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Grazing horses in intensive management face pastures contaminated with infective stages of a number of endoparasites on a daily basis. The management of equine premises as well as the veterinarians have in the past heavily relied on readily available and affordable drugs to treat infestations with parasitic helminths. Due to this fact, the prevalence of the formerly very common Large Strongyles has declined, and the Cyathostomins (CYA) have become the main intestinal parasites of the horse. Their distribution is worldwide, and they carry a high potential of clinical disease. Traditionally, control programmes have been based on anthelmintic drugs, to which CYA are becoming increasingly resistant. Three different drug classes with different modes of action are commercially available, the benzimidazoles (BZ), the tetrahydropyrimidines and the macrocyclic lactones (ML). Resistance of CYA has been detected to all three classes. The objective of this study was to investigate the efficacy of two commonly used anthelmintic drugs, in the German Federal State of Brandenburg: Ivermectin (IVM), a ML, and Pyrantel embonate (PYR), a tetrahydropyrimidine. The efficacy of both drugs was investigated with faecal egg count reduction tests (FECRTs) in two consecutive years. For IVM, the aim was also to detect a possible shortening of the egg reappearance period (ERP), which is considered a first warning sign for the development of resistance. A pre-selected population of horses kept on 24 farms scattered over the Federal State of Brandenburg was considered. These farms had shown a FEC for CYA higher than the average in a previous study, the horses had daily access to pasture or were kept in the field during the summer months. Premises that use anthelmintics infrequently, i.e. twice per year or less often (n=7), were compared with premises that worm their horses frequently, i.e. at least four times per year (n=17), since a risk factor associated with the development of resistance to anthelmintics is the frequent application of these drugs. In 2007, a FECRT with IVM was conducted in late summer/autumn on 23 of these 24 premises, with a total of 428 horses. In 2008, a FECRT with PYR was performed during the summer months on 21 of these 24 premises, and 414 horses were included. Faecal samples were analysed using a modified McMaster technique with a detection threshold of 25 eggs per gram (EPG). The weight of each animal was estimated, and a girth measuring tape was used to validate the estimation. The anthelmintic drugs were then applied according to weight. Horses of all ages with the exception of foals younger than six months were included in the study.
The first FECRT (with IVM) was calculated in analogy to the recommendations of the WAAVP, which for cattle considers a FECR of <95% with a 95% LCL of <90% as ”Resistant”, and a result where only one of the two criteria is met as “Suspect resistant” (Coles et al. 1992). These recommendations propose a value of <90% as indicative of resistance to BZ in horses but give no value for the 95% LCL. Hence, in line with other recent equine AR-studies (Lester et al. 2013; Relf et al. 2014) the cut off was set at 90% FECR, and the 95% LCL was set at 80% in this study. The LCL was calculated according to WAAVP recommendations.
For IVM, a FECR of 100% can be reported for 21 out of 23 farms (91.3%). On the remaining two farms (8.7%), one individual horse each had a faecal egg count (FEC) of 25 EPG, lowering total FECR to 99.73% and 98.33%, respectively. Anthelmintic drugs were used infrequently on both farms. A shortening of the ERP for IVM was not detected.
Controversy exists among parasitologists concerning the calculation and interpretation of FECRTs, with regards to a) at what % of FECR the threshold for resistance should be set, and b), as to which method of calculation should be used. In this study, resistance was defined (i) in analogy to the recommendations for BZ by the WAAVP (mean FECR < 90%), with a lower 95% confidence limit (LCL) of <80% (Lester et al. 2013; Relf et al. 2014); and (ii) with the criteria as above, but also including the upper 95% limit (UCL) – the cut-off was set at UCL <95%, as suggested by Lyndal-Murphy et al. In order to assess the impact of the statistical method used, a total of three methods for the calculation of anthelmintic resistance were applied to the data sets of the FECRT for PYR:
Firstly, the method for calculating FECRTs as currently recommended by the WAAVP was used, and resistance to PYR was detected on four farms (19%). Secondly, a bootstrapping method was employed together with four different equations for the calculation of FECRTs. These four different equations gave varied results: with equation 1, a FECR <90% was obtained on seven farms, with equations 2 and 3 on four farms, and with equation 4 on one farm. Thirdly, a Markov Chain Monte Carlo (MCMC) method was applied to the data. Resistance was detected on 17 farms (73.9%). This accounts for the WAAVP criteria of interpretation. When applying the criteria suggested by Lyndal-Murphy et al., only Farm N° 2 was detected as “Resistant”, by all three methods of calculation.
There was huge variation in inference between statistical methods when using the WAAVP interpretation, but when using the Lyndal-Murphy interpretation all three statistical methods produced very similar recommendations in that there was conclusive evidence for resistance on yard No 2, but none of the other yards.
Of the four farms on which resistance was detected according to the method of calculation as recommended by the WAAVP, two use anthelmintics frequently, and two infrequently. The hypothesis that frequent worming might enhance the development of resistance was therefore not confirmed. However, it was not possible to determine the frequencies in which each drug class had been used in the past, as the horse owners generally did not keep exact records. It is recommended that on these four farms routine FECRTs should be performed to monitor anthelmintic efficacy.
A general agreement needs to be established on the calculation of FECRTs, and thresholds for the detection of resistance need to be created for each drug or drug class. Reports of resistance of equine helminths to all classes of anthelmintics available from all over the world have brought to attention non-chemical approaches to nematode control, as the reliance on anthelmintic treatments alone is not considered sustainable for parasite control in the future. New drug classes are momentarily not expected on the market, as costs for the development of a new drug are high, and the pharmaceutical industry does not regard the market for horses and livestock large enough as to invest in research for new anthelmintic drugs. This implies that the preservation of anthelmintic efficacy has to be of priority. However, it is necessary to involve the expertise of the veterinarian in the adoption of new parasite control programmes, as epidemiological principles need to be incorporated for sustainable control.