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| INTRODUCTION
Equine anoplocephalid cestodes occur worldwide in horses (Gasser et al., 2005 ). There are currently three species of tapeworm known to infect horses: Anoplocephala perfoliata , A. magna and Anoplocephaloides mamillana . The most common of these three is A. perfoliata , with the other two being much more sporadic in appear- ance (Borgsteede & van Beek, 1998 ; Meana et al., 2005 ; Rehbein et al., 2012 ). In the US, A. perfoliata has been reported with infection rates of 52–100% (Chapman et al., 2003 ; Lyons et al., 2000 , 2007 ). In terms of predilection site within the horse, A. mamillana and A. magna follow typical tapeworm behaviour and are located within the small intestine. A. perfoliata , however, instead resides around the ileocaecal junction within the caecum. Currently, only A. perfoliata has been associated with clinical disease in horses. These parasites cause inflammatory reactions around their attachment site, which have been characterised by mucosal thickening, necrotic ulcers and hyperaemia (Kjaer et al., 2007 ; Nilsson et al., 1995 ; Pearson et al., 1993 ; Williamson et al., 1997 ). Common disease manifestations of tapeworm infection include intussusceptions, bowel obstructions, cecal perforations and peritonitis (Barclay et al., 1982 ; Beroza et al., 1983 ; Owen et al., 1989 ). Clinically, A. perfoliata infection has been associated with colic manifestations in several studies (Back et al., 2013 ; Proudman et al., 1998 ; Proudman & Edwards, 1993 ). There are currently two registered and marketed anthelmintic
compounds with documented activity against equine tapeworms, which are pyrantel pamoate/embonate and praziquantel. Pyrantel pamoate/embonate was originally introduced in the 1970s. This an- thelmintic originally reduced A. perfoliata worm burdens by around 80% when given the standard dose labelled for nematode control (6.6 mg base/kg; Lyons et al., 1989 ). However, when the dosage was doubled to 13.2 mg base/kg, the anticestodal efficacy increased to 95–98% (Marchiondo et al., 2006 ; Reinemeyer et al., 2006 ). Praziquantel was introduced later in the 1990s and is usually com- bined with ivermectin or moxidectin in anthelmintic formulations. Praziquantel is administered as a paste at a dosage of 1.0–2.5 mg/ kg and has efficacy against the three equine tapeworm species, with efficacy percentages ranging from 90.8% to 100% against A. perfoliata , and 100% for both A. magna and A. mamillana (Rehbein et al., 2007 ; Slocombe et al., 2007 ). Recently, we have reported evidence of anticestodal treatment
failure in a Central Kentucky Thoroughbred farm with faecal egg count data demonstrating reduced efficacy of pyrantel pamoate and praziquantel (Nielsen, 2023 ). The current study took place on a dif- ferent Thoroughbred farm in an adjacent county. In the previously published study, praziquantel only reduced eggs in 3/17 infected horses, while pyrantel pamoate reduced none (Nielsen, 2023 ). Anecdotal evidence has also previously suggested the possibility of pyrantel- resistant tapeworms in Ontario, Canada, but treatment efficacy data was lacking at the time (Peregrine et al., 2008 ). It is worth noting that in other species of platyhelminthes, evidence of praziquantel resistance has been reported in the canine ces- tode Dipylidium caninum (Jesudoss Chelladurai et al., 2018 ) and the
FINNERTY ET AL.
human trematodes Schistosoma spp. (Vale et al., 2017 ). Due to the evidence of drug resistance in these related parasites, it is entirely possible that drug resistance could develop in other platyhelminth species as well. Currently, there are three methods to diagnose equine tape-
worm infection antemortem: faecal egg counting, enzyme- linked immunosorbent assay (ELISA) saliva testing and ELISA serum testing (Lightbody et al., 2016 ; Proudman & Trees, 1996 ). The ELISA serum and saliva tests are not useful for treatment efficacy evaluation because the anticestodal antibodies do not disappear sufficiently quickly, which can create false positives when tapeworm infections have been eliminated (Abbott et al., 2008 ; Lightbody et al., 2016 ). For faecal egg counting, multiple methods have been described. The McMaster technique has been demonstrated to perform with a diagnostic sensitivity of less than 10% for detecting A. perfoliata (Hreinsdóttir et al., 2019 ; Meana et al., 1998 ; Tomczuk et al., 2014 ). Though modified egg counting techniques exist, they lack sensitiv- ity for detecting burdens under 20 worms (Gawor, 1995 ; Gundlach et al., 2003 ; Proudman & Edwards, 1992 ). Despite these limitations, faecal egg counting techniques have been utilised in field efficacy studies for both pyrantel pamoate and praziquantel providing useful information of field efficacy despite the often sporadic and low pos- itive counts (Marchiondo et al., 2006 ; Proudman & Edwards, 1992 ; Slocombe et al., 2007 ). For pyrantel pamoate, horses were adminis- tered the standard 13.2 base mg/kg with a mean tapeworm faecal egg count reduction of around 95% (Marchiondo et al., 2006 ). For praziquantel, a study demonstrated 90.9–100% egg count reduc- tion, and horses remained negative for tapeworm counts at five weeks post- treatment (Slocombe et al., 2007 ). Similarly, in Kentucky, yearlings were treated with ivermectin- praziquantel and had over 90% reduction of anoplocephalid egg counts, with evidence of reemergence not beginning until 4 weeks post- treatment
(Lyons
et al., 2017 ). This current study aimed to describe a case of apparent treat-
ment failure of pyrantel pamoate and praziquantel against equine tapeworms in a group of Thoroughbred yearlings followed over a 6- month period.
ANIMAL USAGE
Twenty- four yearlings from one Thoroughbred breeding farm in Central Kentucky were enrolled in the study after they were found to have evidence of tapeworm eggs 14 days post- treatment with praziquantel (1 mg/kg) and ivermectin (0.2 mg/kg; Zimectrin Gold, Boehringer Ingelheim) in March 2022 following routine measures in place on the farm. There were 16 female yearlings and 8 male yearlings born between 5 February and 9 May 2021. Yearlings were separated by sex, with no more than 8 yearlings per pasture, with pasture rotations occurring every 2–3 months. Historically, this farm has regularly utilised praziquantel and pyrantel pamoate as part of their protocol for tapeworm management in their yearling herds. The horses were followed from March to August 2022 until they
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