27 Aug 2021
Laura Peachey
Job Title
When the author qualified as a vet in 2007, it was still commonplace to reach for a wormer, without thought, when owners requested it (or they could buy it themselves directly from a supplier). Anthelmintic resistance, although on the author’s radar, seemed a distant threat.
Since then a widespread campaign has been executed to encourage owners to test before treating, and an army of SQPs have been trained to advise on best practice before selling anthelmintics over the counter.
Veterinary courses countrywide have hammered home the principles of refugia and targeted selective treatment (TST). But has it worked? And was it ever really enough to prevent resistance from worsening?
The problem is that we don’t really have adequate surveillance and reporting in place to answer these questions with confidence – and that is a dangerous situation to be in.
We know that resistance to benzimidazoles is widespread in cyathostomins in the UK1,2, hence we can no longer reliably use these products to treat cyathostomins, even at larvicidal doses3. We know that resistance to pyrantel has been reported in a number of studies1,2 – therefore, we should use this drug with caution and on the basis of faecal egg count reduction tests (FECRTs) on each premises.
Moxidectin and ivermectin are still holding their own to a large degree, although shortened egg reappearance rates (especially in youngstock) have been widely reported and are an indicator that cyathostomin populations are on the precipice of resistance4-6.
Indeed, alarmingly, the first reports of resistance in thoroughbred youngstock are now emerging7 (Bull, unpublished data); and these reports are likely just the tip of the iceberg. Nevertheless, macrocyclic lactones remain the “go to” for cyathostomin control and continue to do a reasonable job when used sensibly.
In populations that are receiving regular anthelmintic treatments, the prevalence of Strongylus vulgaris remains, thankfully, very low. Currently, no evidence exists that this parasite has developed resistance against any anthelmintic licensed for roundworm use in horses.
Concerns exist that using TST will increase infection prevalence, although firm evidence that this is the case in the UK has, to date, not been reported.
Where benzimidazoles are all but worthless for cyathostomin control, they remain the main “go to” for prevention of disease due to Parascaris equorum in foals; reports have occurred of resistance globally8,9, but it appears rare at present and is yet to be reported in the UK. As for the cyathostomins, reports have occurred of resistance to pyrantel8,9 and, therefore, this drug should be used with caution.
Unlike cyathostomins, resistance against ivermectin and moxidectin has been frequently reported in P equorum populations10-12 and, consequently, they are not commonly used against it. Efficacy should be checked where they are being used.
Precious little is known about drug efficacy against tapeworm infection – not least because the faecal egg count (FEC) tool is not particularly reliable for detecting tapeworm eggs (let alone providing a quantitative measure of parasite burden).
It is broadly assumed that praziquantel and double dose pyrantel remain effective against tapeworm, and the standard practice is to use one or two tapeworm treatments per year. However, increasing anecdotal evidence indicates that this level of treatment frequency is not effective at quashing transmission on some “problem” establishments (Hodgkinson, unpublished data). Whether this is evidence of reduced drug efficacy is currently unknown, but it is an area that deserves further research.
A number of reports exist of a lack of efficacy of ivermectin against pinworm13, but whether this represents the development of resistance, or a constitutive lack of efficacy, has yet to be definitively established14.
These parasites are also highly infectious, and once the environment is contaminated it can be very difficult to break transmission cycles and eliminate the parasite.
The underlying issues with understanding the resistance problem we have in the UK are that:
So, what are people actually doing in practice? And is there any evidence that what vets are recommending, and owners are doing, is slowing the development and spread of resistant parasite populations in horses?
Sadly, we have very little “official” data on what people are doing in the UK. The existing studies in UK stud farms in 201215, Scotland in 201416 and a mix of establishments in 201917 suggest a trend towards increasing use of targeting treatment using FECs (60.9 per cent in latest survey). Also, in the most recent survey a high level of concern existed regarding resistance and 74.4 per cent using pasture control (that is, poo picking).
Although a responder bias may be present in the data, these are really encouraging results. However, what was concerning in these data was the very low use of FECRTs to assess drug efficacy. Therefore, despite their efforts to prevent resistance, most yards/farms were unaware as to whether the drugs they were using were actually killing the target parasites and, consequently, whether using TST was helping to slow the development of anthelmintic resistance.
Interestingly, a recent study examining attitudes of owners towards anthelmintic use showed that “self-perceived level of knowledge” and peer pressure were among the most influential factors in people using FEC-based approaches, rather than a fear of anthelmintic resistance18. This points towards owner education campaigns as the best route to owner behaviour change.
Ultimately, while owner engagement with vets and SQPs appears to be increasing, a large proportion of owners buy wormers from internet retailers, and the evidence suggests that advice from these retailers alone is likely to result in interval rather than targeted treatment17.
This issue, once again, boils down to the fact that anthelmintics are not POM, which makes it very difficult for the veterinary industry to get a real handle on anthelmintic use, let alone exert influence on owner behaviours.
Current best practice guidelines for parasite control have been summarised recently in the publication of a round table discussion of experts19 – the author recommends vets refer to that for detailed advice when making worm control programme decisions alongside owners.
The important factors to emphasise are that we should encourage owners to:
This means using appropriate diagnostics and regular resistance tests such as FECRTs.
This approach is easier said than done, however, as many parasite diagnostics tests are highly nuanced in their interpretation, which means they cannot always be applied to every circumstance and are often misinterpreted. Furthermore, accurate resistance tests are not available for every parasite.
The FEC test can provide an indication of the presence or absence of strongyles, P equorum, A perfoliata and O equi (although tape strips are better for latter), but their association with parasite burden is weak at best.
Preference of an FEC of greater than 200 eggs per gram (epg) as the threshold for treatment following strongyle egg count analysis is largely based on one study20, which showed that horses with egg counts greater than 200epg had higher worm burdens (on average) than those less than 200epg. By no means, though, does a linear association exist between the two. Therefore, the main aim of treatment is to reduce contamination of pasture, and the consequential upcycling of infection, from high-shedding horses.
For P equorum and A perfoliata, the association between FEC and worm burden has not been shown; therefore, FEC serves more as a presence test for these parasites. Since they are both highly pathogenic, treatment is usually indicated on the basis of a positive test.
Due to the difficultly of diagnosing A perfoliata infection using FEC, ELISAs have been developed to detect antibody responses to these infections; however, these are often misused/misinterpreted. A perfoliata infection seems to be a problem isolated to particular yards/farms; therefore, this test is most useful when applied at a herd level to monitor control programmes on yards with tapeworm problems (Hodgkinson, personal communication).
Similarly, as no test currently exists for encysted cyathostomins, an ELISA has been developed, which gives an indication of recent infection with larval cyathostomins in individual horses21. However, this test has yet to replace routine use of larvicidal treatment in winter, perhaps because it can be hard to interpret and owners may prefer to err on the side of caution.
With regards to resistance testing, aside from critical tests where animals are killed and worms counted at postmortem, no foolproof method exists; FECRTs are currently used as a “best estimate” of whether a drug is working, and this is most easily applied to strongyles (cyathostomins in particular). The future of parasite diagnostics lies in the development of more accurate tests to measure parasite burdens and the identification of markers of resistance in parasite populations. Nevertheless, owners must be encouraged to perform the tests that are currently available so that vets can improve monitoring of drug efficacy and better control anthelmintic resistance in the UK.
In conclusion, convincing evidence exists that anthelmintic resistance is progressing in UK equine parasite populations and our grasp on the situation, both in terms of knowledge and control, is weak. This is not the time for complacency when it comes to equine parasite control. We need to continue to push knowledge transfer and CPD campaigns to increase understanding of anthelmintic and diagnostic use in the UK among equine owners and vets; and importantly, test, test, test for drug resistance.
