Proliferative enteropathy (PE, proliferation of cells within the small intestine, or SI) or ileitis (inflammation of the portion of the SI called the ileum) is a common infectious disease affecting weaned animals of various ages and species. Veterinarians also have documented it in a few adult horses. PE is of special economic importance to the swine industry and is an emerging concern for the horse industry. In all species PE is caused by the obligately intracellular (can only survive in cells) bacterium Lawsonia intracellularis, which causes an unusual pathology: intracellular growth causes proliferation of the cells lining the SI), thickening the mucosa.

Infection has worldwide distribution; PE has been reported in the United States, Canada, Great Britain, Belgium, Switzerland, France, Australia, and South Africa.

There seems to be no gender or breed predisposition to PE. Foals might present with edema (fluid swelling) of the head or abdomen, depression, fever, lack of appetite, weight loss, diarrhea, and colic of varying severity. In a retrospective study of 57 L. intracellularis-infected horses, scientists noted throatlatch and ventral abdomen edema were likely, while diarrhea, lethargy, and fever were variable. A low serum protein level (specifically, albumin, and called hypoalbuminemia) was a consistent finding; fibrinogen and white blood cell count levels were variable. Foals or weanlings were most likely affected, and cases were common August-January. Thus, L. intracellularis should be a differential diagnosis for youngsters with hypoalbuminemia and ventral edema in the fall or early winter.

Lesions suggestive of PE involve marked thickening of the mucosa in parts of the SI, giving it a corrugated appearance. Lesions are often confined to the jejunum and to the ileum, where they're more severe. Edema can thicken other parts of the GI tract