Worming only horses that need it can be cost effective, even allowing for the cost of performing faecal egg counts, research has shown.
Cyathostomins – small strongyles or redworms – are not only responsible for a considerable proportion of worm-related disease in horses, but are also the most likely to develop resistance to deworming drugs.
Among other factors, the development of resistance has been attributed to the regular exposure of susceptible worm populations to deworming agents.
Interval dosing programmes, once recommended, are now being implicated in selecting for anthelmintic resistance.
Rather than concentrating on preventing excretion of worm eggs altogether, the aim now is to ensure parasite populations remain sensitive to the drugs, while minimising the risk of parasite-associated disease.
It is now widely acknowledged that a more targeted approach is preferable. Current recommendations are that only those horses carrying a moderate or high worm burden are treated, thus ensuring that worms are not exposed to deworming drugs needlessly.
Faecal worm egg counts (FECs) are used to determine which horses require treating. To many owners this may seem an unnecessary expense.
However, recent work has shown that using FECs in this way helps reduce the overall cost of worming.
Hannah Lester, with colleagues at the Moredun Research Institute, and the Universities of Bristol, Liverpool and Edinburgh, monitored FECs at three-monthly intervals over a nine-month period.
In all, 368 horses from 16 separate yards were involved in the study, the latest issue of Equine Science Update reports.
Horses with FECs greater than 200 eggs per gram were treated. If treated, horses were given pyrantel (in March and June) and ivermectin in September. All horses received moxidectin/praziquantel in December.
The researchers, whose findings were published in the journal, Veterinary Record, compared the cost with that of a standard interval regime of two treatments with moxidectin and two of moxidectin and praziquantel – which is what had been common practice in the study population.
They estimated the cost of the two approaches by using average prices for anthelmintic products and faecal egg counts that they obtained off the internet.
Even allowing for the cost of faecal egg count reduction tests (that is, repeating the FEC after each treatment to check the anthelmintic had been effective) they found that, over the year, there was an average saving per yard of £294.44.
The study also showed that, overall, 15 percent of horses were responsible for shedding 80 percent of strongyle eggs. So a traditional interval program, in which all horses are treated regardless of FEC results, would mean 85 percent of horses being treated unnecessarily.
They concluded: “These findings support the notion that targeting anthelmintic treatments at those individuals with strongyle FEC of 200 eggs per gram or greater facilitates a reduction in selection pressure for anthelmintic resistance.
“Moreover, the results show that such a strategy has a high chance of reducing the financial cost compared with that associated with more traditional interval treatment regimens, and horse owners should, therefore, be discouraged from the view that it is cheaper to treat all horses prophylactically over time.”