We’re going to focus here on the ectoparasites of horses: lice, mites and all. On June 1st all anti-parasitic medicines licensed for ‘food-producing species’ (which includes horses) become POM - Prescription Only Medicines subject to veterinary prescription.

There’s a temptation to resort to using medicines ‘off-label’ in a manner or species for which they have not been licensed. Legally such use should be under veterinary direction; beware, too, the risk of side-effects. We need to employ anti-parasitic agents in a targeted manner, to selectively and effectively treat the pest but not the rest, thus preserving insect biodiversity. Can we stop midges from biting sweet-itch sufferers? Can we shield dung beetles from blanket, blind treatments with ivermectin?

Which insect parasites matter in equines?

  • Lice - These come in biting and sucking varieties: the former feed on skin and scales, the latter burrow in and suck blood. Irritation causes itching, thus scratching and skin damage. Disturbance from feeding and sleep, plus blood loss leads to ill-thrift. Look for grey/pale pink adults or eggs firmly attached to hairs.
  • Midges - These minute flying insects, known quaintly in some parts as ‘no see-ums’ cause an allergic dermatitis and intense itching in some individuals, most notably native ponies. Repeated midge bites over a period of years leads to permanent skin damage.
  • Flies - Some actively bite, some infest wounds and some simply worry our equines. Bites lead to lumps, itching and cosmetic implications; these can also be confused with other nodules like sarcoids. Some flies lay eggs which become larvae (maggots) in moist tissues necessitating careful wound management. And some of our equines get hot and bothered, no surprise, by swarms of flies buzzing about.
  • Bots - A fly which lives on the wing in summer and is seen as eggs on legs in autumn. These live in stomachs in winter, probably doing little harm to horses.
  • Mites - There are several parasitic species with lovely Latin names, Sarcoptes and Psoroptes to name but two! Some prefer specific sites e.g. face or limbs; some are surface feeders while others burrow deep. Scales and hair loss are key features of infestation; a deep skin scrape might be needed for definitive diagnosis.
  • Ticks - Picked up on an occasional basis by equines on rough grazing, these may act as vectors for such as Lyme disease, affecting both horse and human.

    • Which medicines might we potentially use?

  • Pyrethroids like permethrin: these chemicals are applied topically as spot-ons and sprays e.g. Deosect, also louse powder for equines; some weave cattle-tags into manes or attach to head-collars. There is a licensed pour-on (Z-Itch) for non-food equines; beware hair loss or worse with the non-equine products.
  • Organophosphates like diazinon: predating and similar in use to the above, sale is increasingly controlled due to their long-lasting, adverse effect on environmental insect life.
  • Ivermectin and its cousins: licensed as oral pastes and tablets for horses. Be cautious with pour-on, spot-on and injectables for livestock: consequences may range from the mild (hair loss) and moderate (killing vital insect species) to the disastrous (death of horse and hound, especially collie-types).
  • Fipronil: licensed for dogs and cats (typically for fleas, ticks and lice) the spray may prove effective on a POM basis in equines for now; but for how long before drug resistance becomes widespread?

    • We can enhance our equines’ defence in general ways (garlic or Tea-tree oil anyone?) but also take specific preventative measures. Consider hoods and dawn/dusk stabling for sweet-itch victims; site dung heaps distant from living quarters.

    And we must ask ourelves, can we learn to live with some insect parasites, sometimes?