Polyplax spinulosus, the common rat louse, is easily infected with the virus of typhus by feeding on infected rats. As in the case of Pediculus humanus, such feedings are followed by the appearance of large numbers of Rickettsia prowazeki within the gut of the insect. The virus of Mexican typhus can be transmitted from rat to rat by Polyplax spinulosus by methods of feeding simulating natural conditions. It seems, therefore, that this ectoparasite is an important factor in maintaining an endozoic of Mexican typhus among wild rats.

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