Pet Health Information

HEARTWORM DISEASE IN DOGS

Heartworm disease is a life-threatening disease caused by a blood-borne parasite transmitted to dogs by the bite of an infected mosquito. The infected mosquito injects an immature larval form of the heartworm into the bloodstream. These larvae (or microfilariae) go through 2 stages of development to become adult worms, which can reproduce sexually to produce more micro-filariae. The adult worms live in the pulmonary (lung) arteries where they cause damage to the walls of the arteries which can result in complete blockage of the vessels. When a large number of worms is present some will move into the right ventricle of the heart and cause heart failure and eventually death.

OCCULT HEARTWORM DISEASE - When we test a dog for heartworm disease we can look for the microfilariae (baby heartworms) in a blood smear. However, even if this test is negative your dog could still have occult heartworm disease. This occurs when there are adult worms of only one sex, when there are too few microfilariae to be detected, or when the dog's immune system has killed the microfilariae but not the adult worms. In these situations there are no microfilariae circulating in the bloodstream, but the dog can still become sick from heartworm disease. Fortunately, there is a blood test available to detect proteins produced by adult heartworms. This is the screening test that we perform prior to starting your dog on preventative medication, and every two years thereafter. If you stop the preventative for a period of more than 6 months you also need to have your dog retested.

HEARTWORM TREATMENT - There are two approved treatments for dogs with heartworm infections. Caparsolate is an arsenic compound that must be given in 4 intravenous doses over a 48-hour period. The drug has toxic effects on the liver and kidneys which can sometimes stop treatment temporarily, and is only about 70-75% effective at killing all the worms. With heavy infestations a second treatment is sometimes necessary. Even with low numbers of worms dogs can die from the resulting blockage of pulmonary arteries by the dead worms. The second medication is called Immiticide (melarsomine) which is much less toxic to the liver, can be given by one injection in the muscle (in most cases) and is about 95% effective. This is the treatment we currently recommend. All dogs must also have treatment for the microfilariae (if present) following adulticide treatment. This usually involves using the same type of preventative medications we already use.

HEARTWORM PREVENTION - Because of the risk involved in treating heartworm disease ALL DOGS SHOULD BE MAINTAINED ON HEARTWORM PREVENTATIVE! Even if your dog is rarely outside, mosquitoes can get into your house and infect your dog. In this area we recommend starting preventative in April and continuing through December (or the first HARD frost). Dogs can also be on preventative year-round to help prevent internal parasites such as roundworms and hookworms. Puppies between the ages of 9 weeks and 6 months can be started on heartworm preventative without an initial blood test since it takes a minimum of 6 months to develop a heartworm infection.

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