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Veterinarians recommend heartworm prevention for every dog and cat all year since mosquitoes can live in this climate year round. If a pet is not on heartworm prevention or misses even one dose of monthly prevention it is possible for it to become infected with adult worms living in its heart (heartworm positive).
There are two ways to diagnose heartworm disease. The easiest is a direct smear. This involves looking for babies (microfilaria) in the blood stream. If there are babies then there must be adults to produce the microfilaria. The blood smear test has its limitations. This test cannot be used in cats. Also, many dogs can have adults without babies in the blood stream; if, the dog has immature adults, adults of the same sex, a low number of adults, a long standing infection, or already being on heartworm prevention that has killed the babies in the blood stream.
The second test is an occult antigen test that checks for the presence of adult heartworms. When a dog has missed heartworm prevention a test is required before resuming heartworm prevention and another test is required 6 months later to ensure whether the dog is infected. (Cats do not require testing before heartworm prevention is administered as with dogs. Cats are tested if they show any signs of illness and cats are not treated with arsenic.)
When a dog has a positive heartworm test, the dog needs to immediately and safely be put back on heartworm prevention so no additional worms can develop. When the positive dog is back on prevention, then adulticide (arsenic) treatment is needed to kill the adult worms in the heart and in the blood vessels going to the lungs. Prior to this treatment the dog needs to be on a good quality diet, be free of internal and external parasites and should have a blood profile, urinalysis, fecal exam, and chest X-rays. These tests lets us see if there are any other underlying disease processes going on that need to be addressed before beginning treatment. Treatment involves injections of an arsenic based compound that is designed to kill the worms without killing the dog. Treatment usually involves two injections of arsenic (Immiticide) 24 hours apart, after which the dog is strictly confined for 4-6 weeks. The dog must stay calm and quiet while the worms die, disintegrate and get destroyed by the body. In some cases, if the number of worms in the heart is large or the dog is showing clinical signs such as coughing, exercise intolerance, dull hair coat, weight loss or has some other physical abnormality, the dog is given a single injection. After a months confinement then the two injection series is retried to see if the dog withstands the treatment without adverse reactions. When the dog is sick enough to require the three injection series, the period of strict confinement is repeated for another 4-6 weeks. Six months after heartworm treatment, the blood is retested to determine the success rate of the injections.
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