Heartworm disease is caused by Dirofilaria immitis, a parasitic worm that lives as an adult in the right side of the dog’s heart and large blood vessels leading to the lungs. Heartworms do most of the damage in the adult stage. Dogs are considered the definitive host for heartworms, however, heartworms may infect more than 30 species of animals (e.g., coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, etc.) and humans as well. When a mosquito carrying infective heartworm larvae bites a dog and transmits the infection, the larvae grow, develop and migrate in the body over a period of several months to become sexually mature male and female worms. These reside in the heart, lungs and associated blood vessels. As mature adults, the worms mate and the females release their offspring (microfilariae) into the blood stream. Offspring can be detected in the blood (pre-patent period) about six to seven months after the infective larvae from the mosquito enter the dog. The male heartworms (four to six inches in length) and the females (10-12 inches) become fully grown about one year after infection, and their life span in dogs appears to average up to five to seven years.
The onset and severity of disease in the dog is mainly a reflection of the number of adult heartworms present, the age of the infection and the level of activity of the dog. Dogs with higher numbers of worms are generally found to have more severe heart and lung disease changes. Until the number of mature heartworms exceeds 50 in a 25-kg dog, nearly all of the heartworms reside in the lower caudal pulmonary arteries. Higher numbers of heartworms result in their presence in the right chambers of the heart. In such infections, the most common early pathological changes caused by heartworms are due to inflammatory processes that occur in and around the arteries of the lower portion of the lungs in response to the presence of heartworms. Later, the heart may enlarge and become weakened due to an increased workload and congestive heart failure may occur. A very active dog (e.g., working dog) is more likely to develop severe disease with a relatively small number of heartworms than an inactive one (e.g., a lap dog or couch potato). Occasionally, a dog with a large number of heartworms may not only have worms in the heart, but also in the caudal vena cava between the liver and the heart. If the heartworms are not removed surgically, this syndrome causes sudden collapse and death within two to three days.
Canine heartworm infection is widely distributed throughout the United States. Heartworm infection has been found in dogs native to all 50 states. All dogs, regardless of their age, sex, or habitat, are susceptible to heartworm infection. The highest infection rates (up to 45%) in dogs (not maintained on heartworm preventive) are observed within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries. Other areas of the United States may have lower incidence rates (5% or less) of canine heartworm disease, while some regions have environmental, mosquito population and dog population factors that allow a higher local incidence of heartworm infection. Regions where heartworm disease is common have diagnosed infections in dogs as young as one year of age, with most areas diagnosing infections primarily between the ages of three and eight years. Although there are differences in frequency of infection for various groups of dogs, all dogs in all regions should be considered at risk, placed on prevention programs and frequently examined by a veterinarian.
Heartworm disease may cause a combination of medical problems in the same dog including dysfunction of the lungs, heart, liver and kidneys. Signs of heartworm disease may occur within 6 months of infection or may not appear at all depending upon the number of adult worms that are present. In most cases, signs will begin within 1-2 years after infection. Typical signs include coughing, labored breathing, weakness, and tiring with exercise. Since the signs vary, the disease may be well advanced before the dog begins to show any problems, or signs may be mistaken for another problem. In advanced stages, the heart and lungs can be severely damaged. Eventually, heart failure can occur and the dog can die from damage cause by heartworms unless appropriate treatment is instituted. The disease may have an acute onset but usually begins with barely detectable signs resulting from a chronic infection and a combination of physiologic changes. Dogs with a low number of adult worms in the body that are not exercised strenuously may never have apparent signs of heartworm disease. However, in most dogs, the heart and lungs are the major organs affected by heartworms with varying degrees of clinical signs.
Clinical Signs Associated with Canine Heartworm Disease:
Early Infection- No abnormal clinical signs observed
Mild Disease -Cough
Moderate Disease -Cough, exercise intolerance, abnormal lung sounds
Severe Disease- Cough, exercise intolerance, dyspnea (difficulty breathing), abnormal lung sounds, hepatomegaly (enlargement of the liver), syncope (temporary loss of consciousness due to poor blood flow to the brain), ascites (fluid accumulation in the abdominal cavity), abnormal heart sounds, death
Heartworms are transmitted from dog to dog by mosquitoes. There are three stages in the development of heartworms in the dog.
1. The adult female, living in the right side of the heart and/or major vessels to the lungs, produces immature worms called “microfilariae” that circulate in the blood stream. The microscopic microfilariae can live for up to 3 years.
2. When a mosquito bites an infected dog, it takes in blood containing microfilariae. The microfilariae mature in the mosquito over a period of two weeks to become infective larvae.
3. The mosquito, carrying infective larvae, deposits them in other dogs during blood meals. Larvae develop over 3-6 months and migrate to the right heart. Within 6 months, the larvae develop into adult heartworms that are responsible for the disease process in the heart and lungs. The adult heartworms can live up to 7 years. The adults produce microfilariae, hence completing the life cycle.
To identify heartworm infection, a blood sample is taken from the dog. This test detects specific antigens primarily found in adult female heartworms and are used with much success to detect canine heartworm infection. Currently, tests are available as in-clinic tests as well as at many veterinary reference laboratories. Most commercial tests will accurately detect infections with one or more mature female heartworms that are at least seven or eight months old, but the tests generally do not detect infections of less than five months duration. The identification of the offspring (microfilaria) of heartworms from a blood sample indicates infection with adult heartworms. Identifying offspring can also be accomplished through either one of two concentration tests: the modified Knott’s test (a technique requiring spinning the blood sample in a mechanical device called a centrifuge) or a filter test. Practitioners will often do a quick examination of a blood smear to look for the presence of the offspring (microfilaria), but this procedure is not sensitive enough to rule out heartworms and only verifies the presence of an infection. Another parasitic infection of dogs that is capable of producing circulating microfilariae, detectable upon examination of the blood, is called Acanthocheilonema (Dipetalonema) reconditum. A reconditum is a non-disease-causing parasite that matures in the tissues beneath the skin of dogs. Its offspring can be differentiated from those produced by heartworms through microscopic examination evaluating size, shape and their movement.
Radiographic abnormalities develop early in the course of the disease. Radiographs of the heart and lungs are the best tool available to evaluate the severity of the disease. Typical changes observed are enlargement of the following structures: right-side of the heart, main pulmonary artery, and pulmonary arteries in the lobes of the lung. Blunting and thickening of pulmonary arteries, along with tortuosity is often noted. Evidence of inflammation in the lung tissue that surrounds the pulmonary arteries is often found.
The elimination of heartworms from your dog requires medication to kill the adult heartworms and microfilariae. Most dogs infected with heartworm can be successfully treated. The goal of treatment is to kill all adult worms with an adulticide and all microfilariae with a microfilaricide. It is important to try to accomplish this goal with a minimum of harmful effects from drugs and a tolerable degree of complications created by the dying heartworms. Heartworm infected dogs showing no signs or mild signs have a high success rate with treatment. Patients with evidence of more severe heartworm disease can be successfully treated, but the possibility of complications and mortality is greater. The presence of severe heartworm disease within a patient in addition to the presence of other life-threatening diseases may prevent treatment for heartworm infection.
There is currently one drug approved by the FDA for use in dogs for the elimination of adult heartworms. This drug is an organic arsenical compound. Dogs receiving this drug therapy will typically have had a thorough pretreatment evaluation of its condition and will then be hospitalized during the administration of the drug. Melarsomine dihydrochloride (Immiticide®, Merial) has demonstrated a higher level of effectiveness and safety than any other adult heartworm treatment previously available. It is administered by deep intramuscular injection into the lumbar muscles. One injection is administered intramuscularly and then the dog returns for an additional injection 30 days later. Following treatment with an arsenical compound, the dog must be rested for 4-6 weeks, during which time the dead adult heartworms will slowly be reabsorbed. The primary post-adulticide complication is the development of severe pulmonary thromboembolism. Pulmonary thromboembolism results from the obstruction of blood flow through pulmonary arteries due to the presence of dead heartworms and lesions in the arteries and capillaries of the lungs. If heartworm adulticide treatment is effective, some degree of pulmonary thromboembolism will occur. When dead worms are numerous and arterial injury is severe, widespread obstruction of arteries can occur. Clinical signs most commonly observed include fever, cough, hemoptysis (blood in the sputum) and potentially sudden death. It is extremely important to not allow exercise in any dog being treated for heartworms. Often dogs with severe infections will also require the administration of anti-inflammatory doses of corticosteroids.
The microfilariae must also be eliminated so the dog will not be a source of infection for other dogs. Elimination of the microfilariae is monitored by using blood tests that can easily identify microfilariae in the blood. The most effective drugs for this purpose are the macrocyclic lactone (ML) anthelmintics, i.e.,milbemycin oxime, selamectin, moxidectin and ivermectin. These drugs are the active ingredients in commonly used heartworm preventives. Although their usage as microfilaricides has not been approved by the FDA, they are widely used by veterinarians as there are no approved microfilaricidal drugs currently available. It is recommended that microfilariae positive dogs being treated with these macrocyclic lactones be hospitalized for at least eight hours following treatment for observation of possible adverse reactions, including those resulting from rapid death of the microfilariae. Circulating microfilariae usually can be eliminated within a few weeks by the administration of the ML-type drugs mentioned above. Today however, the most widely used microfilaricidal treatment is to simply administer ML preventives as usual, and the microfilariae will be cleared slowly over a period of about six to nine months. In some cases, not all microfilariae can be eliminated and the veterinarian may recommend retreatment of your dog for adult heartworms and microfilariae.
While treatment of canine heartworm disease is usually successful, prevention of the disease is much safer and more economical. There are a variety of options for preventing heartworm infection, including daily and monthly tablets and chewables and/or monthly topicals. These products are extremely effective and when administered properly on a timely schedule, heartworm infection can be prevented. The American Heartworm Society is now recommending year-round prevention, even in seasonal areas. One reason for this is compliance – to make sure the medicine has been given properly by the pet owner. In addition, most monthly heartworm preventives have activity against intestinal parasites. Many of these same intestinal parasites that infect dogs can also infect people, with estimated infections occurring in three to six million people every year. So this added benefit of monthly deworming makes great sense.
The products listed below are intended to be given on a monthly basis and are highly effective in preventing heartworm disease if given as directed.
Ivermectin (Heartgard® & Heartgard® Plus by Merial, Iverhart® Plus & Iverhart MAX™ by Virbac and Tri-Heart® Plus by Schering-Plough) was the first in this family of drugs to be approved for preventing heartworm infection. An infection with larvae as long as two months prior to the initiation of ivermectin treatment will be blocked from development.
Milbemycin oxime (Interceptor® & Sentinel® by Novartis) has benefits, which are similar to ivermectin.
Selamectin (Revolution® by Pfizer) is applied topically to prevent heartworm disease.
Moxidectin (Advantage Multi™ by Bayer) is available in a topical formulation, in combination with a flea control product, imidacloprid. Moxidectin is also available as a six-month injectable product for dogs (ProHeart®6 (moxidectin) Sustained Release Injectable for Dogs, by Fort Dodge Animal Health).
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