Anthelmintic resistance in Northern Ireland (II): Variations in nematode control practices between lowland and upland sheep flocks

Veterinary Parasitology 192 (2013) 173–182

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Anthelmintic resistance in Northern Ireland (II): Variations in nematode control practices between lowland and upland sheep flocks C. McMahon a , J.P. Barley b , H.W.J. Edgar b , S.E. Ellison a , R.E.B. Hanna b , F.E. Malone b , G.P. Brennan a , I. Fairweather a,∗ a Parasite Therapeutics Research Group, School of Biological Sciences, Medical Biology Centre, The Queen’s University of Belfast, 97 Lisburn Road, Belfast BT9 7BL, United Kingdom b Veterinary Sciences Division, Agri-Food and Biosciences Institute (AFBI) Stormont, Belfast BT4 3SD, United Kingdom

a r t i c l e

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Article history: Received 25 July 2012 Received in revised form 26 October 2012 Accepted 31 October 2012 Keywords: Questionnaire Anthelmintic resistance Northern Ireland Parasite control Farm management Gastrointestinal nematodes

a b s t r a c t A questionnaire to obtain information on nematode control practices and sheep management was sent to over 1000 farmers in Northern Ireland. Replies were received from 305 flock owners, and data from 252 of them were analysed. Farms were divided into lowland and upland areas. Sizes of pasture and stocking rates on lowland and upland farms were 59.5 hectares, 6.99 sheep/hectare and 62.9 hectares and 10.01 sheep/hectare, respectively. Mean drenching rates for lambs and adults were 2.33 and 2.44, respectively, in lowland flocks and 2.73 and 2.71, respectively, in upland flocks. Between 2008 and 2011, the most frequently identified compounds in use were benzimidazoles and moxidectin in lowland flocks, and benzimidazoles and avermectins in upland flocks. Over the same period the most frequently identified commercial formulations were Tramazole® , Panacur® and Allverm® (white drench), Levacide® (yellow drench), Oramec® (clear drench; avermectin), Cydectin® (clear drench; moxidectin) and Monepantel® (orange drench). Most respondents (56.35%) treated their lambs at weaning and the most common time to treat ewes was identified to be pre-mating (67.86% of respondents). The results of the questionnaire survey revealed that lowland annual drench frequency was 2.33 and 2.44 in lambs and ewes, respectively, although drench frequencies were higher in upland flocks: 2.73 and 2.71 for lambs and ewes, respectively. Annual drench rotation was practiced by 43.96% of flock owners, but whether this was true rotation or pseudo-rotation (i.e., substitution of one anthelmintic product by another product belonging to the same chemical group of anthelmintics) could not be explicitly determined. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Anthelmintic resistance is a well recognised and increasing threat to the production and welfare of grazing sheep throughout the world (Jones et al., 2012; Kaplan and Vidyashankar, 2012). It is no longer uncommon to find sheep farms where resistance exists to all available classes

∗ Corresponding author. Tel.: +44 28 90972298; fax: +44 28 90975877. E-mail address: [email protected] (I. Fairweather). 0304-4017/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vetpar.2012.10.022

of anthelmintic drugs (Bartley et al., 2004, 2005; Howell et al., 2008; Cezar et al., 2010; da Cruz et al., 2010; Sargison et al., 2010). Studies carried out across the Americas (including the Caribbean) have indicated the presence of a severe multiple-drug resistance problem in a number of countries: Argentina (Eddi et al., 1996), Brazil (Sczesny-Moraes et al., 2010), Paraguay (Maciel et al., 1996), Trinidad (George et al., 2011) and Uruguay (Nari et al., 1996). Across Africa, anthelmintic resistance has been reported in at least 14 countries (Kaplan and Vidyashankar, 2012), with the

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majority of these reports emanating from Kenya and South Africa and concerning Haemonchus contortus (Boersema and Pandey, 1997; Van Wyk et al., 1997, 1999, 2006; Waruiru et al., 1997; Vatta and Lindberb, 2006). There have been no published reports of anthelmintic resistance in Australia since 2003–2004 (Besier and Love, 2003; Suter et al., 2004), but it is expected that the prevalence of resistance has worsened considerably (Kaplan and Vidyashankar, 2012). Similarly, while only benzimidazole (BZ), levamisole (LV) and BZ–LV multiple-active formulation drench resistance was present in New Zealand some years ago (McKenna, 1995; McKenna et al., 1995), more recent reports suggest a much graver situation, particularly with respect to the macrocyclic lactones (MLs) (Waghorn et al., 2006; McKenna, 2010). Recent studies continue to describe levels of resistance in (continental) Europe which, although they vary between countries, are generally lower than in most other regions of ˇ nanská ˇ the world (Várady et al., 2006; Artho et al., 2007; Cer et al., 2008; Diez-Banos et al., 2008; Höglund et al., 2009; Paraud et al., 2010). In general, BZ resistance is highly prevalent in some, but not all, countries, the levels of LV and ivermectin (IVM) resistance remain relatively low in most areas, and there are only a few reports of moxidectin (MOX) resistance in sheep nematodes. Resistance to the BZs has been reported many times in Great Britain (GB) (Britt, 1982; Cawthorne and Whitehead, 1983; Cawthorne and Cheong, 1984; Taylor and Hunt, 1989; Scott et al., 1991; Hong et al., 1996; Bartley et al., 2003; Mitchell et al., 2010), as has LV resistance (Taylor and Hunt, 1989; Hong et al., 1996; Coles and Simkins, 1996; Coles et al., 1998). More recently, reports have emerged of IVM resistance (Sargison et al., 2001, 2005; Sutherland et al., 2003; Bartley et al., 2004, 2006; Wilson and Sargison, 2007; Yue et al., 2003) and of MOX resistance (Sargison et al., 2005, 2007, 2010). The situation with regard to anthelmintic resistance in Northern Ireland (NI) is largely unknown, possibly because most farmers have not formally tested to determine the resistance status of nematodes on their farms. Defining the resistance status of nematodes has to be the first logical step towards controlling the problem for any farmer (Pomroy, 2006). Previously, we reported that the level of resistance to various anthelmintic classes is 81% to BZs, 14% to LV, and 50% and 62% to avermectins (AVMs) and milbemycins (MILs), respectively. Trichostrongylus is the dominant genus in post-treatment coprocultures (McMahon et al., in press). In the present paper, we extend the analysis of the results from the FECRT survey and examine data from a questionnaire survey conducted in 2011 (covering the period 2008–2011), to determine the extent to which management practices by NI farmers have led to the levels of resistance reported previously (McMahon et al., in press). Specifically, this study concerns the timing of nematocide treatments in lowland and highland sheep flocks and the general patterns of anthelmintic use by NI sheep farmers. As in the rest of the UK, the NI sheep industry is stratified. Upland (hill) farming is a system found in mountainous areas where sheep have access to wide areas of upland grazing, with few enclosures or fences.

Lowland sheep farmers rear sheep in enclosed pastures, often on fairly fertile ground; this typically represents a more intensive farming system. As a result of these fundamental differences, sheep in lowland areas will face parasite challenges different from those faced by their upland counterparts. This means treatment regimes will vary according to the location of the home farm. These regimes may inadvertently enhance the evolution of anthelmintic resistance according to the intensity of the selection pressure associated with them (Sargison, 2011). The intensity of the selection pressure is influenced by the frequency and timing of anthelmintic treatments and drug efficacy which, in turn, is influenced by dose rate, the drug’s inherent efficacy and its pharmacodynamic profile (Pomroy, 2006; Sargison, 2011). Historic anthelmintic resistance surveys suggest that the prevalence of resistant isolates in sheep is lower in upland areas (Mitchell et al., 1991; Hong et al., 1996). This is most likely a reflection of the upland management systems, lower stocking density and less anthelmintic input (Jackson and Miller, 2006). Currently the Sustainable Control of Parasites in Sheep (SCOPS) guidelines report a higher frequency of BZ resistance across England, Scotland and Wales on lowland properties than on upland properties (Abbott et al., 2009). The present study is the first attempt to determine the difference in frequency of resistance across all drench classes for lowland and upland flocks in NI. 2. Materials and methods 2.1. Faecal egg count reduction test (FECRT) survey data A Province-wide resistance survey was carried out, using the postal service, between May and September 2011. For the purposes of this paper, “Province-wide” is defined as all sheep-producing areas within NI. Full details of farm selection and the results obtained can be found elsewhere (McMahon et al., in press). Briefly, suitable packaging material and postage was sent to participating farmers, along with detailed instructions relating to dosing equipment calibration, drenching protocols and faeces collection. Before any anthelmintic treatment was given, the participants were requested to collect and send 10× faeces samples, one each from ten randomly selected animals from the current year’s lamb crop. A second set of 10× samples from as many of the initial 10 animals as possible were to be collected a certain period of time after treatment (the actual time period being dependent on product selection), followed by immediate submission of samples to the Veterinary Science Division, Agri-Food and Biosciences Institute (AFBI), Stormont. Percentage reductions in faecal egg counts were calculated using the method of Kohapakdee et al. (1995). Results of the postal survey (McMahon et al., in press) were interpreted at the Province-wide level, that is to say, without differentiation into lowland and upland areas. Here, the FECRT survey data are split by location of farm holding to ascertain the degree to which resistance varies between lowland and upland flocks in NI (Table 1). Results from the questionnaire conducted in 2011 (described below) are split along similar lines, allowing determination

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Table 1 Differentiation of the survey results into the percentage (%) of flocks tested showing resistance to each anthelmintic class by locale (lowland or upland and Province-wide). Drench

BZ LV AVM MOX AAD

Farms surveyed (N)

Frequency of detected resistance (%)

Lowland

Upland

Province

Lowland

Upland

Province

19 3 7 15 0

12 5 7 5 3

31 8 14 21 3

95 0 57 47 0

75 20 71 80 0

81 14 50 62 0

AD, aminoacetonitrile derivatives (Monepantel); AVM, avermectin; BZ, benzimidazole; LV, levamisole; MOX, moxidectin; N, number of flocks.

of whether the nematode management practices of flock owners in one geographical division may be more selective for anthelmintic resistance than those practiced by their counterparts in the other division.

situated in lowland and upland areas when comparing the numbers of sheep in various categories reared on these farms, the sizes of pastures and the frequency of deworming treatments given to lambs and adults (ewes).

2.2. Questionnaire

3. Results

2.2.1. Topics covered The Questionnaire comprised 51 questions, split into three main sections: parasite control practices (Section 1), farm management (sheep) (Section 2) and farm management (cattle) (Section 3). Section 1 contained questions relating to product selection, refugia, potential under-dosing, the checking of drenching/dosing equipment, quarantine measures, anecdotal reports of resistance and laboratory-confirmed evidence for resistance. Section 2 contained questions relating to flock size, housing (i.e., enclosing sheep, off pasture, over the Winter months) and lambing times, pasture management, timing the frequency of treatment, and product usage over the period 2008–2011, for both nematodes and liver fluke. Section 3 was as described above, but for cattle herds.

3.1. General

2.2.2. Dissemination Between the months of May and September 2011, the questionnaire was sent to 1000 farmers in NI to collect information concerning the control of gastrointestinal nematodes and liver fluke infections in sheep flocks and cattle herds. In order to encompass as many breed and establishment types as possible, names of respondents were obtained from the following lists: (i) sheep breeders (Texel, Suffolk, Lleyn, Jacob, Zwartbles, Hampshire Down and Charolais); Cattle breeders (Limousin, Blonde, Charollais); (ii) owners who submitted diagnostic samples to the Parasitology Laboratories, Veterinary Sciences Division, AFBI, between 2009 and 2011; (iii) owners who attended “the sustainable control of parasites” talk given by the CAFRE group, 2010.

The questionnaire was disseminated to sheep-only farmers, sheep and cattle farmers, and cattle-only farmers. In total, 305 responses were collected from the 1000 distributed, an average of 50.83 ± 20.54 by county throughout the Province. As selection mechanisms favouring the development of anthelmintic resistance in cattle parasites are not under investigation in this study, the responses from cattle-only farmers were not included in the analyses. After filtering out the cattle-only returns, 252 completed questionnaires were suitable for subsequent analyses, with a higher number of returns from the lowland areas than from the upland areas (149 and 103, respectively). The return rate was also higher from lowland areas than from upland areas (59.13% and 40.87%, respectively). 3.2. Differentiated faecal egg count reduction test (FECRT) survey results by enterprise (lowland or upland) When the results of the Province-wide FECRT survey were divided by enterprise type (lowland or upland), the frequency of detected anthelmintic resistance was higher in flocks managed in upland areas for all anthelmintic groups except the BZs (Table 1). The frequency of detected resistance was 20%, 14% and 33% higher in upland areas for LV, the AVMs and MOX, respectively. Resistance to the amino-acetonitrile derivatives (AD) was not detected in either enterprise type, while BZ resistance was 20% higher in lowland areas. Thus, BZ resistance was more prevalent in lowland areas; LV, AVM and MOX resistance was more prevalent in upland areas and AD resistance probably has not yet developed.

2.3. Statistical analysis

3.3. Analyses of questionnaire responses

Descriptive statistics, such as means and standard deviations, were calculated using Microsoft® Excel 2007. The non-parametric Mann–Whitney U-test (GenStat, 12th edition) was used to determine differences between farms

3.3.1. Stocking density Table 2 shows the mean number of ewes, lambs and rams, the mean size of pasture (hectare) and the mean number of sheep reared per hectare.

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Table 2 The mean numbers (±St. Dev., range) of lambs, ewes and tups; mean sizes (±St. Dev., range) of pastures; and mean numbers (±St. Dev., range) of sheep per hectare on sheep farms situated on predominantly lowland and upland holdings. Farm location

Ewes, mean (St. Dev.)

Lambs, mean (St. Dev.)

Tups, mean (St. Dev.)

Pasture (hectares)

Lowland Range Upland Range Total Range

160.1 (156.12) 4–1000 242.7 (344.2) 2–2700 205.9 (258.1) 2–2700

249.9 (244.2) 5–1200 383.5 (476.7) 4–3400 302.7 (364.0) 4–3400

6.0 (7.8) 0–60 8.5 (8.3) 1–50 6.9 (7.9) 0–60

59.5 (97.3) 0.8–900.0 62.9 (72.1) 4.1–485.6 60.9 (87.9) 0.8–900.0

Sheep/hectare 6.99 10.08 8.45

St. Dev., standard deviation.

The Mann–Whitney U-test (Table 3) was performed to assess the significance of the differences found in stock numbers in various categories, pasture area and stock density in lowland and upland areas. Statistically significant differences (P < 0.05) existed between the number of ewes and the number of rams in lowland and upland areas. 3.3.2. Drench frequency The mean numbers of de-worming treatments given to lambs and ewes in lowland areas, upland areas and Province-wide are shown in Table 4. The Province-wide average was 2.2 (±1.4). The majority of flock owners treated twice annually or less: 61.12%, 70.47% and 47.47%, Province-wide and in lowland and upland areas, respectively. 3.3.3. Treatment timing Table 5 shows the distribution of anthelmintic treatments given to ewes at 6 stated points in the sheep production calendar, together with treatment at sign of disease. Each of the 7 options represents a discrete event in the calendar and so each is considered separately. Treatment given pre-mating and at lambing showed the highest frequencies. The distribution of anthelmintic treatments given to lambs at stated points in the sheep production calendar, together with treatment at sign of disease is also shown in Table 5. 3.3.4. Drench rotation The percentage of the population who said that they rotated drench class on an annual basis was approximately 44% in each geographical category (Table 6). Rotation of drench class with each successive treatment was more commonly practiced by flock owners in upland areas (29.41%, compared with 17.21% of lowland flock owners). Concordantly, a larger percentage of the surveyed population in lowland areas never rotated among drench classes, compared with their upland counterparts. Table 3 Mann–Whitney U-test analyses of the significance of the changes observed between lowland and upland farms. Variable

Lowland vs. upland

Pasture Ewes Lambs Rams

0.26 >0.001 0.03 >0.001

Pasture, pasture area (in hectare); P ≤ 0.05, significant difference.

The use of named commercial preparations is shown in Table 7. In the questionnaire, Tramazole® (Tulivin) was the most frequently identified BZ product, Cydectin® (Pfizer Animal Health) was the most used MIL and Oramec® (Merial Animal Health) and Dectomax® (Elanco Animal Health) were the most frequently used products containing IVM and doramectin (DOR), respectively. LV was used sporadically, and Levacide® (Norbrook Pharmaceuticals) was the most often selected of this class of product. 3.3.5. Drenches administered at lambing BZ products accounted for 22.98% of all anthelmintic treatments given at lambing time, with the combination drench containing mebendazole plus closantel being the largest single contributor (Table 8). Products containing MLs accounted for 64.93% of all treatments, with MOX contributing 22.36% to that (64.93%) total. LV usage was the lowest contributor to the total, at 5.30% of all drenches used; the salicylanide, closantel, with its narrow spectrum of activity against nematode infection, was used with greater frequency. Closantel was included in the total as it may have some therapeutic benefit in the 14% of flocks where Haemonchus spp. was identified (McMahon et al., in press), despite the fact it was most likely employed as a fasciolicide. Other fasciolicides such as triclabendazole and nitroxynil, while identified, were not included in the final totals as they are ineffective in treating gastrointestinal nematode infections. Both BZ and LV use was higher in the lowland areas, while ML use (especially DOR use) was higher in the upland areas. 3.3.6. Total (identified) anthelmintic use in lowland areas Table 9 shows the relative contribution of each of the anthelmintic classes to all anthelmintics used on farms over the period 2008–2011. MOX was identified more commonly than the AVMs in each of the 4 years under investigation. The MLs (MOX plus AVMs) were identified with greater frequency than the BZs each year. LV made the lowest contribution of the three main classes of anthelmintic (available before 2010), with the contribution made to their total by LV in combination rather than LV as a single active increasing between 2008 and 2009 and peaking at 4.27% of the total 14.22% of all products used in 2011. 3.3.7. Total (identified) anthelmintic use in upland areas Among all the anthelmintics identified between 2008 and 2011, upland flock owners identified the BZ class with increasing frequency over this period (Table 10), with the

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Table 4 Analyses of the stated frequencies at which flock owners treat their lambs and ewes in lowland and upland areas, compared with the values found at the Province-wide level. The data is shown as the numbers (N) and percentages (%) of owners who indicate that they treat their animals at said frequency.

Treatment of lambs per year 0 1 2 3 4 5 5+ Average St. Dev. Treatment of ewes per year 0 1 2 3 4 5 5+ Average St. Dev.

Lowland, N (%)

Upland, N (%)

Province, N (%)

10 (6.71) 43 (28.86) 52 (34.90) 14 (9.40) 20 (13.42) 4 (2.68) 6 (4.03) 2.32 1.54

3 (2.91) 21 (20.29) 25 (24.27) 26 (25.24) 16 (15.53) 8 (7.77) 4 (3.88) 2.73 1.44

13 (5.16) 64 (25.40) 77 (30.56) 40 (15.87) 36 (14.29) 12 (4.76) 10 (3.97) 2.21 1.43

8 (5.37) 24 (16.11) 55 (36.91) 32 (21.48) 23 (15.44) 6 (4.03) 1 (0.67) 2.44 1.33

6 (5.83) 9 (8.82) 36 (35.29) 25 (24.51) 16 (15.69) 7 (6.86) 3 (2.94) 2.71 1.54

14 (5.56) 33 (13.15) 91 (36.25) 57 (22.71) 39 (15.54) 13 (5.18) 4 (1.59) 2.59 1.51

St. Dev., standard deviation. Table 5 The relative importance of specific times in the sheep production calendar as target times for chemotherapy of ewes and lambs. Data is shown as the numbers (N) and percentages (%) of owners who indicate that they treat their animals at said times. Farm location

Housing, N (%)

Docking/ hoof-trimming, N (%)

Weaning, N (%)

Turn-out, N (%)

Sign of disease, N (%)

Pre-mating, N (%)

Lambing, N (%)

Ewes Lowland Upland Province

47 (31.54) 30 (29.13) 77 (30.56)

16 (10.74) 11 (10.68) 27 (10.71)

30 (20.13) 22 (21.36) 52 (20.63)

38 (25.50) 31 (30.1) 69 (27.38)

24 (16.11) 24 (23.30) 48 (19.05)

102 (68.46) 69 (66.99) 171 (67.86)

79 (53.02) 52 (50.59) 131 (51.98)

Nematodirus dose, N (%) Lambs Lowland Upland Province

78 (52.34) 48 (46.60) 126 (49.21)

Docking/hoof-trimming, N (%) 22 (14.76) 33 (32.04) 55 (21.43)

Weaning, N (%) 80 (53.69) 63 (61.17) 143 (56.35)

Sign of disease, N (%) 50 (33.57) 37 (35.92) 87 (34.13)

Turn-out, the end of housing and the re-introduction of the animals to pasture.

relative contribution from BZ in combination with fasciolicide decreasing over the same interval. Similarly, overall use of LV increased between 2009 and 2011, although a decrease was noted between 2008 and 2009. The contribution to the total LV use from LV in combination products also increased between 2009 and 2011, following a decrease from 2009. MOX was identified more frequently than the collective AVMs in 2009 and 2010, at the same frequency in 2008 and 9.56% less often in 2011. ADs represented less than 1% of all identified products in the 2 years since their commercial release (2010 and 2011).

3.3.8. Formulations The form of anthelmintic most often selected by the farmers is shown in Table 11. Farmers in lowland areas showed a slightly higher percentage of oral formulation use than their upland counterparts (93.95% and 90.29%, respectively) and, conversely, injectable formulations seemed to be favoured more by upland than lowland flock owners (54.37% and 43.62%, respectively). While not many pour-on formulations are available for sheep in NI, approximately 25% of flock owners in lowland and upland areas indicated their use.

Table 6 Drench rotation practices of flock owners in lowland areas, upland areas and at the Province-wide level. Farm location

Never, N (%)

Each time, N (%)

Each year, N (%)

Longer, N (%)

Lowland (122) Upland (85) Province (207)

40 (32.79) 16 (18.82) 56 (27.05)

20 (17.21) 25 (29.41) 45 (21.26)

54 (44.26) 38 (44.71) 92 (43.96)

7 (5.74) 6 (7.06) 13 (6.28)

N, number of flock owners; %, percentage figure.

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Table 7 Commercial preparations routinely identified as used on sheep farms in NI between 2008 and 2011. Active ingredient

Commercial name

BZ LV AVMs MOX AAD

Albensure, Albex, Ovispec, Tramazolea , Valbazen, Panacura , Fenzol, Parafend, Allverma , Rycoben, Supavermb Chanaverm, Levacidea , Ripercol, Downland Fluke and Wormc , Levafas Diamondc , Nilzana,c , Combinexd Dectomax, Bimectin, Ivomeca , Noromectin, Oramec, Virbamec, Closamectine Cydectina , Cydectin Triclamoxf , Zermex Zolvixa

AD, aminoacetonitrile derivative; AVM, avermectin; BZ, benzimidazole; LV, levamisole; MOX, moxidectin. a Most commonly identified product within category where more than one product was routinely named. b Contains mebendazole plus closantel. c Contains levamisole plus oxyclozanide. d Contains levamisole plus triclabendazole. e Contains ivermectin plus closantel. f Contains moxidectin plus triclabendazole. Table 8 Distribution [number (N) and percentage (%)] of anthelmintics used at lambing time, by active ingredient(s) at area and Province-wide levels. Anthelmintic

Lowland (97), N (%)

Upland (73), N (%)

Province (170), N (%)

BZs BZsa LV LVa AVMs AVMsa MOX MOXa CLOS

7 (7.21) 17 (17.53) 1 (1.03) 5 (5.15) 30 (30.93) 7 (7.22) 20 (20.62) 2 (2.06) 8 (8.25)

7 (9.59) 8 (10.96) 0 (0.00) 3 (4.11) 30 (41.10) 4 (5.48) 16 (21.92) 0 (0.00) 5 (6.85)

14 (8.23) 25 (14.71) 1 (0.59) 8 (5.71) 60 (35.30) 11 (6.47) 36 (21.18) 2 (1.18) 13 (7.65)

AVM, avermectin; BZ, benzimidazole; CLOS, closantel; LV, levamisole; MOX, moxidectin a As part of a combination product.

4. Discussion 4.1. Geographical differences Regional and geographical differences have been found in previous surveys of anthelmintic resistance in mainland Britain. For example, Coles (1997) reported a higher prevalence of BZ resistance on lowland farms compared to upland and hill farms (13, 3 and 2%, respectively), and Hong et al. (1996) highlighted differences in BZ resistance between selected farms from the north-east and the south-west of England, with prevalence of 15 and 44%, respectively. The Wales Worm Watch Project (2011) showed that BZ resistance was present on 83% of farms tested. Higher levels of LV resistance were found in upland areas compared to lowland areas (49% and 32%, respectively). More recently, the prevalence of LV resistance in Wales was also recorded

as higher in upland areas (Mitchell et al., 2010). At the time of writing, no information is available about the current levels of ML resistance in Wales. Observed differences may have been due to a difference in the length of grazing time, to an increased reliance on anthelmintics, to stocking rates and/or to a warmer climate (Hong et al., 1996; Bartley et al., 2003). Analysis of the questionnaire of 2011 allowed us to explore some of these factors and determine how they may relate to the geographical differences in prevalence of resistance revealed in the survey. 4.2. Stocking rate The stocking rate in NI showed significant changes between 2008 and 2011. Thus, there was a reduction in lowland areas in the number of ewes, lambs and rams, and this finding is supported by the annual statistical reports of

Table 9 Distribution of product used among the main anthelmintic categories per year in lowland areas. Anthelmintic

2011 (211), N (%)

2010 (165), N (%)

2009 (140), N (%)

2008 (131), N (%)

BZs BZsa LV LVa AVMs AVMsa MOX AAD

63 (29.86) 24 (11.37) 21 (9.95) 9 (4.27) 35 (16.59) 6 (2.84) 51 (24.17) 2 (0.95)

48 (29.09) 17 (10.30) 12 (7.27) 7 (4.24) 32 (19.39) 5 (3.03) 43 (26.06) 1 (0.61)

39 (27.86) 17 (12.14) 8 (5.71) 1 (0.71) 24 (17.14) 3 (2.14) 48 (34.29) –

35 (26.72) 14 (10.69) 9 (6.87) 1 (0.76) 24 (18.32) 2 (1.53) 46 (35.11) –

AD, aminoacetonitrile derivative; AVM, avermectin; BZ, benzimidazole; LV, levamisole; MOX, moxidectin. a As part of a combination product.

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Table 10 Distribution of product used among the main anthelmintic categories per year in upland areas. Anthelmintic

2011 (136), N (%)

2010 (108), N (%)

2009 (97), N (%)

2008 (91), N (%)

BZs BZsa LV LVa AVMs AVMsa MOX AAD CLOS

32 (23.53) 9 (6.62) 12 (8.82) 6 (4.41) 42 (30.88) 2 (1.47) 31 (22.79) 1 (0.74) 1 (0.74)

21 (19.44) 8 (7.41) 7 (6.48) 5 (4.63) 30 (27.78) 1 (0.93) 35 (32.41) 1 (0.93) 0 (0.00)

19 (19.59) 7 (7.22) 5 (5.15) 2 (2.06) 29 (29.90) 0 (0.00) 35 (36.08) – 0 (0.00)

16 (17.58) 7 (7.69) 7 (7.69) 7 (7.69) 27 (29.67) 0 (0.00) 27 (29.67) – 0 (0.00)

AD, aminoacetonitrile derivative; AVM, avermectin; BZ, benzimidazole; CLOS, closantel; LV, levamisole; MOX, moxidectin. a As part of a combination product.

the Department of Agriculture and Rural Development NI, which indicate that the national flock has decreased in size every year since 1993 (www.dardni.gov.uk). The stocking density in upland areas is 1.43 times higher than in lowland areas (10.01 and 6.99 animals per hectare in upland and lowland areas, respectively). In Scotland, the stocking densities a few years ago were reported to be 11.9 sheep per hectare on lowland farms and 6.1 sheep per hectare on upland farms (Bartley et al., 2003). Stocking densities in the Slovak Republic were reported to be 6.3 sheep per hectare on lowland farms and 3.6 sheep ˇ nanská ˇ per hectare on upland farms (Cer et al., 2006). The effects of stocking rate on anthelmintic resistance (particularly thiabendazole resistance) were investigated by Bartley et al. (2003). It was discovered that within a locality there was limited evidence to suggest that stocking rate influenced resistance directly when compared to other factors, such as breed and the immune status of the animals. It has been suggested that the relative level of acquired immunity to nematode infection interacts with anthelmintic treatment to influence selection for anthelmintic resistance (Waghorn et al., 2010). High stocking density, however, does contribute to pasture degradation and soil erosion in certain marginal areas, as it forces the animal to graze closer to faecal material, which increases the exposure to nematode infections (Thamsborg, 1999). The latter inevitably results in the uptake of higher numbers of infective larvae and therefore a greater level of infection. Consequently, farmers may have a tendency to treat sheep more frequently to avoid high parˇ nanská ˇ asitic infection (Cer et al., 2006). The stocking rate on NI farms suggests that treatment frequencies might be higher than those seen in Scotland and the Slovak Republic, which in turn might drive up the rate of development of anthelmintic resistance. According to SCOPS principles (Abbott et al., 2009), a higher frequency of dosing is likely to

Table 11 Distribution, number (N) and percentage (%) of flock owners identifying preferred delivery method for anthelmintic treatment: either oral drench, injectable endectocide or pour-on product. Farm location

Oral, N (%)

Injection, N (%)

Lowland Upland Province

140 (93.95) 91 (90.29) 231 (91.67)

65 (43.62) 55 (54.37) 120 (47.61)

Pour-on, N (%) 37 (25.50) 29 (28.16) 66 (26.19)

increase the rate of development of anthelmintic resistance in nematode populations. 4.3. Drench frequency Elsewhere in the world, varying drench frequencies have been reported. For example, Coles (1997) reported that lambs and ewes in England were drenched at a frequency of 4.39 and 2.43 treatments per year, respectively. Ewes in Scotland were drenched at a frequency of 2.7 and lambs at a frequency of 3.2 times per year (Bartley et al., 2003). The mean annual rate of drench usage for lambs in Denmark was 1.9 and for ewes 2.3 treatments per year (Maingi et al., 1996). In France, Chartier et al. (1998) stated that the mean frequency of drenching was 5.2 times per year and Papadopoulos et al. (2003) reported that sheep in Greece were drenched one to two times per year. Frequencies of 1.76 and 1.70 in lambs and ewes, respectively, ˇ nanská ˇ have been reported in the Slovak Republic (Cer et al., 2006). In NI, drenching frequency was shown to be 2.21 and 2.59 in lambs and ewes, respectively. This means that in the recent past only lambs in Denmark had a lower treatment frequency than those in NI and frequency of treatments given to ewes is second only to France. 4.4. Timing of treatments – ewes Anthelmintic use at mating (Autumn) showed the highest frequency, but was more common in lowland areas. Any nematode surviving treatment at mating would experience reproductive advantage over a prolonged period, because at this time any reduction in worm burden would be detrimental to the acquired immunity to nematode infection in the ewe, making this both highly selective for anthelmintic resistance and counterproductive in terms of sheep production (Abbott et al., 2009). Therefore, there is a case for limiting any anthelmintic treatments at mating to those individuals requiring treatment based on condition scoring. Untreated adult ewes present a source of unselected parasite genotypes, increasing the size of the population in refugium and thus slowing the development of anthelmintic resistance in most, but not all, parasite species by dilution of the pool of resistant eggs on pasture (Waghorn et al., 2010). The capacity for adult

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ewes to remove more eggs/larvae from pasture than they add through faecal contamination provides a measure of suppression of pasture contamination. This may be of particular benefit for lambs exposed to anthelmintic resistant nematodes (Leathwick et al., 2008). 4.5. Timing of treatment – lambs The percentage of respondents who treat their animals in April for Nematodirus spp. infection was higher in lowland areas (52.54%) than in upland areas (46.60%), which may suggest that the severity of spring nematodirosis is less in upland areas. Treating at sign of disease was a practice adopted by approximately one third of the farmers surveyed in the questionnaire (Table 5). FEC monitoring provides information about the worm infection status of a flock of sheep and can help in the decision-making process about the need for treatment with anthelmintics. That is, if grazing sheep have high FECs, and the faecal samples have been collected appropriately, one can safely assume that worm burdens are high and that treatment is justified (Abbott et al., 2009). Monitoring will also reduce the confusion between nematodosis and coccidiosis on the one hand, and nutritional scouring when animals graze on lush pasture on the other, thus reducing the potential number of treatments necessary, which will reduce the rate of evolution of anthelmintic resistance. 4.5.1. Use of persistent anthelmintics Of all drenches identified as used at lambing, approximately 64% (Table 8) were MLs. The emergence of resistance to the ML anthelmintics is of particular concern because they may become the only useful class of anthelmintic in some UK sheep flocks (Sargison et al., 2005). Factors identified as being contributory to the development of ML (IVM) resistance on farms include the use of long-acting anthelmintic formulations in ewes prelambing, the reduction of unselected parasites in refugia on the farm, the presence of sheep breeds with a high requirement for anthelmintic treatment, and the importation of anthelmintic-resistant parasites with purchased stock (Lawrence et al., 2006). There are anthelmintic products on the UK market with persistent action against nematode species, which contain MOX or closantel (MOX usage and resistance on NI farms will be discussed further in Section 4.6). 4.6. Product use and product rotation Between 2008 and 2011, the ML MOX was the most commonly identified single anthelmintic used in each year (Tables 9 and 10). In NI, avermectin resistance was present in 50% of flocks tested and MIL resistance is at comparable levels (62%) (McMahon et al., in press). It is likely that this almost unprecedented level of resistance to the MLs reflects the on-going heavy use of long-acting ML formulations, particularly those containing MOX. It should also be noted that pour-on formulations of MLs are frequently used in NI to control ectoparasite infestations in sheep, and

they are likely to contribute significantly to the emerging spectrum of ML resistance. In the face of developing resistance, IVM shows lower efficacy than MOX (Craig et al., 1992; Ranjan et al., 2002). This can be ascribed to lower potency and a shorter period of protection (Rendell et al., 2006). Reductions in efficacy of MOX have also been noted in recent years (Sutherland and Leathwick, 1999; Wooster et al., 2001; Tyrrell et al., 2002), as has the capacity for MOX to select for IVM resistance (Molento et al., 1999). It was noted by Rendell et al. (2006) that a long-acting MOX treatment given at lambing or as a first treatment in a summer infection reduced the necessity for a second treatment, which was consistent with the findings of Tengrove (2003). In cases where IVM was used as a first treatment, a high percentage of flocks required follow-up treatments. This was also consistent with the findings of Rhodes et al. (2006) in New Zealand, who demonstrated that the use of long-acting ML products for ≥3 years was associated with an increased likelihood of resistance. The rapid development of ML resistance, associated with the use of MOX and other long-acting formulations, may be a result of “tail-selection” (Sutherland and Scott, 2010). The mechanism of this phenomenon involves a decline in plasma concentration of ML contemporary with a reduction of the in refugia population as time passes. Those larvae surviving the subclinical exposure to MOX would then establish in the lamb, forming a substantial part of the parasite population. Computer modelling of the dynamics of populations of parasites has recently become important in the development of strategies for the use of anthelmintics in livestock (Smith, 1990; Dobson et al., 1996; Kao et al., 2000). Such studies have shown that a greater percentage of the parasite population in refugia critically decreased the selection for anthelmintic resistance, and suggest that the use of anthelmintics should be minimised and restricted to highly effective, short-acting treatments. The high efficacy of MOX led to it being recommended as the ML subclass of choice for delaying the onset of resistance. This recommendation was based on the high potency of this subclass, leading to reduced ‘head’ selection (direct effect at the time of treatment) (Abbott et al., 1995; Dobson et al., 1996; Le Jambre et al., 1999; Sutherland and Leathwick, 1999). Abamectin may pose less risk than either IVM or MOX as it is shorter-acting than MOX and more potent than IVM (Dobson et al., 2001; Hughes et al., 2004; Suter et al., 2005). Unfortunately, at the current time no drenches containing abamectin are available commercially in the UK. Although there is no strong evidence that rotating anthelmintic classes is an effective strategy to delay the appearance of resistance (Sargison, 2008), it is possible that rotation could delay the appearance of resistance on holdings where genes for resistance are absent, or at very low levels. Anthelmintic class rotation should not take precedence over other factors determining product selection in certain cases, in particular for quarantine treatments. Repeated exposure to the same anthelmintic is likely to be highly selective for resistance. There is the possibility that some farmers who indicated that they rotate drench

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class frequently are simply changing products, while the active ingredient remains the same (or belongs to the same anthelmintic class as the product used previously). 4.7. Conclusions Further investigations into the specific practices of flock owners are required. Specifically, there is a need to establish whether or not the current recommendations of the SCOPS working group (Abbott et al., 2009) are being followed and to what extent. This will complete the picture of nematocide use in NI and indicate what changes in farm management practices are necessary to effect real change and slow the development of resistance to new active classes. The implementation of SCOPS recommendations in NI will be the subject of a separate publication. Acknowledgements We wish to thank Desmond Irwin (AFBI) who helped compile the 2011 questionnaire, Sophie Tynan, Maria Guelbenzu, Robert Walker and David McCoubrey (AFBI) who helped disseminate the questionnaire, Mary Devlin and Jennie Finlay from the School of Biological Sciences, QUB, Don Morrow and Steven Johnston, from CAFRE, William Sherrard from Pfizer, and all the farmers who participated in the survey, without whom this work would not have been possible. References Abbott, K.A., Cobb, R.M., Glass, M.H., 1995. Duration of the persistent activity of moxidectin against Haemonchus contortus in sheep. Aust. Vet. J. 72, 408–410. Abbott, K.A., Taylor, M.A., Stubbings, L.A., 2009. Sustainable Control of Parasites in Sheep (SCOPS), A Technical Manual for Veterinary Surgeons and Advisors, 3rd edition. www.nationalsheep.org.uk Artho, R., Schnyder, M., Kohler, L., Torgerson, P.R., Hertzberg, H., 2007. Avermectin resistance in gastrointestinal nematodes of Boer goats and Dorper sheep in Switzerland. Vet. Parasitol. 144, 68–73. Bartley, D.J., Jackson, E., Johnston, K., Coop, R.L., Mitchell, G.B.B., Sales, J., Jackson, F., 2003. A survey of anthelmintic resistant nematode parasites in Scottish sheep flocks. Vet. Parasitol. 117, 61–71. Bartley, D.J., Jackson, F., Jackson, E., Sargison, N., 2004. Characterisation of two triple resistant field isolates of Teladorsagia from Scottish lowland sheep farms. Vet. Parasitol. 123, 189–199. Bartley, D.J., Jackson, E., Sargison, N., Jackson, F., 2005. Further characterisation of a triple resistant field isolate of Teladorsagia from a Scottish lowland sheep farm. Vet. Parasitol. 134, 261–266. Bartley, D.J., Donnan, A.A., Jackson, E., Sargison, N., Mitchell, G.B.B., Jackson, F., 2006. A small scale survey of ivermectin resistance in sheep nematodes using the faecal egg count reduction test on samples collected from Scottish sheep. Vet. Parasitol. 137, 112–118. Besier, R.B., Love, S.C.J., 2003. Anthelmintic resistance in sheep nematodes in Australia: the need for new approaches. Aust. J. Exp. Agric. 43, 1383–1391. Boersema, J.H., Pandey, V.S., 1997. Anthelmintic resistance of trichostrongylids in sheep in the highveld of Zimbabwe. Vet. Parasitol. 68, 383–388. Britt, D.P., 1982. Benzimidazole resistant nematodes in Britain. Vet. Rec. 118, 343–344. Cawthorne, R.J.G., Whitehead, J.D., 1983. Isolation of benzimidazole resistant strains of Ostertagia circumcincta from British sheep. Vet. Rec. 112, 274–277. Cawthorne, R.J.G., Cheong, F.H., 1984. Prevalence of anthelmintic resistant nematodes in sheep in south-east England. Vet. Rec. 114, 562–564. ˇ nanská, ˇ ˇ Cer D., Várady, M., Corba, J., 2006. A survey on anthelmintic resistance in nematode parasites of sheep in the Slovak Republic. Vet. Parasitol. 135, 39–45.

181

ˇ nanská, ˇ ˇ Cer D., Várady, M., Cudekva, P., Corba, J., 2008. Worm control practices on sheep farms in the Slovak Republic. Vet. Parasitol. 154, 270–276. Cezar, A.S., Toscan, G., Camillo, G., Sangioni, L.A., Ribas, H.O., Vogel, F.S.F., 2010. Multiple resistance of gastrointestinal nematodes to nine different drugs in a sheep flock in southern Brazil. Vet. Parasitol. 173, 157–160. Chartier, C., Pors, I., Hubert, J., Rocheteau, D., Benoit, C., Bernard, N., 1998. Prevalence of anthelmintic resistant nematodes in sheep and goats in Western France. Small Rumin. Res. 29, 33–41. Coles, G.C., Simkins, K., 1996. Resistance to levamisole. Vet. Rec. 139, 124. Coles, G.C., 1997. Nematode control practices and anthelmintic resistance on British sheep farms. Vet. Rec. 141, 91–93. Coles, G.C., Rhodes, A.C., Glover, M.G., Preston, G.D., Coles, E.M., 1998. Avoiding introduction of levamisole-resistant nematodes. Vet. Rec. 143, 667. Craig, T.M., Hatfield, T.A., Pankavich, J.A., Wang, G.T., 1992. Efficacy of moxidectin against an ivermectin-resistant strain of Haemonchus contortus in sheep. Vet. Parasitol. 41, 329–333. da Cruz, D.G., da Rocha, L.O., Arruda, S.S., Palieraqui, J.G.B., Cordeiro, R.C., Santos Junior, E., Molento, M.B., de Paula Santos, O.C., 2010. Anthelmintic efficacy and management practices in sheep farms from the state of Rio de Janeiro, Brazil. Vet. Parasitol. 170, 340–343. Diez-Banos, P., Pedreira, J., Sanchez-Andrade, R., Francisco, I., Suarez, J.L., Diaz, P., Panadero, R., Arias, M., Painceira, A., Paz-Silva, A., Morrondo, P., 2008. Field evaluation for anthelmintic-resistant ovine gastrointestinal nematodes by in vitro and in vivo assays. J. Parasitol. 94, 925–928. Dobson, R.J., Besier, R.B., Barnes, E.H., Love, S.C.J., Vizard, A., Bell, K., LeJambre, L.F., 2001. Principles for the use of macrocyclic lactones to minimize selection for resistance. Aust. Vet. J. 79, 756–761. Dobson, R., LeJambre, L., Gill, J., 1996. Management of anthelmintic resistance-inheritance of resistance and selection with persistent drugs. Int. J. Parasitol. 26, 993–1000. Eddi, C., Caracostantogolo, J., Pena, M., Schapiro, J., Marangunich, L., Waller, P.J., Hansen, J.W., 1996. The prevalence of drench resistance in nematode parasites of sheep in southern Latin America: Argentina. Vet. Parasitol. 62, 189–197. George, N., Persad, K., Sagam, R., Offiah, V.N., Adesiyun, A.A., Harewood, W., Lambie, N., Basu, A.K., 2011. Efficacy of commonly used anthelmintics: first report of multiple drug resistance in gastrointestinal nematodes of sheep in Trinidad. Vet. Parasitol. 183, 194–197. Höglund, J., Gustafsson, K., Ljungstrom, B.L., Engstrom, A., Donnan, A., Skuce, P., 2009. Anthelmintic resistance in Swedish sheep flocks based on a comparison of the results from the faecal egg count reduction test and resistant allele frequencies of the ␤-tubulin gene. Vet. Parasitol. 161, 60–68. Hong, C., Hunt, K.R., Coles, G.C., 1996. Occurrence of anthelmintic resistant nematodes on sheep farms in England and goat farms in England and Wales. Vet. Rec. 139, 83–86. Howell, S.B., Burke, J.M., Miller, J.E., Terrill, T.H., Valencia, E., Williams, M.J., Williamson, L.H., Zajac, A.M., Kaplan, R.M., 2008. Prevalence of anthelmintic resistance on sheep and goat farms in the southeastern United States. J. Am. Vet. Med. Assoc. 233, 1913–1919. Hughes, P.L., McKenna, P.B., Murphy, A., 2004. Resistance to moxidectin and abamectin in naturally acquired Ostertagia circumcincta infections in sheep. N. Z. Vet. J. 52, 202–204. Jackson, F., Miller, J., 2006. Alternative approaches to control – quo vadit? Vet. Parasitol. 139, 371–384. Jones, J., Pearson, P., Jeckel, S., 2012. Suspected anthelmintic resistance to macrocyclic lactones in lambs in the UK. Vet. Rec., 59–60. Kao, R.R., Leathwick, D.M., Roberts, M.G., Sutherland, I.A., 2000. Nematode parasites of sheep: a survey of epidemiological parameters and their application in a simple model. Parasitology 121, 85–103. Kaplan, R.M., Vidyashankar, A.N., 2012. An inconvenient truth: global worming and anthelmintic resistance. Vet. Parasitol. 186, 70–78. Kohapakdee, S., Pandey, V.S., Pralomkarn, W., Choldumrongkul, S., Ngampongsai, W., Lawpetchara, S., 1995. Anthelmintic resistance in goats in Southern Thailand. Vet. Rec. 137, 124–125. Lawrence, K.E., Rhodes, A.P., Jackson, R., Leathwick, D.M., Heuer, C., Pomroy, W.E., West, D.M., Waghorn, T.S., Moffat, J.R., 2006. Farm management practices associated with macrocyclic lactone resistance on sheep farms in New Zealand. N. Z. Vet. J. 54, 283–288. Leathwick, D.M., Miller, C.M., Atkinson, D.S., Haack, N.A., Waghorn, T.S., Oliver, A.-M., 2008. Managing anthelmintic resistance: untreated adult ewes as a source of unselected parasites, and their role in reducing parasite populations. N. Z. Vet. J. 56, 184–195. Le Jambre, L.F., Dobson, R.J., Lenane, I.J., Barnes, E.H., 1999. Selection for anthelmintic resistance by macrocyclic lactones in Haemonchus contortus. Int. J. Parasitol. 29, 1101–1111.

182

C. McMahon et al. / Veterinary Parasitology 192 (2013) 173–182

Maciel, S., Gimenez, A.M., Gaona, C., Waller, P.J., Hansen, J.W., 1996. The prevalence of anthelmintic resistance in nematode parasites of sheep in Southern Latin America: Paraguay. Vet. Parasitol. 62, 207–212. Maingi, N., Bjørn, H., Thamsborg, S.M., Dangolla, A., Kyvsgaard, N.C., 1996. Worm control practices on sheep farms in Denmark and implications for the development of anthelmintic resistance. Vet. Parasitol. 66, 39–52. McKenna, P.B., 1995. The identity of nematode genera involved in cases of ovine anthelmintic resistance in the southern north island of NewZealand. N. Z. Vet. J. 43, 225–227. McKenna, P.B., Allan, C.M., Taylor, M.J., Townsend, K.G., 1995. The prevalence of anthelmintic resistance in ovine case submissions to animal health laboratories in New Zealand in 1993. N. Z. Vet. J. 43, 96–98. McKenna, P.B., 2010. Update on the prevalence of anthelmintic resistance in gastrointestinal nematodes of sheep in New Zealand. N. Z. Vet. J. 58, 172–173. McMahon, C., Bartley, D.J., Edgar, H.W.J., Ellison, S.E., Barley, J.P., Malone, F.E., Hanna, R.E.B., Brennan, G.P., Fairweather, I. Anthelmintic resistance in Northern Ireland (I): prevalence of resistance in ovine gastrointestinal nematodes, as determined through faecal egg count reduction testing, in press. Mitchell, G.B., Jackson, F., Coop, R.L., 1991. Anthelmintic resistance in Scotland. Vet. Rec. 129, 58. Mitchell, E.S.E., Hunt, K.R., Wood, R., McLean, B., 2010. Anthelmintic resistance on sheep farms in Wales. Vet. Rec. 166, 650–652. Molento, M.B., Wang, G.T., Prichard, R.K., 1999. Decreased ivermectin and moxidectin sensitivity in Haemonchus contortus selected with moxidectin over 14 generations. Vet. Parasitol. 86, 77–81. Nari, A., Salles, J., Gil, A., Waller, P.J., Hansen, J.W., 1996. The prevalence of anthelmintic resistance in nematode parasites of sheep in Southern Latin America: Uruguay. Vet. Parasitol. 62, 213–219. Papadopoulos, E., Arsenos, G., Sotiraki, S., Deligiannis, C., Lainas, T., Zygoyiannis, D., 2003. The epizootiology of gastrointestinal nematode parasites in Greek dairy breeds of sheep and goats. Small Rumin. Res. 47, 193–202. Paraud, C., Pors, I., Rehby, L., Chartier, C., 2010. Absence of ivermectin resistance in a survey on dairy goat nematodes in France. Parasitol. Res. 106, 1475–1479. Pomroy, W.E., 2006. Anthelmintic resistance in New Zealand: a perspective on recent findings and options for the future. N. Z. Vet. J. 54, 265–270. Ranjan, S., Wang, G.T., Hirschlein, C., Simkins, K.L., 2002. Selection for resistance to macrocyclic lactones by Haemonchus contortus in sheep. Vet. Parasitol. 103, 109–117. Rendell, D.K., Rentsch, T.E., Smith, J.M., Chandler, D.S., Callinan, A.P.L., 2006. Evidence that moxidectin is a greater risk factor than ivermectin in the development of resistance to macrocyclic lactones by Ostertagia spp in sheep in south eastern Australia. N. Z. Vet. J. 54, 313–317. Rhodes, A.P., Leathwick, D.M., Waghorn, T.S., Pomroy, W.E., West, D.M., Jackson, R., Lawrence, K., Moffat, J., 2006. Management of Internal Nematode Parasites on Sheep Farms in New Zealand. Part 2b of a Series. Sustainable Farming Fund, MAF, and Meat and Wool New Zealand, Wellington, New Zealand http://www.meatnz.co.nz/main Sargison, N., Scott, P., Jackson, F., 2001. Multiple anthelmintic resistance in sheep. Vet. Rec. 149, 778–779. Sargison, N.D., Jackson, F., Bartley, D.J., Moir, A., 2005. Failure of moxidectin to control benzimidazole-, levamisole- and ivermectin-resistant Teladorsagia circumcincta in a sheep flock. Vet. Rec. 156, 105–109. Sargison, N.D., Jackson, F., Bartley, D.J., Wilson, D.J., Stenhouse, L.J., Penny, C.D., 2007. Observations on the emergence of multiple anthelmintic resistance in sheep flocks in the south-east of Scotland. Vet. Parasitol. 145, 65–76. Sargison, N., 2008. Sheep Flock Health: A Planned Approach. Blackwell Publishing Ltd., Oxford. Sargison, N.D., Jackson, F., Wilson, D.J., Bartley, D.J., Penny, C.D., Gilleard, J.S., 2010. Characterisation of milbemycin-, avermectin-, imidazothiazole- and benzimidazole-resistant Teladorsagia circumcincta from a sheep flock. Vet. Rec. 166, 681–686. Sargison, N.D., 2011. Pharmaceutical control of endoparasite infections in sheep. Vet. Clin. North. Am. Food Anim. Pract. 27, 139–156. Scott, E.W., Duncan, J.L., McKellar, Q.H., Coop, R.L., Jackson, F., Mitchell, G.B.B., 1991. Benzimidazole resistance in sheep nematodes. Vet. Rec. 128, 618–619.

Sczesny-Moraes, E.A., Bianchin, I., da Silva, K.F., Catto, J.B., Honer, M.R., Paiva, F., 2010. Anthelmintic resistance of gastrointestinal nematodes in sheep, Mato Grosso do Sul, Brazil. Pesqui. Vet. Braz. 30, 229–236. Smith, G., 1990. A mathematical model for the evolution of anthelmintic resistance in a direct life cycle nematode parasite. Int. J. Parasitol. 20, 913–921. Suter, R.J., Besier, R.B., Perkins, N.R., Robertson, I.D., Chapman, H.M., 2004. Sheep farm risk factors for ivermectin resistance in Ostertagia circumcincta in Western Australia. Prev. Vet. Med. 63, 257–269. Suter, R.J., McKinnon, E.J., Perkins, N.R., Besier, R.B., 2005. Effective life of ivermectin on Western Australian sheep farms – a survival analysis. Prev. Vet. Med. 72, 311–315. Sutherland, I.A., Leathwick, D.M., 1999. Moxidectin: persistence and efficacy against drug-resistant Ostertagia circumcincta. J. Vet. Pharmacol. Ther. 22, 2–5. Sutherland, I.A., Brown, A.E., Leathwick, D.M., Bisset, S.A., 2003. Resistance to prophylactic treatment with macrocyclic lactone anthelmintics in Teladorsagia circumcincta. Vet. Parasitol. 115, 301–309. Sutherland, I., Scott, I., 2010. Gastrointestinal Nematodes of Sheep and Cattle, Biology and Control. Blackwell Publishing, Oxford. Taylor, M.A., Hunt, K.R., 1989. Field observations on the control of ovine parasitic gastroenteritis in south-east England. Vet. Rec. 123, 241–245. Tengrove, C.L., 2003. How to determine optimum time for the first summer drench. Australian Veterinary Association Conference of the Sheep Veterinary Society 13, 144–150. Vet. Parasitol. 115, 301–309. Thamsborg, S.M., 1999. Integrated and biological control in organic and conventional production systems. Vet. Parasitol. 84, 169–186. Tyrrell, K.L., Dobson, R.J., Stein, P.A., Walkden-Brown, S.W., 2002. The effects of ivermectin and moxidectin on egg viability and larval development of ivermectin resistant Haemonchus contortus. Vet. Parasitol. 107, 85–93. Van Wyk, J.A., Malan, F.S., Randles, J.L., 1997. How long before resistance makes it impossible to control some field strains of Haemonchus contortus in South Africa with any of the modern anthelmintics? Vet. Parasitol. 70, 111–122. Van Wyk, J.A., Stenson, M.O., Van der Merwe, J.S., Vorster, R.J., Viljoen, P.G., 1999. Anthelmintic resistance in South Africa: surveys indicate an extremely serious situation in sheep and goat farming. Onderstepoort J. Vet. Res. 66, 273–284. Van Wyk, J.A., Hoste, H., Kaplan, R.M., Besier, R.B., 2006. Targeted selective treatment for worm management-how do we sell rational programs to farmers? Vet. Parasitol. 139, 336–346. ˇ nanská, ˇ ˇ Várady, M., Cer D., Corba, J., 2006. Use of two in vitro methods for the detection of anthelmintic resistant nematode parasites on Slovak sheep farms. Vet. Parasitol. 135, 325–331. Vatta, A.F., Lindberb, A.L.E., 2006. Managing anthelmintic resistance in small ruminant livestock of resource-poor farmers in South Africa. J. S. Afr. Vet. Assoc. 77, 2–8. Waghorn, T.S., Leathwick, D.M., Rhodes, A.P., Lawrence, K.E., Jackson, R., Pomroy, W.E., West, D.M., Moffat, J.R., 2006. Prevalence of anthelmintic resistance on sheep farms in New Zealand. N. Z. Vet. J. 54, 271–277. Waghorn, T.S., Oliver, A-M.B., Miller, C.M., Leathwick, D.M., 2010. Acquired immunity to endoparasites in sheep interacts with anthelmintic treatment to influence selection for anthelmintic resistance. N. Z. Vet. J. 58, 98–102. Waruiru, R.M., Weda, E.H., Otieno, R.O., Ngotho, J.W., Bogh, H.O., 1997. Comparative efficacies of closantel, ivermectin, oxfendazole, thiophanate and levamisole against thiabendazole resistant Haemonchus contortus in sheep. Trop. Anim. Health Prod. 28, 216–220. Wilson, D., Sargison, N., 2007. Anthelmintic resistance in Teladorsagia circumcincta in sheep in the UK. Vet. Rec. 161, 535–536. Wooster, M.J., Woodgate, R.G., Chick, B.F., 2001. Reduced efficacy of ivermectin, abamectin and moxidectin against field isolates of Haemonchus contortus. Aust. Vet. J. 79, 840–842. “Wales Worm Watch”, 2011. Wormer Resistance and the Need to Change. Hybu Cig Cymru – Meat Promotion Wales www.hccmpw.org.uk Yue, C., Coles, G., Blake, N., 2003. Multi-resistant nematodes on a Devon farm. Vet. Rec. 153, 604.