Masterclass - The problems of equine parasite control
The bewildering array of information and recommendations coming from drug companies indicates that there is no single, simple solution to these problems. The specific problems that horse owners face are:
- Resistance to worming drugs
Worms can develop resistance to worming drugs in just the same way that bacteria can develop resistance to antibiotics. Resistance occurs across chemical groups of worming drugs, if the worms are resistant to one drug in that group, they will have cross-resistance to all chemically similar drugs. This is a worrying situation because it may mean that in the future some or all of the worming drugs presently available will be ineffective. At present, we have widespread resistance to benzimidazole drugs in the UK but not to pyrantel or ivermectin/moxidectin. We must delay the onset of resistance to these drugs by using them responsibly and sparingly. Diagnostic tests can help us to do this.
- Worming drugs are not all the same
Just as different antibiotics are active against different bacteria, so worming drugs each have their own spectrum of activity. To control all the major parasites of horses, different drugs must be used. Each type of wormer also has its own recommended dosing interval. This is because the period of suppression of the passage of worm eggs is different for each drug.
- Livery yards with many horse owners
Many horses are kept on livery yards where individual owners are responsible for worm control in their horses. Under conditions of shared grazing, owners must realise that if their horse is shedding lots of worm eggs, other horses in the paddock are likely to become infected. The paddocks available for grazing are often smaller than is ideal for the number of horses. This leads to close-cropped grass and horses grazing close to piles of dung. Most livery yards also have a high turnover rate of horses. New horses, with unknown worming histories, are a particular threat. They might bring worms into a previously “clean” population of horses; they might bring resistant strains of worm into the population. Diagnostic tests can help assess the parasite status of new horses so they can be treated appropriately.
- Cyathostomes living in the gut wall
Not only do cyathostome parasites live inside the intestine, larval stages of the parasite can live in cysts in the gut wall. These encysted parasites can “hibernate” in the gut wall, only to re-activate at some time in the future. At present it is impossible to detect these encysted parasites with diagnostic tests.
Due to the complexity of this subject, horse owners need expert advice on their particular situation and potential problems. Your veterinary surgeon is the person with the greatest understanding of this subject and is best placed to give accurate advice.