We have narrowed it down to one of two sources. This virus has a 21 day incubation period and we did not start having problems until we introduced welp hatchery chicks to the group when they were 8weeks old. We lost several from the group and also from our starter flock at that time starting with the buff polish we got in. They seemed to show the most symptoms of this when i looked over them. However I believe we had coccidia in the group before and that did not help any, which combined caused birds to die. The last chicks I introduced were all from this hatchery. However we could have gotten the ms from wild birds flying over as well.