To learn more about heartworm, visit the American Heartworm Society's website HERE.

What is dog heartworm?

Dog heartworm is a disease caused by a roundworm (Dirofilaria immitis) which lives in the heart and lungs of dogs. Heartworm disease has become a serious health problem for dogs throughout the United States. The disease is transmitted into the bloodstream of dogs, coyotes, foxes and even cats through the infective bite of a western treehole mosquito (Aedes sierrensis). These microscopic larvae move into the heart and lungs of the animal where they complete its development. Mature adult worms measure 6 to 12 inches in length. They interfere with heart and lung functions, making the animal very ill.

Though heartworm has become a health concern for canines, it is not usually a threat to humans or other domestic animals.

What are the signs and symptoms of dog heartworm?

Severe infections of adult worms results in coughing, labored breathing, fatigue, loss of appetite and general weakness. In advanced stages of the disease, the heart becomes badly damaged, as does the liver and lungs. At this stage, recovery is difficult. Unfortunately, visible symptoms of the disease do not present themselves until the worms have reached the adult stage. The presence of immature worms (microfilariae) can only be detected by microscopic examination of blood samples.

How can I protect my dog from getting heartworm?

Once established, heartworm is difficult and expensive to treat. However, preventative treatments are available from your veterinarian. These are pills that are administered daily to kill the microfilariae in the dog’s bloodstream before they can enter the heart and lungs. Mosquitoes infected with dog heartworm are present in San Mateo County so it is important that dogs receive preventative treatment.

The mosquito that transmits dog heartworm develops in treeholes, making it very difficult to control. In wooded areas, there may be hundreds of hidden treeholes. These can be anywhere from ground level to over 100 feet above, depending on the height of the trees. Treehole mosquitoes are active during the day with peak activity at dawn and dusk. The adult mosquitoes prefer to stay in shady areas and do not venture far into bright sunlight. Larvae begin developing in winter, emerging as adults from May through July with a peak in June. The timing varies with climate.

How do I know if my dog has dog heartworm?

Your veterinarian can administer a blood test to detect the infection.

Is there a treatment if my dog has heartworm?

Treating dogs for the adult stage of heartworm is difficult and expensive. Complications are not uncommon and may require several treatments using different prescription compounds to rid your dog of both adult heartworms and microfilariae.

If your dog tests negative for the circulating microfilariae or it has been returned to the negative state through treatment, as described above, your veterinarian can recommend a suitable prophylaxis to be placed in your dog’s food. The prophylaxis must not be administered to dogs without first testing for the presence of an existing infection.

What is the most likely time my dog could acquire this disease?

The highest risk is during the summer months in California, from May through August, yet may vary due to weather.