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XX] in certain rats the disease may assume a chronic form, the bacillus becoming more or less latent in the lymphatic glands, at the same time being capable of resuscitation into active pathogenicity. Rocher states that in Yunnan the mortality among the rats is particularly noticeable. Other animals also die, he says; oxen, sheep, deer, pigs, and dogs are all attacked at times, the dog less severely and less frequently than the others. Pringle says that in Shurwal, Himalayas, where in 1864 plague was epidemic, the rats quitted the various villages in anticipation of the advent of the disease, and that the people, taught by experience, on seeing this exodus recognized it as a warning. Clemow has pointed out the connection of Mongolian and Siberian plague epidemics with the occurrence of the disease in a species of marmot, called tarbagan (Arctomys bobac), common and much hunted in these regions. This animal can harbour the plague bacillus in its body without apparently suffering any ill effects during hibernation, thus constituting a more or less permanent reservoir of the plague virus.

Schurupoff reports the ground squirrel of the Caucasus (Spermophilus guttatus) as being extremely susceptible to plague infection, and as probably concerned in the spread of the disease in that region. In California the ground squirrel (Citellus beechyi), although it does not live near human habitations, infects rats that do, and thereby acts as an important reservoir of B. pestis. It has been ascertained that in this instance the flea, Hoplopsyllus anomalus, which infests both animals, acts as the carrier of the plague bacillus from one to the other, the infection being ultimately transmitted to man by the rat- fleas (see below). These and many similar facts observed in Bombay, Sydney, Cape Town, and Hong Kong with regard to plague in the lower animals throw important light on one of the ways in which the disease is spread. They, together with the results of the several experiments already alluded to, have to be reckoned with in the