Heartworm

Heartworm Disease

General Information

Heartworm disease is becoming more common in many parts of the United States and can infect dogs and cats. It is caused by the heartworm, Dirofilaria immitis. This parasite lives in the right side of the heart, the nearby large vessels (pulmonary arteries), and lungs (especially cats). The female worm produces large numbers of microscopic, immature heartworms that circulate in the blood. These immature worms (microfilariae) are taken up with the blood by a mosquito feeding on an infected pet. After living in the mosquito for 10 to 14 days, the microfilariae can then infect another dog or cat that the mosquito feeds on. The feeding mosquito deposits infective microfilariae into the skin of another pet, and these enter the body through the mosquito bite wound. The microfilariae eventually travel to the heart where they develop into adult heartworms. The adult heartworms produce new microfilariae within 3 months. It takes at least 190 days from the time the dog (or cat) is bitten by an infected mosquito until the animal becomes a new source of infective microfilariae.

Most cases of heartworm are diagnosed by a blood test that detects the body's immune response to the adult heartworm. Finding the microfilariae in the blood can confirm the diagnosis. Sometimes, however, no microfilariae are found in the blood (occult heartworm disease). These cases are diagnosed by a combination of blood tests and chest radiographs (x-rays). Failure to treat heartworm disease may result in heart failure and/or serious disease of the liver and kidneys. Untreated heartworm disease is usually fatal.

Prevention

Preventive Medication

Ivermectin: This drug is a highly effective heartworm preventative available as a monthly chewable treat. The drug is also available combined with pyrantel pamoate to provide control of roundworm and hookworm in addition to heartworm. It is safely given to young, growing puppies and kittens as well as adult dogs and cats. The trade name for this product is Heartgard Plus and Heartgard for Cats.
Milbemycin: This drug is another highly effective once-a-month heartworm preventative for puppies and adult dogs. In addition to heartworm, milbemycin controls hookworms and removes and controls roundworms and whipworms. Milbemycin by itself is provided under the trade name Interceptor and comes as either a plain or chewable tablet. Sentinel, which is made by the same company, provides additional control of flea eggs. (If during the course of treatment live adult fleas are seen, it is recommended that an adulticide such as Frontline be used.)
Selamectin: This is a convienient product that is applied as a "spot-on" treatment between the shoulder blades of dogs and cats to prevent a broad range of parasites including heartworms, fleas, ticks, ear mites, sarcoptic mange and hookworms and roundworms, depending on the species. It goes by the trade name Revolution and is also very safe for puppies and kittens.
Dogs (and cats at high risk- especially outdoor cats) should be treated year-around to prevent heartworm infection, especially in geographic areas where mosquitoes are present throughout the year. This is especially important when pets are traveling to a warmer climate for the winter.

Important Points in Treatment

Heartworm preventives generally should not be given to dogs harboring microfilariae. Your dog must be blood tested before starting the preventive medication. Cats are not routinely tested unless heartworm disease is suspected.
Weigh your dog monthly during the treatment period. A change in your pet's weight might require a change in dosage.
If you give the drug in your pet's food, mix the medication in a small amount of food and give this first. If you mix the medication in a whole bowl of food, your pet may not eat everything and you will not know if the proper dose was given.
These drugs must be given on the prescribed schedule to be effective. You must not skip doses. Call the doctor if you are having trouble adhering to the prescribed schedule.

Mosquito Control

Screen outdoor kennels, especially in areas where heartworm is prevalent.
Keep your pet indoors during early morning or evening hours, when mosquitoes are most active.
Electronic insect killers can be positioned near kennels. Premise sprays are available for treatment of the environment.
Mosquito repellents can be used for short periods of exposure. Be sure your pet does not lick off the repellent.