Guinea-pigs as Indicators of Infected Houses.—The following experiment shows strikingly the necessity for disinfecting houses where human or animal plague cases have occurred.
Upon December 17, 1912, Dr. O. Schöbl, of the Bureau of Science, and myself, placed two healthy guinea-pigs, free from fleas, in a wire trap cage in the house at No. 4 Calle Barraca, a few hours before the house was disinfected, a patient with plague from this house having died within the preceding twelve hours. The cage containing the guinea-pigs was placed exactly where the patient had slept upon the floor, as indicated by the other tenants of the house. Disinfection was delayed for a few hours and the guinea-pigs were left in the house for one day. Upon December 21 one of the guinea-pigs died from typical bubonic plague—anatomically and bacteriologically positive—other inoculated experimental animals also developing the disease.
Other guinea-pigs placed in plague houses on Calle Cabildo and Calle San Fernando, after disinfection of the premises, failed to acquire plague.
Natural Enemies of the Flea.—It was observed during the studies in Java that certain natural enemies of fleas exist and operate against their laboratory cultivation and their natural reproduction.
Ants of several varieties, large and small red ants and small black ones, were found to be very antagonistic to fleas, both in the larval and adult states, destroying them actively.
Fleas in the laboratory were found to be affected with mites, with a resultant high mortality among the insects. The same parasites were not found upon wild fleas. On account of the prevalence of mites upon the laboratory fleas certain experiments concerning the transmission of plague were vitiated.
The activity of ants in attacking and disposing of rat cadavers found in our antiplague work in Manila was frequently brought to my attention. We invariably included an attack upon ants in treatment of houses known to harbor, or suspected of harboring, plague rats. The combination of kerosene and cresols, elsewhere referred to, was found to be perfectly satisfactory in the destruction of ants; assuming, of course, that the necessary procedure of exposing the ants, by the moving of merchandise, boards or other protecting materials, was performed, so that contact, by spraying the insecticide mixture, was secured.
Activity of Fleas.—It was also observed during the Java studies that the rat flea, while rather lazy, may and does cover distances of five metres and that he sometimes covers eighteen centimetres at a single leap.
In addition to this, of course, there must be considered the possibility of his falling considerable distances.
Zoölogic Classification of Rats.—The matter of accurately, systematically and scientifically cataloguing and classifying rats is one of great difficulty and is not to be undertaken by anyone but a trained naturalist. However, some of the notes we have at our disposal, gathered from many sources, may be set before the reader. It is extremely difficult to find exact correspondence of statement in the various classifications offered by writers upon plague and rats.