The present study comprises observations on the course and outcome of experimentally produced Type I pneumococcus lobar pneumonia in dogs. It was found that within certain limits of dosage the outcome of the disease was closely related to the amount of culture and the volume of the starch inoculum employed. The much higher mortality rate resulting from simply increasing the volume of starch in which the pneumococci were suspended, appeared to be due chiefly to the greater area of lung tissue initially involved. Certain striking relationships were observed to exist between the leucocytic response, extent of pulmonary involvement, bacteremia and outcome. Marked changes in the number of white blood cells at end of the first 24 hours of the disease provided more valuable prognostic information than either the blood culture or the size of the lesion. Dogs showing high white counts at this stage seldom succumbed, while absence of leucocytosis and leucopenia were associated with a mortality which increased with the diminishing numbers of circulating white blood cells. Over 90 per cent of the animals with counts of less than 2,000 cells per c. mm. died. The extent of the pulmonary lesion was likewise observed to bear a direct relationship to death rate. With the lesion confined to a single lobe the mortality was only 1 per cent. Spread to other lobes was accompanied by a sharply rising death rate until it reached 98 per cent in dogs with three-fourths the lung field involved. Increasing occurrence of bacteremia paralleled both the decreasing white counts and the extending pulmonary lesions and subsequent to the 24 hour stage of the disease the presence or absence of pneumococci in the blood appears to be of more significance, in relation to outcome, than either of these two other factors. Even in the presence of leucopenia and an extensive lesion the animal might survive provided the blood remained sterile.

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