. Animal parasites and human disease. Parasites; Medical parasitology; Insects as carriers of disease. 412 FLEAS 1906 that the rat flea was the principal means of transmission of the bubonic form of the disease. As far as is known the plague bacilli live only in the digestive tract of the fleas and do not infect either the saliva or the body cavity. From this fact it is evident that the germs are inoculated into the wound made by the flea, either with the excrement which is commonly voided while sucking blood or with regurgitated blood. It has been pointed out that a rat flea's stomach will ho
. Animal parasites and human disease. Parasites; Medical parasitology; Insects as carriers of disease. 412 FLEAS 1906 that the rat flea was the principal means of transmission of the bubonic form of the disease. As far as is known the plague bacilli live only in the digestive tract of the fleas and do not infect either the saliva or the body cavity. From this fact it is evident that the germs are inoculated into the wound made by the flea, either with the excrement which is commonly voided while sucking blood or with regurgitated blood. It has been pointed out that a rat flea's stomach will hold about one-half a cubic centimeter of blood and could there- fore take 5000 germs with a single feed from an infected animal. These often multiply to such an extent as to form a solid mass of organisms, blocking the digestive tract of the insect (Fig. 182). It has been stated that when the stomach and intestine of a flea are plugged with plague germs the normal action of the valves of the digestive tract is lost, and the pumping move- ments of the pharynx ^result in regurgitating infected material into ^the wound instead of sucking fresh blood from it. Fleas were found by the British Plague Commission to remain infective for 15 days during the height of an epidemic, though during the non-epidemic tive "tract" of flea season no individual remained infective for plugged with solid more than seven days. In Java the Indian growth (in black) of . plague bacilli. (After rat fleas have been found to remain infective Manson.) for 33 ^ays anc[ Bacot has found the European rat flea, Ceratophyllus fasciatus, to remain infective, when kept away from a host, for 47 days. The Indian rat flea, Xenopsylla cheopis (Fig. 177), is the species most intimately associated with plague transmission. This spe- cies has been introduced with rats on ships into all parts of the tropics, and into seaports in many temperate countries, especially such ports as San Francisco, which trade extensive
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