Efficacy of ivermectin against cutaneous Draschia and Habronema infection (summer sores) in horses.

Thirty-one horses with naturally occurring summer sores were given a single IM injection of 0.2 mg of ivermectin (22,23-dihydroavermectin B1)/kg during the summer of 1980. Larvae of Draschia and/or Habronema spp were recovered from biopsy samples taken from 21 of 25 horses (84%) on the day of anthelmintic treatment. There was a marked clinical improvement in 26 horses (84%) 7 days after ivermectin was given. The typical summer sore was replaced by healthy pink granulation tissue at 7 days and this healed after 1 to 5 weeks. Biopsy samples were taken from 21 of these horses 1 to 6 weeks after treatment, and the samples in 18 horses (86%) were negative for larvae. Five of the 31 infected horses did not respond to a single dose and appeared to have become reinfected. Biopsy samples taken from 3 of these horses were positive for larvae 2 to 4 weeks after the 1st dose, but became negative and healed 1 to 2 weeks after a 2nd dose with ivermectin. Histopathologic examination of sections from lesions before anthelmintic treatment showed granulation tissue with marked eosinophilic infiltration, multiple eosinophilic abscesses, and transverse sections of nematodes. In sections examined after anthelmintic treatment, there were marked reduction in the number of eosinophils and absence of eosinophilic abscesses or nematodes, and the granulation tissue was more mature, as indicated by the appearance of dense fibrous connective tissue.