Relation of Humidity to Resistance to Infection in Rachitic Rats

Abstract

McDowell, 1 showed that the mortality following an intraperitoneal injection of pneumococci in adult rats kept for 2 weeks in a room at a temperature of 84°F., was much higher when the relative humidity was 84% than when it was 44%. When the temperature varied between 65° and 72°F., a larger percentage of all the rats died, but variations in the humidity had no effect. Kligler and Olitzki 2 reported very similar results with adult mice, but, in their work, the exposure to the special environment took place only 3 days before and for one month after an oral infection with Salmonella enteritidis. At 68°F. differences in humidity did not influence the mortality rate.

The following experiments differ from those above mentioned in that (1) young rats were used; (2) a rickets-producing diet was fed; (3) the exposure to the various temperatures and humidities was for the 4 weeks preceding infection; (4) the effect of air drawn from inside and outside the building was compared. Litters of rats which had just been weaned were divided into 4 equal groups, and put in divided cages in 4 closed wooden boxes through which the conditioned air was passed. The air for 2 of the boxes, one high and one low humidity, was drawn from outside the building, and that for the other 2, also one high and one low humidity, was drawn from the large animal room which housed the boxes. The air for each pair of boxes was drawn from its source by a pump and passed through a tank of anhydrous crude calcium chloride, through an electric heater, and into the box. The heaters were under the control of thermo-regulators in the boxes, and the temperature of the conditioned air surrounding the rats was kept constant to within a few degrees.