ORIGINAL PAPER Serum Strongylus vulgaris-specific antibody responses to anthelmintic treatment in naturally infected horses Martin K. Nielsen & Anand N. Vidyashankar & Jennifer Bellaw & Holli S. Gravatte & Xin Cao & Emily F. Rubinson & Craig R. Reinemeyer Received: 28 August 2014 /Accepted: 21 October 2014 /Published online: 31 October 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Strongylus vulgaris is the most pathogenic hel- minth parasite of horses, causing verminous endarteritis with thromboembolism and infarction. A serum enzyme-linked immunosorbent assay (ELISA) has been validated for detec- tion of antibodies to an antigen produced by migrating larvae of this parasite. The aim was to evaluate ELISA responses to anthelmintic treatment in cohorts of naturally infected horses. Fifteen healthy horses harboring patent S. vulgaris infections were turned out for communal grazing in May 2013 (day 0). On day 55, horses were ranked according to ELISA titers and randomly allocated to the following three groups: no treatment followed by placebo pellets daily; ivermectin on day 60 followed by placebo pellets daily; or ivermectin on day 60 followed by daily pyrantel tartrate. Fecal and serum samples were collected at ∼28-day intervals until study termination on day 231. Increased ELISA values were observed for the first 53 days following ivermectin treatment. Titers were signifi- cantly reduced 80 days after ivermectin treatment. Horses receiving daily pyrantel tartrate maintained lower ELISA values from 137 days post ivermectin treatment until trial termination. These results illustrate that a positive ELISA result is indicative of either current or prior exposure to larval S. vulgaris infection within the previous 5 months. Keywords Strongylus vulgaris . Ivermectin . Pyrantel tartrate . SvSXP . ELISA Introduction The large strongyle Strongylus vulgaris is considered to be the most pathogenic helminth parasite of horses. Larvae of this parasite migrate in the cranial mesenteric arterial tree, where they cause fibrinous endarteritis with pronounced thrombosis, formation of aneurysms, and other pathologic alterations of the vessel structures (Duncan 1974). Upon infection, exsheathed third stage larvae penetrate the large intestinal mucosa, where they molt to the fourth larval stage (L4) within the first 5 days. The L4s then migrate proximally toward the cranial mesenteric artery (CMA), where they arrive about 11 to 14 days post infection (PI). In the CMA, larvae molt to the immature adult stage at about 90 days PI, and migration back to the large intestine begins around day 120 PI (Duncan and Pirie 1972). Thromboembolism caused by L4s and immature adults has been associated with a painful and often fatal colic syndrome, characterized by ischemia and infarction of intes- tinal segments (Enigk 1951; Duncan and Pirie 1975). In the late 1980s, observations suggested a significant decline in prevalence and abundance of S. vulgaris, which was attributed to the intensive anthelmintic treatment regi- mens typically implemented for horse populations (Herd 1990; Love and Duncan 1991). Nevertheless, subsequent surveys of managed horses across the world continue to document that S. vulgaris is still encountered on a regular basis (Höglund et al. 1997; Boxell et al. 2004; Pilo et al. 2012). One recent study reported a statistical association be- tween the occurrence of S. vulgaris on Danish horse farms and usage of targeted selective therapy regimens in which treat- ment decisions were based on fecal egg counts (Nielsen et al. 2012). These egg count-based parasite control programs are M. K. Nielsen (*) : J. Bellaw : H. S. Gravatte : E. F. Rubinson M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA e-mail: martin.nielsen@uky.edu A. N. Vidyashankar : X. Cao Department of Statistics, George Mason University, Fairfax, VA, USA C. R. Reinemeyer East Tennessee Clinical Research, Inc, Rockwood, TN, USA Parasitol Res (2015) 114:445–451 DOI 10.1007/s00436-014-4201-5