Evaluation of the efficacy of monthly oral administration of afoxolaner plus milbemycin oxime (NexGard Spectra®, Merial) in the prevention of adult Spirocerca lupi establishment in experimentally infected dogs

The nematode Spirocerca lupi (Rudolphi, 1809) is widely distributed but mostly occurs sporadically with stable populations only in certain geographic areas. This helminth mainly infects dogs and wild canids. Primary pathology relates to migration of third stage larvae (L3) damaging the thoracic aorta and establishment of adults in nodules in the oesophagus. The objective of the present study was to evaluate the efficacy of milbemycin oxime in combination with afoxolaner (NexGard Spectra®, Merial), administered monthly, in preventing establishment of adult worms after experimental infection. Two groups consisting of eight animals each were experimentally infected with 15 L3 on Days −28, −14 and −2, respectively (45 L3 per animal in total). Group 1 dogs served as untreated (negative) control, whereas animals in group 2 were treated with NexGard Spectra® at a minimum dose of 0.5mg/kg milbemycin oxime on Day 0 and from then onwards every 28 days up to Day 140 (six treatment occasions). Endoscopy was performed on Day 112 and for some animals also Day 140. Necropsy for worm recovery and nodule/lesion scoring was performed on Day 168. All eight animals in the control group (group 1) presented with 1–3 nodules and worm counts ranging from 9 to 41. Six animals in the NexGard Spectra® group presented with 1–4 nodules and worm counts ranging from 1 to 5. Significantly (p<0.05) fewer worms were collected from treated animals in the treated group (geometric mean 1.7) versus the negative control group (geometric mean 22.0) with 92.3% efficacy calculated. There was no significant (p>0.05) difference between groups with reference to number of nodules in the oesophagus. However, nodules in the control group were significantly (p<0.05) larger than those in the treated group.Number and size of lesions in the dorsal aorta did not differ statistically between groups 1 and 2. Because NexGard Spectra® was administered 28 days after onset of inoculation, migrating and developing L3 caused damage to the aorta wall of animals in the treated group. Milbemycin oxime (administered as NexGard Spectra®) demonstrated effectiveness in reducing infection with adult Spirocerca lupi worms in the oesophagus.