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There are many factors that contribute to the clinical relevance of endoparasites in adult horses and much research has been undertaken into the factors that cause disease in susceptible equine populations. Many control strategies (for example faecal egg counts) and anthelmintic treatment programmes have been recommended in order to limit the clinical effects of these intestinal parasites; however due to the lack of clinical signs seen in horses most owners have relied on the routine prophylactic use of anthelmintics, and as a result anthelmintic resistance has become a growing concern. Veterinary nurses can play an important role in the continuing education of horse owners by providing advice on such topics as pasture management, targeted dosing and faecal egg counts.
Control of intestinal parasites (worms) in horses is fundamental to their health and welfare. In the past worm burdens were commonly associated with the occurrence of colic (Proudman and Matthews, 2000) and thus the use of routine prophylactic anthelmintics (wormers) was encouraged in an attempt to prevent a worm burden and the risk of colic. However, parasite control is a complex process as most burdens are not caused by a single parasite, but more often, co-infected with many different species (Nielsen, 2012). In the majority of cases, unless a horse has a significant worm burden, the signs are often of a subclinical nature (Nielsen et al, 2010). One of the major obstacles with regards to provision of advice is that horses are kept under a variety of conditions and in most cases advice is based on an individual horse rather than the population of horses within an area. Livery yards present probably the greatest risk with regards to the ever changing population of horses, due to high stocking densities, mixed and changing populations of horses and an environment where individual horse owners have less control over parasite control programmes due to livery yard policy (Chapman, 2013).
Target worms
According to a study by Nielsen (2012) it is estimated that 95–100% of the total parasite burden of individual horses is due to nematode species, in particular, cyathostomins. Nematode species (Table 1) include:
Cyathostomins (small red worms) — considered to be the most clinically significant parasite to infect horses
Strongylus vulgaris (large red worms) — in the past, this species was a significant contributor to parasite problems and a major cause of colic in horses. As they are susceptible to all the recommended anthelmintics (wormers) they are not currently regarded as significant in worming programmes
Oxyuris equi (pinworm) is probably the least significant with regards to clinical disease, however, their presence can cause intense irritation (pruritus) in the perineal region
Parascaris equorum (roundworm) is a particular problem in foals as adult horses develop immunity to the effects of this parasite. Poor growth, ill-thrift, weight loss, colic and death (subsequent to intestinal impaction or perforation) are associated with burdens by this parasite. In addition, coughing is often present, as this parasite has a migratory life cycle through the lungs.
Nematode species
Age
Life cycle
Key points
Parascaris equorum (ascarid; roundworm)
Immature horses — foals and yearlings
Infection via ingestion of eggs; larvae emerge from eggs in small intestine and migrate through liver and lungs
L3 development requires 10 days at 25–35°C; larvated eggs survive in the environment 5–10 years; L4 patent 75–80 days post infection
Cyathostomins (small redworm)
All ages
Adults live in the large intestine; undergo a period of arrested development as larvae in the large intestinal mucosa
Most common; non-migratory life cycle; over 50 different species; most horses carry a mixed burden of 5–10 species; horses do not acquire a strong immunity to these parasites therefore infected animals are a source of pasture contamination
S. vulgaris
All ages
Adults live in the large intestine; migratory phase
Larvae most pathogenically relevant as they live in the cranial mesenteric arteries
Cestodes, e.g. Anoplocephala perfoliata (tapeworm) — most infections are asymptomatic, however, heavy infection can cause ulceration leading to colic. Susceptible to pyrantel pamoate and praziquantel
Gasterophilus intestinalis (bots) — the larvae of flies that have become highly specialised parasites of the horse. These are most active during the summer months and the eggs stick to the skin of the horse (most notably the legs) and horses becomes infected when they lick the eggs off the area. The larvae then hatch in the mouth of the horse and eventually migrate to the stomach. Infection is usually asymptomatic unless heavy infestations occur. Susceptible to all recommended anthelmintics.
Laboratory tests
In comparison with dogs, cats and ruminants, there is limited diversity of parasite eggs and oocysts found in equine faeces and the most common finding is the strongylid egg. The following tests can therefore be utilised for testing:
Faecal egg counts (FECs) are used in order to determine which individual horses have a significant worm burden; however the tests that are available are modified from cattle and sheep (Presland, et al, 2005). The McMaster or Modified McMaster technique has been utilised for many years and is used to estimate the nematode (e.g. Strongyle spp. P. equorum, and O. equi) worm burden of individual horses. However, there is no universally recognised cut off point with regards to the number of eggs that are regarded as a significant burden (Brady and Nicols, 2009) and this is usually left to the individual laboratory to determine.
Faecal egg count reduction tests (FECRTs) are used to evaluate drug efficacy. This is of particular importance when anthelmintic resistance in strongylid nematodes is increasing worldwide. This is the only test that is currently practical for field use in all host species, i.e. the horse.
The testing for tapeworm has always been challenging as the use of FECs is not reliable (Nielsen, 2015). In the past, serum antibody enzyme-linked immunosorbent assay (ELISA) testing has been used. A newer technique has now become available that is non-invasive and can be done by the owner, this is the:
Tapeworm saliva test — a saliva antibody ELISA (EquiSal, Westgate Laboratories) test. According to the literature from the manufacturing laboratory, the best time to carry out the first test is 4 months after treatment for tapeworm (Westgate Laboratories, 2015). This is to allow for the fact that antibody levels in the saliva reduce over time after treatment and will help to prevent against a false result. The recommendation is that horses are tested twice a year, with a 6 month gap between tests. It is suggested that the diagnostic sensitivity and specificity of the test is above 80% and this is at, or above, the level of serum ELISA. However, the test is in its early stages of use and no research is yet published in the peer-reviewed literature with regards to any limitations of the test.
How does a worm programme work?
The majority of equine yards still use routine anthelmintic prophylaxis as a means to reduce the parasite burden of individual horses. While advice regarding the use of the ‘wormers’ that are available has undergone many changes over the years, the standard advice to owners has been to rotate the drug classes that are used (Chapman, 2013). Most horse owners continue to use anthelmintics on a regular basis, regardless of the individual worm burden of the horses. This is in spite of the growing problem of anthelmintic resistance that has been noted to the three major drug groups that are used for the prevention of nematodes (Brady and Nichols, 2009; Nielsen et al, 2010).
Refugia is the term used to describe a population of worms that have not been exposed to drug treatments and therefore have not developed resistance. This population of worms ‘in refugia’ is then able to dilute the population of resistant worms, with a pool of sensitive genes (Molento, 2012). Refugia is influenced by three factors (Cook, 2008):
Number of larvae on the pasture
Percentage of animals treated with anthelmintics
The ability to kill all developmental stages within the host.
Thus, advice should be based on rational therapeutic considerations and not based on the ‘perceived threat’ that parasites pose (Allison et al, 2011).
Therefore, a successful worming programme should include:
Routine FECs to determine the parasite burdens for individual horses
Most of the worms live within a few susceptible horses; identify and treat the individual
Good stable hygiene, e.g. mucking out of stables and regular disinfection of stables and horseboxes
Minimising the number of anthelmintic drug doses through the year and slow rotation between different drug classes (not brand) (Table 2)
Correct dosing according to the individual horse's weight and body condition score; the majority of horses are under dosed
Effective treatment of new horses, i.e. quarantine, FEC, anthelmintic and quarantine before introduction to resident horses
Regular monitoring for resistance (FECRT).
Class
Anthelmintic
Key points
Resistance
Benzimadazoles
Fenbendazole FBZ
Most common drug used in horses; drug of choice for foals less than 6 months oldFive day regimen is not effective against cyathostomin populations shown to be resistant to a single dose of FBZ
Cyathostomins
Macrocyclic Lactones (avermectin/milbemycin)This group share common structural features but differ in pharmacokinetic profiles
Ivermectin (avermectin)
Ivermectin has no efficacy against encysted or hypobiotic larval stages of cyathostomes
Cyathastomins reported in UK, USA, Italy, Germany, BrazilP. equorum resistance reported worldwide
Moxidectin (milbemycin)
Effective against encysted or hypobiotic larval stages of cyathostomesRestricted to use in horses over the age of 6 months of age due to reported adverse reactions
Over-reliance on this drug for worming programmes associated with increased risk of resistance
Pyrimidines
Pyrantal embonate
Effective against susceptible populations of most gastrointestinal nematodes
Cyathostomins
Heterocyclic Compounds
Pipirazine
Narrow spectrum when used alone (adult cyathostomins and ascarids); more often combined with a benzimadazole
This is one of the key areas that horse owners and yard owners can utilise in order to ensure that their worming programme is efficient (Nielsen et al, 2010; Hinney et al, 2011; Relf et al, 2012). Key points to consider include:
Reduced parasite burdens when pasture hygiene (removing manure) is performed at least once a week
Alternate grazing with ruminants
Rotational grazing and resting of pasture
Overcrowding on restricted pasture can force horses to graze ‘roughs’ harbouring high levels of infective larvae
Higher prevalence of P. equorum associated with using horse manure as a paddock fertiliser
Movement of horses to ‘clean’ grazing after treatment is not recommended due to the fact that any potentially resistant parasites excreted post treatment, will represent the majority of the population on the ‘clean’ grazing.
Practical advice for the veterinary nurse
Ask the owner what current parasite control programme (if any) they are following including questions on the number and signalment of the horses; type of yard where the horses are kept and how they are housed; types of anthelmintics used and when they are used; the use of grassland management strategies and if any faecal testing is routinely undertaken. This ensures that the information provided is not just generic advice, but targeted therapeutic advice.
It is also important to ask the owner if they are responsible for the control programme or if somebody else controls it, and if so whom.
Faecal egg testing is an integral part of parasite control and veterinary nurses can play a key role in the provision of this within a veterinary practice.
Ensure that knowledge regarding parasites, parasite control and anthelmintic resistance in horses is current. Attendance at continuing professional development courses/workshops and reading of current literature is important in order to stay informed.
It is important to ensure that all species of endoparasites are included in a parasite control programme and it may be necessary to provide further information to owners with regards to the new tapeworm saliva test, as owners will be able to purchase this without the veterinary surgeon's knowledge.
The use of client evening lectures where horse owners can be informed of the most current advice with regards to parasite control is a good way of maintaining good client–practice communication.
Conclusion
In conclusion, while there is a growing body of evidence to suggest that anthelmintic resistance is a problem in horses, it is clear that owners and the veterinary profession are still relying heavily on the use of anthelmintics for the prevention and treatment of endoparasites. A greater emphasis needs to be placed on the use of FECs and indeed, many owners could be encouraged to do this themselves. In recent years sheep farmers have also been encouraged to undertake FECs before administering anthelmintics. This would allow for a more targeted approach to worming and indeed, would undoubtedly lead to owners having a better understanding of worming in horses. Faecal egg reduction tests should also be encouraged by the veterinary profession to ensure that the efficacy of the drugs that are used is taken into consideration with a worming protocol for a yard.
Key Points
Most horse owners use anthelmintics on a regular basis, regardless of the individual worm burden of the horse.
The problem faced by the veterinary profession with regards to prevention and treatment of endoparasites is that anthelmintic resistance has now been noted to the three major drug groups.
The availability of new endoparasite tests, i.e. the tapeworm saliva test, makes testing more cost effective for owners; however research is lacking on the efficacy of this test.
The use of faecal egg counts need to be encouraged so that a more targeted approach to worming can be made.
Prevention should focus on reducing the reliance on anthelmintics and the use of strategic intestinal parasite management (SIPM).
