How to Diagnose and Treat Lawsonia intracellularis

Proliferative enteropathy (PE) caused by Lawsonia intracellularis has emerged as a significant equine pathogen in recent years. Early diagnosis of this disease allows prompt initiation of appropriate therapy to help decrease the severity of this disease. Lawsonia intracellularis is an obligate intracellular, gram-negative, curve-shaped bacteria. In pigs, the organism results in lesions in the lower ileum and sometimes, the large intestine. Edema occurs in the serosal layer of the intestine, which results in the necrosis and mucosal thickening. Crypt cells expand and elongate, and the bacteria is usually seen in the apical cytoplasm of these cells. Mitotic cells are abundant, whereas inflammatory cells and goblet cells are reduced or absent. The junction between normal and abnormal tissue is clearly demarcated. Affected areas usually involve the epithelial layer but not the lamina propria or muscularis mucosae; however, these deeper layers may occasionally be affected. Immature animals around weaning age are most often affected. Affected older animals are more likely to have PE with intestinal hemorrhage. In these cases, the mucosal thickening is not as apparent. Additionally, congestion of the mucosal blood vessels occurs, and inflammatory cells are consistently present in the lamina propria. As in the pig, L. intracellularis in the horse affects the crypt cells. This causes increased mitosis and thus, ileal mucosal hyperplasia. The lack of normal crypt cells and therefore, lack of a functional brush border eventually leads to intestinal malabsorption and associated clinical findings. Like the pig, affected horses are most often weanlings 6–7 mo of age. The stress of weaning and other procedures that are done at several months of age (deworming, vaccinations, training, and increased population density) may contribute to the increased incidence in this age group. A decline in maternal antibodies around this age may also increase susceptibility to the organism in these young animals. Also, environmental or seasonal factors cannot be ruled out. The method of infection and transmission of the disease in the horse is still largely unknown, although the fecal-oral route is probable. The main source of the disease in pigs is the mixing of chronic carriers with naı̈ve pigs. This seems less likely with the horse, because, unlike the pig and the hamster, clinical equine cases usually occur as isolated cases or a few sporadic cases on a farm as opposed to a herd outbreak. However, three herd outbreaks have been described in Canada, and