Researchers who studied small strongyles at a large British Thoroughbred stud found signs of invermectin resistance, saying it will probably plainly emerge in the future unless the treatment programme changes.
Small strongyles, also known as cyathostomines or small redworm, are very common.
The parasite has an encysted stage, in which they hibernate in the lining of the gut. They usually emerge in spring, and a mass release can cause serious gut problems.
Standard faecal egg counts cannot identify the larvae during this encysted stage. Moxidectin is the only dewormer that can target them during their encysted stage.
University of Cambridge researcher Rebecca Molena and her colleagues described their work to determine the cyathostomine egg reappearance period following treatment, which can provide evidence of resistance. They also identified the cyathostomine species present.
The study team, writing in the journal Parasites & Vectors, said their evidence suggests that populations of ivermectin-resistant cyathostomines are indeed developing on the stud farm where the investigation was carried out.
It appeared that the most prevalent species of cyathostomines were responsible for the shortened egg reappearance periods seen in the study.
“Overt anthelmintic resistance will likely emerge in future, should the current regime of ivermectin administration continue,” they wrote.
The researchers said that the stud in question regularly administered, in rotation, ivermectin and moxidectin to horses found to have a faecal eggs-per-gram count of over 50.
“While aimed to provide ‘targeted’ treatments to horses with relatively high infection intensity, this practice may facilitate the emergence of anthelmintic resistance by preventing the development and maintenance of refugia of susceptible parasites.”
They instead proposed a threshold of 200 eggs per gram for treatment administration, combined with strategies of environmental control aimed to reduce the numbers of free-living larvae on the pasture, including twice-weekly removal of dung.
This, they said, may help in slowing development of resistance.
In the study, 54 yearlings were screened, with the 11 found to have a fecal count of over 75 eggs per gram enrolled in the study. Their cyathostomine counts were performed at the start of the experiment, before being dosed with ivermectin, with counts carried out following treatment on days 14, 28, 25, 42 and 49.
At day 14, no cyathostomine eggs had reappeared. However, the numbers of eggs in individual faecal samples exceeded the set threshold for the egg reappearance period at day 28 in one of the horses, at day 35 in six of the horses, at day 42 in two horses, and day 49 in the remaining two.
Before invermectin treatment, 11 cyathostomine species were detected in faecal samples: Cyathostomum catinatum, Cylicostephanus longibursatus, Cylicostephanus goldi, Cylicocyclus nassatus, Cylicostephanus calicatus, Cyathostomum pateratum, Cylicocyclus radiatus, Paraposteriostomum mettami, Coronocyclus labratus, Cylicocyclus insigne and Cylicocyclus radiatus variant A.
Of these, eggs of Cya. catinatum, Cys. longibursatus, Cyc. nassatus and Cyc. radiatus could be detected 28 days after treatment, while from day 42 onwards, cyathostomine species composition reflected data obtained before treatment, with the exception of eggs of Cor. labratus and Cyc. insigne which could no longer be detected.
The cyathostomine subfamily currently includes 52 recognised species.
Discussing their findings, the researchers said the data obtained from the faecal egg count reduction test suggested ivermectin was effective in eliminating adult populations of cyathostomines 14 days after treatment.
Nevertheless, the egg reappearance period was shorter than that originally reported for an ivermectin-susceptible population of parasites, which is 8–13 weeks.
“While this study provides valuable data on the occurrence of ivermectin-resistance in the UK, further investigations are needed to shed light on the prevalence and incidence of drug-resistance in this country, as well as other areas of the world where equine trade is substantial.
“Primarily, it should be established whether our observation that the most prevalent species of cyathostomines are responsible for shortened egg reappearance periods, are consistent across different geographical areas, or whether emergence of is dependent on specific micro-climatic and/or epidemiological conditions.
“This information is indeed crucial to inform future strategies aimed to mitigate the occurrence and spread of anthelmintic resistance.”
The study team comprised Molena, Laura Peachey, Angela Di Cesare, Donato Traversa and Cinzia Cantacessi.
Cyathostomine egg reappearance period following ivermectin treatment in a cohort of UK Thoroughbreds
Rebecca A. Molena, Laura E. Peachey, Angela Di Cesare, Donato Traversa and Cinzia Cantacessi
Parasites & Vectors 2018 11:61 https://doi.org/10.1186/s13071-018-2638-6