Please cite this article in press as: Reinemeyer, C.R., et al., Effects of daily pyrantel tartrate on strongylid population dynamics and performance parameters of young horses repeatedly infected with cyathostomins and Strongylus vulgaris. Vet. Parasitol. (2014), http://dx.doi.org/10.1016/j.vetpar.2014.05.034 ARTICLE IN PRESS G Model VETPAR-7270; No. of Pages 9 Veterinary Parasitology xxx (2014) xxx–xxx Contents lists available at ScienceDirect Veterinary Parasitology jo u r nal homep age: www.elsevier.com/locate/vetpar Effects of daily pyrantel tartrate on strongylid population dynamics and performance parameters of young horses repeatedly infected with cyathostomins and Strongylus vulgaris C.R. Reinemeyer a,∗ , J.C. Prado a , U.V. Andersen b , M.K. Nielsen c , B. Schricker d , T. Kennedy d a East Tennessee Clinical Research, Rockwood, TN, USA b Department of Large Animal Science, Faculty of Health and Medical Sciences, University of Copenhagen, Denmark c M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA d Central Life Sciences, Phoenix, AZ, USA a r t i c l e i n f o Article history: Received 12 September 2013 Received in revised form 14 May 2014 Accepted 16 May 2014 Keywords: Horse Anthelmintic Pyrantel tartrate Cyathostomins Strongylus vulgaris Performance a b s t r a c t Strongylid infections are ubiquitous in grazing horse populations. Infections with cyathos- tomin (small strongyle) and strongylin (large strongyle) nematodes have long been associated with clinical disease in horses, but little is known about their subclinical impact. A masked, randomized, controlled study was conducted to evaluate the effects of daily admin- istration of pyrantel tartrate on body condition scores, weight gain, fecal egg counts, and total worm counts of young horses repeatedly inoculated with strongylid larvae. Twenty eight immature horses were treated with larvicidal anthelmintic regimens and randomly allocated to two groups. Group 1 horses were given a pelleted placebo product once daily, and those in Group 2 received pyrantel tartrate once daily at ∼2.64 mg/kg body weight. On five days during each week, ∼5000 infective cyathostomin larvae were administered to each horse. In addition, horses received ∼25 infective Strongylus vulgaris larvae once weekly. Horses were maintained on pasture for 154 days and had ad libitum access to grass hay throughout. At approximate, 14-day intervals, body weights were measured, body con- dition scores were assigned, fecal samples were collected for egg counts, and blood samples were collected for measurement of S. vulgaris antibodies and various physiologic parame- ters. After 22 weeks at pasture and 14–17 days in confinement, horses were euthanatized and necropsied. Nematodes were recovered and counted from aliquots of organ contents, representative samples of large intestinal mucosa, and the root of the cranial mesenteric artery. Daily treatment with pyrantel tartrate at the recommended dosage significantly reduced numbers of adult cyathostomins in the gut lumen and early third-stage larvae in the cecal mucosa, increased the proportions of fourth-stage larvae in the gut contents, and was accompanied by significant improvements in body condition scores. Fecal egg counts of horses receiving daily pyrantel tartrate were significantly reduced, with percentages of efficacy ranging from 84.4% to 98.9%, but egg counts of both groups increased signifi- cantly over the course of the study. Treatment also significantly reduced the numbers of S. vulgaris larvae in the cranial mesenteric artery by 99.2%. Serum antibodies to S. vulgaris apparently persisted from pre-enrollment infections, but ELISA values gradually declined ∗ Corresponding author. Tel.: +1 865 354 8420; fax: +1 865 354 8421. E-mail address: crr@easttenncr.com (C.R. Reinemeyer). http://dx.doi.org/10.1016/j.vetpar.2014.05.034 0304-4017/© 2014 Elsevier B.V. All rights reserved.