C. perfringens virulence factors along the road to necrotic enteritis in chickens

Avian necrotic enteritis (NE) is an enteric disease that mainly affects broilers, but is also seen in turkeys (mostly young meat-type), layer pullets, and some other avian species. In broilers, NE can present itself as either a haemorrhagic or non-haemorrhagic necrotic enteritis. Both diseases are characterized by necrosis and inflammation in the small intestine. Non-haemorrhagic NE is widespread around the globe, and is caused by specific NetB toxin-producing C. perfringens strains. On the other hand, haemorrhagic NE is not observed in Europe, but is reported by research groups from China, Canada and the USA. The NetB toxin is not involved in this necro-haemorrhagic enteritis, and it’s still unclear which toxin(s) are causing these haemorrhagic necrotic lesions. In addition to the disease in broilers, NE has also been observed in layer pullets. Macroscopically, the disease resemble NetB toxin-induced non-haemorrhagic NE in broilers. However, no NetB-producing strains could be isolated from the necrotic lesions, indicating that different pathogenic C. perfringens strains are responsible for the disease in layer pullets and broilers. Also in layer pullets, the causative toxins are unknown.

As specific toxins are essential to cause disease, they have long been the main focus in NE-related research. However, other virulence factors might be equally important in disease development. In order to fully understand the pathogenesis of NE, research should also focus on the different enzymes and virulence factors that are needed by C. perfringens to overcome the protective mucus layer and break through the epithelial barrier, to eventually cause disease. During initial colonization, bacteriocin production by a virulent C. perfringens strain inhibits the growth of commensal C. perfringens bacteria, leading to overgrowth of the virulent strain in the small intestine. This virulent C. perfringens strain interacts with the intestinal mucus layer through the secretion of various mucolytic enzymes. These enzymes can either be general C. perfringens virulence factors, such as sialidases, galactosidases, hexaminidases, or linked to a specific pathogen, such as chitinases and the ZmpB mucinase that are specific for NetB-producing strains. Breakdown of the protective mucus layer will release nutrients for further C. perfringens growth, thereby uncovering the intestinal epithelium. This enables C. perfringens to reach the epithelial cells and facilitates bacterial attachment. Once the barrier of the protective epithelial layer is broken, the C. perfringens bacteria and its toxins can move deeper into the tissue layers, thereby producing collagenases and other matrix lysing enzymes to break down the epithelial basement membrane and the lamina propria, allowing further penetration of toxins in the intestinal tissue, eventually resulting in the typical tissue necrosis and inflammation.

Many research is focussed on the development of strategies to prevent NE. Nowadays, the development of preventive measures is highly focussed on the toxins that cause tissue necrosis. However, these necrotic toxins are only expressed in late NE pathogenesis, and strategies targeting these toxins are not able to block the initial stage of disease development. In order to develop effective strategies to tackle NE, special attention should go to inhibition of the early phases of intestinal colonization by virulent C. perfringens strains.

Presented at the 7th International Conference on Poultry Intestinal Health, Cartagena, Colombia, 2022. For information on the next edition, click here.