By Dr Carl Eden BVM&S MRCVS
Worms would not be a problem in horses if they lived in perfect harmony with their hosts. At low levels of infection, such as those seen in wild horses and Zebra, they cause little harm and are well tolerated. With domestication and intensification however we have turned the balance in favour of the worms. Our horses, now faced with a greater worm challenge, develop significant worm burdens. These burdens will, in a percentage of horses, result in a significant risk of suffering from disease due to parasitism.
It has been many years since Glasgow veterinary school made the link between Strongylus vulgaris, verminous arteritis and colic, Oliver et al reported on cases of Duodenal rupture due to the tapeworm Anoplocephala magna1 and Beroza et al reported on cases of caecal perforation and peritonitis associated with A. perfoliata.2
It is more recent however that research from the U.K. has highlighted once again the role of tapeworms, (Anoplocephala perfoliata), in 22% of spasmodic colic cases and 81% of cases of ileal impactions.3 Surgical colics such as iliocaecocolic intussusceptions4 and caecal rupture5 and peritonitis have also been attributed to infection with A. perfoliata. With the poor sensitivity of coprological methods of diagnosis6 for tapeworms, the prevalence in Australia of these parasites is undoubtedly being underestimated.
Tapeworms are still causing many diseases in our horse population and their eggs are seen more frequently in animals treated with wormers containing macrocyclic lactones alone,1,2,7 (ML’s and BZ’s do not have any efficacy against cestodes). Without accurate clinical studies into their prevalence and distribution using either data collected at necropsy or the more recent ELISA tests developed in the U.K. the best estimates we have at present are prevalence rates of roughly 29-32% in most states of Australia.8,9 In reality their prevalence may be higher.
Research in USA has shown that both cyathastomin and tapeworms are associated with colics and that the better the worming strategy, the lower the incidence of colic attributable to those parasites.10
Diarrhoea is a common presenting sign in many equine patients, particularly young horses towards the end of winter or beginning of spring. This diarrhoea is commonly associated with larval cyathastomiasis.
Thousands of migrating larvae will arrest their development during the grazing season becoming hypobiotic. The emergence of these previously “encysted” or hypobiotic larvae (normally in early spring), cause massive damage to the gut lining and a resultant diarrhoea. The stimulus for their emergence ‘en mass’ is poorly understood, the damage caused when they do however results in severe diarrhoea, weight loss and sometimes death. Treatment is often unrewarding despite intensive therapy.
A less severe form of the disease caused when the hypobiotic larvae do not excyst ‘en mass’, manifests itself as weight loss, poor condition and failure to grow. It is not understood why some horses develop the diarrhoeal form of the disease and others do not.
The goals of anthelmintic control strategies
Gone are the days of building a veterinary practice on the sale of wormers but instead a practice can succeed through advice given on how and when to use them.
Of course there will be variations in advice given relating to changed geographical locations, management practices and weather, but the objectives will remain. Those objectives are to bring the level of parasitism down to a level where there is no clinical disease whilst allowing a degree of acquired immunity to develop over the years. The transitory nature of the equine industry makes it neither practicable nor desirable to achieve worm free horses. The goals of a comprehensive anthelmintic control strategy should therefore be built around minimising the burdens of tapeworms, the cyatostomins and maintaining our control over Strongylus vulgaris and Parascaris equorum.
Individual worm control strategies are achievable through the judicious use of anthelmintics, monitoring programs and pasture management.
Although both are based on the use of the same anthelmintic types of drug, it is important to separate the treatment of parasitic disease from the prevention of parasitic infection. Knowledge of parasitic biology and pathogenic mechanisms should be applied to treatment whilst prevention should be based fundamentally upon epidemiology. With the release of new and novel anthelmintics being unlikely in the near future the relevant use of those categories in use today is a major issue in veterinary medicine.
Despite over 20 anthelmintics being on sale in Australia there are only three principal classes with broad spectrum activity in common use, namely, the macrocyclic lactones, benzimidazoles and the tetrahydropyrimadines. The combination of these drugs with others of narrower spectrum is employed to encompass the treatment of cestodes or arthropod larvae creating broad spectrum or “all wormers”. (Eg Praziquantel with Abamectin in Equimax and Praziquantel with Ivermectin in Equimax LV).
The fundamental aspects in designing a parasite control strategy are based on the need to minimise transmission of parasites between hosts. Despite non-chemical methods being helpful in this means, the use of chemical compounds remains the mainstay of any worming program. The program needs to be formulated with reference to current management practices, number of horses, current parasitic burden and environmental considerations (geography and weather).
As a starting point the current parasitic burden can be measured using Faecal Egg Counts (whilst their use in diagnosis is limited they are invaluable in monitoring parasitic control strategies). In cases where any individual FEC or the average in a group is over 200 epg those horses should receive a dose of a broad-spectrum anthelmintic (such as Equimax). Performing a repeat FEC 10-14 days post treatment can then assess the efficacy of that treatment (termed a Faecal Egg Count Reduction Test).
FECRT % = 1 – Mean FEC post treatment x 100
Mean FEC pre treatment
Percentage efficacy of less than 90% is indicative of the emergence of resistance or other problems (inadequate pasture hygiene, stocking density etc). Any indication of inefficacy should be investigated and if indicated alternative anthelmintic classes employed and tested for efficacy. At this stage current advice dictates either monitoring for the emergence of resistance and/or rotating anthelmintic on a slow (ie-annual) basis. The fundamental basis for rotation must be from one class of active to another and not within the same class of anthelmintic. For example a change to oxfendazole from ivermectin would be warranted but to moxidectin from ivermectin would not.
Subsequent FEC’s can be used to modify the treatment intervals based upon the recurrence of a FEC greater than 200 epg. As a general rule of thumb, in wet years or climates more doses of anthelmintic will be required than in hot, dry years or climates. This is due to parasitic life cycles favouring wetter years or climates.
Although resistant strains of cyathastomins have been reported to the benzimidazoles and to a lesser extent tetrahydopyrimidine families of drug, resistance to the macrocyclic lactones has not been documented as yet. The current thinking on why resistance to ML’s has not yet emerged is based on a significant proportion of the cyathastomin population being ‘in refugia’ (hypobiotic) at times of dosing and as such not ‘seeing’ the drug. With the advent of moxidectin the emergence of resistance to ML’s is likely to be accelerated due to the persistent effect of this drug at concentrations which favour the survival of the cyathastomins.9 Leading veterinarians are advising that moxidectin is not used as a routine anthelmintic for this reason, and is reserved for cases of clinical disease on a maximum of one occasion per annum.11
Pitfalls in preventative strategies
- Young animals have a lower level of immunity to parasites and therefore when grazed in close proximity to other horses will become reinfested more rapidly than older animals. This can lead to higher than expected FEC post treating and may not be an indication of resistance. Stocking densities of one horse per acre should not be exceeded.
- All animals on an establishment must receive doses of anthelmintic at the same time to minimise pasture contamination. It is no good treating some animals whilst others continue to contaminate the pasture. By the same token it is false economy to under dose, and all horses should receive accurate doses erring on the side of a slight overdose (if weight has been estimated rather than accurately measured).
- Newly introduced animals should be quarantined and treated upon arrival to avoid immediate spread of parasites to other horses. Despite these treatments the survival of hypobiotic cyathostomin larvae may commence pasture contamination months to years after introduction of the new stock, and therefore regular treatments must be maintained to decrease levels of pasture contamination.
- The continuous use of an anthelmintic containing only one class of drug may predispose the horses to development of certain species of parasite. For example the exclusive use of “BZ” based wormers may predispose a horse population to significant parasitism by tapeworms.
- Harrowing of fields is commonly practiced but if performed at wet or cool times of the year this merely spreads manure along with the parasites to all corners of a field. It is far more rewarding to collect faeces from pastures or employ mixed grazing with cattle and sheep (decreasing parasitic challenge levels by acting as ‘biological hoovers’).
- Mares should be wormed one month prior to expected foaling date. Subsequently both the foal and mother should be dewormed on regular 6-8 week intervals until weaning. This decreases the cycling of parasites between mother and her foal.
The increased costs of anthelmintic products, the vet’s time in devising and monitoring strategies, and the increased labour for administration of the anthelmintic and pasture management are outweighed by the benefits in improved performance, better feed utilisation and reduced incidence of diseases such as colic and diarrhoea.
Treatment of parasitic infestations
With regard to therapy and diagnosis of clinical disease attributable to parasites, confirmation of diagnosis is fraught with difficulties. Faecal tests are unreliable when it comes to both cyathastomins and tapeworms, due to the poor sensitivity and specificity of these tests at the time of disease. For this reason disease attributable to parasites is often misdiagnosed or under diagnosed.
Remembering that the tapeworm life cycle is an indirect one taking roughly 6 months to complete, the likelihood of significant burdens is reduced in animals receiving biannual doses of either praziquantel (Equimax and Equimax LV), or doses of pyrantel salts. In cases of suspected tapeworm associated colic, such as spasmodic colic or ileal intussusception and ileal impaction, a thorough worming history should evaluate the likelihood of Anoplocephala spp involvement. Treatment with a broad-spectrum wormer containing praziquantel, (Equimax, and Equimax LV) after the colic symptoms have subsided is indicated.
The development of diarrhoea within 14 days of administering an anthelmintic dose is highly suggestive of larval cyathastomiasis. This syndrome is not widely recognised in Australia, (it is the authors opinion that this may be due to under diagnosis and also to different weather patterns in Australia relative to the rest of the world.) Due to the high prevalence rates of cyathastomins and difficulty in diagnosing the disease the incidence of larval cyathastomiasis may be higher than thought. Larval cyathostomiasis should be considered in any equid with a protein losing enteropathy that can not be attributable to infiltrative bowel disease (eg. lymphosarcoma).
Clinical signs are those of a protein loosing enteropathy resulting from the emergence ‘en mass’ of hypobiotic larvae. The removal of negative feed back from adults in the intestinal lumen is considered a possible mechanism for the emergence ‘en mass’ of the arrested larvae. Small red worms (L4 larvae) may occasionally be found on a rectal sleeve enabling confirmation of diagnosis. More commonly cases are diagnosed with a combination of history of recent deworming, clinical signs and ruling out other causes of diarrhoea or weight loss and response to treatment.
Diagnosis of one case in grazing groups faces the clinician with a dilemma. There is a strong likelihood that other similarly managed horses have heavy burdens of cyathastomins. Anthelmintic dosing may therefore induce clinical disease. In the majority of situations I would recommend treatment of all animals using a protocol such as the one suggested below.
Clinically affected animals
Combination of ivermectin (Eraquell) and benzimidazole (Fencare 100, Panacur 100):
Ivermectin, 0.2 mg/kg (5g per 100kg) on days 1, 16, 31, 61 and 91
Fenbendazole, 10mg/kg (5mls/50kg of Fencare 100 or Panacur 100) on days 2-6, 17-21, 32-36, 62-66 and 92-96
Concomitant symptomatic treatment with codeine phosphate (oral) to control the diarrhoea (1-2 mg / kg daily dependant on effect. Unlicensed, only use if diarrhoeic) and prednisolone (Preddy Granules1mg/kg days 1-20 and 1mg/kg e.o.d. on days 21-40)
It should be noted that the use of moxidectin in thin or debilitated animals is contraindicated and it is best not used for treatment of cyathastomiasis.
Rest of grazing group
Owners should be advised that treatment may precipitate overt disease.
Combination of ivermectin (Eraquell) and benzimidazole (Fencare 100, Panacur 100):
Ivermectin, 0.2 mg/kg (5g of Eraquell per 100kg) on days 1, 31, 61 and 91
Fenbendazole, 10mg/kg (5mls/50kg of Fencare 100 or Panacur 100) on days 2-6, 32-36, 62-66 and 92-96
(The use of Fenbendazole as described above is “off label” in Australia, but licensed in other countries of the world)
Strongyloides westeri (Verminous arteritis)
With the regular use of anthelmintics such as Equimax and Equimax LV these parasites will not cause disease and colic. In cases where a regular anthelmintic has not been administered, colic due to verminous arteritis is a possibility. Surgical resection of devitalised bowel and fluid therapy are indicated.
Parascaris equorum (Large roundworms)
Parascaris equorum is a pathogen of young horses and foals. A degree of immunity to this helminth has usually developed by the time a horse reaches 1 year of age. When allowed to reach large numbers in naive animals they can cause life-threatening obstruction of the small intestine necessitating surgical intervention. Due to the rapid completion (in under 10 weeks) of this parasite’s life cycle it is recommended to ensure all foals are treated on a regular basis from one month of age.
- Oliver DF, Jenkins CT, Walding JP. (1977) Duodenal rupture in a nine month old colt due to Anoplocephala magna. Vet Rec, 101(4):80
- Beroza G.A et al. (1983) Caecal perforation and peritonitis associated with Anoplocephala perfoliata infection in three horses. J. Am Vet Med Assoc, 183(7): 804-6
- Proudman CJ, French NP, Trees AJ. (1991) Tapeworm infection is a significant risk factor for spasmodic colic and ileal impaction colic in the horse. Equine Vet J, 30(3):194-9
- Barclay WP, Phillips TN, Foerner JJ. (1982) Intussusception associated with Anoplocephala perfoliata infection in five horses. J. Am Vet Med Assoc, Apr 1;180(7): 752-3
- Ryu SH et al. (2001) Caecal rupture by Anoplocephala perfoliata infection in a thoroughbred hose in Seoul Race Park, South Korea. J Vet Sci, 2(3):189-93
- Mathews JB, Hodgkinson JE, Dowdall SM, Proudman CJ. (2004) Recent developments in research into the Cyathastomes and Anoplocephala perfoliata. Vet Res, 35(4):371-81
- French DD, Chapman MR, Klei TR. (1994) Effects of treatment with ivermectin for five years on the prevalence of Anoplocephala perfoliata in three Louisiana pony herds. Vet Rec, 135(1):63-65
- Mfitilodze MW, Hutchinson GW. (1989) Prevalence and intensity of non-strongyle intestinal parasites of horses in northern Queensland. Aust Vat J,66(1):23-6
- Bucknell DG, Gasser RB, Beveridge I. (1995) The prevalence and epidemiology of gastrointestinal parasites of horses in Victoria, Australia. Int J Parasitol, 25(6):711-24
- Urhlinger C. (1990) Effects of three anthelmintic schedules on the incidence of colic in horses. Equine Vet J, 29(4):251-255
- Love S. (2003) Treatment and prevention of intestinal parasite-associated disease. Vet Clin Equine, 19:791-806
- Proudman CJ, Holdstock NB. (2000) Investigation of an outbreak of tapeworm-associated colic in a training yard. Eq Vet J Suppl, (32):37-41
Dr Carl Eden BVM&S MRCVS