Immunizing action of in vitro-attenuated Histomonas meleagridis in chickens and turkeys

Histomonas meleagridis was cultivated more than 6 years in Tissue Culture Medium 199, fortified with serum and antibiotic-killed bacteria of cecal origin. The histomonads first lost their ability to produce disease but retained some immunizing ability. During the sixth year of in vitro cultivation, representing serial passages 730–835, the immunizing ability declined. At the same time, these attenuated histomonads were also losing their ability to survive in the presence of freshly isolated, antibiotic-killed cecal bacteria. Meantime, freshly isolated histomonads were unable to live in the culture medium with only the limited flora that had survived the numerous in vitro passages. Apparently, long continued cultivation operated selectively on both the bacteria and Histomonas, leaving only histomonads with no ability to penetrate the host's tissues.