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Dog heartworm

What is dog heartworm?

Heart infected with heartworm
Heart infected with heartworm

Heartworm is a potentially fatal condition caused by a filarial nematode (Dirofilaria immitus) that invades and occupies the canine heart and pulmonary arteries. Dogs and coyotes are most susceptible to infection, but cats and occasionally other mammals may also be affected. The parasitic worms are transmitted by the bite of an infected western treehole mosquito (Aedes sierrensis).  Although certain areas may have a higher incidence of heartworm than others, the disease has been reported in all 50 states.

Dog heartworm is primarily a disease of veterinary importance. Although human infections have been occasionally reported, humans are not the natural hosts for heartworms.

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 Marin and Sonoma counties 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.

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.

Useful links: 

American Heartworm Society

  • Resources for pet owners and veterinarians

CDC: Life Cycle Page (Dirofilaria immitis)

  • Life cycle graphic of Dog Heartworm and explanation of graphic


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