That the ovum of Heterakis papillosa is an important source of the virus in the natural transmission of blackhead in turkeys is suggested by experimental evidence obtained in the present investigation. Confirmatory morphological evidence has not thus far been obtained to establish definitely that this worm serves as the invertebrate host of Histomonas meleagridis.

It is possible occasionally to produce blackhead in turkeys by feeding large amounts of the virus alone, as obtained in liver lesions. This artificial procedure is, however, not duplicated in nature since the lesions of the disease are not eaten by young turkeys.

The intentional contamination of the food of young turkeys with dirt taken from hen yards has in the present experiments invariably produced blackhead. The conditions provided are essentially those that prevail in many farmyards.

The identity of blackhead infection in common fowls with that of turkeys is indicated by the production of the typical disease in a turkey by the inoculation of liver lesions obtained from a diseased chicken.

It is not known whether variation in the period between the ingestion of Heterakis ova and the onset of the disease is dependent on the absence of the virus for a time or on the absence of the conditions necessary for invasion.

Under experimental conditions a large proportion of the Heterakis ova fail, at least when fed by hand, to hatch out in the intestine, but, passing through, may later be taken up with contaminated food. This is shown by the occurrence in the ceca of larvæ in various stages of development long after the Heterakis material is fed.

Ipecac, like other drugs previously tested, is found to be of no practical value in the control of blackhead. It has no deleterious effect on Histomonas in the tissues and does not prevent the development of Heterakis. Its daily administration to the limit of tolerance fails to prevent infection or a fatal outcome in the naturally acquired disease.

The fact that ipecac may delay the onset of the infection may perhaps be due to some physiological property of this drug. Sulfur administered daily in large amount likewise, possibly on account of its evacuant action, tends to delay infection.

Recovery from inoculated blackhead is associated with a degree of immunity. That this is not permanent is indicated by the appearance of symptoms of blackhead several months after recovery from the inoculated disease. The fact that blackhead sometimes appears in mature turkeys that have been kept constantly on infested ground suggests that these birds may suffer from repeated attacks.

Observations made during the course of the present investigations confirm previous conclusions as to the flagellate character of Histomonas meleagridis.

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