Multiple anthelmintic resistance at a goat farm in Slovakia

Anthelmintic resistance (AR) to the main classes of anthelmintics is rapidly increasing in small ruminants at farms throughout the world. Multidrug resistance is common on sheep farms in humid tropical areas of Central and South America where was reported lack of in vivo efficacy for recommended dosages of ivermectin, albendazole and levamisole (Herrera-Manzanilla et al., 2017; Chaparro et al., 2017) or reduced effectiveness of macrocyclic lactones and benzimidazoles (Santiago-Figureoa et al., 2019). In Europe, multidrug resistance has been detected in Haemonchus contortus in The Netherlands (Van den Brom et al., 2013) and France (Cazajous et al., 2018) and in Teladorsagia circumcincta in Ireland (Keegan et al., 2015). Information has been lacking in recent decades about the prevalence of AR in goat herds, because most surveys have focused on sheep. A comparison of parasitism with gastrointestinal nematodes (GINs) found that only 20 – 25 % of studies included goats (Hoste et al., 2010). Cases of AR at goat farms in Europe have been reported from France (Paraud et al., 2009), Italy (Cringoli et al., 2007; Zanzani et al., 2014), Demark (Holm et al., 2014), and Poland (Mickiewicz et al., 2017; 2019). Despite these reports, more surveys need to be conducted at goat farms. The owner of the farm in our study suspected poor efficacy of benzimidazoles (BZs) due to low weight gains and high morbidity of yearlings. The main goal of this study was thus to test the presence of BZ resistance on this farm. Two additional anthelmintics, ivermectin (IVM) and levamisole (LEV) were included to test their efficacies. Another objective was to morphologically identify species of nematodes from larval cultures before and after treatment.

All procedures performed in this study were in accordance with the ethical standards of the Ethics Committee on 20-April-2015, meets the requirements of the Ethics Committee of the Institute of Parasitology of the Slovak Academy of Sciences in accordance with the national legislation in Slovakia - Animal Welfare Act No. 23/2009 and was approved on 1-January-2016.

We confirmed the presence of a multidrug-resistant strain of Teladorsagia and a BZ-resistant strain of H. contortus. The efficacy of the FECRT was lowest (approximately 60 %) in the IVM group, which was also confirmed by LDT, despite the suspicion of the ineffectiveness of BZs. This low efficacy was probably the consequence of frequent use and incorrect dosing for many years and can be expected on most goat farms where similar approaches of farm management have been applied. Nationwide data on the prevalence of IVM resistance in goat herds in Slovakia, however, is still lacking. IVM has been used for decades to prevent and treat ectoparasites and may lead to selection pressure on gastrointestinal parasites that can in turn lead to the development of AR at farms (Cazajous et al., 2018).

The first national survey of goat farms, which focused only on BZs, was not carried out until 2014 – 2016 and detected AR on 30 farms (Babják et al., 2018), long after multidrug-resistant strains were detected in goats imported from New Zealand (Várady et al., 1993). A similar national survey would also be desirable for macrocyclic lactones (MLs) due to the critical situation of the prevalence of BZ resistance. Our results for the IVM group support this statement. Egg output in the in vivo FECRT after treatment and MIC 1.0μg/ ml from LDT indicated the presence of a sensitive GIN population, perhaps because LEV-based products have not been available on the local market for more than two decades and have therefore not been used. The presence of almost 10 % L₃ larvae of H. contortus in the larval cultures for the LEV group after therapy however, indicates the continued monitoring of efficacy. Discriminating dose of 0.5 μg/ml according to Coles et al. (2006) was exceeded, but due to the presence of H. contortus larvae after therapy as well as in the LDT at concentration of 0.5 μg/ml, a threshold used by Taylor et al. (1989) 2.5 μg/ml has been applied. The population would be assessed as susceptible even if we used the threshold 1.0 μg/ ml established by Coles et al. (1988) for Trichostrongylus colubriformis. Despite these differences in thresholds for each species, the value of 1.0 μg/ml can be considered as a borderline between susceptible and resistant population, and it is necessary to control the effectiveness of LEV in the future and use the correct dosing of drug in therapy.

An effective anthelmintic strategy needs the administration of the correct dose of drug tailored to the metabolic and physiological characteristics of each host species. Underdosing is one of the most important factors responsible for accelerating the development of AR (Torres-Acosta & Hoste, 2008). A survey of goat farms conducted in northern Italy found that recommended doses for sheep were applied at six of eight farms tested, and only three farms had an adequate efficacy of ML (Lambertz et al., 2019). Eprinomectin (EPM) is currently the only anthelmintic with no milk-withdrawal period authorized for goats. The development of an optimal pour-on dose of EPM for dairy goats is currently being discussed due to differences in reduction between lactating and dry goats and to strong individual variability with the pour-on formulation (Rostang et al., 2020). The importance of correct dosing was demonstrated by a case in Poland, where EPM resistance developed after two years of use (Mickiewicz et al., 2019). Murri et al. (2014) conducted a survey of 43 goat farms in the canton of Berne, Switzerland, where 39 farms had faecal egg reductions <90 % after the application of the recommended dose of EPM (1 mg/kg bw). More clinical studies are needed to confirm the recommended dose of EPM in goats, especially in countries where similar studies have not been yet performed. A reduced efficacy of EPM can be also expected at Slovak farms, because its mechanism of action is the same as for other MLs.

Compared to sheep, goats require a different approach in drug dosing due to higher metabolic rate. Most of anthelmintics commonly applied on farms still do not have a licensed dose for use in goats (Várady et al., 2011). Differences in drug bioavailability between sheep and goats were demonstrated by Sharma et al. (2014), where treatment with the same doses of LEV and fenbendazole reduced faecal egg counts by 100 % in sheep but only by 82.60 and 78.87 %, respectively, in goats. Using double doses of anthelmintics would be appropriate in goats from the point view of farm management and the prevention of the development and spread of AR. Cringoli et al. (2004) observed significant differences in percentage egg reduction between single (0.5 mg /kg bw) and double dose (1 mg/kg bw) of EPM in naturally infected goats. The survey of 30 goat farms in Slovakia by Babják et al. (2018), however, found that the use of a double dose in the FECRT could underestimate the presence of resistant strains. The dose recommended for sheep for the diagnosis of AR using the in vivo FECRT would therefore be more suitable, which could also indicate the low level of AR at goat farms and for this reason was this approach also applied in the current study.

Faecal examination before treatment and subsequent selective therapy is another approach that is still not applied in many countries. Valcárcel et al. (2015) applied three systems of targeted selective treatment (egg output, clinical signs, and criteria of live weight) for two consecutive years to reduce the frequency of treatment from 37.95 to 100 % without affecting health or production. The approaches of targeted selective therapies are costly but can prevent the development of resistance, which can cause greater production and financial losses. The application of anthelmintics at many farms is often used as the first solution without prior clinical or laboratory diagnosis. Factors such as underdosing, poor estimation of weight, and ignoring differences between sheep and goats were also reported by Mickiewicz et al. (2017) for the first case of BZ resistance in goats in Poland. BZs and MLs in our study were frequently applied more than twice a year. The owner also did not quarantine newly purchased animals, which could serve as sources of resistant parasites, as has been described in several studies (Várady et al., 1993; Himonas & Papadopoulos, 1994; Requejo-Fernandez et al., 1997). All new animals in our study were from Slovakia, so we can expect the presence of multidrug-resistant parasites on several other farms if the animals were sources of AR parasites. This case is an example of how the most common faults in farm management can lead to the development of resistance to multiple anthelmintics.