In cattle and sheep, blood flow to the peripheral tissues decreased and body temperature increased when tall fescue seed was included in the diet (Rhodes et al., 1991). The reduction in blood flow to the peripheral tissues probably is related to increased body temperature because the animal is less efficient in cooling itself. Unlike cattle and sheep, pregnant mares exhibited no increase in body temperature when exposed to the endophytic toxins (Monroe et al., 1988; Putnam et al., 1991). However, horses sweat more freely than cattle and are more capable of cooling themselves. Putnam et al. (1991) observed increased sweating in gravid mares grazing E+ tall fescue. In cattle, peripheral vasoconstriction, that is, reduction in size of the blood vessels, caused by the alkaloids of E+ tall fescue results in fescue foot (see Chapter 18) (Solomons et al., 1989). Rohrbach et al. (1995) reviewed data from 185,781 horses, of which 5536 had a diagnosis of laminitis. Although these data were preliminary, they concluded that there appeared to be a relationship between laminitis in horses and consumption of E+ tall fescue. Abney et al. (1993) observed a vasoconstrictive effect of ergot alkaloids on equine vessels in vitro. Carbohydrate overload and aqueous extract of black walnut (Juglans nigra L.) (Galey et al., 1990) are associated with the development of laminitis in horses. The aqueous extract of black walnut caused post-capillary venoconstriction (reduction in size of the vein), increased capillary hydrostatic pressure, and transvascular fluid movement (movement through the capillary wall), resulting in increased tissue pressure, edema, vascular collapse, and ischemia (reduced blood perfusion) in the equine digit (Eaton et al., 1995). It is possible that through interaction of the ergot alkaloids with the adrenergic receptors of the sympathetic nervous system, similar responses may be occurring in horses consuming E+ fescue, but direct evidence for this hypothesis does not exist.


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