Efficacy of oral, topical and extended-release injectable formulations of moxidectin combined with doxycycline in Dirofilaria immitis naturally infected dogs

Abstract

Background: Several studies in both experimentally and naturally infected dogs have reported the adulticide effect of a combination of macrocyclic lactones and doxycycline against Dirofilaria immitis, showing that these protocols can be used as an alternative to melarsomine. The present study evaluated the efficacy of oral, topical and extended-release injectable formulations of moxidectin when combined with doxycycline in dogs naturally infected with D. immitis from a shelter located in southern Italy.

Methods: Thirty dogs with naturally acquired D. immitis infection were divided in three groups (G) and treated with oral moxidectin (G1) once a month for 9 consecutive months, topical moxidectin (G2) once a month for 9 consecutive months or extended release moxidectin injectable (G3) at enrolment and again at 6 months (Day 180). All treatment groups received doxycycline for the first 30 days. Microfilarial concentrations in 1 ml (mff/ml) blood were determined monthly for 9 months with the modified Knott's test. A clinical scoring system was employed for each dog enrolled in the study based on thoracic radiography and cardiac ultrasound (CU) examinations performed at Day - 15 (before treatment) and at Day 180.

Results: Results from the present study suggest that the majority of dogs from all treatment groups became antigen negative, as evaluated at Day 270: 9/10 dogs (90.0%) from G1, 6/10 dogs (60.0%) from G2 and 8/10 dogs (80.0%) from G3. Improvement of radiographic alterations was observed in all treatment groups, and almost all dogs were cleared of pulmonary abnormalities by 6 months from the beginning of treatment (P = 0.000). Cardiac ultrasound examination showed a progressive improvement of cardiac function in a limited number of animals (4/30).

Conclusions: The combination of doxycycline and three different formulations of moxidectin leads to antigen-negative status in naturally infected dogs.

Keywords: Dirofilaria immitis; Doxycycline; Injectable; Moxidectin; Oral; Spot-on.

Fig. 1

Study design. DOXY = 10 mg/kg SID (Ronaxan®, Boehringer Ingelheim Animal Health, Italy); MOX ORAL = oral moxidectin (3 µg/kg SID; Afilaria®, Fatro-Italy); MOX SPOT-ON = topical 10% imidacloprid + 2.5% moxidectin (Advocate®, Elanco, Italy); MOX INJ = extended release moxidectin injectable (Afiliara SR®, Fatro, Italy)

Fig. 2

Reduction of circulating microfilariae of D. immitis (a) and D. repens (b) in the three treatment groups

Fig. 3

Right-lateral radiographs pre- (A) and post-treatment (B) of the same dog. A A diffuse mixed alveolar/unstructured interstitial pattern. This increase in pulmonary opacity involves especially the caudal lung lobes where multiple air bronchograms are visible. In B, there is a partial resolution of the radiographic alterations involving the lung lobes, although there is a residual unstructured interstitial pattern associated with mild to moderate thickening of the bronchial walls. This dog was initially assigned in score 2, but after the 6 months of therapy was assigned a score of 1

Fig. 4

Dorsoventral (DV) radiographs pre- (A) and post-treatment (B) of the same dog. In both views, there is a mild, diffuse, unstructured interstitial pattern associated with mild dilatation of the right atrium (black arrowheads). Furthermore, several caudal lobar arteries (white arrows) have a convoluted shape. This dog was assigned a pre-treatment score of 1 and, after 6 months of therapy, was re-assigned in the same score. Legend: R = right, L = left