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
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
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.