RESUMEN
On the basis of positive skin snips for Onchocerca cervicalis microfilariae (MF), 45 horses were chosen from 48 in a total of 257 screened on 12 locations in the northeast Province of Formosa (Argentina), and randomly assigned to two treatment groups of 20 horses each, and a nontreated control group of five horses. On Day 14 post-treatment (PT), skin snip samples in the ivermectin-treated (0.2 mg/kg) group were negative for normal viable microfilariae (MF), while horses in the control group maintained their pretreatment level of infection. On the same Day in the moxidectin-treated (0.4 mg/kg) group, 18 horses were negative for MF, but the remaining two had a total of 1 and 2 MF, respectively (equivalent to 10 and 20 MF/g of skin), but all three parasites showed marked cuticular and structural damage. Both horses were negative in a repeat biopsy on Day 21. From Day 3 PT, one ivermectin-treated horse (5%) evidenced an approximate 15 x 2 x 3 cm-sized, apparently nonpainful, oedematous swelling on the ventral midline, 20 cm in front of the navel, which remained unchanged on Day 14 PT. Adverse reactions were not observed in the moxidectin-treated group. Parasitaemia was found in 18.7% of sampled horses (48 of 257), and the number of MF varied between 10-1820/g of skin snip (mean 172). Similar prevalence and total counts had been described previously in 1985 and 1986 in cattle-farm horses in the same area of Argentina; in surveys in Texas (1974) and Louisiana (1995) in the USA, infection rates were also similar, but total counts much higher. It is concluded that moxidectin 2% equine oral gel and ivermectin 2% equine oral paste, were equally 100% effective in the control of O. cervicalis MF. Contrary to ivermectin, moxidectin did not cause post-treatment dermal reactions.