ANTHELMINTIC-resistant equine nematodes continue to pose a real threat to the health and welfare of our horses.
Cyathostomins, commonly known as “small strongyles”, are now considered to be the most important helminth of equidae in terms of their prevalence and pathogenicity. Cyathostomin infections are complex, and horses with a high level of infection can develop clinical symptoms of disease.
Most notable is the life-threatening syndrome known as “larval cyathostominosis”, whereby encysted larvae emerge en masse from the walls of the colon and caecum, causing proteindepriving enteropathy, chronic diarrhoea, oedema, weight loss, colitis (Traversa, 2008) and possible fatality in up to 50% of cases (Love et al, 1999).
Currently there are three classes of broad-spectrum anthelmintic licensed for use in horses; these are the benzimadazoles (BZ), tetrahydropyrimidines (THP) and the macrocyclic lactones (ML).
The ML group is split into two ML derivatives known as the avermectins and the milbemycins to which ivermectin (IVM) and moxidectin (MOX) belong respectively. Praziquantel is a short-acting anthelmintic specifically licensed for use against cestodes.
In the past few decades, drug resistance in cyathostomins has become widespread, particularly to BZ, where reports of resistance have been recorded in more than 21 countries (Lyons et al, 1999; Kaplan et al, 2002; Kaplan et al, 2004).
Resistance to the THP is also relatively common, particularly in the USA, with a reported prevalence of more than 40% in horse farms (Kaplan et al, 2004).
Reduced IVM efficacy has been reported in the UK (Campbell et al, 2007), the US (Lyons et al, 2008), Germany (von Samson-Himmelstjerna et al, 2007) and Australia (Edward et al, 2008). Most
worrying, however, is the reported failure of MOX to control cyathostomins in Brazil (Molento et al, 2008) and in donkeys in the UK (Trawford et al, 2005).
Moreover, in a recent study, single or multi-drug resistance was present in equine cyathostomins and is considered widespread in Italy, the UK and Germany, particularly to BZ and/or THP with one report of resistance in the UK to BZ, THP and IVM (Traversa et al, 2009).
This short communication describes a situation in which horses imported from horse farms in the USA to the UK were found to be harbouring significant gastrointestinal nematode burdens.
It aims to highlight the importance of quarantine measures for imported horses in order to reduce the threat of introducing resistant nematodes, and the need for a sound risk management strategy consisting of administration of a quarantine treatment and performing worm egg counts (WEC) to ensure it is effective.
At present there is an absence of information about imported worm burdens, but also no measures in place to deal with the risk of introducing worms that may potentially be resistant to the three available classes of anthelmintics.
The importance of this issue is highlighted when one considers the source of the horses imported into the UK and the history of anthelmintic resistance in the USA.
A total of 98 yearlings were imported from the United States in November 2009 to a stud in the UK. All the horses were imported from thoroughbred breeding farms, and before importation had followed a typical worming regimen adopted in the US.
Anthelmintics are used excessively by many breeding farms, particularly in the US, where it is a common practice to administer ivermectin for treatment of suspected Strongyloides infection as routine prophylactic treatment in the absence of clinical signs when foals are less than one month of age (Craig et al, 2007).
Thereafter, frequent anthelmintic rotation is implemented, and juvenile horses are often de-wormed at monthly intervals until their first birthday. Many farms use ML at least bimonthly in juvenile horses (Craig et al, 2007). Another typical parasite control practice adopted for juvenile horses at many breeding farms essentially constitutes excessive and frequent use of a single drug class, thus increasing the selection pressure for anthelmintic resistance (Kaplan, 2004).
After a settling-in period of approximately three to four weeks, the horses were reported to have diarrhoea and signs of ill thrift. All horses had been kept stabled and had not had access to pasture.
Initial investigations led the veterinarians at the yard to perform WECs to establish whether parasites were a possible cause. A fresh faecal sample was collected from the stable floor of each horse, placed in a sealable bag with the air removed to prevent larval development and sent to Ridgeway Research Ltd on the same day for analysis for coprological examination.
Faecal samples were analysed using the modified McMaster technique with a sensitivity of 50 eggs per gram (epg). All horses had been stabled since arrival in the UK; there was therefore no opportunity for them to acquire worms in the UK.
A total of 82 samples had a positive WEC (83.6%) ranging from 50-3,450 epg. All eggs seen were strongyles; no ascarid eggs were detected.
Following the WEC analysis, all horses were immediately treated with IVM (Eqvalan Paste for Horses at 200mg/kg, Merial) and the diarrhoea and ill thrift resolved.
Fourteen and 56 days post-treatment, faecal samples were taken from all horses with an initial WEC of greater than 1,000 epg. A total of 25 samples were taken and a faecal egg count reduction test (FECRT) was performed.
Results demonstrated 100% reduction in WECs at both time points, indicating that these particular populations of nematodes were susceptible to ivermectin.
Whilst the results demonstrate that this population of nematodes was susceptible to IVM, FECRT were not performed using BZ or PYR; therefore, if either of these anthelmintics had been used as a quarantine treatment and their efficacies not evaluated, potentially this could result in eggs from resistant worms being passed onto pasture.
No way of knowing…
Similarly, if quarantine measures are ignored, and horses are put straight onto pasture, there is no way of knowing what parasites they are harbouring or their resistance profile, which could result in the introduction of potentially multi-drug resistant parasites.
In the face of ever increasing reports of anthelmintic resistance and with there being no new equine anthelmintics under development, this poses a huge threat to the health and welfare of our horses.
Many studs in the UK import horses from farms in the USA and other countries throughout the world. While strict screening and quarantine measures are imposed for limiting the importation of diseases such as dourine and glanders, the relevant legislation, Directive 93/197/EEC (which is concerned with imports of registered equidae and equidae for breeding and production) does not state the requirement for worming prior to importation.
This short communication highlights the risk of importation of potentially resistant worms, and in light of growing widespread evidence of worldwide resistance, consideration on how to prevent the importation of resistant worms should be discussed.