14 Nutritional disorders
It is easy to forget that the skin, like other organs, requires the normal nutrients if it is to behave in a normal fashion. Almost any inappropriate food material and any gross deficiency of the basic requirements can affect the skin to some extent (Hintz 1993). However, even prolonged malnutrition may have surprisingly little material effect on the skin and hair.
Dietary minerals are known to affect coat and skin health as well as skin healing ability.
1. Zinc in particular appears to have a major influence on skin health and an adult horse requires a regular daily intake of around 500 mg of zinc. Deficiency of zinc develops within a short time as reserves are low and manifests as generalized alopecia with extensive surface scaling and flaking giving an appearance of severe dandruff (Harrington et al 1973). Initially the lesions may appear on the thighs and ventral abdominal wall. Severe and prolonged deficiencies result in generalized exudation and flaking with extensive and severe loss of hair (Fig. 14.1). Any remaining hair is easily pulled out by light grooming. The histological appearance is characteristic (hyperkeratosis, acanthosis and parakeratosis). Daily dietary supplementation with zinc methionine or other zinc salts has a curative effect over some weeks.
2. Iodine deficiency during pregnancy has obvious effects on the hair coat of the foal at birth. Affected foals have a sparse coat and an obvious goitre is often found at birth. Other deformities such as contracted tendons and fused joints (arthrogryposis) may be associated with iodine deficiency. Chronic excessive iodine fed to adult horses (often in the form of seaweed powder supplements, etc.) may be responsible for a sparse, short hair coat (Fadok & Wild 1983). Iodism can also develop during treatment of fungal (and other) infections with potassium or sodium iodide. The earliest signs include lacrimation and a scurfy, dry coat quality (see below).
3. Copper deficiency causes a loss of black pigment because it is essential for the production of melanin (McLean & Jones 1983). Affected horses develop a coarse, harsh hair quality and a russet-brown hue to the darker areas of the coat. Alopecia and browning of the hair around the eyes gives the animal a ‘spectacled’ appearance, but these signs are not as prominent in the horse as in cattle and may easily be missed. Confirmation of the diagnosis can be made from blood and liver assays for copper, but response to careful supplementation may be as useful. A more serious tendency to arterial rupture and chronic anaemia is possibly associated with copper deficiency. Horses may be more resistant to copper deficiency than other domestic animals but there have been few studies on the status of horses grazed on known copper-deficient land.
