Ancillary drug therapy

Ancillary drug therapy is advocated in patients with pulmonary thromboemboli, eosinophilic pneumonitis, eosinophilic pulmonary granulomatosis, or lymphomatoid granulomatosis. Pulmonary thromboemboli are a common complication of heartworm disease and adulticide treatment and usually develop five to 21 days after treatment is initiated. Heparin therapy has been recommended for patients with symptomatic pulmonary thromboemboli subsequent to adulticide treatment or prophylactically to prevent pulmonary thromboemboli in dogs with severe pulmonary arterial disease.⁷ Heparin (75 to 150 IU/kg subcutaneously t.i.d. with a target activated partial thromboplastin time [aPTT] of one and a half to two times normal) is continued for five to 21 days after the resolution of clinical signs associated with pulmonary thromboemboli and then slowly tapered over several days. In one study, the survival rate was 97.5% in heparin-treated Class 3 dogs compared with 73.5% in Class 3 dogs treated with antiplatelet drugs.⁷ And postadulticide complications were reduced in heparin-treated patients compared with aspirin-treated patients.⁷

Corticosteroid therapy remains controversial, but some clinicians still advocate its use for eosinophilic pneumonitis, eosinophilic granulomatosis, and pulmonary infiltrates.^8,21 If indicated, prednisone (0.5 to 1 mg/kg orally daily) can be administered until radiographic evidence of the disease shows improvement. Routine prophylactic administration is not recommended as corticosteroids are procoagulant and decrease pulmonary blood flow.⁸ Oxygen therapy is also recommended if available, as it reduces pulmonary arterial pressures and improves perfusion.²² Oxygen chambers that are temperature-controlled and percent-saturation-controlled are ideal. Nasal insufflation is adequate. The oxygen promotes pulmonary arteriole dilation, decreasing the severity of pulmonary hypertension.

The choice of prophylactic is often based on a preference for administration frequency (daily [for diethylcarbamazine only] vs. monthly) and whether the dog is at risk of intestinal parasites or ectoparasites. Many excellent prophylactics are available.

The most common prophylactic drugs administered are avermectin- or milbemycin-based. The approved prophylactic ivermectin dose for dogs is 6 to 12 μg/kg orally monthly,²³ and selamectin can be applied to the skin monthly (6 mg/kg) for heartworm prophylaxis.²⁴ Moxidectin (ProHeart 6—Fort Dodge Animal Health) is currently not available in an injectable form in the United States. Milbemycin oxime (0.5 to 1 mg/kg) can also be administered orally on a monthly basis for prophylaxis.²⁵

Ivermectin and milbemycin oxime are effective against early L3 infestations. Studies have shown that three-month-old infestations can be greatly reduced with monthly prophylactic doses of either ivermectin or milbemycin oxime, so missing a month or two of prophylaxis is not likely to lead to severe heartworm disease when the patients are receiving these forms of prophylaxis.