unable to demonstrate, a bacterial pathopen. 

 Although no infectious agent was directly asso- 

 ciated with the mortalities, disease signs such 

 as mantle retraction, pale digestive gland, 

 muscle degeneration, and pustules on the shell 

 and mantle were seen. Ulcerations and pustules 

 on the body and mantle, and shell pustules con- 

 taining dead or moribund leucocytes were ob- 

 served in oysters from England and the Neth- 

 erlands during periods of mortality (Orton, 

 1937). These signs often result from disease. 

 The exhaustive studies of Orton were supple- 

 mented by those of Eyre (1923, 1924), who iso- 

 lated a number of species of bacteria from sick 

 and healthy oysters, but doubted that any were 

 true pathogens. 



Korringa (1952a) gave an excellent hist(ni- 

 cal account of these mortalities of oysters in 

 Europe. Cultured-oyster beds in France, Eng- 

 land, Denmark, Germany, and the Netherlands 

 were affected almost simultaneously. Many nat- 

 ural beds were also destroyed. A few isolated 

 populations — Helgoland and Brittany — were 

 not affected until several years later. Mortali- 

 ties did not occur in Portuguese oysters during 

 this time. Although environmental factors such 

 as poor food supply and low temperatures were 

 held by .some to be causes of the catastrophic 

 mortalities (Gaarder and Alvsaker, 1941; 

 Sparck, 1950), the available evidence strongly 

 indicates an infectious disease (Cole, 1951b; 

 Fischer, 1951; Korringa, 1952a). 



A mortality, with characteristics very simi- 

 lar to those seen in the European oyster, was 

 described by Roughley (1926) in populations 

 of rock oysters, Crassost7-ea conimercialL'i (Ire- 

 dale and Roughly), from Austi'alia. Oysters 

 died in 1924 and 1925 in Georges River, New 

 South Wales. Disease signs, such as abscesses 

 and ulcerations, were observed, and a bacterial 

 pathogen was suspected — possibly combined 

 with winter environmental stresses. 



Disease-associated mortalities, with a history 

 of long and frustrating scientific study, were 

 first observed in 1915 in American oysters of 

 Prince Edward Island, Canada (Needier and 

 Logie, 1947). In the period 1915-33, the dis- 

 ease (commonly known as "malpeque disease") 

 spread around the Island and destroyed most 

 of the oyster stocks — some of which required 

 20 years to return to previous levels of abun- 



dance (Logie, 1956). During the outbreak, oys- 

 ters apparently developed resistance to the 

 causative organism, whose identity remains un- 

 determined. Beginning in 1955, mortalities, 

 probably due to the same disease, began in 

 waters o^ the adjacent mainland of New Bruns- 

 wick across Northumberland Strait. Oyster 

 populations along the entire northern coast of 

 New Brunswick and Nova Scotia were deci- 

 mated, but mass transfer of disease-resistant 

 oysters from Prince Edward Island waters, be- 

 ginning in 1957, has hastened the recovery of 

 the fishcr.y (Logie, Drinnan, and Henderson, 

 1960; Drinnan and England, 1965). 



Pacific oysters imported as seed from Japan 

 and planted in waters of the States of Washing- 

 ton and Ciilifornia began to die in significant 

 numbers in the late 1950's. Oysters in their 

 second year after introduction were most com- 

 monly killed; peaks of mortalities occurred in 

 late summer; and deaths were most often ob- 

 served at the heads of bays. In the absence of 

 other obvious environmental changes, and be- 

 cause of the selective nature of the mortalities, 

 it seems logical to suspect disease. A pathologi- 

 cal condition described by us as "focal necro- 

 sis," has been found in seed from Japan and in 

 several samples of larger oysters from beds in 

 Washington. As many as 30 percent of the indi- 

 viduals in a sample were affected. In addition, 

 a haplosporidan parasite, morphologically simi- 

 lar to the pathogen Minchiuia xchoni associ- 

 ated with recent mortalities on the U.S. east 

 coast, has been seen by staff members of the 

 BCF Biological Laboratory, Oxford, Md., in 

 Pacific oysters from the State of Washington ; 

 a similar organism was recognized recently in 

 a sample of seed oysters from Taiwan. Pereyra 

 (1964) mentioned a "multinucleated MSX-like 

 organism, possibly pathogenic" in a dying oys- 

 ter from Oyster Bay, Wash. No clear associa- 

 tion has yet been made, however, of specific 

 pathogens with mortalities of C. gigas on the 

 Pacific coast of the United States, and it is 

 quite possible that other environmental factors 

 are operative in the mortality areas. 



The Japanese literature contains numerous 

 historical accounts of mass mortalities of oys- 

 ters dating back to 1915. Although disease was 

 often suspected, specific pathogens were usually 

 not identified. Takeuchi et al. (1960) mentioned 



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U.S. FISH AND WILDLIFE SERVICE 



