TREATMENT FAILURE EQUINE TAPEWORMS
went to the September and October sales. Faecal collections from March to June 2022 were done 14 days post- anthelmintic treatment. Also, for July and August, smaller groups of horses had faeces col- lected prior to and 14 days post- anthelmintic treatment to determine tapeworm faecal egg count reduction. Six horses were sampled in July; 4 females and 2 males and 14 were sampled in August; 7 fe- males and 7 males. All yearlings were monitored for general health by an on- site veterinarian for the duration of the study.
ANTHELMINTIC ADMINISTRATION
Yearlings enrolled in the study were weighed by the on- site vet- erinarian using a livestock scale for horses. Anthelmintics were administered by the farm veterinarian, following the standard dosing protocols for that anthelmintic according to the yearling ' s weight. Three anthelmintic products were used in the study; (1) A combination of ivermectin (0.2 mg/kg) and praziquantel (1 mg/kg; Zimectrin Gold, Boehringer Ingelheim), (2) A combination of mox- idectin (0.4 mg/kg) and praziquantel (2.5 mg/kg; Quest Plus, Zoetis) and (3) Pyrantel pamoate (13.2 mg base/kg; Strongid P, Zoetis). Praziquantel/ivermectin was administered in March 2022 and April 2022. Moxidectin/praziquantel was administered in May 2022, July 2022 and August 2022, and pyrantel pamoate was administered in June 2022. Fresh faeces were collected from stalls, placed in sepa- rate containers and stored refrigerated until laboratory analysis. Samples were analysed within one week of collection.
FAECAL EGG COUNTING
All faecal egg counting was done at the University of Kentucky ' s Maxwell H. Gluck Equine Research Center using the Mini- FLOTAC method of egg suspension and collection, with a multiplication fac- tor of 5 and glucose- NaCl solution as the flotation medium at 1.24– 1.26 specific gravity (Noel et al., 2017 ).
RESULTS
Results from the post- treatment samples analysed during March– June are presented in Table 1 . Out of 24 yearlings sampled from March 2022 to June 2022, 19.8% were tapeworm egg count posi- tive, and only 9/24 (37.5%) of foals remained egg count negative for the entirety of the study. In March, 11/24 (45.8%) were posi- tive at 2 weeks post- ivermectin/praziquantel treatment with posi- tive counts ranging from 5 to 10 eggs per gram (EPG). In April, 3/24 (12.5%) were positive in the post- ivermectin/praziquantel treatment samples with counts in the 5–15 EPG range. Two of 24 yearlings (8.3%) were egg count positive following treatment with moxidectin/ praziquantel, and 3/24 remained positive following pyrantel pamo- ate administration. Two horses were positive on two post- treatment occasions, while one horse was positive on three samples.
DISCUSSION
This study suggested treatment failure of both praziquantel and py- rantel pamoate against anoplocephalids in Thoroughbred yearlings. The data were collected over multiple treatment timepoints with yearlings being anoplocephalid egg count positive pre- and post- treatment. Additionally, some yearlings that initially were negative for tapeworm eggs became positive at subsequent post- treatment time points and some maintained tapeworm egg positivity across multiple timepoints. These data contrast with previously published work that suggested that praziquantel and pyrantel pamoate were effective with faecal egg count reductions in the 90–100% range (Lyons et al., 2017 ; Marchiondo et al., 2006 ; Slocombe et al., 2007 ). The data presented herein suggest that neither praziquantel nor py- rantel pamoate were fully effective against equine anoplocephalids. Although the Mini- FLOTAC egg counting technique employed
herein is demonstrated to perform with high accuracy and preci- sion for equine strongylid egg counts (Noel et al., 2017 ), it should be acknowledged that it has not been validated for detection of anoplocephalid eggs. However, the diagnostic sensitivity of fae- cal egg counting techniques is generally low for detecting equine tapeworm eggs (Gawor, 1995 ; Gundlach et al., 2003 ; Hreinsdóttir et al., 2019 ; Meana et al., 1998 ; Proudman & Edwards, 1992 ; Tomczuk et al., 2014 ), this is less of a concern when testing for ev- idence of tapeworm clearance following anthelmintic treatment. A low diagnostic sensitivity may result in a high proportion of false negative samples post- treatment, which would, in theory, provide false evidence of cestode clearance. Considering this, the number of positive post- treatment samples in this study was remarkable, and it is possible that several of the egg count negative horses may have harboured tapeworm infections as well. More work with refined egg counting techniques is warranted to further investigate and quantify anticestodal treatment efficacy in equine establishments around the world. One limitation of this study was the lack of pre- treatment egg
counts for the months of March to June. This was due to the na- ture of using client- owned animals, as it was initially suspected that the tapeworm egg persistence was due to faulty anthelmintics or improper administration, so administration of
| 581 Results from the tapeworm egg counts determined pre- and
post- administration of moxidectin/praziquantel are presented in Table 2 . For the July treatment, 2/6 horses (33.3%) were tapeworm positive, with one being positive both pre- and post- treatment and the other being positive in the post- treatment sample only. For the August treatment, four of 14 horses (28.6%) were tapeworm egg count positive pre- and post- treatment, and one horse was positive in the post- treatment sample only. Overall, 6/11 (54.5%) of horses were egg count positive in August, with egg counts in the 5–25 EPG range. All horses remained healthy for the duration of the study.
different antices-
todal anthelmintic products was suggested, with researchers per- forming the egg counts via Mini- FLOTAC to rule out the potential
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