Diagnostic tests

Heartworm antigen test kits are widely used in private practices to screen for heartworm infestation. A number of sensitive and specific antigen tests are available. Heartworm antigen tests are based on the presence of mature female worms.¹¹ After successful postadulticide treatment, most antigen test results will be negative by 12 weeks, but occasionally the antigen remains longer (up to five months).¹² A study comparing three commercial heartworm antigen test kits found the ELISA Snap RT test (IDEXX Laboratories) had significantly higher sensitivity, accuracy, and negative predictive value than the two other test kits evaluated (VetScan CHAT—Abaxis, Solo Step CH—HESKA).¹¹

Another screening technique is microfilariae testing, though it has largely been replaced by the heartworm antigen test kits. Microfilariae testing is mainly recommended for screening dogs 6 or 7 months of age that have not yet received prophylactic therapy but are ready to start. These patients are screened because heartworm antigens can take up to eight months to appear in the blood after infection. Microfilariae testing must be done in dogs before diethylcarbamazine administration because severe adverse reactions are likely to occur if this drug is given to microfilariae-positive dogs.

Figure 4: A short-axis right parasternal echocardiogram obtained at the heart base in a dog. Note the linear echo artifacts (arrows) representing the heartworms (PA = pulmonary artery, AO = aorta).

Treatment

Adulticides

The adulticide most frequently administered is melarsomine dihydrochloride. Melarsomine is effective against immature and young adult D. immitis.^15,16 Melarsomine is administered by deep intramuscular injections of 2.5 mg/kg in the lumbar epaxial muscles. The recommended therapy for Class 1 and mild-to-moderate Class 2 patients is two injections in alternating epaxial muscle bellies 24 hours apart.¹⁷ Certain Class 2 and all Class 3 patients require a modified treatment protocol. A single intramuscular injection is given, followed four to six weeks later with two injections 24 hours apart. About half the heartworms are killed with the initial injection, reducing thromboembolic showering and allowing for the initial pulmonary inflammatory response to subside before the complete worm kill.¹⁸ Some researchers advocate treating Class 1, 2, and 3 patients with the split protocol.¹⁸

Alternating sides for melarsomine injections is highly recommended. The manufacturer recommends using a 23-ga, 1-in needle for dogs 10 kg or less and a 22-ga, 1.5-in needle for dogs weighing more than 10 kg. Some patients may benefit from sedation for proper deep epaxial intramuscular injections. No more than 4 ml/injection site is recommended. Injection site reactions may arise if the drug migrates out of the injection site along the fascial planes. To help prevent the drug from migrating, use light sedation and limit movement. However, the drug can still migrate and cause ascending inflammation along nerve roots and vasospasm or vasculitis.¹⁹ The local inflammation can lead to necrosis and spinal cord compression. Adverse effects of melarsomine include localized swelling, tenderness at the injection site, and possibly mild anorexia. Overdosed patients can be treated with dimercaprol (British antilewisite, or BAL) (3 mg/kg two or three times three hours apart).²⁰ To avoid pulmonary thromboemboli after adulticide therapy, strict rest for three or four weeks is crucial. Exercise restriction one week before adulticide treatment is also recommended.

Microfilaricidal treatment

If circulating microfilariae still exist after adulticidal therapy, microfilaricidal treatment is recommended, but not until four to six weeks after adulticidal therapy. Give milbemycin oxime at a prophylactic dose (0.5 to 1 mg/kg) or ivermectin (50 μg/kg) as a one-time dose, and retest the patient for microfilariae three weeks later. Microfilariae testing is performed rather than immunodiagnostic testing because heartworm antigen can remain for up to three months after elimination of adult heartworms. If microfilariae still exist, repeat the microfilaricidal treatment. If the microfilariae are successfully eliminated, institute prophylactic therapy. Ivermectin at prophylactic doses (6 to 12 μg/kg) is safe to start immediately after adulticidal therapy, and it will slowly eliminate the remaining microfilariae and any remaining adult heartworms.³