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Canine Heartworm

Dirofilaria immitis or heartworm (HW) has been found around the world in Asia, Europe, North and South America as well as Australia. In North America it occurs primarily in the southeastern areas. HW is found throughout the U.S. and Canada including Hawaii. However, with the exception of the southeast the incidence is low. The incidence in the southeast and along the Gulf of Mexico is greater than 50% and persists 12 months of the year. Whereas, in many of the more northern areas the disease is only of concern in the warmer 6 months.

Heartworm is a parasitic helminth that lives in the dog, fox, wolf and cat. It has also been diagnosed in humans in the more southeastern part of the US. Canidae are the natural host for the worm and the incidence is low in other species. The worm is quite long (25-30cm). The mosquito is essential for the transmission of the disease. The disease is spread by mosquitoes when a blood meal is taken from a host¹s tissues. Within an aliquot of blood, immature microscopic HW (microfilaria) are found. These microfilaria mature in the digestive tract of the mosquito and molt into an infective stage. This process of molting is dependent on daily temperatures greater than 64 degrees F over a month’s time and, is the reason why the disease is primarily seasonal in North America. After several weeks in the mosquito and 2 molts, the third larval stage can infect the dog with the next blood meal. Upon infecting the new canine host, the microfilaria migrate to major vessels and by 5 months are entering the large vessels of the lungs. By six months adult heartworms are discharging their own microfilaria to begin the cycle again at the next mosquito bite. A host can act as its own reservoir where a mosquito bites the infected dog and 3 weeks later returns to bite and reinfest the same dog for a superimposed infection increasing the adult HW burden. Signs of the disease are related to the number of adult worms found in the dog, and burdens of 200+ worms have been reported. As the number of worms increase in the host, worms migrate into the right heart and major vessels obstructing blood flow. Obstruction to blood flow is associated with many deleterious effects: pulmonary hypertension, hemorrhage, kidney disease and pulmonary thromboembolism.

Testing for HW is done approximately 6 months after the previous mosquito season; in the spring. Dogs in climates where the mosquito season is extended late into the year have the option of continuous prophylaxis or repeat testing in the middle summer. Two basic tests for HW exist: direct microfilaria tests and antigen tests. Antigen tests are dependent on the microfilaria being greater than 6 months old. Antigen tests are extremely sensitive and require no more than 1 adult female worm. Almost all adult female worms produce an antigen response and thus, antigen tests are extremely reliable. However, it is possible to only have male adult worms. Additionally, approximately 1% of female worms produce no antigen response resulting in false negative antigen tests. Microfilaria without adults are possible in early stages of infection and in certain stages of ongoing HW therapy. Radiographs of the lungs can detect HW thromboembolisms but should only be used as a follow up to a confirmed case or an occult suspect. Microfilaria tests are very quick and inexpensive. They only test for circulating microfilaria and can miss HW infections that are not shedding microfilaria. Also, dogs using the once a month prophylaxis are free of microfilaria but may still have adult worms. Antigen testing is quickly becoming the screening test of choice. Certain instances exist where both tests should be employed.

Signs of heartworm disease are not apparent until several years post infection. They include coughing, difficulty breathing, occasionally coughing blood, fainting, and, exercise intolerance. Advanced disease can show abdominal fluid and severe bleeding.

Treatment for the disease involves first destroying the adult worms and then eliminating the microfilaria. Risks of therapy are directly related to the quantity of the HW burden and lung involvement which increase the incidence of pulmonary thromboembolism. Adulticide therapy involves arsenic compounds and hence, potential complications. Complications include caval syndrome and pulmonary artery infections where the HW burden has been extended into the heart and main pulmonary arteries. Usually prior to adulticide treatment the adult heartworms are surgically retrieved from the heart. Historically intravenous administration of thiacetarsamide sodium has been the mainstay of therapy. Four intravenous injections are given over 2 days. The toxicity of the drug is high to the patient and therefore, hospitalization and monitoring are important. Complications of severe vomiting and jaundice require suspension of treatment for several months before resuming the full treatment.

Recently, melarsomine has been released as an adulticide. Although this drug is also an arsenical it provides several advantages over thiacetarsamide. It is less toxic to the patient and seems to be more toxic to adult HW. Additionally, the administration is via an intramuscular injection in the lumbar muscles. Previous concerns with phlebitis from thiacetarsamide have been eliminated with melarsomine. The drug is administered twice over 2 days. Each adulticide can be associated with complications up to 4 weeks post therapy that result from the pulmonary thromboemboli from the dead worms. A patient should be restricted to mild activity for one month after adulticide therapy.

A microfilaricide is administered about 4 weeks after completing adulticide therapy. Some dogs with heavy burdens may experience side effects to the therapy which include: pale membranes, anorexia, vomiting and salivation. It is unlikely that these signs will progress to serious effect but, not impossible. Some dogs may need to be hospitalized for reaction to sudden death of large numbers of microfilaria. Two microfilaricides exist: ivermectim and milbemycin oxime. A third drug will soon be on the market. In addition to being used to clear the microfilaria of an existing HW infection they are also the mainstay in HW prophylaxis.

HW prophylaxis is given after a negative HW test has been performed. Prophylactic drugs include the monthly use of ivermectim or milbemycin oxime or the daily administration of diethylcarbamazine citrate (DEC). DEC is administered daily starting just prior to the beginning of the mosquito season and extending one month beyond the mosquito season. The drug is decreasing in use since the introduction of monthly microfilaricides. DEC is very inexpensive and is safe providing the dog is tested negative prior to its use each year. It can cause very serious reactions, including shock and death, if given to dogs with current microfilaria. Annual retesting is important using this drug and should include in addition to the common antigen test, a direct test for microfilaria in the blood.

Monthly microfilaricides are extremely safe and easy to use but they are significantly more expensive than DEC. The price difference is most pronounced in large dogs. Both ivermectim and milbemycin oxime also are effective dewormers for roundworms and hookworms. Milbemycin oxime also is effective against whipworms. Additionally, these drugs are highly effective in their use as microfilaricides. Aside from the initial HW test prior to their use, retesting beyond the first year is not essential if the owner has been diligent. Veterinarians may want to retest after the first year of use to insure successful prevention. Retesting every other year or third year is probably adequate.



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