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Worming Horses in the Beginning
Worms are parasites! Parasites are creatures that have evolved to live in or on another creature and they have been in existence for many millions of years. There is evidence that even the earliest civilisations were aware that parasites existed, at least in people.
Worms have existed in horses for as long as horses have existed, it is, of course, a ‘which came first scenario – the chicken or the egg?’ Treating horses for worm infestations became necessary because we domesticated them; a horse became a valuable working animal that we needed to keep in good health. An article about the history of worming horses, published in the scientific journal “Veterinary Parasitology”, describes how horses were treated for worms in bygone centuries. The methods used were crude and strange, treatments included administering such things as soap, liquorice, linseed oil, tobacco and even mercury. These substances would have had little effect, except to make the horse ill, especially mercury which of course is highly toxic. People who had no training in medicines or pharmaceuticals experimented with herbs and the recipes they used were probably ineffective or harmful to the horse.
A book published in 1891, entitled ‘Veterinary Country Practice’ was an early attempt by veterinarians to establish, presumably what were thought at the time to be, relatively ‘safe’ compounds. The ingredients for these compounds included herbs such as garlic, ginger and valerian; as well as poisons such as arsenic and turpentine.
Chemicals used to worm horses between 1940 and 1970 were administered by veterinarians and were highly toxic to people as well as horses.
In 1966, two scientists called Drudge and Lyons recommended that horses should be treated with anthelmintic (wormer) chemicals every 8 weeks, all year round.
The first type of Benzimidazole drugs used for worming horses became available in the 1970’s. The worm that was the main target of this drug was Strongylus vulgaris, which at that time, was causing the greatest concern within the equine industry. It was known in 1966 that it was possible for worms to become resistant to Benzimidazole but no one recognised that this was a problem that could or would occur on a worldwide basis. Benzimidazole was a safer chemical group than any that had been used before and resulted in a paste being produced and sold commercially. Horse owners could give their horse a worm control medicine, without consulting a vet. This seemed like a step forward in the fight against preventing illness and death in horses but removing the expertise of the veterinarian may have contributed to the situation to date, of worms being resistant to worming chemicals.
The drug Pyrantel (pyrimidine group) became available as an alternative to Benzimidazole, in the 1970’s. Advice to horse owners, at this stage was to rotate the type of chemical given to the horse, in the hopes of reducing the progress of resistant worms.
Subsequently Ivermectin and Moxidectin became available and have been used in the rotation regime to date.