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contained plague microbes. Fleas attacking rats were then suspected, and experiments proved that they were capable of transmitting the disease to human beings. Rats and ground squirrels die in large numbers from the plague, which is said to be primarily a disease of rats.

So, working in the light of the knowledge previously gained, Dr. Blue, of the Public Health and Marine Hospital Service, in charge of the outbreak of plague in San Francisco in 1907, directed the warfare against rats, killing more than a million in the city, and disinfecting them as soon as they could be caught. At first the work was difficult. Cases of plague were kept secret. The prejudices of ignorance and superstition had to be overcome. City officials as well as the people had to be educated, and laws made and enforced. But the work was finally performed so thoroughly that San Francisco made a new record in sanitation, and only about 140 cases developed. Except for this effective campaign not only San Francisco and California were endangered, but the whole country imperiled.

Destroy all rats by the use of baited traps and poisons, dipping them immediately in a solution of corrosive sublimate to disinfect them. This immediately kills the rats and the fleas, with the plague germs on them.

Fumigate buildings to kill fleas and disease germs.

Build solid concrete foundations, floors and walls where possible in buildings and wharves to keep out rats.

There are certain other diseases, mostly tropical, that are known to be carried by insects, and still others that are believed to be similarly transmitted, though the connecting evidence forming the proof is not yet complete. Elephantiasis is caused by worm-like parasites transmitted through the bites of certain mosquitoes of the genus Culex in the East and West Indies. Dengue fever and malta fever are probably disseminated by mosquitoes.

It is thought that the germs of leprosy are transmitted by the bites of mosquitoes, flies, fleas, lice, mites or bedbugs. Dr. Patton, of the Indian Medical Service, has demonstrated that the fatal and infectious tropical disease called "kala-azar" is transmitted by the bites of the