DESCRIPTIVE EPIDEMIOLOGY OF MARINE ANEMIA IN SEAPEN-REARED SALMON IN SOUTHERN BRITISH-COLUMBIA

Marine anemia, also known as plasmacytoid leukemia, is a recently desc ribed disease of farmed Pacific salmon in British Columbia. Most of wh at is known about the disease has been generated through laboratory st udies or field investigations of severely affected farms. The goals of this study were to determine the range of the spatial and temporal di stribution of naturally occurring marine anemia, identify potential ri sk factors, and provide an initial description of the impact of the di sease on commercial salmon farms in British Columbia. Data were obtain ed from mail surveys, farm visits, and reviews of clinical and laborat ory records. An attempt was made to evaluate negative, as well as mild ly, moderately, and severely affected sites. The results showed marine anemia to be widely distributed throughout the major salmon farming r egions in British Columbia. The disease was most commonly diagnosed in August and September, when water temperatures were at their seasonal peaks. A wide variety of lineage's and fish sources were associated wi th the disease. The average mortality rate attributed to marine anemia was 6% (range 2.5% to 11%). The peak occurrence of the disease was as sociated with a peak in the occurrence of other infectious and inflamm atory diseases. The broad demographic distribution of marine anemia, c oupled with its endemic nature, indicated that the disease is unlikely to be due to the recent introduction of a new pathogen and that causa l factors are widespread in southern British Columbia. It is concluded that the significance of diagnosing marine anemia is not that it is p redictive of an impending epidemic of mortality, but that it is an ind icator of the general pattern of disease on a farm.