Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and occasionally in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are filarids, one of many species of roundworms. Dogs and cats of any age or breed are susceptible to infection.
Where is Heartworm Disease? Heartworm disease has been reported in all 50 states. The map below shows particularly endemic areas based on the number of cases reported by clinics.
History The first published description of heartworm in dogs in the United States appeared more than 100 years ago in an issue of "The Western Journal of Medicine and Surgery." 1 Heartworm in cats was first described in the early 1920's. 2, 3 Since then, naturally acquired heartworm infection in cats and dogs is identified as a worldwide clinical problem. Despite improved diagnostic methods, effective preventives and increasing awareness among veterinary professionals and pet owners, cases of heartworm infection continue to appear in pets around the world.
How Heartworm Happens: The Life Cycle First, adult female heartworms release their young, called microfilariae , into an animal's bloodstream. Then, mosquitoes become infected with microfilariae while taking blood meal from the infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. After that, the mosquito bites another dog, cat or other susceptible animal, and the infective larvae enter through the bite wound. It then takes a little over 6 months for the infective larvae to mature into adult worms. In dogs, the worms may live for up to 7 years. Microfilariae cannot mature into adult heartworms without first passing through a mosquito.
Within the mosquito, the microfilariae mature into the infective larval stage. When the mosquito then bites another dog, cat, or susceptible animal, the larvae are deposited on the skin and actively migrate into the new host. For about 2 months the larvae migrate through the connective tissue, under the skin, then pass into the animal's venous blood stream and are quickly transported to the arteries of the lung. It takes a total of approximately six months for the infective larvae to mature into adult worms that begin producing offspring, microfilariae . Adult heartworms can live for five to seven years in the dog.
In the dog, the larvae progress in their development to an adult form of the worm, and live in the pulmonary vessels, where they continue the life cycle and cause extensive injury. The period of time when heartworms are reproductively capable is referred to as patency. In cats, it takes seven to eight months before adult worms potentially reach patency in the pulmonary vessels, and this is referred to as transient patency, as reproductive capability in the cat is usually very short (months) compared to that of dogs (years). In most cases the cat is not an effective reservoir host, since microfilaria are produced in less than 20% of the cats.
What Are the Signs of Heartworm Disease? For both dogs and cats, clinical signs of heartworm disease may not be recognized in the early stages, as the number of heartworms in an animal tends to accumulate gradually over a period of months and sometimes years and after repeated mosquito bites. Recently infected dogs may exhibit no signs of the disease, while heavily infected dogs may eventually show clinical signs, including a mild, persistent cough, reluctance to move or exercise, fatigue after only moderate exercise, reduced appetite and weight loss. Cats may exhibit clinical signs that are very non-specific, mimicking many other feline diseases. Chronic clinical signs include vomiting, gagging, difficulty or rapid breathing, lethargy and weight loss. Signs associated with the first stage of heartworm disease, when the heartworms enter a blood vessel and are carried to the pulmonary arteries, are often mistaken for feline asthma or allergic bronchitis, when in fact they are actually due to a syndrome newly defined as H eartworm A ssociated R espiratory D isease ( HARD ).
Heartworm disease may cause a combination of medical problems in the same dog including dysfunction of the lungs, heart, liver and kidneys. 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
How Do You Detect Heartworm Disease? Heartworm infection in apparently healthy animals is usually detected with blood tests for a heartworm substance called an " antigen " or microfilariae , although neither test is consistently positive until about seven months after infection has occurred. Heartworm infection may also occasionally be detected through ultrasound and/or x-ray images of the heart and lungs, although these tests are usually used in animals already known to be infected.
Antigen Tests Antigen tests detect 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.
Antibody Tests Since the late 1970's and early 1980's, several canine heartworm antibody tests have been developed and introduced, but such tests for dogs have been largely replaced by the more useful antigen tests. This lack of utility of the antibody tests is due to the fact that these tests detect the antibody response to exposure to infection, but not necessarily actual disease. This is important because not all infections fully mature.
Prevention Because heartworm disease is preventable, the AHS recommends that pet owners take steps now to talk to their veterinarian about how to best protect their pets from this dangerous disease. Heartworm prevention is safe, easy and inexpensive. While treatment for heartworm disease in dogs is possible, it is a complicated and expensive process, taking weeks for infected animals to recover. There is no effective treatment for heartworm disease in cats, so it is imperative that disease prevention measures be taken for cats. There are a variety of options for preventing heartworm infection in both dogs and cats, including daily and monthly tablets and chewables and monthly topicals. All of these methods are extremely effective, and when administered properly on a timely schedule, heartworm infection can be completely prevented. These medications interrupt heartworm development before adult worms reach the lungs and cause disease. It is your responsibility to faithfully maintain the prevention program you have selected in consultation with your veterinarian.
Treatment Usually, all but the most advanced cases of heartworm disease can be successfully treated in dogs. Currently, there are no products in the United States approved for the treatment of heartworm infection in cats. Cats have proven to be more resistant hosts to heartworm than dogs, and often appear to be able to rid themselves of infection spontaneously. Unfortunately, many cats tend to react severely to the dead worms as they are being cleared by the body, and this can result in a shock reaction, a life-threatening situation. Veterinarians will often attempt to treat an infected cat with supportive therapy measures to minimize this reaction; however it is always best to prevent the disease.
Adult heartworms in dogs are killed using a drug called an adulticide that is injected into the muscle through a series of treatments. Treatment may be administered on an outpatient basis, but hospitalization is usually recommended. When the dog is sent home, exercise should be limited to leash walking for the duration of the recovery period, which can last from one to two months. This decreases the risk of partial or complete blockage of blood flow through the lungs by dead worms.
Post-Adulticide Complications 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.
Re-infection during treatment is prevented by administration of a heartworm preventive. These preventives may also eliminate microfilariae if they are present. Dogs in heart failure and those with caval syndrome require special attention
Immiticide: Usual Dose and Administration Given intramuscularly (IM) deep in the lumbar (back) muscle. Do not administer at any other site. Alternate sides. Usually given twice 24 hours apart for animals with Class 1 or 2 heartworm disease. May need second round of treatment 4 months later depending on repeated test results. Dogs with other classes of heartworm disease may receive different doses/timing to decrease risk of complications. Given in a hospital setting to allow for the necessary supervision of the patient.
Side Effects May see pain, swelling, and tenderness at the injection site or reluctance to move due to pain at injection site. Firm nodules can persist indefinitely. May also see coughing, gagging, depression, lethargy, lack of appetite, fever, lung congestion, and vomiting. Less commonly seen are excessive drooling, panting, diarrhea, coughing up blood, abnormal heart rhythms, and death.
Caparsolate: Indications for Use An arsenic-containing compound used to kill immature (4+ month old) and adult heartworms ( Dirofilaria immitis ).
General Information FDA approved for use in dogs for the removal of adult heartworms. Used in a hospital setting. Although not FDA approved for use in cats, it has been used in cats to treat heartworm disease. Thiacetarsamide is used in a hospital setting.
Usual Dose and Administration Given directly into a vein twice a day for 2 days. Given in a hospital to allow for the necessary supervision of the patient. Side Effects Vomiting is a common reaction. May also see loss of appetite, depression, liver or kidney damage, and reaction to the dying heartworms (coughing, fever, loss of appetite, and lethargy). Thrombocytopenia (lack of platelets, causing bleeding/bruising) is seen also. If the medication gets outside the vein during injection, pain, swelling, and severe sloughing may be seen as the skin and underlying tissues die. Less commonly seen are severe liver damage and severe pulmonary embolism . Death can occur from reaction to the medication and/or from reaction to the dying heartworms.