In a flock of artificially reared turkeys originally consisting of 85 birds and reduced during the summer and fall by deaths and withdrawals for experimental purposes to 42 birds, five cases of blackhead occurred. These appeared during the months of July, September, and October. In four, Heterakis was searched for and found. In 38 birds from this flock killed for food during November and December, five harbored no Heterakis, and the rest carried light infestations.
Of sixteen healthy birds withdrawn from the above flock during July and placed with a flock of older birds which had passed through this disease in former seasons, all contracted blackhead and fourteen died of the disease. The infestation with Heterakis was, as a rule, high, reaching a hundred specimens in some cases. In general, it appears that a high infestation with Heterakis is correlated with a high incidence of blackhead, a relation that had already been inferred in feeding experiments. In both of these groups no other species of worm was found in the ceca, and in instances in which examinations for coccidia were made none was found.
Pheasants have been incriminated as a source of infestation with Heterakis papillosa in artificially reared flocks.
In an artificially reared flock 38 birds that had never been ill, when killed in November and December, failed to show lesions of blackhead or evidence in the nature of scars that they had passed through an attack of the disease.
Infectious soil that had remained unoccupied by turkeys and chickens for a period of 5 months beginning in the depth of a severe winter still harbored viable ova of Heterakis and proved highly dangerous to young poults.
These experiments and observations fail to throw any light on the source of the protozoan parasite (Amœba meleagridis) which causes the fatal lesions of blackhead.