The main lesions detected in chest radiographs of heartworm-infected dogs are: increased diameter of the pulmonary arteries (IDPCA); increased pulmonary trunk (IPT); right ventricular enlargement (RVE); diffuse densification and reticular interstitial lung of the diaphragmatic lobes (DDRILDL) and micronodular marking (MM). To compare chest radiograph patterns of 12 microfilariae and antigen-positive (Snap 4DX, IDEXX) dogs before treatment and after monthly topical moxidectin (topical moxidectin + imidacloprid at 2.5 to 6.25 mg moxidectin/kg/month) associated with semi-annual doxycycline (10 mg/kg/BID - 30 days), dogs had chest radiographs taken before and after treatment. No dogs presented with clinical signs, although all became antigen negative at six months and remained negative at 12 months of treatment. After treatment, the only lesion that had the frequency increased was IDPCA; four lesions became less frequent, and one remained unchanged (Table). According to the radiographs, there was no worsening of lesions. These results show that worsening of pulmonary artery and lung damage when using slow-kill may be an overstatement, although it must be considered that if a faster elimination of worms were achieved, resolution of those severe lesions could lead to complete recovery faster. Therefore, slow-kill may be beneficial for patients where the adulticide treatment is unavailable.

Table. Chest radiographic findings in heartworm-infected dogs submitted to the slow-kill treatment for one-year period