Etiology and Pathology of Equine Placentitis

Abstract

Placentas from aborted, stillborn, and premature foals were examined during the 1988 and 1989 foaling seasons, and 236 of 954 (24.7%) had placentitis. Microorganisms associated with placentitis were isolated or demonstrated from 162 of 236 (68.6%) placentitis cases. Leptospira spp. and a nocardioform actinomycete were 2 important, newly emerging bacteria associated with equine placentitis. Major pathogens identified in decreasing order were Streptococcus zooepidemicus, Leptospira spp., Escherichia coli, a nocardioform actinomycete, fungi, Pseudomonas aeruginosa, Streptococcus equisimilis, Enterobacter agglomerans, Klebsiella pneumoniae, and alpha-hemolytic Streptococcus. Pathogens were not recovered in 64 cases (27.1%) and overgrowth by saprophytic bacteria was recorded in 10 cases (4.2%). Twenty-seven cases (16.6%) had mixed bacterial growth and 93 cases (57.4%) had bacteria cultured from both placenta and fetal organs. The majority of the placentitis cases caused by bacteria, with the exception of Leptospira spp. and the nocardioform actinomycete, occurred in 2 forms. One was acute, focal or diffuse; had an infiltration of neutrophils in the intervillous spaces or necrosis of chorionic villi; was associated with bacteremia; and frequently occurred in the placenta from fetuses expelled before or at midgestation. The other was observed from foals expelled at late gestation, was mostly chronic and focal or focally extensive, and occurred mostly at the cervical star area. Chronic placentitis was characterized by the presence of 1 or a combination of the following lesions: necrosis of chorionic villi, presence of eosinophilic amorphous material on the chorion, and infiltration of mononuclear inflammatory cells in the intervillous spaces, villous stroma, chorionic stroma, vascular layer, and allantois. Chorangiosis, hyperplasia with or without squamous metaplasia of the chorionic epithelim, and adenomatous hyperplasia of the allantoic epithelium were frequently associated with chronic placentitis. Leptospira spp. induced, regardless of gestational age, a diffuse placentitis with the presence of a large number of spirochetes in the stromal tissues. The nocardioform acti-nomycete, a gram-positive, filamentous, and branching bacillus, induced an unique chronic-active focally extensive placentitis located at the base of the horn or at the junction between the body and horn of the allantochorion. Fungi, with the exception of Histoplasma sp. and Candida sp., induced a chronic focally extensive placentitis at the cervical star area similar to that seen in chronic bacterial placentitis. Histoplasma sp. induced a multifocal granulomatous placentitis, and Candida sp. induced a diffuse necrotizing and proliferative placentitis.