Isolation of Clostridium Perfringens from Necrotic Enteritis in Chickens

I am looking for standard protocol for isolation of clostridium perfringens from necrotic enteritis in chickens. Is the enrichment step necessary for isolation? What is the most efficient enrichment broth that I could use? I had observed in some articles I read that they subject the test sample in anerobic broth to heat shock prior incubation. Why do they do that?