Pruritus is the technical term for itchiness, the unpleasant sensation that leads horses to bite, scratch or rub at their skin. Sometimes the sensation is so strong that horses will cause severe damage to themselves or their environment, while in extreme cases, horses cannot tolerate tack on their skin, let alone a rider.
Although poorly understood, pruritus is known to result from the stimulation of special nerve endings and receptors in the skin. In the horse, the three main factors inducing itchy skin are ectoparasites (such as biting insects), allergies and some infections. Biting insects including lice, midges, black flies and horse flies can trigger cases of pruritus, but milder cases can occur simply as a horse sheds its coat.
In most cases a severe itch is made up of a number of smaller itches. The point at which a horse responds to an itch will vary from animal to animal and is known as the pruritus threshold. Below this threshold the horse may have potentially itchy stimuli present, but not respond to them.
One example is that a horse can carry a burden of parasites such as lice, which cause damage to the skin, without showing any clinical signs. However, once the damaged skin becomes infected, this additional level of discomfort can push the horse beyond its pruritus threshold, causing it to rub or bite itself.
Sweet itch in horses is the term for the itchiness caused by midges, which typically appears in spring and settles down to virtually disappear during the winter provided it is cold enough for the midges not to be flying, which is usually less than about 4°C. For this reason, purchasers need to be aware of the potential risk of buying a horse with no symptoms during the winter which, by mid-summer, could turn into a major sweet itch case. Affected horses and ponies, which are more commonly affected, are sensitive to the irritants in midge saliva.
Signs of sweet itch in horses
- Mild to severe itching and rubbing, usually along the mane, back and tail
- Loss of tail and mane hair
- Bald patches, which can look ugly and grey due to permanent hair loss and skin damage
- Areas of sore, open, broken skin, which tend to bleed
- In some cases, itching along the legs and under the belly