Sub-clinical necrotic enteritis in broiler chickens: Novel etiological consideration based on ultra-structural and molecular changes in the intestinal tissue

The present study revealed several previously not recognized etiological details in the development of necrotic enteritis (NE) in broilers. We provide evidence that the pathological process leading to mucosal epithelium necrosis follows morphologically distinct phases commencing at the basal domain of the mucosal epithelium and then progressively invading the entire lamina propria. Initially mucosal epithelium appears normal, but as the pathological changes progress throughout the lamina propria, the adjacent enterocytes begin to show features of necrotic cell death and the necrotic process of the epithelium progresses from being focal to locally extensive.Ultra-structural examination showed that primary changes occur at the level of basal and lateral domains of the enterocytes, whereas the apical domain of enterocytes remains intact even in the face of advanced necrotic changes. This indicates that the mucosal necrosis does not result from direct damage to the mucosal epithelium. Rather, the necrotic death of enterocytes is a consequential effect of the destruction of lamina propria, the extra-cellular matrix, and intercellular junctions.The nature of these morphological changes indicates that initiation of the pathological process leading to NE involves proteolytic factors affecting the extra-cellular matrix and cellular junctions. Further studies revealed that, indeed, the elevated activity of collagenolytic enzymes in the mucosal milieu and in intestinal tissue represents an integral component of the pathological process leading to NE. In the first instance we discovered that Clostridium perfringens strains isolated from field cases of NE secrete several potent collagenolytic enzymes. In the second instance we observed that, in comparison to controls, broilers challenged with C. perfringens isolated from field cases of NE show high levels of several collagenolytic enzymes in the intestinal tissue. A major component of the overall collagenolytic activity detected in the intestinal tissue was identified by zymography as matrix metalloproteinases (MMPs). Dominant activity was associated with MMP-2. We confirmed using immuno-histochemistry that this enzyme is expressed at high levels in mucosal tissue showing signs of NE.The high levels of collagenolytic activities, in particular associated with MMP-2, demonstrated in our studies are consistent with the nature of morphological changes observed primarily in extra-cellular matrix (ECM) at the basal domain of enterocytes, as well lateral domains of enterocytes. The lack of changes at the level of apical domain of mucosal epithelium indicates that the lipolytic aspect of alpha toxin in NE is not an essential factor in primary lesions development. Taken together, our findings indicate that the early lesions leading to NE are associated with virulence factors that induce proteolytic activity, rather than lipolytic activity.