Ulcerative dermatitis outbreaks caused by thorns of Mimosa setosa, M. debilis and M-pudica (Fabaceae) in horses

Mechanic natural skin lesions in horses caused by thorns of Mimosa spp. are described. Between the three plant species identified as responsible for the lesions, Mimosa setosa was present in greater quantity (80%) in the pasture, whilst M. debilis and M. pudica existed in lower proportion. Three ulcerative dermatitis outbreaks were observed during rainy periods of April to May 2013, December 2013 to February 2014 and April to May of the same year. Twenty-five horses from the Sector of Animal Reproduction, Federal Rural University of Rio de Janeiro, showed ulcerative skin lesions with irregular borders, hemorrhagic exudate, sometimes covered with scabs, located mainly in the regions of the pastern, fetlock, scapular-humeral joints, upper and lower lips, nose, nostrils, cheeks and chamfer. Seven horses were biopsied and histopathological examination revealed ulceration of the skin with inflammatory infiltrate by macrophages and neutrophils, delimited by granulation tissue. In some cases, microspicules of these plants (hirsute trichomes) were found throughout the inflammatory reaction. The diagnosis of skin dermatitis, caused by traumatic action of the plants, was based on the presence of Mimosa spp. in the pasture, on the characteristic clinic-pathological features and on recovery of the horses after their removal from the pasture. This appears to be the first report of the occurrence of ulcerative dermatitis caused by Mimosa setosa, as dermatitis caused by the others has been described before.