
ba EPID A OO eee 



germs are discharged from the mouth and nose as in the case of a 
common cold. Pneumonic plague, therefore, spreads directly from 
man to man by contact, but in the ordinary or ““bubonic”’ plague the 
éerms are not discharged in any of the excretions of the body and 
can only be transmitted by the flea. In northern Asia the rodent 
host of the plague bacillus is the Siberian marmot or tarbagan, 
Arctomys bobac, and the first human victims in Manchuria were 
trappers and dealers in marmot skins. In California, the ground 
squirrel, Citellus variegatus beecheyi, is infected, and the United 
States Public Health Service up to September, 1913, had isolated 
plague bacilli from 1891 different individuals. A number of human 

Fig. 30. SHIP EQUIPPED WITH GUARDS ON THE 
HAWSERS TO PREVENT LANDING 
OF INFECTED RATS 
cases in California were traced to infection from these animals. The 
most important carriers of plague germs, however, are the various 
rats, the black rat (us rattus), the common rat of the middle ages 
in Europe, the brown rat (V/. norvegicus), which has now generally 
supplanted the smaller and less ferocious black rat, and the roof rat 
(M. alexandrinus) which has established itself at many seaports. 
The modern campaign against plague depends mainly upon the 
control of the rodent host. Human cases must of course be isolated 
but the great essential is that possibly infected rats on incoming ships 
should be excluded from the wharves by rat-Suards on the hawsers, 
and rats on board destroyed by fumigation. In seaports, or other 
54 
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