The disease in fleas also has a distinctive pattern. Small mammals, such as urban and sylvatic (or wood) rats, as well as squirrels, prairie dogs, rabbits, voles, coyotes, and domestic cats, are the principal hosts for Y. pestis. More than 80 different species of fleas are involved as plague vectors. Fleas are bloodsucking insects, and when a flea bites a plague-infected host (at the bacteremic/septicemic stage) it ingests the rod-shaped bacteria; these multiply in the blood clot in the proventriculus (foregut) of the flea. This bacteria-laden clot obstructs the flea's bloodsucking apparatus and, as a consequence, the flea is unable to pump blood into the midgut, where normally it would be digested. The flea becomes hungrier and in this ravenous state bites the host repeatedly; with each bite, it regurgitates plague bacteria into the wound. In this way, infection is initiated. Y. pestis can also be pathogenic for the flea, and fleas with their foregut blocked rapidly starve to death. If the mammalian host dies, its body cools down, and fleas respond by moving off the corpse to seek another live warm-blooded host. However, if there is an extensive die-off of rodents, the fleas move on to less preferred hosts such as humans, and so an epidemic may begin.
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