Ostrea edulis and Crassostrea virginica have been shown to contract a serious disease under conditions of low temperature, crowding, and recirculation of water in holding basins of Holland. This disease, known to the Dutch oystermen as “pit disease” is here
designated as Hexamitiasis from the etiological agent, Hexamita. The flagellates are parasitic in the blood stream of their hosts, developing very heavy infections resulting in embolism and subsequent widespread histologic damage complicated by bacterial invasion and often resulting
in death of the host. The disease occurs in America, as well as in Europe, and has been reported (unpublished mimeographed manuscript) from Prince Edward Island, Canada, and, in this paper, from Louisiana, where it is quite rare. Certain published data by Orton (1924) and Needler and Logie
(1947) suggest that the disease may have caused extensive mortalities of oysters in the past.
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