Lawsonia: Emerging Concern for Horse Industry

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.

Diagnosis is based on clinical signs, low serum protein level, SI wall thickening, positive serum titer, and DNA analysis of feces for the bacterium. Taken individually, none of these are diagnostic, so when PE is suspected, the vet should rule out common intestinal conditions. If the animal dies, clinicians can confirm the bacterium by microscopically examining cells or immunohistochemistry staining of fixed tissues.

If a vet diagnoses PE early and administers appropriate antimicrobial therapy, the outcome can be good. Oxytetracycline, doxycycline, or chloramphenicol alone, or azithromycin, clarithromycin, or erythromycin, alone or combined with rifampin, appear to be effective for infection control. Michelle Frazer, DVM, Dipl. ACVIM, of Hagyard Equine Medical Institute, in Lexington, Ky., evaluated sale price and race records in Thoroughbreds diagnosed with L. intracellularis. She found that L. intracellularis horses do not draw comparable sales prices to others by 1 year of age, but PE doesn't seem to adversely affect athletic potential. Foals with L. intracellularis therefore may have decreased initial (up to 1 year of age) weight gain, compared to unaffected animals.

L. intracellularis can cause outbreaks and become endemic on farms. This raises questions about origin of contamination and the financial impact of PE on the industry. It's unknown if foals or asymptomatic adults contribute to propagation of foal infection.

Wildlife and possibly other domestic animals might serve as L. intracellularis reservoirs. Scientists have detected the bacterium in mice, rats, guinea pigs, wild pigs, hamsters, hedgehogs, rabbits, ferrets, dogs, wolves, fox, calves, fallow deer, white-tailed deer, giraffes, monkeys, emus, and ostriches. Skunks, jack rabbits, Virginia opossums, coyotes, dogs, and deer could be sources of contamination for foals.

Further studies are needed to determine route of infection. Current research is under way at Hagyard Equine Medical Institute, the University of California, Davis, the University of Minnesota, and Boehringer Ingelheim on preventing the disease with a novel modified-live vaccine.

This information was discussed at the Boehringer Ingelheim-sponsored L. intracellularis Workshop, which was held at Hagyard Equine Medical Institute in June. Participants included the author; Michelle Frazer, DVM, Dipl. ACVIM; Nicola Pusterla, DVM, PhD, Dipl. ACVIM; Connie Gebhart, PhD; Jean-Pierre Lavoie, DVM, Dipl. ACVIM; Julie Wilson, DVM, Dipl. ACVIM; Unneeda Bryant DVM; Alan Loy-nachan, DVM, PhD, Dipl. ACVP; Allen Page, DVM; Bob Stenbom, DVM; Ann Chapman, DVM; Marian Graves Little, DVM; Eran Raizman, DVM, PhD; and Nora Nogradi, DVM.

About the Author

Nathan M. Slovis, DVM, Dipl. ACVIM, CHT

Nathan M. Slovis DVM, Dipl. ACVIM, CHT, received his DVM from Purdue University. He is board certified in large animal internal medicine and he is currently the Director of the McGee Medical Center at Hagyard Equine Medical Institute in Lexington, Ky. His special interests are in neonatology, infectious diseases, and hyperbaric medicine (in which he is certfied as a hyperbaric technologist).

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