The successful experimental induction of necrotic enteritis in chickens by Clostridium perfringens: a critical review

Abstract

Necrotic enteritis (NE) is one of the most important enteric diseases in poultry and is a high cost to the industry worldwide. It is caused by avian-specific, Necrotic Enteritis Beta toxin (NetB)-producing, strains of Clostridium perfringens that also possess in common other virulence-associated genes. In Europe the disease incidence has increased since the ban on in-feed "growth promoting" antibiotics. Because of this, many recent studies of NE have focused on finding different ways to control the disease, and on understanding its pathogenesis. Frustratingly, reproduction of the disease has proven impossible for some researchers. This review describes and discusses factors known to be important in reproducing the disease experimentally, as well as other considerations in reproducing the disease. The critical bacterial factor is the use of virulent, netB-positive, strains; virulence can be enhanced by using tpeL- positive strains and by the use of young rather than old broth cultures to increase toxin expression. Intestinal damaging factors, notably the use of concurrent or preceding coccidial infection, or administration of coccidial vaccines, combined with netB-positive C. perfringens administration, can also be used to induce NE. Nutritional factors, particularly feeding high percentage of cereals containing non-starch polysaccharides (NSP) (wheat, rye, and barley) enhance disease by increasing digesta viscosity, mucus production and bacterial growth. Animal proteins, especially fish meal, enhance C. perfringens proliferation and toxin production. Other factors are discussed that may affect outcome but for which evidence of their importance is lacking. The review compares the different challenge approaches; depending on the aim of particular studies, the different critical factors can be adjusted to affect the severity of the lesions induced. A standardized scoring system is proposed for international adoption based on gross rather than histopathological lesions; if universally adopted this will allow better comparison between studies done by different researchers. Also a scoring system is provided to assist decisions on humane euthanasia of sick birds.

Figure 1

Critical factors influencing the development of necrotic enteritis. Summary of different factors important for the successful reproduction of necrotic enteritis; the most critical factor is presence of netB and the netB plasmid, but other factors summarized in the figure all affect the outcome of experimental infection. They can be manipulated by researchers to vary the severity of the disease produced and the outcome desired.

Figure 2

Different lesions of necrotic enteritis in chickens, used to illustrate the scoring system (Table 2 ).a: Necrotic enteritis score 0, everted jejunal segment. No gross lesions are present. b: Necrotic enteritis score 1, everted jejunal segment. There are no obvious ulcers in the mucosa, but the entire mucosal surface is covered with a layer of loosely adherent fibrin. c: Necrotic enteritis score 2–4, everted jejunal segment. There is an excavated ulcer of the mucosa with acute, bright red hemorrhage within the ulcer bed and scant crusting of fibrin around the periphery. d: Necrotic enteritis score 2–4, everted jejunal segment. There is an excavated ulcer of the mucosa with dark green-black pigment within the ulcer bed and scant crusting of fibrin over the surface. e-f: Necrotic enteritis score 2–4, everted jejunal segments. There are excavated ulcers of the mucosae, the periphery of which are covered by thick, tightly-adherent layers of fibrin, necrotic tissue, and inflammatory cells. g-h: Necrotic enteritis score 5–6, everted jejunal segment. The mucosae are covered by large, confluent plaques of fibrin, necrotic tissue, and inflammatory cells (g) to the point where they extend over broad regions of the intestinal mucosa (h).