Immunizing Action of a Nonpathogenic Strain of Histomonas against Blackhead in Turkeys

SYNOPSIS. A nonpathogenic strain of Histomonas was used in an attempt to immunize young turkeys against blackhead. Rectal inoculation of several thousand nonpathogenic histomonads on 2 or 3 consecutive days afforded considerable protection against modest rectal challenges with pathogenic histomonads 3 to 6 weeks later, but was much less effective against pathogenic histomonads introduced by feeding eggs of Heterakis gallinae. It is believed that an immune barrier limited to the surface of the cecal mucosa was established, and that the larvae of the cecal worms often penetrated this barrier before liberating their histomonads, thus permitting blackhead to develop. Immunization by the introduction of nonpathogenic histomonads with Heterakis eggs was not satisfactory. Apparently, the nonpathogenic organisms introduced in this way were too few to assure formation of an intact barrier.