It has been reported that Plasmodium isolated from fowls in Japan is less virulence.The authors have found exoerytlarocytic forms in chickens inoculated with a strain ofPlasmodium (P. juxtanucleare) isolated from a chicken.The present paper deals with examination made on the pathogenicity of this strain.1. Infected chickens showed a high fatality rate (66.7%). They died sporadicaFlyduring a period from 14 to 141 days after inoculation. The rnajority of deaths occurredwithin one month after inoculation. Many exoerythrocytic forms were detected fromdead birds. The inoculated chickens presented anemia, depression, or anorexia on the10th day and greenish droppiuags on the 15th day after inoculation, when the red bloodcell count ranged from 2 to 9 X 105.Half the number of experimental chickens died from inoculation with splenic emul-sion. The blood virulent was not infective after it had been filtered through Seitz fitterpads.2. When chickens 21, 34, and 70 da)s old were inoculated with virulent blood, asmall number of them died, but adults showed only slight anemia and suspended eggproduction.3. Preliminary experiments made it clear that sulfadimethoxine inhibited bothendoerythrocytic and exoerythrocytic forms. Pathogenicity was compared between theendoerythrocytic and exoerythrocytic forms in chickens administered with suITadi-methoxine at a concentration of 0.I% with a diet. The medication was continued for 5days from 6 to 10 days and 12 to 16 days after inoculation in order to inhibit growth ofthe endoerythrocytic and exoerythrocytic forms, respectively.In consequence, the fatality of chickens medicated for exoerythrocytic forms (l5.3%)was lower than that for endoerythrocytic forms (4O.O%). It is suggested that those formsmay have a higher pathogenicity than these forms.4. Microscopic examination revealed that all cases suffered from anemia, enlargedspleen, and hemosiderosis of liver and spleen, and that many cases had hemorrhagictpetechiae in the subcutaneous tissue, muscle, thymus, and myocard. Microscopic cha
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