Small strongyle infection: Consequences of larvicidal treatment of horses with fenbendazole and moxidectin Tanja Steinbach a,b , Christian Bauer a , Hermann Sasse b , Wolfgang Baumga ¨rtner c,1 , Cecilia Rey-Moreno a , Carlos Hermosilla a , I Made Damriyasa a , Horst Zahner a, * a Institute of Parasitology, Justus Liebig University Giessen, Rudolf-Buchheim-Strasse 2, D-35392 Giessen, Germany b Clinic for Horses, Justus Liebig University Giessen, Germany c Institute of Veterinary Pathology, Justus Liebig University Giessen, Germany Received 23 November 2005; received in revised form 21 February 2006; accepted 1 March 2006 Abstract The study was undertaken to evaluate adverse effects of larvicidal treatment in horses naturally infected with cyathostomins. Out of 24 ponies kept on pasture, four animals were housed in September and anthelmintically cured to serve as worm-free controls (group C-0). The others were housed in December. Eight animals each were treated 8 weeks later with 5 Â 7.5 mg/kg fenbendazole (FBZ) or 1 Â 0.4 mg/kg moxidectin (MOX). Four animals remained untreated (group C-i). Two, 4, 6 and 14 days after the end of treatment two animals of each of the treated groups were necropsied together with group C-0 and C-i animals. Infected animals before treatment showed weight loss, eosinophilia, increased plasma protein and globulin contents. Treatment was followed by weight gain and temporal plasma protein and globulin increase. Proportions of CD4 + and CD8 + T lymphocytes in the peripheral blood did not differ between the groups before treatment but dropped significantly temporally after FBZ treatment. Group C-0 was worm-free at necropsy. Group C-i animals contained variable numbers of luminal and tissue cyathostomins. Histological sections showed larval stages in the lamina propria und submucosa surrounded by macrophages. Either treatment was effective against luminal parasites and reduced the number of larvae in the bowel wall beginning 4–6 days after FBZ and 6–14 days after MOX treatment. Histologically, as a first reaction after FBZ application T lymphocytes accumulated around morphologically intact L4 in the submucosa. Subsequently T lymphocytes associated with eosinophils infiltrated the submucosa. Parasites became enclosed by granulomas with eosinophils adhering to and invading the larvae which started to disintegrate on day 4. Later on, particularly on day 14 inflammation extended into the mucosa and was frequently associated with ulcerations. Third stage larvae in general and L4 in the lamina propria, however, seemed not to be affected until day 14 and even then, parasites did usually not generate extensive inflammation. After MOX treatment severe morphologically detectable alterations of tissue larvae could not be observed earlier than day 14. Different from FBZ treatment, larvae disintegrated and were obviously resorbed without causing severe inflammation in the gut wall. In conclusion treatment with either drug was efficacious against tissue larvae of cyathostomins but there may be different clinical consequences: in contrast to www.elsevier.com/locate/vetpar Veterinary Parasitology 139 (2006) 115–131 * Corresponding author. Tel.: +49 641 99 38460; fax: +49 641 99 38469. E-mail address: horst.zahner@vetmed.uni-giessen.de (H. Zahner). 1 Present address: Institute of Pathology, Hannover School of Veterinary Medicine, Bu ¨nteweg 17, D-30559 Hannover, Germany. 0304-4017/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vetpar.2006.03.028