Not enough horse owners are thinking about the parasite burden of individual horses when it comes to giving them worming treatments, an expert in the field suggests.
Not all horses carry similar worm burdens, explains Professor Jacqui Matthews, chair of veterinary immunobiology at the University of Edinburgh and Moredun Research Institute.
A good rule of thumb, she told the recent Thoroughbred Racing and Breeding Seminar at Cheltenham Racecourse, was that 20 per cent of horses carried 80 per cent of the parasite burden.
This meant horse owners should look toward targeted treatment, she said, based on the type and stage of worm present and the level of the burden.
"Not enough of us are thinking about the level of burden individual horses have in terms of giving them worm treatments," she said.
"When we treat horses with viral disease we don't usually treat the whole group. And we don't do that with antibiotics."
Only horses that need deworming should be treated, she said.
The cyathostomins (small strongyles) are the most important group of intestinal parasites of the horse - both numerically, and through their ability to cause disease.
Controlling these parasites is becoming increasingly difficult as they are developing resistance to the drugs used against them, she told delegates.
Resistance has been reported to all three groups of dewormers currently available. As no new anthelmintics are being developed, it is important to retain the effectiveness of those available for as long as possible.
Explaing how drug resistance developed, Matthews said worm populations were extremely large and genetically diverse, and able to adapt under pressure. Drug treatments provided a very potent trigger for adaptation.
Faecal egg counts at an appropriate time (spring/summer) can be used to identify which horse to treat. But encysted worms in the gut wall in winter cannot be detected with faecal egg counts.
The key was to detect horses with high larval counts. "We've been working to develop a blood test that will enable us to measure antibody response to proteins released by these encysted stages within the gut wall."
Researchers have identified the worm proteins that are targets for these antibody responses. Making these proteins is a time-consuming process requiring a regular supply of infected guts.
However, by studying the genome of the worms, researchers have been able to identify the genes responsible.
They can now clone those genes and insert them into bacteria, which then produce a plentiful supply of the proteins.
"We get a good correlation between antibody response to the protein and the level of burden.
"Now with cyathostomins being cyathostomins we have to deal with multiple species - potentially 50 - but most horses have a common mixed bag of 8-10 species with a few rarer species thrown in.
"Our job now is to make proteins from a number of different cyathostomin species to give us wide species coverage with our blood test."
These proteins were isolated from the later larval stages (late L3 and developing L4). Now the researchers want to go back and look at early L3 encysted larvae. Hopefully within the next few years they will be able to develop a test to detect all stages of encysted larvae.
Matthews stressed the importance of detecting resistance. She explained how they have been able to identify genetic differences between fenbendazole-resistant and sensitive worms.
"What we can do is take eggs or L3 from the faeces, lyse them and amplify the DNA of the worm and analyse the segment of DNA. This kind of mutation scanning could be used to tell you whether use of fenbendazole in your horses would be appropriate.
"The faecal egg count reduction test (FECRT) is a good way of assaying the sensitivity of the worms in your horses to a particular drug. But it's a very crude assay and it does need updating and refinement."
She added that the research team at Edinburgh had just obtained funding from the Horserace Betting Levy Board to look at this test and to make it more robust and provide recommendations for vets and parasitology labs to give better thresholds on resistance levels to individual drugs.
She is looking for studs with large numbers of young horses to help in the research. The project will involve a questionnaire, treatment with ivermectin and a FECRT, which will be provided free of charge.
Horse owners need to be more aware of the possibility of resistance within worms, she concluded. "We need to target those horses with high worm burdens; primarily through the use of faecal egg counts. We also have to think about drug sensitivity of the population for the different drugs."
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