All horse owners know exactly what it is when their horses develop itchy hairless patches on their heads – they’re suffering from ‘sweet itch’ or ‘grass mange’. In reality, though, these are two different conditions with differing causes.
‘Sweet itch’ is the name given to a skin disease caused by hypersensitivity to the bites of midges – the same Culicoides that transmit horse sickness.
The disease is well-recognised worldwide and the chemical molecules (allergens) in midge saliva that give rise to the allergy have been described in detail. The reaction is blamed on the release of immunoglobulin E, an antibody that binds to the allergen with a cascade of histamines and cytokines. The result is an itchy reaction in the skin. Hairless patches will also be found along the side of the neck and at the base of the tail.
Grass mange has an entirely different cause.The word ‘mange’ refers to an infestation of the skin and hair by a biting mite, a tiny eight-legged creature that also causes mange in other species, such as dogs. Several different species of mite are blamed for mange in horses. Of these, the most serious is sarcoptic mange. It often starts on the neck and face, but may extend to the whole body. Itchy patches become hairless and, over time, the spots coalesce and the skin becomes bare and wrinkled and covered with crusts.
The condition is frequently related to poor nutrition or other diseases that decrease immunity. The probable cause of ‘true grass mange’, however, is the harvest mite, also known as ‘chiggers’. This is a trombiculid mite that normally infests small rodents and the larvae feed on plant material. Horses can be infected while grazing or even when eating hay.
Often a vet will prescribe antihistamine tablets, which will take away the itch. This works well for genuine sweet itch, as it’s an allergy. If measures are used at the same time to repel midges, such as face masks and even insect-proof rugs, the problem will be sorted out fairly quickly. However, bear in mind it’s likely that, as in the case of humans, allergic responses are hereditary and it may be a good idea not to breed with these horses.
Mange, on the other hand, is caused by a parasite. Antihistamines will decrease itching in the short term, but action also has to be taken to kill the mites. Sarcoptic mites can be diagnosed from skin scrapings. Lime-sulphur washes, insecticidal shampoos and ivermectin given orally are usually needed. Just to confuse matters further, there’s a possibility of mixed causes. These include tick larvae, biting flies and ringworm, as well as mange and midge bites.
In general, for itchy faces and necks for horses in South Africa, the first thing to do is put on a fly mask, then de-worm with ivermectin oral paste and spray or sponge with a repellent or dip containing the insecticide cypermethrin, registered for horses. If this doesn’t work, consult a vet.
Contact Dr Mac at [email protected]. Please state ‘Horse talk’ in the subject line of your email.