A 4-year-old (estimated) Pit Bull cross intact female (25 kg) was presented for routine evaluation after it was adopted from the streets. The results showed that she had asymptomatic D. immitis infection as indicated by the presence of microfilariae in Knott's modified test and a positive D. immitis antigen test, although seronegative for Anaplasma spp., Ehrlichia spp., and Borrelia burgdorferi (SNAP 4DX Plus , IDEXX Laboratories Inc., Westbrook, ME, USA). She was seronegative for Leishmania infantum (TR DPP  canine visceral leishmaniasis, Bio-Manguinhos, Brazil). The routine physical examination showed no clinical signs of the disease and the hematological and urine examination results were normal.
The heartworm pre-treatment tests included blood work (blood urea nitrogen test, creatinine, alanine transaminase, and alkaline phosphatase levels and complete blood count), Doppler echocardiogram and chest X-rays. The blood work and Doppler echocardiogram were normal; however, the X-rays showed mild right atrioventricular enlargement and mild increase in interstitial and bronchial pulmonary pattern (Figures 1, 2).
Since the organic arsenical drug is unavailable in Brazil, the treatment option was the use of slow-release injectable moxidectin (ProHeart  SR-12, Zoetis, Campinas, Brazil) biannually (0.5 mg/kg) instead of annually in combination with 30 days of doxycycline (Doxifin , Ourofino, Cravinhos, Brazil) (10 mg/kg/BID). The owner was duly informed and clarified about the off-label alternative treatment and after her formal consent the administration of both drugs was initiated on the same day.
When the dog was presented for evaluation 6 months following the first moxidectin injection it was informed that she could be pregnant as she had been in heat approximately 30 days before the visit and she lived with an intact male dog. Abdominal ultrasound was performed, and pregnancy was confirmed. Therefore, chest X-rays for lungs evaluation were precluded. All the other pre-treatment examinations were repeated, and the results were within the reference range. No microfilariae were detected by Knott's modified test and D. immitis antigen test was negative. Since a D. immitis-infected dog receiving alternative treatment must present microfilariae and D. immitis antigen test negative results 6 months apart to confirm the elimination of the parasite, a second moxidectin injection (0.5 mg/kg) was administered 6 months after the first and the animal was kept under observation. Within 1 week following the second injection, the animal gave birth to three healthy puppies.
Blood work, Doppler echocardiogram, and chest X-rays were unchanged when she was presented 6 months after the second moxidectin dose and serological tests with the heat pre-treatment sample confirmed that heartworm had been successfully eliminated. Since the animal was free of infection, the owner was advised to continue with the moxidectin injections annually.