In a busy veterinary practice time is a precious commodity and the nature of equine practice has evolved significantly over the past few decades. Owners rely on veterinary staff to provide them with the most current and cost-effective advice regarding parasite control in their horses and while the term evidence based veterinary medicine (EBVM) is recognised, difficulties arise with regards to the types and levels of evidence and how to evaluate the rigour of different sources (Kaplan and Nielsen, 2010). In addition the wealth of horse-related magazines that offer advice on worming leads to the dissemination of information that is at times outdated and with a lack of peer review (Hinney, 2011). When trying to evaluate the numerous studies that are available with regards to anthelmintic resistance the problem arises in that there is a lack of universal agreement as to a definitive definition of what is anthelmintic resistance (Brady and Nichols, 2009). The literature suggests that owners are not getting the best advice, and the levels of resistance to the available anthelmintics provides evidence that veterinary practices need to be doing more with regards to both their own education and that of horse owners. One of the major obstacles with regards to provision of advice is that horses are kept under a variety of conditions and in particular there could be more than one veterinary practice providing advice on parasite control and, as such, advice can be based on an individual horse rather than the population of horses within an area. Livery yards present probably the greatest risk with regards to this due to high stocking densities, mixed and changing populations of horses and an environment where individual horse owners have less influence over parasite control programmes due to livery yard policy, and it has been suggested that the current interface between veterinary practices and livery yards is insufficient (Allison et al, 2011).
Kaplan and Nielsen (2010) suggest that there needs to be a move towards the provision of advice based on rational therapeutic considerations and not based on the ‘perceived threat’ that parasites pose. The questions utilised in this article outline their suggested approach to the provision of advice on the subject of parasite control.
Is there clinical justification for treating the horse?
In the past worm burdens have been attributed to the occurrence of colic in the horse (Proudman and Matthews, 2000) and with this disorder being of significant risk to the health of the horse, owners have been encouraged to use routine prophylactic anthelmintics in an attempt to prevent a worm burden and the risk of colic. An important distinction to make is with regards to prevention of endoparasites as opposed to treatment, as while they both require the use of anthelmintics, prevention should be based on parasite epidemiology and the prevention of transmission of parasites between individual animals; whereas treatment should be based on the known pathogenicity of a diagnosed parasite (Love, 2003). This is a complex process as most burdens are not caused by a single parasite, but moreover horses are co-infected with many different species (Nielsen, 2012). The main problem with infestation is that unless a horse has a significant worm burden the signs are often of a subclinical nature (Nielsen et al, 2010) and thus a variety of diagnostic methods are employed for both prevention and treatment of parasites, namely:
What parasite am I trying to eliminate and what stages of that parasite are likely present?
Clinical relevance of species
It is estimated that 95–100% of the total parasite burden of individual horses is due to cyathostomes (Nielsen, 2012) with the rest of the burden dependent on age and being made up of the nematode species outlined above and shown in Table 1.
| 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 |
| Cyathostomes (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 Strongylus species (large redworm) | All ages | Adults live in the large intestine; migratory phase | Larvae most pathogenically relevant as they live in the cranial mesenteric arteries |
| Oxyuris equi (pinworm) | All ages | Adults live in the caecum, colon and rectum | Extremely common; development to L3 is rapid |
Which anthelmintic to recommend, will this drug kill the desired parasite(s) and stage(s), and how confident am I that this drug will work as expected?
Routine anthelmintic prophylaxis aims to reduce the parasite burden of individual horses and thus limit contamination of pasture (Presland et al, 2005). Only 11 new endoparasiticides (five classes) have been developed for use in the horse since 1917 (Love, 2003) and no new anthelmintic drugs with a different mode of action have been introduced since ivermectin in 1983 (Nielsen, 2012). Advice regarding the use of anthelmintic drugs has undergone many changes over the years; however the standard advice to owners has been for some time to rotate the drug classes that are used (Allison et al, 2011).
With the introduction of the benzimadazoles in the 1960s owners were advised to de-worm their horses on a regular basis every 8 weeks (commonly known as ‘interval dosing’) and by the 1970s and 80s with the introduction of new drug classes the strategy changed to one whereby the drug types used were determined based on the season and when populations of nematodes would be prevalent in the horse, and thus the rotation of drug classes became the recommended strategy (Kaplan and Nielsen, 2010). Despite the wealth of information that is available a survey study conducted by Allison et al (2011) found that most horse owners used anthelmintics on a regular basis, regardless of the individual worm burden of the horse; in addition the study also reported owner confusion regarding strategic anthelmintic use as opposed to the use of the manufacturers' recommendations of the use of calendar-based strategies. Relf et al (2012) also noted that there appeared to be confusion with horse owners with regards to the different brands of anthelmintics available and to which class of anthelmintics they belonged.
In recent years due to the growing problem of anthelmintic resistance (Brady and Nichols, 2009; von Samson-Himmelstjerna, 2012) there has been a move to only use a drug when a worm burden has been suggested as significant by the use of a faecal egg count. Recommendations have been suggested in order to try to preserve the efficacy of anthelmintic drugs including (Lind et al, 2007):
The major problem faced by the veterinary profession with regards to prevention of nematodes is that resistance (including cross resistance and side resistance) has now been noted to the three major drug groups (Table 2) that are marketed for use in horses (Brady and Nichols, 2009; Nielsen et al, 2010). This is in spite of manufacturers marketing current anthelmintics as having good efficacy (>90%) against large strongyles, cyathostomins, ascarids and pinworms (Reinemeyer, 2009). It is interesting to note that due to concerns regarding anthelmintic resistance Denmark (1999), Sweden (2007), Netherlands (2008) and Finland (2009) introduced legislation restricting anthelmintic drugs to prescription-only (POM-V) thus requiring an individual diagnosis by a veterinarian (Nielsen, 2012).
| Class | Anthelmintic | Key points | Resistance |
|---|---|---|---|
| Benzimadazoles | Fenbendazole | Most common drug used in horses; drug of choice for foals less than 6 months old | Cyathostomes |
| 5 day regimen is not effective against cyathostomin populations shown to be resistant to a single dose of fenbendazole | |||
| Macrocyclic lactones (avermectin/milbemycin) | Ivermectin (avermectin) | Ivermectin has no efficacy against encysted or hypobiotic larval stages of cyathostomes | Cyathostomes reported in UK, USA, Italy, Germany, Brazil |
| This group share common structural features but differ in pharmacokinetic profiles | Parascaris equorum resistance reported worldwide | ||
| Moxidectin (milbemycin) | Effective against encysted or hypobiotic larval stages of cyathostomes | Over reliance on this drug for worming programmes associated with increased risk of resistance | |
| Restricted to use in horses over the age of 6 months of age due to reported adverse reactions | |||
| Pyrimidines | Pyrantal embonate | Effective against susceptible populations of most gastrointestinal nematodes | Cyathostomes |
| Heterocyclic compounds | Pipirazine | Narrow spectrum when used alone (adult cyathostomes and ascarids); more often combined with a benzimadazole |
Are there any adjunct management techniques that might help to achieve the ultimate goal of decreased transmission?
Prevention should focus on reducing the reliance on anthelmintics and the use of strategic intestinal parasite management (SIPM) can be utilised (Allison et al, 2011). Consideration should include frequency of anthelmintic use and overuse of the same drug class, stocking rates and correct dosing according to actual weight (Fritzen et al, 2010). Control strategies should also be based on regional climate, host demographics and pasture management (Love, 2003). Table 3 presents recent published results outlining control practices that either contribute to or can aid reduction of parasite burdens.
| Pasture management | Anthelmintic-related control | Yard hygiene |
|---|---|---|
| Higher prevalence of Parascaris equorum associated with using horse manure as paddock fertiliser | Weighing of horses to prevent underdosing; most established estimate weight of horses | Reduced prevalence of ascarids noted with daily cleaning and regular disinfection of horse boxes |
| Reduced parasite burdens when pasture hygiene is performed at least weekly or biweekly | Faecal egg counts (FECs) and faecal egg count reduction tests (FECRTs) on a routine basis | Mucking out of stable boxes on a daily basis aids control of strongyles |
| Alternate grazing with ruminants — rotational grazing; resting of pasture | Confinement should be considered after P. equorum treatment in order to lower contamination of environment | |
| Overcrowding on restricted pasture can force horses to graze roughs harbouring high levels of infective larvae | No scientific evidence for confinement following strongyle treatment | |
| 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 | Quarantine, FEC; anthelmintic and confinement for new horses into a yard |
Practical advice for the veterinary nurse
The veterinary nurse can contribute in a number of ways:
Conclusion
Parasite control of nematodes in horses is fundamental to their health and welfare. Anthelmintic resistance has been documented to the three main classes of drugs that are available to prevent/treat strongyles, ascarids, cyathostomes and pinworms. Despite this, owners still appear to be relying on the manufacturer recommendations of using a calendar-approach strategy, whereas a strategic, targeted approach should be used. The use of evidence based medicine provides a research based approach that can be utilised in a clinical setting and veterinary staff need to ensure that they are up to date with current recommendations by relying more on informed research rather than reliance on manufacturers' websites. More needs to be done in order to encourage owners to seek advice from veterinary practices with regards to the best approach to parasite control in their horses and client evenings can provide a useful forum for this.