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INSECTS THAT HAVE BEEN SUSPECTED IN THE TRANSMISSION OF 



PLAGUE. 



It is probable that all insects capable of sucking blood will take 

 the Bacillus pestis into their alimentary canal if they feed on a 

 septicaemic plague animal. Ogata suggested that not only the 

 flea but the mosquito also may be responsible for the transmission 

 of plague. Yersin, Hankin, and Nuttall have each demonstrated 

 the presence of Bacillus pestis in the dejecta of flies and ants; and 

 Nuttall and Verjbitski in the stomach and dejecta of the bedbug. 

 Hertzog found the bacilli in the Pediculus capitis taken from a child 

 which died of plague, and McCoy (1) has found the organism in lice, 

 Haematopinus columbianus, taken from a plague-infected squirrel. 

 The plague bacilli have been frequently demonstrated in rat fleas 

 taken from plague rats, and McCoy has shown its presence in the 

 flea (CeratopJiyllus acutus) of the California ground squirrel (Citellus 

 beecheyi). The cockroach has also been thought to be instrumental 

 in spreading the infection by contaminating food. The presence 

 of bacilli in the stomach and dejecta of insects has not only been 

 proven microscopically but by animal inoculation as well. 



Assuming that the relation between rat plague and human plague 

 has been proven without a doubt that is, that an outbreak of human 

 plague is associated with an infection in rats, or, in other words, that 

 plague is primarily a disease of rats and secondarily a disease of 

 man the theory that it is conveyed through an intermediate para- 

 sitic host is the only one which will fulfill all the requirements, and 

 after a study of their habits we are able to exclude all of the parasites 

 but the flea as the active agent in its transmission. 



Plague is rarely or never contracted either in rat or in man by 

 eating contaminated food. Therefore those insects like flies and 

 cockroaches, which are supposed to spread the infection by contam- 

 inating food with their dejecta, need not be considered. 



The habits of the domestic mosquitoes are such that while they 

 occasionally do bite animals they usually feed on the blood of man, 

 and are not known to feed where there is much hair, as there is on the 

 rat. This also applies to the bedbug. Verjbitski has shown experi- 

 mentally that bedbugs would not feed on rats until the animals were 

 shaved. 



Pediculi are degenerate insects, their powers of locomotion being 

 limited. Their eggs are laid on and are attached to the hair of the 

 host. They are born, live, and die on the same host, and rarely 

 pass from one animal to another of a different species. It can not 

 be denied, however, that this parasite occasionally may be instru- 

 mental in spreading plague from rat to rat. The Pediculus capitis, 

 if placed on a rat, will feed with avidity, but these insects are rarely 

 found upon rats in nature. 



