Pet First
Canine Heartworm Disease
By Dr. John Varnado, DVM

Heartworm disease in dogs is caused by the nematode or roundworm, Dirofilaria immitis. Heartworm disease is found in all 50 states in the United States. Mosquitos are involved in the transmission of heartworms, so states with warmer climates and heavier mosquito populations have more heartworm disease.

A dog becomes exposed to heartworms when bitten by an infected mosquito. When a mosquito bites a dog infected with heartworms, the mosquito ingests microscopic larvae called microfilaria that are present in the infected dog’s blood. These microfilaria undergo development within the mosquito and then become infective to dogs. Then, when the mosquito bites a dog, the infective larvae are injected into the dog’s tissue. These larvae migrate throughout the dog’s body and undergo development until they arrive at the heart as immature adult heartworms. The immature adult heartworms grow into adult worms 10-12” in length. The adult worms live in the heart and the blood vessels between the heart and lungs called the pulmonary arteries. The presence of heartworms causes an inflammatory reaction in the vasculature that over time leads to heart and lung damage. Eventually permanent heart and lung damage can occur. Signs of heartworm disease vary depending upon stage and severity of infection and include weight loss, coughing, exercise intolerance, shortness of breath, abdominal swelling, and collapse.

There is a severe form of heartworm disease called post caval syndrome. This is a rapidly progressing and highly fatal form of heartworm disease. In this condition, there are often an excessively large number of heartworms that migrate to unusual locations in the dog’s blood system. The heartworms restrict the blood flow in the vein called the caudal vena cava resulting in rapid heart and lung failure. Surgery is necessary to remove the heartworms from the vena cava in order to save these dogs. Medical complications to caval syndrome are frequent and severe, leading to a poor prognosis for survival.

Fortunately for your dog, heartworm disease is readily preventable. There are several excellent products available for heartworm prevention. Most of these preventatives are given monthly and they are highly effective in preventing heartworm disease. They work by killing the heartworm larvae before they mature to adult heartworms. Puppies are started on preventative medication at approximately 2 months old before they are old enough to have heartworms. Heartworm preventatives are safe to use in any stage of the dog’s life and are also safe on pregnant dogs and nursing females. If your dog is over 6 months old and has never been on heartworm preventative, then a heartworm test should be performed before starting preventative to see if your dog may already be infected. Heartworm preventative should be given year round in warm climates where mosquitoes are prevalent throughout the year. In northern states with longer winters and shorter mosquito seasons, heartworm preventative can be stopped after the end of the mosquito season. Consult with your veterinarian to determine how best to administer preventative for your area. When it comes to heartworms, the old adage, “an ounce of prevention is worth a pound of cure” definitely applies.