When an epidemic begins, rats must be destroyed by trapping and poisoning. Various so-called biological poisons have not proved practicable. Sources of food supply should be cut off by thorough cleaning up, by use of rat-proof garbage cans and similar measures. Hand in hand with these, must go the destruction of breeding places, and the rat-proofing of dwellings, stables, markets, warehouses, docks and sewers. All these measures are expensive, and a few years ago would have been thought wholly impossible to put into practice but now they are being enforced on a large scale in every fight against the disease. Rats and other rodents are regularly caught in the danger zone and examined for evidence of infection, for the sequence of the epizootic and of the human disease is now understood. In London, rats are regularly trapped and poisoned in the vicinity of the principal docks, to guard against the introduction of infected animals in shipping. During the past six years infected rats have been found yearly, thirteen having been found in 1912. In Seattle, Washington, seven infected rats were found along the water front in October, 1913, and infected ground squirrels are still being found in connection with the anti-plague measures in California. The procedure during an outbreak of the human plague was well illustrated by the fight in San Francisco. The city was districted, and captured rats, after being dipped in some fluid to destroy the fleas, were carefully tagged to indicate their source, and were sent to the laboratory for examination. If an infected rat was found, the officers in charge of the work in the district involved were immediately notified by telephone, and the infected building was subjected to a thorough fumigation. In addition, special attention was given to all the territory in the four contiguous blocks. By measures such as these, this dread scourge of the human race is being brought under control. Incidentally, the enormous losses due to the direct ravages of rats are being obviated and this alone would justify the expenditure many times over of the money and labor involved in the anti-rat measures. CHAPTER VII ARTHROPODS AS ESSENTIAL HOSTS OF PATHOGENIC ORGANISMS We now have to consider the cases in which the arthropod acts as the essential host of a pathogenic organism. In other words, cases in which the organism, instead of being passively carried or merely accidentally inoculated by the bite of its carrier, or vector, is taken up and undergoes an essential part of its development within the arthropod. [Illustration: 113. Dipylidium caninum. The double pored tapeworm of the dog.]