Necrotizing Enterocolitis in Foals
Necrotizing enterocolitis (damage and death of cells in small intestine and colon) is a serious disease of sudden onset in foals less than 4-6 days of age associated with a high death rate, despite therapeutic interventions. Clinical manifestations of enterocolitis in foals vary from being found dead to a spectrum of clinical manifestations.

The most common of these include failure to suckle or nurse, fever, depression, severe colic, and diarrhea. Although the association of Clostridium difficile, Neorickettsia risticii (which causes Potomac horse fever) and Salmonella infection with enterocolitis in adult horses is clearly established, many cases of fatal necrotizing enterocolitis in foals have no known risk factors.

Clostridium perfringens is the most commonly isolated clostridial species worldwide and is part of the normal intestinal flora. Following acquisition of this bacterium by the foal from the mare’s teats or the environment, the organism multiplies rapidly in the stomach and intestines. C. perfringens numbers are reduced fairly quickly, so that by the time foals are several months of age, the organism is found in relatively low numbers within the large intestine. The almost universal presence of C. perfringens type A in the intestine of healthy young animals has complicated the understanding of its role in enterocolitis. Although most strains don’t cause intestinal disease, there are two types that do so in foals.

One of these, C. perfringens type C, is a well-established but relatively uncommon cause of necrotizing enteritis. The other is C. perfringens type A, which includes a small subset that produces a novel pore-forming toxin called NetF (Necrotizing enteritis toxin, Foal). This novel toxin is related to the beta toxin of type C strains that causes severe enteric disease in foals, other species, and in humans. We found NetF-positive C. perfringens in 74% (11/15) of foals with necrotizing enteritis but not in 11 foals with undifferentiated diarrheal illness. In another study, NetF-positive C. perfringens was identified in 6 of 23 isolates from foals in Kentucky with severe enteritis. In adult horses with undifferentiated diarrhea, the detection rate of NetF among C. perfringens isolates was low (4/58). This would suggest this toxin is primarily associated with severe enteritis in neonatal foals.

One explanation why type C and NetF-positive type A C. perfringens cause necrotizing enteritis in very young foals is because of the trypsin inhibiting effect of colostrum. Trypsin is a protein-degrading enzyme secreted by the pancreas during digestion, and its inhibition by colostrum prevents the breakdown of protein toxins, such as NetF. The reservoir for NetF-producing C. perfringens is not yet known.

Real-time PCR can be used to rapidly diagnose necrotizing enteritis caused by NetF-positive C. perfringens. An alternative but slower approach is to culture C. perfringens and confirm the presence of toxin genes (cpb for type C and NetF for type A) by PCR.

In terms of prevention, an autogenous bacterintoxoid vaccine has been used for mare immunization in Kentucky in an attempt to prevent type A C. perfringens enteritis in foals. This vaccine likely includes NetF toxin since mares immunized with the vaccine have antibodies to NetF. Hyperimmune plasma is commercially available for the prevention or treatment of necrotizing enteritis in foals caused by C. perfringens types A (including NetF), C, and D.

Although the discovery of NetF has furthered our understanding of C. perfringens enterocolitis in foals, further work is required to fully understand how NetF-producing type A C. perfringens causes disease. Research studies are slowly chipping away at C. perfringens enterocolitis, and the discovery of NetF has been another important step forward.

CONTACTS—Iman Mehdizadeh Gohari, DVM, MSc—

John F. Prescott, MA, Vet. MB, PhD——519/824-4120 ext. 54716

University of Guelph Ontario Veterinary College Department of Pathobiology, Canada

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.