Blackhead disease (histomoniasis) is a disease that affects turkeys, chickens, pheasants, quail, and some other birds. It is caused by the protozoa Histomonas meleagridis that are spread to the bird by the roundworm Heterakis gallinarum.
While chickens are mostly immune to the pathogen, it’s fatal to turkeys. And when living together, the turkeys often get it from the chickens that carry the protozoa without being infected. This makes the chicken coop and yard a deadly place for the big birds.