is caused by long, thread-like parasitic worms (Dirofilaria immitis) that can grow up to 14 inches and live in a dog's heart and the arteries of the lungs. Dogs catch heartworm only through mosquito bites: a mosquito picks up microscopic larvae when it bites an infected dog or coyote, the larvae mature inside the mosquito during warm weather, and the next bite passes them into a new dog. Because worms live in the heart, they block blood flow and damage the heart and lungs — heavy infections can pack the entire right side of the heart. Many infected dogs show no signs at first. Later signs include a soft cough, tiring easily, and eventually weight loss, persistent coughing, and difficulty breathing. A dangerous complication called caval syndrome — collapse and dark brown urine — is an emergency. The good news: heartworm is easy to prevent with year-round monthly medication and a yearly blood test. Treatment exists (a series of deep-muscle injections, plus antibiotics and a month of strict rest), but it is painful, expensive, and hard on the dog. Call your vet if doses are missed, before starting prevention, or if your dog coughs, tires easily, or loses weight.