on these antigen-antibody complexes and their significance is ongoing. At this time, it is not recommended to routinely heat-treat samples for heartworm testing (American Heartworm Society, 2014). However, it is important to keep in mind that false negatives may be more common than once thought. Veterinarians should consider reporting heartworm tests as “no antigen detected” (NAD) instead of negative, as there is the possibility of heartworm disease even in the case of a negative antigen test. The color intensity of a positive heartworm antigen test has a direct, but imprecise, relationship to the number of mature female heartworms. Keep in mind, however, that the heartworm antigen test does not take into account numbers of male heartworms. For this reason, the color intensity of a positive test cannot be used to reliably determine the level of worm burden (American Heartworm Society, 2014). Microfilaria tests In addition to testing for adult female heartworm antigen, microfilaria testing is also beneficial in characterizing a dog’s heartworm status. When performing microfilaria testing, however, it is important to remember that up to 20% of heartworm-infected dogs may be negative for microfilaria; this number is even higher for dogs on heartworm prevention (American Heartworm Society, 2014). Still, microfilaria testing remains an important adjunct to antigen testing. The most commonly performed microfilaria test is a direct microscopic exam of a drop of canine blood. If microfilariae are present, it is important to determine whether observed microfilariae are truly D. immitis or whether they belong to the species Acanthocheilonema reconditum (previously Dipetalonema reconditum ). A number of characteristics can be used to distinguish these two species. First, D. immitis infections are often associated with the presence of many microfilariae, while A. reconditum infections are only associated with small numbers of microfilariae. The microfilariae of D. immitis largely remain stationary, while A. reconditum microfilariae demonstrate progressive, forward motion. The two species also differ in size and shape. Finally, the microfilariae of D. immitis are larger, at 300-322 µm, with a straight body and tapered head. In contrast, the microfilariae of A. reconditum are smaller, at 250-288 µm, with a curved body, “button-hook” tail, and blunt head. If D. immitis microfilariae are seen on a direct blood sample, this confirms the presence of heartworms. Although more technically complicated than a direct blood examination, there are two tests that are more reliable for detecting the presence of microfilariae, especially if low numbers of microfilariae are present (Blagburn, 2013). The Modified Knott’s test is the most commonly used, but a filtration (Difil ® ) test can also be performed. The Modified Knott’s test is performed by mixing 1.0 mL of EDTA blood with 9.0 mL of 2% formalin, in a centrifuge tube. When mixed thoroughly, the formalin solution lyses the red blood cells, allowing microfilariae to be visualized more easily. The tube is then placed in a centrifuge, spun at 1100 to 1500 rpm for 5 to 8 minutes, and the liquid is poured off, leaving the sediment. A drop of methylene blue is added to the sediment and the stained sediment is placed on a glass slide, with a cover slip applied. The slide is examined under low power (100X) for the presence of microfilariae (American Heartworm Society, 2014). Centrifugation concentrates the microfilaria, while lysing the red blood cells allows for increased visibility. The Difil ® test also uses a solution to lyse red blood cells. Instead of concentrating microfilariae through centrifugation, however, the Difil ® test relies on the use of a specialized filter
to collect microfilaria. This test is less commonly used than the Modified Knott’s test. Rationale for appropriate testing Current recommendations for heartworm testing are that all dogs be tested using both an antigen test and a microfilariae test, due to the risk of false-positive and false-negative antigen tests. Discordant results should be further investigated (American Heartworm Society, 2014). All dogs receiving regular heartworm prevention should be tested for heartworms once yearly. Although treatment failures are uncommon, early detection of these failures will ensure timely identification and treatment, minimizing the chronic effects of heartworm disease. Additionally, dogs should be tested any time that the owner wishes to change their brand of heartworm prevention, in order to facilitate evaluation should the dog ever come up heartworm-positive (American Heartworm Society, 2014). Dogs must also be tested any time that the owner has been noncompliant in administering heartworm prevention. It can take approximately six months for a heartworm-infected dog to be become antigen-test positive, therefore, dogs should also be tested prior to resuming heartworm prevention and then retested six months later (after six months of continuous prevention). This test ensures that the prepatent period of infection has passed and that the dog was truly not infected during the lapse in heartworm prevention (American Heartworm Society, 2014). Imaging Imaging tests are frequently performed in the confirmation or pre-treatment assessment of heartworm disease. Imaging most commonly includes thoracic radiography, although echocardiogram may also be beneficial in some cases. Thoracic radiographs of dogs with heartworm infection typically demonstrate enlarged, tortuous, and truncated peripheral intralobar and interlobar branches of the pulmonary arteries. In the early stages of heartworm disease, these vascular abnormalities are typically observed in the caudal lung lobes. As the disease progresses, however, these pulmonary arterial changes may be seen throughout the remainder of the lung. Infected dogs may also demonstrate varying degrees of pulmonary parenchymal disease, evident as an interstitial radiographic pattern on thoracic radiographs. In severe cases of canine heartworm disease, right heart enlargement eventually develops pulmonary opacities may be observed (American Heartworm Society, 2014). Echocardiography can also be used to assess dogs for the presence of heartworms. This modality is not very effective in diagnosing dogs with low worm burdens because adult worms prefer to lodge in the peripheral pulmonary vessels. These vessels are beyond the view of ultrasound, and therefore, worms in this location may go undetected. In dogs with heavy worm burdens, however, heartworms are more likely to be found in the main pulmonary artery, right and proximal left interlobar branches, or within the right side of the heart, where they can be imaged easily. Heartworms have a highly echogenic body wall, which produces distinct, short, parallel-lines images on ultrasound, possessing the appearance of an “equals sign” (American Heartworm Society, 2014). This can confirm the presence of heartworms. Ultrasound is especially beneficial in the diagnosis of caval syndrome. Heartworm-positive dogs with hemoglobinuria should be suspected of having caval syndrome. In these cases, adult heartworms can often be visualized with ultrasound in the orifice of the tricuspid valve, providing conclusive confirmation of caval syndrome (American Heartworm Society, 2014).
TREATMENT
There are two approaches to treating heartworm disease. The safest and most effective treatment method is adulticide therapy using melarsomine. Melarsomine kills the adult heartworms,
preventing ongoing cardiopulmonary damage. In situations where adulticide therapy is not an option, the “slow-kill” method of treatment has also been discussed as a means of managing
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