“No virus is known to do good. It has been well said that a virus is a piece of bad news wrapped up in protein.” Medawar and Medawar

Kuwait – 28 – 29 – 30 September 2014

Virus

Virus infections of the gastrointestinal tract occur commonly in chickens and turkeys. These infections occur in birds of all age groups but tend to predominate in young birds. Clinically, these infections result in a broad range of outcomes from inapparent, economically insignificant effects to those that are severe and economically devastating. The outcome of these infections is determined by a variety of interacting factors not least of which are age and immune status of affected birds, and virulence of the involved virus(es). In field situations, these infections almost always are complicated by other infectious agents as well as management, nutrition and environmental factors, thus the true role of these agents in naturally occurring gastrointestinal diseases often is difficult to assess.

JAMES S. GUY

Department of Microbiology, Pathology and Parasitology, College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina 27606

Virus

Intestinal integrity is compromised when the mucus layer is degraded; epithelial cells are effaced or destroyed, the vascular supply is interrupted, or the immune system is compromised. Intestinal integrity as considered specific to the epithelial layer, can be damaged by viruses, bacteria, fungi, myriad parasites, and toxins; reviewed by Moon, 1997

Viruses: Infection and replication of an enteric virus usually kills an epithelial cell. In contrast to bacteria, viruses do not produce toxins. Each of the intestinal viruses has a tropism for cells in a specific state of differentiation along the villus. Viruses preferentially infect cells in the crypts, on the tips and sides of the villus, or only on the tip, respectively. The severity of the clinical disease and the course of the uncomplicated viral infection are a reflection of target cells destroyed by the virus. For a virus that destroys cells on the tip of the villus, the absorptive function of the gut is lost and surviving epithelium is secretory. Watery diarrhea occurs until the villi are repaired with mature functional cells on the tips. A torovirus-like virus isolated from turkeys with Poult Early Mortality Syndrome (PEMS) does not cause death vof the host cell (intestinal epithelial cells. Rather, the cell is stimulated to release messengers (cytokines) that interact with the immune system, and perhaps a more complex series of reactions with the inflammatory response, and nerves in the wall of the intestine. Each of these, in turn, releases lymphokines or mediators that produce cyclic amplification of responses by the intestinal epithelium. The result is increased fluid secretion in the gut that overrides its absorptive capacity, causing diarrhea and decreased digestive efficiency.