In the meantime you might like to have a read of some coursework I've written about wriggly red worms and a predatory fungus that eats them (or maybe not, if it's teatime).
Could a predatory fungus be the solution to anthemlintic resistance in small redworms in horses?
A big issue for veterinary medicine is the growing resistance to antiparasitic drugs in parasites of livestock. In the horse industry specifically resistance of small redworms to currently available classes of anthelmintic drugs is of great concern [1].
Over the last four decades there has been a change in the helminth considered to be the most important in horses. Widespread use of the highly effective anthelmintics that became available in the late 1960s [2] has resulted in Strongylus vulgaris (2-5cm large redworms [3]) no longer being the main parasite of concern. The Cyathostominae group (cyathostomins / small strongyles, <1.5cm small redworms [3]) are now the most common pathogenic endoparasite in the domestic horse. Resistance has been detected in this group of nematodes worldwide.
Anthelmintic resistance is a problem because horses treated with currently available worming preparations can still harbour harmfully high worm burdens. This can result in severe pathological consequences, such as chronic weight loss, diarrhoea, and death [4], with potentially high financial as well as emotional costs to owners.
The variety of branded worming products on the market is misleading. There are actually only three chemical classes of wormers available for use against Cyathostominae in horses, shown in table 1. This maps some of the branded products on the market to the class of drug to which they belong [5] [6] [7].
| Chemical class | Drug names | When introduced | Brand names |
| Benzimidazoles | Thiabendazole Fenbendazone Mebendazone Oxibendazole | Early 1960s | PanacurTM TelminTM EquitacTM |
| Nicotinic agonists (tetrahydropyrimidines, imidazothiaoles ) | pyrantel tartrate Pyrantel embonate | Late 1960s | PyratapePTM Strongid-P TM |
| Macrocyclic lactones (avermectins, milbemycins) | Ivermectin | Early 1980s | EqvalanTM, VectinTM, FurexelTM, PanomecTM, EquimaxTM, EraquellTM, EquestTM, IvomecTM, EprinexTM |
| and | Moxidectin | 1997 | CydectinTM QuestTM |
Table 1: classes of anthelmintic currently in use against cyathostominae in horses in the UK
The widespread, long-term misuse of these anthelmintics has placed a large selection pressure on the Cyathostominae populations [2]. Some worms, through random mutation, will have alleles that give them anthelmintic resistance. This reproductive advantage will allow the resistant worms to survive worming treatments and reproduce, passing on their advantageous alleles to their offspring. This results in an increase in allele frequency of these advantageous alleles in the gene pool of the treated population. If the selection pressure continues, the proportion of resistant individuals in the population increases over time – meaning that worming treatments become less and less effective.
The summary data below for UK yards is taken from a large European study [8] done in 2008 to evaluate effectiveness of wormers currently in use. The data show that Fenbendazole, in the oldest class of wormer, had the least reduction in eggs in faeces after treatment, with the majority of results falling below a 90% reduction. This indicates resistance in the cyathostomin population to this class of wormer at these yards. (In some yards this wormer was not tested, accounting for the blank results). The data show more success for the next-oldest class, the nicotinic agonist Pyrantel pamoate, but a less than 90% efficacy here in several yards indicates resistant populations at some yards and suspected resistance in yards with reductions of between 90 and 100%. The newer macrocyclic lactones have the greatest efficacy, but some resistance is suspected in one yard, and indicated in two yards. Only Moxidectin, the newest drug, showed 100% effectiveness in this survey.
This data illustrates the development of resistance over time – the oldest drugs are now the least effective, since the worms have had that selection pressure exerted for longer, and more time to adapt. The same pattern is evident in Italy, Germany, USA and other countries [1][8][12]. It is only a matter of time before the worm populations adapt further to become resistant to the macrocyclic lactones too. New classes of drugs are currently being researched [10][11] but there are no accurate estimates yet for when they could be market-ready. The eventual introduction of new drugs will in any case merely postpone the inevitable resistance of the parasites to anthelmintics [2].
There are potential solutions other than the development of new anthelmintic drugs. Research is ongoing into vaccinations against endoparasites, although with little success to date [11]. Development and maintenance of refugia through more judicious use of anthelmintics is a recognised way to preserve efficacy of existing drugs for a time. Refugia can be described as “the maintenance of worm populations that have not been exposed to a particular drug and hence still contain a large proportion of worms that are not resistant” [13]. A third approach, in my opinion more promising if widely adopted, is the use of a predatory fungus with nematocidal properties, currently undergoing long-term laboratory and field research in horses and other livestock[12][13] [14] [15][16][24].
An understanding of the lifecycle of the cyathostomins has allowed scientists to target the parasites at the infective stage of development using a biological method of control: a fungus that can trap and kill nematodes on pasture. This is a diversion from the typical strategy of attacking parasites inside the host.
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| Figure 1: Lifecycle of cyathostomins |
The lifecycle can be interrupted at the infective L3 stage using the predatory fungus, Duddingtonia flagrans. This fungus has been found naturally occurring in horse faeces [17] where it can attack and kill the free-living larval stages of nematodes, reducing the number of infective larvae on the pasture [18]. The chlamydospores (thick-walled, resistant spores) of the fungus are ingested by the host animal along with normal feed. They have no effect on the animal, and are passed out unharmed in the faeces. Once outside they develop into ‘nematode-trapping fungal nets’ [18] in which the worm larvae become ensnared, killed and digested by the fungus. This approach has been shown to greatly reduce the numbers of cyathostomin eggs found in subsequent faecal egg counts (FECs) of livestock fed the fungus compared with controls [15]. Trials using horses [12], cattle [15], and small ruminants [16] as hosts have had similarly positive results: reducing the number of larvae ingested by the animals grazing the pasture leads to a reduction in the worm burden in the animals, quantified by the reduction in faecal egg counts after treatment.
One advantage of this approach is that it is a highly effective method of reducing faecal egg counts when compared with the efficacy of current drug treatments. Another advantage is being able to use a natural substance as a worm treatment instead of exposing horses and handlers to chemicals. Horses are sometimes farmed for human consumption, and this treatment is likely to be approved for use in organic production units. The fungus is also effective against other parasitic nematodes in other livestock [15][20], so the investment in research and development will yield rewards across different sectors of the farming industry.
Research is ongoing into the potential environmental and ecological impact of increasing the abundance of D. Flagrans in pasture habitats, with results indicating “no detectable negative environmental impacts of D. flagrans use in a typical improved pasture” [19]. The dung beetle and the earthworm are both important factors in the normal break-down of manure on pasture. Effective levels of D. flagrans in the animal faeces have been shown to have no detrimental effect on dung beetle populations [20]. The fungus remains contained within the area of the droppings, not spreading onto surrounding soil. Earthworm populations in the surrounding soil have also been studied and no harmful effects determined [19].
| Figure 2: trapping mechanisms of nematophagal fungi |
At present the most significant disadvantage of this approach is that the treatment is still in the research phase and is not yet commercially viable. As yet it is not possible for horse owners to implement a treatment program using D. flagrans, and it is likely to be some years before commercial products based on current research reach the market.
The other solution to be discussed here, of maintaining refugia, requires re-training for both equine vets and owners in order to bring about a shift in behaviour. The following traits of a traditional worming program have favoured the development of resistance:
- Interval dosing programs where all horses are wormed regardless of worm burden to the same schedule, e.g. every 6-8 weeks
- Too rapid rotation of the class of anthelmintic drug used
- Under dosing due to underestimation of horses’ weights
- No veterinary involvement in worming program design – anthelmintics are freely available without prescription
A major advantage of this approach to solving the problem of resistance is that the horse industry can continue using existing treatments for some time. Maintaining a large unresistant proportion of the cyathostomin population will ensure that the classes of wormer available today will remain effective for longer than if resistance is allowed to grow unchecked.
An advantage for owners is lower expenditure on wormers, from treating only those horses with high enough worm burdens to be potentially hazardous to their health or performance. However, a disadvantage is the added cost of regular FEC testing. Currently costs vary from around £7-£13 per test, recommended quarterly. If tests prove that horses need treatment each time then the cost of testing is not countered by lower worming costs. Owners may opt to continue their previous regular worming instead to minimise potential costs.
There are also potentially higher financial costs to yards due to more complex and labour-intensive grazing management practices. The removal of droppings from all grazing is recommended at least weekly, a practice not yet common in all yards. This is an essential part of the strategy for maintaining refugia.
There are parallels between the development of anthelmintic resistance in parasites to the resistance in bacterial to antibiotics, which has lead to the evolution of multi-drug-resistant ‘superbugs’ such as MRSA due to overuse of antibiotics in humans [23]. The growing public awareness of the problem of antibiotic resistance will hopefully hasten acceptance of the idea of anthelmintic resistance, and the adoption of new worming practices across the horse industry.
An approach to fighting anthelmintic resistance where success depends on changing people’s habits and long-held beliefs about worming makes this a risky approach, in my opinion. I think that restricting anthelmintic availability to prescribed use only, as antibiotics largely are in humans, would help to enforce BEVA’s guidelines and preserve the efficacy of the currently available drugs for longer.
I believe that without this control the change in behaviour in the horse industry is likely to be too slow and resistance will continue to grow. The consequence could be a lack of reliably effective treatment for cyathostomins in as soon as a few decades if resistance grows at the rate it has done since the introduction of the benzimidazoles. My preference therefore is for the introduction of treatments based on naturally nematocidal fungi for cyathostomin control in horses as soon as possible. Developing this method of treatment alongside the current behavioural changes will ensure that there is a sustainable and successful treatment available whether the chemotherapeutic approach fails or not.
References
1 Anthelmintic resistance in nematodes of horses Author: Ray M. Kaplan Publication: Veterinary Research, 10/2002, Vol. 33, no. 5,Page 491-507 Publisher: INRA, EDP Sciences,
2 http://www.sheepandgoat.com/articles/anthelminticswork.html Accessed: 12 March 2011 14:37 PM
3 Pathology and parasitology for veterinary technicians, Volume 1 Author: Leland Shapiro Chapter: Endoparasites of Large Animals, Page 183 Publisher: Cengage Learning, Inc
4 Recent developments in research into the Cyathostominae and Anoplocephala perfoliata Authors: Jacqueline B. Matthews, Jane E. Hodgkinson, Samantha M.J. Dowdall and Christopher J. Proudman Publication: Veterinary Research, volume 35 number 4, 2004, pages 371-381
5 http://www.surreycc.gov.uk/sccwebsite/sccwspages.nsf/LookupWebPagesByTITLE_RTF/Worming+advice?opendocument Accessed: 11 March 2011, 11:42 PM
6 Leaflet “Confused About Worming?” Publisher: The Ashbrook Equine Hospital, Knutsford, WA16 9JG
7 http://www.aber.ac.uk/~mpgwww/Edu/AntiDrug/DrugTxt.html
Accessed: 13 March 2011 17:39 PM
8, 9 Anthelmintic resistance in cyathostomin populations from horse yards in Italy, United Kingdom and Germany Authors: Donato Traversa, Georg von Samson Himmelstjerna, Janina Demeler, et al.Piermarino Milillo, Sandra Schurmann, Helen Barnes, Domenico Otranto, Stefania Perrucci, Antonio di Regalbono, Paola Beraldo, Albert Boeckh, Rami Cobb Publication: Parasites & Vectors, vol. 2, 2009, Supplement 2
10 Leaflet: Guidance on the Use of Anthelmintics Publisher: British Veterinary Association leaflet
11 http://www.aber.ac.uk/~mpgwww/Edu/Vaccine/VaccTxt.html Accessed: 13 March 2011 17:42 PM
12 Biological control of cyathostomin (Nematoda: Cyathostominae) with nematophagous fungus Monacrosporium thaumasium in tropical southeastern Brazil. Authors: Tavela Ade O, Araújo JV, Braga FR, Silva AR, Carvalho RO, Araujo JM, Ferreira SR, Carvalho GR. Publicaton: Veterinary Parasitology 08/10/2010
13 Veterinary Clinic article Publication: Horse and Hound magazine, 17th March 2011, Page 19
14 Predatory activity of the nematophagous fungus Duddingtonia flagrans on horse cyathostomin infective larvae. Authors: Braga FR, Araújo JV, Silva AR, Carvalho RO, Araujo JM, Ferreira SR, Benjamin LA. Publication: Tropical Animal Health and Production, Vol 42, No. 6, 08/2010
15 Duddingtonia flagrans: biological control of cattle nematodes in the field (Duddingtonia flagrans: controle biológico de nematodeos de bovinos a campo) Authors: Marta Bañolas Jobim; Janio Morais Santurio; Mario Luiz De La Rue Publication: Ciência Rural, versão impressa vol.38 no.8, 11/2008
16 Top Dressing of Feed with Desiccated Chlamydospores of Duddingtonia flagrans for Biological Control of the Pre-Parasitic Stages of Ovine Haemonchus contortus Authors: Sanyal P.K.1; Mukhopadhyaya P.N. Publication: Veterinary Research Communications, Volume 27, Number 5, 07/2003, pages 381-390
17 Biological control of Helminths Author: M. Larsen Publication: International Journal for Parasitology, Volume 29, Issue 1, 01/1999, Pages 139-146,
18 http://www.equinescienceupdate.co.uk/worm4.htm A ccessed: 12/03/2011 23:09
19 Deployment of Duddingtonia flagrans in an improved pasture system: dispersal, persistence, and effects on free-living soil nematodes and microarthropods Authors: M. R. Knox, P. F. Josh and L. J. Anderson Publication: Biological Control Volume 24, Issue 2, 02/06/2002, Pages 176-182
20 Lack of effect of the nematophagous fungus Duddingtonia flagrans on the development of the dung beetle, Aphodius constans Authors: C. Paraud, J.-P. Lumaret, C. Chartier Publication: Small Ruminant Research 70 (2007) page 276–279 04/04/2006
21 21st Century Guidebook to Fungi Author: David Moore, Geoffrey D. Robson and Anthony P. J. Trinci, http://sbli.ls.manchester.ac.uk/fungi/21st_Century_Guidebook_to_Fungi/Ch15_06.htm Accessed: 19/03/2011 16:27
22 http://www.bva.co.uk/newsroom/1698.aspx Accessed: 12 March 2011 2:28 PM
23 http://www.nhs.uk/news/2007/january08/pages/antibioticresistancemeasures.aspx Accessed: 19/03/2011, 6:27 PM
24 Sustainable Worm Control Strategies for Sheep 3rd Edition. A Technical Manual for Veterinary Surgeons and Advisors, Authors: Dr. K. A. Abbott, Prof. M. Taylor, L. A. Stubbings 04/2009, Pages 5, 24
25 Prevalence of anthelmintic resistant cyathostomes on horse farms Ray M. Kaplan, DVM, PhD Thomas R. Klei, PhD Eugene T. Lyons, PhD Guy Lester, DVM, PhD Charles H. Courtney, DVM, PhD Dennis D. French, DVM Sharon C. Tolliver, MS Anand N. Vidyashankar, PhD Ying Zhao, MS Publication: Journal of the American Veterinary Medical Association, 15/09/2004, Vol. 225, No. 6, Pages 903-910

