Veterinary Parasitology 185 (2012) 16–24 Contents lists available at SciVerse ScienceDirect Veterinary Parasitology jou rn al h om epa ge: www.elsevier.com/locate/vetpar Resistance to avermectin/milbemycin anthelmintics in equine cyathostomins – Current situation Marcelo Beltrão Molento a,* , Martin K. Nielsen b , Ray M. Kaplan c a Laboratory of Parasitic Diseases, Department of Veterinary Medicine, Federal University of Parana, Curitiba, PR, Brazil b Department of Large Animal Sciences, Faculty of Large Animal Sciences, University of Copenhagen, Denmark c Department of Infectious Diseases, College of Veterinary Medicine, University of Georgia, Athens, GA, USA a r t i c l e i n f o Keywords: Horse Cyathostomins Avermectin Milbemycin Drug resistance a b s t r a c t Avermectins and milbemycins (AM) are potent compounds against all major nematode parasites, but their continuous usage has led to the development of widespread resis- tance in many of the important species of ruminant and equine parasites. The exception to this has been the cyathostomins, where AM resistance was recently first reported only after decades of drug exposure. Data from a Brazilian study suggests that AM resistance has developed in cyathostomins and reports of shortened egg reappearance periods after ivermectin treatment have been published recently from USA and Germany. Thus, AM resis- tance in cyathostomins is an emerging worldwide concern, but there is only limited amount data on the extent of this problem. To limit the development and spread of AM-resistant cyathostomins the equine industry must implement new strategies for worm control, and the veterinary parasitology community must develop and validate improved protocols for detecting anthelmintic resistance in the field. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Cyathostomins are the most prevalent parasites affect- ing horses and can be found in virtually all grazing horses worldwide (Torbert et al., 1986; Silva et al., 1999; Lichtenfels et al., 2001). Early third stage larvae (EL 3 ) of these parasites undergo encystment as a part of the life cycle, and numbers of encysted larvae often reach more than 200 thousand in clinically healthy horses (Bucknell et al., 1995; Barbosa et al., 2001; Martins et al., 2001; Pook et al., 2002). When the encysted larvae emerge and enter the lumen, inflammatory changes occur in the large intesti- nal wall. When this occurs in very high numbers, larvae are capable of causing serious illness with signs of weight loss, * Corresponding author at: Universidade Federal do Paraná. R: dos Fun- cionários, 1540, Cabral., CEP: 80035-050 Curitiba, Paraná, Brazil. Tel.: +55 41 3350 5618; fax: +55 41 3350 5623. E-mail address: molento@ufpr.br (M.B. Molento). diarrhea, anaemia, hypoalbuminemia and colic episodes (Love et al., 1999; Peregrine et al., 2006). Most horse-owners almost everywhere, have easy access to relatively inexpensive commercially available anthelmintics without restrictions, thus there is a major concern regarding over usage and the high selection pressures placed on parasite populations for resistance (Sangster, 1999; Kaplan, 2002; Molento, 2005). With increasing levels of resistance to benzimidazoles and pyrantel being reported worldwide (reviewed by Kaplan, 2002, 2004), equine parasite control now relies heavily on the avermectin/milbemycin (AM; also referred to as macro- cyclic lactones) drug class. Over the past couple of decades, parasitologists have recommended changing parasite con- trol regimens from the intensive interval-dose treatment regime into a more sustainable approach, which has a sec- ondary goal of also preserving the effective lifespan of the drugs (Duncan and Love, 1991; Gomez and Georgi, 1991; Lloyd et al., 2000; Kaplan, 2002; Kaplan et al., 2004). Despite this, most equine establishments continue to rely 0304-4017/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.vetpar.2011.10.013