This article was downloaded by: [Selcuk Universitesi] On: 06 February 2015, At: 13:23 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
New Zealand Veterinary Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tnzv20
Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand? a
b
I Scott , RM Bishop & WE Pomroy
a
a
Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North, New Zealand b
Vetlife Alexandra, 123 Centennial Avenue, Alexandra 9320, New Zealand Accepted author version posted online: 22 Jan 2015.
Click for updates To cite this article: I Scott, RM Bishop & WE Pomroy (2015): Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand?, New Zealand Veterinary Journal, DOI: 10.1080/00480169.2014.987840 To link to this article: http://dx.doi.org/10.1080/00480169.2014.987840
Disclaimer: This is a version of an unedited manuscript that has been accepted for publication. As a service to authors and researchers we are providing this version of the accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proof will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to this version also.
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Publisher: Taylor & Francis & New Zealand Veterinary Association Journal: New Zealand Veterinary Journal DOI: 10.1080/00480169.2014.987840
Review Article
Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand?
I Scott*§, RM Bishop† and WE Pomroy*
Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
*
Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North, New Zealand † Vetlife Alexandra, 123 Centennial Avenue, Alexandra 9320, New Zealand § Author for correspondence. Email: [email protected]
Abstract There is growing concern that given the high frequency with which anthelmintics are being administered to many horses, anthelmintic resistance amongst equine helminth populations will be an increasing problem, rendering many of the currently available products unusable with little prospect of new products becoming available at least in the near future. Worldwide, much reliance has been placed on the macrocyclic lactone (ML) group of anthelmintics, but resistance has been reported to these products as well as to the two other major anthelmintic classes used in this species, the benzimidazoles (BZ) and the tetrahydropyrimidines (e.g. pyrantel). In New Zealand, resistance has been reported to the ML and BZ groups, but not yet to pyrantel. As an alternative to intervalbased anthelmintic regimens, the highly overdispersed nature of parasite populations in horses can be utilised to decide whether treatment is required, based on whether or not animals exceed a predetermined level of shedding of parasite eggs. If well managed, such a targeted and selective approach can be utilised to eliminate the majority of egg output whilst still providing a refuge for susceptible parasites to persist. Such a system would require that an adequate standard of monitoring be in place and cognisance needs to be taken of parasites or their lifecycle stages that cannot be diagnosed by routine methods. At the same time, using anthelmintics with high levels of efficacy, avoiding practices such as under-dosing, as well as utilising non-chemical means of parasite control when possible, e.g. regular removal of dung from pasture, should all be considered. Combinations of anthelmintics, specifically of anthelmintics that target the same or a similar spectrum of parasite species, should play an important role in parasite control in horses. As well as
1
providing arguably the highest levels of efficacy, combinations may also slow the rate at which anthelmintic resistance develops.
KEY WORDS: Horse, helminth, anthelmintic, resistance
Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
BZ ERP FECRT L3 L4 ML
Benzimidazole Egg reappearance period Faecal egg count reduction test Third-stage larvae Fourth-stage larvae Macrocyclic lactone
Introduction The alimentary tract of New Zealand horses is home to at least 27 species of helminth parasite; 26 nematodes and the one cestode, Anoplocephala perfoliata (McKenna 2009). Of the nematode species, half are small strongyle species (cyathostomins), of which little is known individually. The other nematodes that may be present include the large strongyles such as the highly pathogenic Strongylus vulgaris, the ascarid nematode Parascaris equorum more typically found in foals, and the pinworm, Oxyuris equi. Strongylus vulgaris was long considered the most important helminth pathogen of horses (Drudge 1979) although ill health due to this parasite has probably been overstated (Kaplan and Nielsen 2010). In the 1980s, the advent of the macrocyclic lactone (ML) anthelmintics allowed much better control of S. vulgaris, especially its larval stages, and thereafter its importance declined dramatically. Concomitant with this decline, the harmful effects of other parasites such as the cyathostomins and A. perfoliata became more appreciated. The cyathostomins are now recognised as the principle helminth pathogens of horses (Love et al. 1999), in terms of prevalence across age groups, proportion of the total helminth population present in animals, as well as in clinical importance. Cyathostomin infections are often not associated with marked adverse effects, but the syndrome of larval cyathostominosis is recognised as a significant cause of mortality for the minority of horses that become afflicted (Love et al. 1999). In total, 40 separate species of cyathostomin spread across 14 genera have been recorded as parasitic in the horse (Lichtenfels et al. 2008). Consideration of such a diverse number of species as one group rather than individually undoubtedly has its advantages, but may ultimately mask much 2
of the complexity and inherent variability of such a large group. These organisms differ in size (Lichtenfels et al. 2008) and this may relate strongly to many aspects of their biology such as pathogenicity and female fecundity (Kuzmina et al. 2012). These organisms vary in their pre-patent periods, from 5–6 weeks to more than twice that (Ogbourne 1978), and will also probably vary in their susceptibility to anthelmintics and the rate at which anthelmintic resistance develops. An additional difficulty of working with this group of parasites in horses is that faecal egg counts, which tend to be dominated by the eggs of cyathostomins, bear a poor relationship to actual worm burden and certainly none to the numbers of larval stages present (Nielsen et al. 2010). The tapeworm A. perfoliata has also been linked to equine disease, with some studies showing particular associations to spasmodic colic, impaction and intussusception (Owen et al. 1989; Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
Proudman et al. 1998; Back et al. 2013), but at least one other study failed to detect any risk (TrotzWilliams et al. 2008). In the study of Back et al. (2013), horses with colic were 16 times more likely to have tapeworm eggs in their faeces than control horses, but there was no association detectable when infection was diagnosed using a serological test. It is worth pointing out that younger horses are often considered more likely to suffer ill health due to parasitism than more mature animals. Larval cyathostominosis tends to occur in animals 99.9%, consistent with full efficacy for this drug (Kaplan et al. 2004). When efficacy has declined completely, there is probably little need for a statistician, as egg counts will remain largely unchanged and may even rise following treatment. There is however greater difficulty in interpreting efficacy figures that are more equivocal. Several authors have examined the utility of different statistical models for analysing FECRT data (Denwood et al. 2010; Vidyashankar et al. 2012). In particular focusing on providing better confidence intervals for the observed efficacy, but it is perhaps unlikely that many would apply such approaches in the field, unless they could be provided in the form of a spreadsheet that simply needed the egg count data to be entered. There are, however, some things that could be easily done to improve the reliability of FECRT. These include increasing sample sizes to include as many positive animals as possible, collecting more than one sample pre- and post-treatment, and using an egg counting method capable of detecting lower egg counts. The current American Association of Equine Practitioners guidelines for conducting FECRT in horses recommend that at least six horses be included, selecting those with the highest egg counts . The egg counting method used should also be capable of detecting counts of 25 epg or less 7
(Anonymous 2013; Lyons and Tolliver 2013). With many egg counting methods used for ruminants, each egg counted in the technique equates to a count of 50 epg, but it is generally technically straightforward to adapt these methods to detect an egg count of 99% efficacy of ivermectin against adult and L4 O. equi, respectively. This was based on actual worm counts of naturally infected horses and efficacy was calculated using geometric means. Those authors included the worm burdens of the individual horses in their paper, thus allowing calculation of efficacy using arithmetic means, resulting in values of 62% for adults and 90% for L4, suggesting that a problem may well be present after all. Following reports of suspected pinworm resistance in two horses in New Zealand, the horses were treated with either ivermectin or abamectin (both at 200 µg/kg), and their faeces collected for the next 48 hours to allow the number of killed parasites shed in faeces to be counted. A week after ML treatment the horses were retreated with oxfendazole (at 10 mg/kg) and the faecal collection repeated. One horse passed no O. equi after ivermectin and five after treatment with oxfendazole, whilst the other horse passed nine after abamectin and 11 after oxfendazole (Rock et al. 2013). Whilst limited in the number of animals and in methodology, these results suggest that MLresistance in O. equi may also be present in New Zealand.
What should be done? Things to avoid; practices that may promote resistance
Resistance can be seen as the inevitable consequence of anthelmintic use. Treating animals imposes just another evolutionary bottleneck, through which it would be foolhardy to believe that any 11
parasite could not eventually emerge. A treatment interval of 6–8 weeks is very close to and in some cases shorter than the minimum pre-patent periods of the cyathostomins, and is well within the longer pre-patent periods of P. equorum and the large strongyles such as S. vulgaris. Thus under these circumstances, the only parasites to ever lay eggs are much more likely to be resistant. The obvious answer to this is to reduce anthelmintic use in horses and thus prolong the interval between treatments. Another factor that might accelerate the development of resistance is underdosing. Whilst visual estimation of the weight of horses can prove remarkably accurate (Reavell 1999) there is clearly scope for weight to be underestimated by owners or practitioners and for underdosing to occur. Likewise, off-label use of anthelmintics, through unpredictable pharmacology, may also result in Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
exposure of parasites to less than adequate drug concentrations. Underdosing may be important in promoting the development of resistance by allowing even partially resistant worms to survive treatment whilst still killing susceptible ones (Smith et al. 1999). Indeed, underdosing was used experimentally to generate ivermectin-resistance in the cattle nematode Ostertagia ostertagi (Van Zeveren et al. 2007). The use of persistent drugs has been seen as a factor in the development of resistance in the nematodes of ruminants (Sutherland et al. 1997) and this may also be important in horses (Sangster 1999; Schumacher and Taintor 2008). Moxidectin is detectable in the plasma of treated horses for at least 75 days (Perez et al. 1999). For some of this time incoming resistant larvae may preferentially be able to establish whilst susceptible ones are still excluded, so-called tail selection. Likewise, tail selection may also be applied to cyathostomin larvae that were inhibited at the time the horse was treated, were not killed at that time, but later resume development and advance to a stage when they become vulnerable to the anthelmintic. Cyathostomin stages encysted in the mucosa include third-stage larvae (L3), which can be classed as either early or late L3, and L4. The late L3 and L4 are regarded as larvae that are developing normally, whilst early L3 are often identified as being inhibited in development, although some early L3 may be recently acquired and developing normally. In many horses, mucosal stages outnumber the luminal stages (emerged L4 and adults) many times over (Bairden et al. 2001, 2006). Ivermectin has little (zero) efficacy against any mucosal stages (Monahan et al. 1996). Thus in ivermectin-treated animals there is a significant additional refuge of parasites that will not be selected by treatment. Moxidectin has greater efficacy against mucosal stages (Bairden et al. 2001, 2006), and thus use of moxidectin can be expected to reduce the magnitude of this population in refugia posing an additional selection pressure on cyathostomins. Whether the potential for 12
selection for resistance posed by the persistence of moxidectin is offset by the greater interval permitted between treatments has not been investigated. It is worth pointing out that there has been some debate over the exact efficacy of moxidectin against inhibited cyathostomin larvae. Many of the early studies examined efficacy perhaps too early for killed larvae to have been eliminated from their tissue site (Bairden et al. 2006) thus underestimating efficacy. The two studies that showed better efficacy (>90%), delayed post-mortem examination to 8 weeks after treatment (Bairden et al. 2001, 2006). Nevertheless one of these studies (Bairden et al. 2006) showed that the efficacy against early L3 calculated on the basis of geometric means was indeed high (92.2%), but when arithmetic means were used efficacy was only 55%, showing that a reasonable number of inhibited larvae may remain in some animals after Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
treatment. There is one major facet of parasite control largely unique to horses that could theoretically have an impact on the development of resistance, the removal of dung from pastures. The impact of this practice has not yet been investigated, but, if done thoroughly enough, the numbers of eggs and larvae in the environment should be dramatically diminished, meaning that a greater proportion of the total parasite-population could reside inside the animals. A smaller parasite population on pasture may well be compensated for by the smaller population this generates inside the animals, thus not accelerating resistance at all. The reduced burdens should be reason enough to cut back on anthelmintic use, however there is evidence from overseas that many establishments continue to treat animals regularly despite removing dung (Lloyd et al. 2000; Relf et al. 2012). What must be considered a very real threat for many horse owners is the ease with which resistant nematodes may be and probably have been transported between properties. Movement of animals may allow the transport of resistant genotypes from one property to the next. Whilst the concept of quarantine treatment may be gaining traction in the sheep industry it is questionable, given the difficulty of killing encysted cyathostomin stages, whether a true quarantine treatment of horses is even feasible. Reducing anthelmintic use in horses
Reliance on frequent anthelmintic use in all horses is believed to be unsustainable as well as being inappropriate so perhaps the simplest way to reduce anthelmintic use is to take advantage of the over-dispersed distribution of parasite populations. There is evidence that many horses are treated when their egg counts are low if not zero. In the work of Kaplan et al. (2004), 31% of the overall population, with individual egg counts of ≥500 epg, were responsible for 88% of the total egg output. More than 33% of the horses sampled had counts of 20 epg or less. In similar findings, 13
Kaplan and Nielsen (2010) presented data from 261 horses across 12 farms and showed that using a cut-off for treatment of 200 epg would result in only 45% of horses needing treatment, but total egg output would be reduced by 96%. The eggs released by the untreated animals (now 98% of total egg output) would then dilute the eggs continuing to be released by the treated animals (now only 2%) that were more likely to be from resistant parasites. In the context of providing a population in refugia, Kaplan and Nielsen (2010) also pointed out that the efficacy of the anthelmintic being used was important. The above changes occurred when anthelmintic efficacy was 99.9%, but if efficacy was reduced to 90%, treated horses would continue to release the majority of the egg output (now 69%).
Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
A question that is frequently asked is “what is an appropriate cut-off value for egg counts in horses?” In general egg counts in horses bear a poor relationship to actual worm burden, but low counts (95
90–95
90
85–90
98
95–98
New Zealand Veterinary Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tnzv20
Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand? a
b
I Scott , RM Bishop & WE Pomroy
a
a
Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North, New Zealand b
Vetlife Alexandra, 123 Centennial Avenue, Alexandra 9320, New Zealand Accepted author version posted online: 22 Jan 2015.
Click for updates To cite this article: I Scott, RM Bishop & WE Pomroy (2015): Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand?, New Zealand Veterinary Journal, DOI: 10.1080/00480169.2014.987840 To link to this article: http://dx.doi.org/10.1080/00480169.2014.987840
Disclaimer: This is a version of an unedited manuscript that has been accepted for publication. As a service to authors and researchers we are providing this version of the accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proof will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to this version also.
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Publisher: Taylor & Francis & New Zealand Veterinary Association Journal: New Zealand Veterinary Journal DOI: 10.1080/00480169.2014.987840
Review Article
Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand?
I Scott*§, RM Bishop† and WE Pomroy*
Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
*
Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North, New Zealand † Vetlife Alexandra, 123 Centennial Avenue, Alexandra 9320, New Zealand § Author for correspondence. Email: [email protected]
Abstract There is growing concern that given the high frequency with which anthelmintics are being administered to many horses, anthelmintic resistance amongst equine helminth populations will be an increasing problem, rendering many of the currently available products unusable with little prospect of new products becoming available at least in the near future. Worldwide, much reliance has been placed on the macrocyclic lactone (ML) group of anthelmintics, but resistance has been reported to these products as well as to the two other major anthelmintic classes used in this species, the benzimidazoles (BZ) and the tetrahydropyrimidines (e.g. pyrantel). In New Zealand, resistance has been reported to the ML and BZ groups, but not yet to pyrantel. As an alternative to intervalbased anthelmintic regimens, the highly overdispersed nature of parasite populations in horses can be utilised to decide whether treatment is required, based on whether or not animals exceed a predetermined level of shedding of parasite eggs. If well managed, such a targeted and selective approach can be utilised to eliminate the majority of egg output whilst still providing a refuge for susceptible parasites to persist. Such a system would require that an adequate standard of monitoring be in place and cognisance needs to be taken of parasites or their lifecycle stages that cannot be diagnosed by routine methods. At the same time, using anthelmintics with high levels of efficacy, avoiding practices such as under-dosing, as well as utilising non-chemical means of parasite control when possible, e.g. regular removal of dung from pasture, should all be considered. Combinations of anthelmintics, specifically of anthelmintics that target the same or a similar spectrum of parasite species, should play an important role in parasite control in horses. As well as
1
providing arguably the highest levels of efficacy, combinations may also slow the rate at which anthelmintic resistance develops.
KEY WORDS: Horse, helminth, anthelmintic, resistance
Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
BZ ERP FECRT L3 L4 ML
Benzimidazole Egg reappearance period Faecal egg count reduction test Third-stage larvae Fourth-stage larvae Macrocyclic lactone
Introduction The alimentary tract of New Zealand horses is home to at least 27 species of helminth parasite; 26 nematodes and the one cestode, Anoplocephala perfoliata (McKenna 2009). Of the nematode species, half are small strongyle species (cyathostomins), of which little is known individually. The other nematodes that may be present include the large strongyles such as the highly pathogenic Strongylus vulgaris, the ascarid nematode Parascaris equorum more typically found in foals, and the pinworm, Oxyuris equi. Strongylus vulgaris was long considered the most important helminth pathogen of horses (Drudge 1979) although ill health due to this parasite has probably been overstated (Kaplan and Nielsen 2010). In the 1980s, the advent of the macrocyclic lactone (ML) anthelmintics allowed much better control of S. vulgaris, especially its larval stages, and thereafter its importance declined dramatically. Concomitant with this decline, the harmful effects of other parasites such as the cyathostomins and A. perfoliata became more appreciated. The cyathostomins are now recognised as the principle helminth pathogens of horses (Love et al. 1999), in terms of prevalence across age groups, proportion of the total helminth population present in animals, as well as in clinical importance. Cyathostomin infections are often not associated with marked adverse effects, but the syndrome of larval cyathostominosis is recognised as a significant cause of mortality for the minority of horses that become afflicted (Love et al. 1999). In total, 40 separate species of cyathostomin spread across 14 genera have been recorded as parasitic in the horse (Lichtenfels et al. 2008). Consideration of such a diverse number of species as one group rather than individually undoubtedly has its advantages, but may ultimately mask much 2
of the complexity and inherent variability of such a large group. These organisms differ in size (Lichtenfels et al. 2008) and this may relate strongly to many aspects of their biology such as pathogenicity and female fecundity (Kuzmina et al. 2012). These organisms vary in their pre-patent periods, from 5–6 weeks to more than twice that (Ogbourne 1978), and will also probably vary in their susceptibility to anthelmintics and the rate at which anthelmintic resistance develops. An additional difficulty of working with this group of parasites in horses is that faecal egg counts, which tend to be dominated by the eggs of cyathostomins, bear a poor relationship to actual worm burden and certainly none to the numbers of larval stages present (Nielsen et al. 2010). The tapeworm A. perfoliata has also been linked to equine disease, with some studies showing particular associations to spasmodic colic, impaction and intussusception (Owen et al. 1989; Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
Proudman et al. 1998; Back et al. 2013), but at least one other study failed to detect any risk (TrotzWilliams et al. 2008). In the study of Back et al. (2013), horses with colic were 16 times more likely to have tapeworm eggs in their faeces than control horses, but there was no association detectable when infection was diagnosed using a serological test. It is worth pointing out that younger horses are often considered more likely to suffer ill health due to parasitism than more mature animals. Larval cyathostominosis tends to occur in animals 99.9%, consistent with full efficacy for this drug (Kaplan et al. 2004). When efficacy has declined completely, there is probably little need for a statistician, as egg counts will remain largely unchanged and may even rise following treatment. There is however greater difficulty in interpreting efficacy figures that are more equivocal. Several authors have examined the utility of different statistical models for analysing FECRT data (Denwood et al. 2010; Vidyashankar et al. 2012). In particular focusing on providing better confidence intervals for the observed efficacy, but it is perhaps unlikely that many would apply such approaches in the field, unless they could be provided in the form of a spreadsheet that simply needed the egg count data to be entered. There are, however, some things that could be easily done to improve the reliability of FECRT. These include increasing sample sizes to include as many positive animals as possible, collecting more than one sample pre- and post-treatment, and using an egg counting method capable of detecting lower egg counts. The current American Association of Equine Practitioners guidelines for conducting FECRT in horses recommend that at least six horses be included, selecting those with the highest egg counts . The egg counting method used should also be capable of detecting counts of 25 epg or less 7
(Anonymous 2013; Lyons and Tolliver 2013). With many egg counting methods used for ruminants, each egg counted in the technique equates to a count of 50 epg, but it is generally technically straightforward to adapt these methods to detect an egg count of 99% efficacy of ivermectin against adult and L4 O. equi, respectively. This was based on actual worm counts of naturally infected horses and efficacy was calculated using geometric means. Those authors included the worm burdens of the individual horses in their paper, thus allowing calculation of efficacy using arithmetic means, resulting in values of 62% for adults and 90% for L4, suggesting that a problem may well be present after all. Following reports of suspected pinworm resistance in two horses in New Zealand, the horses were treated with either ivermectin or abamectin (both at 200 µg/kg), and their faeces collected for the next 48 hours to allow the number of killed parasites shed in faeces to be counted. A week after ML treatment the horses were retreated with oxfendazole (at 10 mg/kg) and the faecal collection repeated. One horse passed no O. equi after ivermectin and five after treatment with oxfendazole, whilst the other horse passed nine after abamectin and 11 after oxfendazole (Rock et al. 2013). Whilst limited in the number of animals and in methodology, these results suggest that MLresistance in O. equi may also be present in New Zealand.
What should be done? Things to avoid; practices that may promote resistance
Resistance can be seen as the inevitable consequence of anthelmintic use. Treating animals imposes just another evolutionary bottleneck, through which it would be foolhardy to believe that any 11
parasite could not eventually emerge. A treatment interval of 6–8 weeks is very close to and in some cases shorter than the minimum pre-patent periods of the cyathostomins, and is well within the longer pre-patent periods of P. equorum and the large strongyles such as S. vulgaris. Thus under these circumstances, the only parasites to ever lay eggs are much more likely to be resistant. The obvious answer to this is to reduce anthelmintic use in horses and thus prolong the interval between treatments. Another factor that might accelerate the development of resistance is underdosing. Whilst visual estimation of the weight of horses can prove remarkably accurate (Reavell 1999) there is clearly scope for weight to be underestimated by owners or practitioners and for underdosing to occur. Likewise, off-label use of anthelmintics, through unpredictable pharmacology, may also result in Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
exposure of parasites to less than adequate drug concentrations. Underdosing may be important in promoting the development of resistance by allowing even partially resistant worms to survive treatment whilst still killing susceptible ones (Smith et al. 1999). Indeed, underdosing was used experimentally to generate ivermectin-resistance in the cattle nematode Ostertagia ostertagi (Van Zeveren et al. 2007). The use of persistent drugs has been seen as a factor in the development of resistance in the nematodes of ruminants (Sutherland et al. 1997) and this may also be important in horses (Sangster 1999; Schumacher and Taintor 2008). Moxidectin is detectable in the plasma of treated horses for at least 75 days (Perez et al. 1999). For some of this time incoming resistant larvae may preferentially be able to establish whilst susceptible ones are still excluded, so-called tail selection. Likewise, tail selection may also be applied to cyathostomin larvae that were inhibited at the time the horse was treated, were not killed at that time, but later resume development and advance to a stage when they become vulnerable to the anthelmintic. Cyathostomin stages encysted in the mucosa include third-stage larvae (L3), which can be classed as either early or late L3, and L4. The late L3 and L4 are regarded as larvae that are developing normally, whilst early L3 are often identified as being inhibited in development, although some early L3 may be recently acquired and developing normally. In many horses, mucosal stages outnumber the luminal stages (emerged L4 and adults) many times over (Bairden et al. 2001, 2006). Ivermectin has little (zero) efficacy against any mucosal stages (Monahan et al. 1996). Thus in ivermectin-treated animals there is a significant additional refuge of parasites that will not be selected by treatment. Moxidectin has greater efficacy against mucosal stages (Bairden et al. 2001, 2006), and thus use of moxidectin can be expected to reduce the magnitude of this population in refugia posing an additional selection pressure on cyathostomins. Whether the potential for 12
selection for resistance posed by the persistence of moxidectin is offset by the greater interval permitted between treatments has not been investigated. It is worth pointing out that there has been some debate over the exact efficacy of moxidectin against inhibited cyathostomin larvae. Many of the early studies examined efficacy perhaps too early for killed larvae to have been eliminated from their tissue site (Bairden et al. 2006) thus underestimating efficacy. The two studies that showed better efficacy (>90%), delayed post-mortem examination to 8 weeks after treatment (Bairden et al. 2001, 2006). Nevertheless one of these studies (Bairden et al. 2006) showed that the efficacy against early L3 calculated on the basis of geometric means was indeed high (92.2%), but when arithmetic means were used efficacy was only 55%, showing that a reasonable number of inhibited larvae may remain in some animals after Downloaded by [Selcuk Universitesi] at 13:23 06 February 2015
treatment. There is one major facet of parasite control largely unique to horses that could theoretically have an impact on the development of resistance, the removal of dung from pastures. The impact of this practice has not yet been investigated, but, if done thoroughly enough, the numbers of eggs and larvae in the environment should be dramatically diminished, meaning that a greater proportion of the total parasite-population could reside inside the animals. A smaller parasite population on pasture may well be compensated for by the smaller population this generates inside the animals, thus not accelerating resistance at all. The reduced burdens should be reason enough to cut back on anthelmintic use, however there is evidence from overseas that many establishments continue to treat animals regularly despite removing dung (Lloyd et al. 2000; Relf et al. 2012). What must be considered a very real threat for many horse owners is the ease with which resistant nematodes may be and probably have been transported between properties. Movement of animals may allow the transport of resistant genotypes from one property to the next. Whilst the concept of quarantine treatment may be gaining traction in the sheep industry it is questionable, given the difficulty of killing encysted cyathostomin stages, whether a true quarantine treatment of horses is even feasible. Reducing anthelmintic use in horses
Reliance on frequent anthelmintic use in all horses is believed to be unsustainable as well as being inappropriate so perhaps the simplest way to reduce anthelmintic use is to take advantage of the over-dispersed distribution of parasite populations. There is evidence that many horses are treated when their egg counts are low if not zero. In the work of Kaplan et al. (2004), 31% of the overall population, with individual egg counts of ≥500 epg, were responsible for 88% of the total egg output. More than 33% of the horses sampled had counts of 20 epg or less. In similar findings, 13
Kaplan and Nielsen (2010) presented data from 261 horses across 12 farms and showed that using a cut-off for treatment of 200 epg would result in only 45% of horses needing treatment, but total egg output would be reduced by 96%. The eggs released by the untreated animals (now 98% of total egg output) would then dilute the eggs continuing to be released by the treated animals (now only 2%) that were more likely to be from resistant parasites. In the context of providing a population in refugia, Kaplan and Nielsen (2010) also pointed out that the efficacy of the anthelmintic being used was important. The above changes occurred when anthelmintic efficacy was 99.9%, but if efficacy was reduced to 90%, treated horses would continue to release the majority of the egg output (now 69%).
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A question that is frequently asked is “what is an appropriate cut-off value for egg counts in horses?” In general egg counts in horses bear a poor relationship to actual worm burden, but low counts (95
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