ANTHELMINTIC RESISTANCE IN EQUINE WORMS

Transcription

1 Vet Times The website for the veterinary profession ANTHELMINTIC RESISTANCE IN EQUINE WORMS Author : Gerald coles Categories : Vets Date : December 28, 2009 Gerald coles explains how the very real prospect of anthelmintic resistance could prove to have dire consequences for dealing with worms REGULAR use of any chemical to kill infective organisms almost inevitably results in the development of drug resistance; you will be familiar with antibacterial resistance. Resistance is also developing in equine nematodes, and we could shortly have no anthelmintics left for two of the most important groups of worms. Because of their value, thoroughbreds are dosed frequently, and this group of animals are most at risk from anthelmintic-resistant nematodes. However, other horse owners should be aware of the problems and take the necessary actions to slow the development of resistance. Present situation Parascaris equorum is a large roundworm found in foals. Moxidectin is not licensed for use in foals. Resistance to a macrocyclic lactone (ML), ivermectin, is common in the USA (Kaplan R, personal communication) and it has also been reported in the UK1 (and unpublished), but we do not know how widespread it has become. It must not be assumed treatment with ivermectin will keep foals clear of infection. In the USA2, Brazil3 and Japan (Tsuda and Coles, submitted), resistance to pyrantel has also been found, and the reduced efficacy of benzimidazoles has been reported in the USA2. 1 / 10

2 Multiple resistance could easily develop in the UK. Since heavy P equorum infections can kill foals, it is essential that owners who are breeding foals are aware of the need to know the resistance status of P equorum in their animals. Large strongyles Large strongyles are now relatively rare, except in poorly maintained animals and in wild ponies. The only reduced efficacy we have found is to pyrantel in Strongylus edentatus4. Cyathostomins (small strongyles) Cyathostomins are recognised as the most important horse nematodes. A survey in the UK and Europe, sponsored by Fort Dodge5, showed that benzimidazole resistance was present on 14 out of 17 yards (82 per cent) and pyrantel resistance on four of 22 (18 per cent). However, of the greatest concern was the finding of ivermectin resistance in two out of 22 (nine per cent) using egg counts two weeks after treatment. It has been suggested that resistance has begun, based on sensitive faecal egg counts four weeks after treatment with ivermectin6. In The Donkey Sanctuary, there is moxidectin resistance in cyathostomins. This is a closed system and does not pose a risk to horse owners, but it does illustrate what can happen. In Brazil, resistance has been reported to all anthelmintic groups3. Triple resistance (benzimidazole/ pyrantel/ml) is developing in the UK, and this could soon mean there will be nothing left to treat horses. This poses a serious risk to equine health and welfare. Other parasites There have been no reports of resistance in bots or tapeworms, although it has been suggested Anoplocephala perfoliata had changed its predeliction site as a way of avoiding the action of pyrantel7. How far this could show as poor efficacy is not known. The situation with pin worms, Oxyuris equi, is not clear. Managing resistance. Obviously, the control of worms has to be changed to slow the development of resistance and, as I see it, the role of veterinary surgeons is to be proactive in trying to reduce the development and spread of anthelmintic resistance. How often do you discuss worm control and anthelmintic resistance when you visit horse yards? Refugia s role 2 / 10

3 It is now recognised that the most important way of slowing the development of resistance is to leave some worms in refugia (in effect, unexposed to an anthelmintic)8. As ivermectin does not kill inhibited cyathostomins, this effectively means populations of inhibited worms are in refugia, so at least in theory resistance should develop more slowly than with moxidectin, which does remove inhibited larvae. In practice, leaving worms in refugia usually means leaving some animals untreated so the next generation of worms comes from the untreated worms. Put bluntly, the habit of treating all animals regularly has to stop, and drug companies should no longer be promoting this method of worm control. Studies in Denmark9 have shown that horses prone to worms tend to remain with them from year to year, while other horses have few worms. Horses with worms need to be identified and provided with concentrated treatment. In Denmark, anthelmintics are only available by prescription and can only be prescribed after diagnosis. This has reduced the sales of equine anthelmintics, but increased veterinary income from diagnosis10. It is now generally recommended that horses should only be treated when faecal egg counts reach 200epg, although more research is required on this rather arbitrary value. Remember that young animals (up to three years) may not have developed immunity, so they need more frequent attention and possibly treatment. The best way of counting nematode eggs for deciding on whether treatment is required is the FECPAK system (sensitive to 25epg), which is more accurate than the McMaster technique (this was developed for sheep nematodes, not horses)11. The FECPAK system can be bought commercially from Innovis and, with a little education, can be used by yard staff. Failing that, the University of Bristol offers FECPAK tests. Testing for resistance Every yard or horse farm should know which anthelmintics are effective on their yard by running an annual faecal egg count reduction test. This means an egg count at treatment, correct administration of the anthelmintic to be tested and egg counts 14 days later. This will show whether there is obvious anthelmintic resistance. A more sensitive test that will give early warning of resistance is to run egg counts using the FLOTAC technique, a new and improved counting method developed in Italy that is accurate to 1.0epg12. Used at one week after treatment with pyrantel, two weeks after treatment with benzimidazoles and four weeks after treatment with ivermectin, it will detect resistance before it becomes a practical problem, giving time to alter the way worms are controlled on the yard or farm. The cells for the FLOTAC method are not sold on the open market, and tests using FLOTAC are 3 / 10

4 only available commercially in the UK from Langford Veterinary Services. At Bristol, we shall be using results from the testing to monitor how resistance is developing in both P equorum and cyathostomins in the UK. Resistance management Know what anthelmintics are effective in the yard or farm and look out for the development of ivermectin/moxidectin resistance in P equorum and cyathostomins. Yards should be tested annually for resistance. Use effective products when required (200epg), and not on a regular basis. Since ivermectin does not kill inhibited larvae, it will leave some worms untreated. Therefore, resistance should develop more slowly than if moxidectin is used for every treatment. Since moxidectin is valuable because of its activity against inhibited larvae, it is best confined to use for an autumn or early winter treatment. The movement of animals is probably the most important cause of resistant worm spread. There is a problem with treating incoming horses because of the difficulty of removing all inhibited larvae. Test incoming horses for ML resistance by running FLOTAC egg counts four weeks after treatment with ivermectin (not moxidectin). Keep the horses separately until the results are known. If ML resistance is present, either graze the horses permanently on a separate paddock or, if possible, do not keep the animals. Use biological methods of control. Pick up faeces twice weekly in summer and once weekly during the winter. Graze sheep to clean the pasture, either with or after the horses. Do not graze foals on the same pasture each year, and ensure stable hygiene is good to reduce the risks of infection with P equorum. Mares can be dosed before giving birth, but the anthelmintic used must be known to be effective against the large roundworm. If possible, do not breed from wormy horses. New anthelmintics Two new drugs being developed for sheep are monepantel and a combination of derquantel and abamectin. I do not know whether either will be marketed for horses, so for the sake of horse health and welfare, we have a duty to try to maintain the efficacy of existing anthelmintics, particularly ivermectin and moxidectin. For queries about anthelmintic-resistant worms in horses, telephone Gerald Coles on or him on gerald.c.coles@bristol.ac.uk 4 / 10

5 To discuss this article with other veterinary professionals on dedicated interactive forums, or to download published Veterinary Times articles, visit References 1. Stoneham S and Coles G (2006). Ivermectin resistance in Parascaris equorum, Veterinary Record 158: Lyons E T, Tolliver S C, Ionita M and Collins S S (2008). Evaluation of parasiticidal activity of fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate in horse foals with emphasis on ascarids (Parascaris equorum) in field studies on five farms in central Kentucky, Parasitology Research 103: Molento M B, Antunes J, Bentenes R N and Coles G C (2008). Anthelmintic-resistant nematodes in Brazilian horses, Veterinary Record 162: Comer C, Hillyer M H and Coles G C (2005). Anthelmintic use and anthelmintic resistance on thoroughbred training yards in the UK, Veterinary Record 158: Traversa D, von Samson-Himmelstjerna G et al (2009). Anthelmintic resistance in cyathostomin populations from horse yards in Italy, United Kingdom and Germany, Parasites and Vectors 2(Suppl 2): Dudeney A, Campbell C and Coles G C (2008). Macrocyclic lactone resistance in cyathostomins, Veterinary Record 163: Yue C, Coles G C and Lawrence S (2003). Changing behaviour in the common horse tapeworm, Veterinary Record 153: Coles G C (2002). Sustainable use of anthelmintics in grazing animals, Veterinary Record 151: Nielsen M K, Haaning N and Olsen S N (2006). Strongyle egg shedding consistency in horses on farms using selective therapy in Denmark, Vet Parasitol 135: Nielsen M K (2009). Restrictions of anthelmintic usage: perspectives and potential consequences, Parasites and Vectors 2(Supplement 2): Presland S L, Morgan E R and Coles G C (2005). Counting nematode eggs in equine faecal samples, Veterinary Record 156: Cringoli G (2006). FLOTAC, a novel apparatus for a multivalent faecal egg count technique, Parassitologia 48: / 10

6 Owners of all horse types should be aware of the dangers of resistance. Photo: SXC/JUSTYNA FURMANCZYK. 6 / 10

7 The FLOTAC cell is used for sensitive egg counts. 7 / 10

8 Parascaris equorum found in a foal s small intestine. 8 / 10

9 Examples of adult cyathostomins (small strongyles) on a large intestinal ulcer. 9 / 10

10 Cyathostomins are recognised as the most important horse nematodes. Photo: SXC? HAZEL MOORE. 10 / 10 Powered by TCPDF (