The Epidemiology of Hemorrhagic Kidney Syndrome - Infectious Salmon Anemia in Atlantic Salmon in Atlantic Canada

Hemorrhagic Kidney Syndrome (HKS) is characterized by high mortality rates in Atlantic salmon seawater sites with unique histological kidney lesions. Virus isolation results were positive for the orthomyxovirus responsible for Infectious Salmon Anemia (ISA) experienced by Norway and also a new virus tentatively classified in the Togaviridae family was isolated in some, but not all outbreaks. This study examined the associations between husbandry and environmental factors and the risk of high mortality rates in Atlantic salmon transferred to seawater in the spring of 1996. The study population consisted of over 2.2 million fish in 218 cages in three bays in the Bay of Fundy. The mean duration of an outbreak was 37 days and ranged from seven days to 125 days. Median total mortality during an outbreak period was 6,600 per 100,000 fish, while ten percent of case cages lost more than 29,000 per 100,000 fish. Cages containing larger fish populations, cages which had high mortality rates in the 1996 season, and cages at sites which had one or more positive cages, had increased risk of experiencing an outbreak of HKS/ISA in 1997. Collection of data in this type of aquaculture field study presented some unique challenges, such as adequacy of farm records, maintaining farm confidentiality, and relating mortality rates to diagnostic surveillance results. Although the orthomyxovirus is believed to be the etiologic agent, there remain many questions about the interaction of farm factors and the presence of other infectious agents which may alter the presentation and severity of the disease. Considerations for Investigation of Emerging Diseases in Atlantic Canada