Dirofilaria immitis.txt

Dirofilaria immitis

Clinical signs of infection in dogs
Dogs show no indication of heartworm infection during the six-month prepatent period prior to the worms' maturation, and current diagnostic tests for the presence of microfilariae or antigens cannot detect prepatent infections. Rarely, migrating heartworm larvae get "lost" and end up in aberrant sites, such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizures, and lameness, but normally, until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness.
Many dogs show little or no sign of infection even after the worms become adults. These animals usually have only a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections may show the classic signs of heartworm disease. Early signs include a cough, especially during or after exercise, and exercise intolerance. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss, fainting, coughing up blood, and finally, congestive heart failure.
There are 4 different classes of symptoms:

Diagnosis in dogs
Microfilarial detection is accomplished by the using one of the following methods:
Direct blood smear
A blood sample is collected and viewed under the microscope. The direct smear technique allows examination of larval motion, confirming the presence of microfilaria. It also helps in the distinction of D. immitis from Acanthocheilonema reconditum. This distinction is important because the presence of the latter parasite does not pose a health risk to the host. D. immitis usually has stationary body movement, while A. reconditum has progressive movement. However, this method often misses light infections because only a small amount of blood sample is used.
Hematocrit tube method
This method uses a microhematocrit (or capillary tube) filled with a blood sample that has been centrifuged, separating the plasma from the red blood cells. These layers are divided by the buffy coat. The buffy coat consists of the leukocytes and platelets that are in the sample. The tube is snapped at the buffy coat and added to a slide for microscopic examination. Adding methylene blue stain to the sample may allow greater visibility of any microfilariae. However, the hematocrit tube method will not allow for species differentiation.
Modified Knott's test
The modified Knott's test is more sensitive because it concentrates microfilariae, improving the chance of diagnosis. A blood sample is mixed with 2% formalin and centrifuged in a tube. The supernatant is removed and methylene blue stain is added to the pellet remaining in the tube for microscopic examination. It allows microfilariae species differentiation based on morphology. Microfilariae can be differentiated between D. immitis and Acanthocheilonema reconditum because of small differences in morphology. The Modified Knott's test is the best method of visual examination when determining presence of microfilaria because it preserves their morphology and size. It is easy to perform, quick, and inexpensive.
The potential for a microfilaremic infection is 5 – 67%. The number of circulating microfilariae does not correlate with the number of adult heartworms, so is not an indicator of disease severity.
Antigen testing
In most practices, antigen testing has supplanted or supplemented microfilarial detection. Combining the microfilaria and adult antigen test is most useful in dogs receiving diethylcarbamazine or no preventive (macrolides like ivermectin or moxidectin typically render the dog amicrofilaremic). Up to 1% of infected dogs are microfilaria-positive and antigen-negative. Immunodiagnostics (ELISA, lateral flow immunoassay, rapid immunomigration techniques) to detect heartworm antigen in the host's blood are now regularly used. They can detect occult infections, or infections without the presence of circulating microfilariae. However, these tests are limited in that they only detect the antigens released from the sexually mature female worm's reproductive tract. Therefore, false-negative results may occur during the first five to eight months of infection when the worms are not yet sexually mature. The specificity of these tests is close to 100%, and the sensitivity is more than 80%. A recent study demonstrated a sensitivity of only 64% for infections of only one female worm, but improved with increasing female worm burden (85%, 88%, and 89% for two, three, and four female worms, respectively). Specificity in this study was 97%. False-negative test results can be due to low worm counts, immature infections, and all-male infections.
X-rays
X-rays are used to evaluate the severity of the heartworm infection and develop a prognosis for the animal. Typically, the changes observed are enlargement of the main pulmonary artery, the right side of the heart, and the pulmonary arteries in the lobes of the lung. Inflammation of the lung tissue is also often observed.

Treatment in dogs
If an animal is diagnosed with heartworms, treatment may be indicated. Before the worms can be treated, however, the dog's heart, liver, and kidney function must be evaluated to determine the risks of treatment. Usually, the adult worms are killed with an arsenic-based compound. The currently approved drug in the US, melarsomine, is marketed under the brand name Immiticide. It has a greater efficacy and fewer side effects than the previously used drug thiacetarsamide, sold as Caparsolate, which makes it a safer alternative for dogs with late-stage infections.
After treatment, the dog must rest, and exercise is to be heavily reduced for several weeks so as to give its body sufficient time to absorb the dead worms without ill effect. Otherwise, if the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure and sudden death. According to the American Heartworm Society, the administering of aspirin to dogs infected with heartworms is no longer recommended due to a lack of evidence of clinical benefit, and aspirin may be contraindicated in several cases. Aspirin had previously been recommended for its effects on platelet adhesion and the reduction of vascular damage caused by the heartworms.
The course of treatment is not completed until several weeks later, when the microfilariae are dealt with in a separate course of treatment. Once heartworm tests are negative and no surviving worm is detected, the treatment is considered a success, and the patient is effectively cured.
Surgical removal of the adult heartworms as a form of treatment may also be indicated, especially in advanced cases with substantial heart involvement and damage.

Signs and symptoms
Acute heartworm disease in cats can result in shock, vomiting, diarrhea, fainting, and sudden death.  Chronic infection can cause loss of appetite, weight loss, lethargy, exercise intolerance, coughing, and difficulty breathing.  Some cats' immune systems are able to clear a heartworm infection, though the immune system response can cause many of the same symptoms.  Also, even if the infection resolves, respiratory damage can cause some symptoms to persist beyond it.
Diagnosis
Diagnosis of heartworm infection in cats is problematic. Like in dogs, a positive ELISA test for heartworm antigen is a very strong indication of infection. However, the likelihood of a positive antigen test depends on the number of adult female worms present. If only male worms are present, the test will be negative. Even with female worms, an antigen test usually only becomes positive seven to eight months after infection. Therefore, a cat may have significant clinical signs long before the development of a positive test. Heartworm-associated respiratory disease can be found in cats that never develop adult heartworms and therefore never have a positive antigen test.
An antibody test is also available for feline heartworm infection. It will be positive in the event of exposure to D. immitis, so a cat that has successfully eliminated an infection may still be positive for up to three months. The antibody test is more sensitive than the antigen test, but it does not provide direct evidence of adult infection. It can, however, be considered specific for diagnosing previous larval infections, and therefore fairly specific for heartworm-associated respiratory disease.
X-rays of the chest of a heartworm-infected cat may show an increased width of the pulmonary arteries and focal or diffuse opacities in the lungs. Echocardiography is a fairly sensitive test in cats. Adult heartworms appear as double-lined hyperechoic structures within the heart or pulmonary arteries.
Treatment and prevention
Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) topical solution.  Ivermectin, milbemycin, and selamectin are approved for use in cats in the US.
Arsenic compounds have been used for heartworm adulticide treatment in cats, as well as dogs, but seem more likely to cause pulmonary reactions.  A significant number of cats develop pulmonary embolisms a few days after treatment.  The effects of melarsomine are poorly studied in cats.  Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended, and no drugs are approved in the US for this purpose in cats.
Treatment typically consists of putting the cat on a monthly heartworm preventive and a short-term corticosteroid.  Surgery has also been used successfully to remove adult worms.  The prognosis for feline heartworm disease is guarded.

Diagnosis
Diagnosis of heartworm infection in cats is problematic. Like in dogs, a positive ELISA test for heartworm antigen is a very strong indication of infection. However, the likelihood of a positive antigen test depends on the number of adult female worms present. If only male worms are present, the test will be negative. Even with female worms, an antigen test usually only becomes positive seven to eight months after infection. Therefore, a cat may have significant clinical signs long before the development of a positive test. Heartworm-associated respiratory disease can be found in cats that never develop adult heartworms and therefore never have a positive antigen test.
An antibody test is also available for feline heartworm infection. It will be positive in the event of exposure to D. immitis, so a cat that has successfully eliminated an infection may still be positive for up to three months. The antibody test is more sensitive than the antigen test, but it does not provide direct evidence of adult infection. It can, however, be considered specific for diagnosing previous larval infections, and therefore fairly specific for heartworm-associated respiratory disease.
X-rays of the chest of a heartworm-infected cat may show an increased width of the pulmonary arteries and focal or diffuse opacities in the lungs. Echocardiography is a fairly sensitive test in cats. Adult heartworms appear as double-lined hyperechoic structures within the heart or pulmonary arteries.
Treatment and prevention
Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) topical solution.  Ivermectin, milbemycin, and selamectin are approved for use in cats in the US.
Arsenic compounds have been used for heartworm adulticide treatment in cats, as well as dogs, but seem more likely to cause pulmonary reactions.  A significant number of cats develop pulmonary embolisms a few days after treatment.  The effects of melarsomine are poorly studied in cats.  Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended, and no drugs are approved in the US for this purpose in cats.
Treatment typically consists of putting the cat on a monthly heartworm preventive and a short-term corticosteroid.  Surgery has also been used successfully to remove adult worms.  The prognosis for feline heartworm disease is guarded.

Treatment and prevention
Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) topical solution.  Ivermectin, milbemycin, and selamectin are approved for use in cats in the US.
Arsenic compounds have been used for heartworm adulticide treatment in cats, as well as dogs, but seem more likely to cause pulmonary reactions.  A significant number of cats develop pulmonary embolisms a few days after treatment.  The effects of melarsomine are poorly studied in cats.  Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended, and no drugs are approved in the US for this purpose in cats.
Treatment typically consists of putting the cat on a monthly heartworm preventive and a short-term corticosteroid.  Surgery has also been used successfully to remove adult worms.  The prognosis for feline heartworm disease is guarded.