Persistence of Sylvatic Plague

Abstract

In June, 1916, sylvatic plague was first demonstrated in San Mateo County by gross anatomical examinations of squirrels (Citellus beecheyi). During the next 4 years infected rodents were again discovered by dissection. Between 1921 and 1931 no surveys were conducted. In 1932 to 1935 annually, a few hundred squirrels and rats were shot and autopsied; no gross anatomical lesions suggestive of plague were observed. During August and September of 1936, a total of 863 beecheyi squirrels were again examined with negative results. The fleas, which had been killed with chloroform, were removed from the animals and collected in physiological salt solution. A total of 18,729 fleas were thus assembled in 355 pools, which were stored at 3°C. for approximately 10 to 11 months since suitable facilities to examine them were not available. In 1937 the insects collected from the squirrels of the same area were pooled in 155 lots, carefully washed with salt solution, ground in a mortar and then injected subcutaneously into guinea pigs. Of the 155 pools so tested 6 produced fatal plague infections within 6 to 8 days. Two of the locations in the County where infected fleas were found in 1936 were recognized as the same colony or series of burrows proven to harbor diseased squirrels in the summer of 1916.

These and similar observations indicate that sylvatic plague persists probably indefinitely in an area once invaded and that the gross anatomical examinations fail to detect rodent infections. Squirrels, just as rats, may harbor P. pestis without visible lesions. The biology of these latent infections is the subject of further investigations. Although the viability of P. pestis in stock cultures for many years is well known, the fact that the bacteria remained alive in the dead fleas soiled with a variety of microörganisms (B. coli, B. proteus, Cocci, etc.) held at ice box temperature is indeed noteworthy.