Photosensitization in sheep fed Ammi majus (Bishop's weed) seed.

Ammi majus (bishop's weed) grows on the coastal region of southern United States and in other parts of the world. This plant causes severe photosensitization in livestock and probably contributes to the severe photosensitization outbreaks seen in Texas. Sheep were fed finely ground seed of A majus via stomach tube at dose rates of 1, 2, 4, and 8 g/kg of body weight and exposed to sunlight. The single dose of 8 g/kg produced severe clinical signs (in 24 to 48 hours): cloudy cornea, conjunctivokeratitis, photophobia, and edema of the muzzle, ears, and vulva. Daily dosing at 2 and 4 g/kg produced (in 72 to 96 hours) similar signs, whereas the smallest dose (1 g/kg) produced mild irritation of the muzzle. Pathologic changes included (1) corneal edema and marked neutrophilic infiltration of the cornea and corneal/ciliary process, (2) subacute ulcerative and exudative dermatitis of the skin of ears, muzzle, and vulva, and (3) mild focal tubular degeneration (vacuolar type) of the kidney.