Atlantic salmon site in Chile
Typical ISA signs in Atlantic salmon
Chiloe Island is focus of ISA cases in Chile
22 November 2007
Sergio Vasquez, Intervet Chile
Infectious salmon anemia (ISA) is an infectious disease caused by an orthomyxo-like-virus. It was reported for the first time 24 years ago in Norway. Later ISA was also reported in Canada (New Brunswick and Nova Scotia), the United Kingdom (including Scotland and the Shetlands), the Faeroe Islands and in the USA, Maine. A non-clinical infection has also been detected in sea-reared rainbow trout (Onchorhynchus mykiss) in Ireland (2002).
In Chile the virus was first isolated in 1999 from Coho salmon (Oncorhynchus kisutch) stock cultivated in the south of Chile, and identified later as ISA virus (ISAV) by Dr. Kibenge at the University of Prince Edward Island. In this case, the fish exhibited atypical signs of ISA infection and the condition was locally called Jaundice Syndrome. The first cases started at the end of summer and initially affected the bigger fish on the site. The main external findings were anemia and jaundice of the mucosa, eyes and abdomen, while internally, the liver, spleen and kidney were swollen. The clinical and pathological picture was complicated by subsequent Piscirickettsiosis. The ISAV diagnosis was done using polymerase chain reaction (PCR) techniques.
After this first discovery of ISAV in Chile in 2001, the country was officially designated by the OIE as positive for ISAV affecting Coho salmon but with atypical ISA symptoms. Since the first ISAV infections affecting non-susceptible species in Chile, there have been some asymptomatic ISAV infections affecting Coho and Atlantic salmon. During July 2007, the ISAV reappeared causing six typical ISA disease outbreaks in Atlantic salmon cultivated in seawater on Chiloe Island, south of Puerto Montt.
The recent cases started as an unexpected rise in mortality on affected sites. The sick fish ranged in size between 0.4 & 2.5 kg and were cultivated at an average water temperature of 10 °C. The fish showed the classical external and internal signs of ISA such as pale gills, skin petechiae and hemorrhages, anemia, ascites, congestion and enlargement of liver and spleen. Presence of hemorrhagic ascites in the abdominal cavity and pericardium was also observed. Tissue samples taken for histopathology revealed diffuse hemorrhagic necrosis in the tissues and was highly suspicious for ISAV infection
The ISAV was subsequently isolated with a cytopathic effect (CPE) occurring in Chinook Salmon Embryo–214 (CHSE–214), Epithelioma papulosum cyprinid (EPC) ( the 1st time ISa has grown in this cell lie) and Salmon Head Kidney-1(SHK) cell lines after 4–7 days using tissue homogenates of internal organs of sick fish. The presence of the virus was also identified using an indirect fluorescent antibody test (IFAT). Polymerase chain reaction (PCR) was performed using Devold’s and Mjaaland’s methods, which are OIE approved. The disease was therefore confirmed in Chile using OIE diagnostic methods on 6 sites and was reaffirmed by Norwegian and Canadian experts. A further 10 cases have been confirmed and most of the affected fish have been slaughtered.
Preliminary sequencing of segments 6 & 7 of the ISA virus from the first outbreak in Atlantic salmon indicates at this time that the virus is similar to Norwegian strains of the virus. Once the disease and the causative agent was officially identified and declared, the Chilean aquaculture agency (Sernapesca) took control of the situation and leads all the subsequent biosecurity and epidemiological measures to ensure the notification of the disease and avoid the spread of the agent in Chilean waters.
Chilean aquaculture regulations require all new diseases and or agents to be notified to the authorities by the company and or the fish health laboratory involved. With this information the Chilean authorities declare an epidemiological alert and implement contingency measures. They aim to identify the ISA virus positive sites and take samples to re confirm the presence of the virus and to identify adjacent sites for quarantine and surveillance measures. The sites in the affected areas have been classified as follows:
• Positive or confirmed sites: with or without mortality rises, presence or absence of clinical signs, but with two positive results for ISA virus, checked with the OIE accepted techniques.
• Suspicious: for sites that have only one positive isolate or identification and with positive serology to ISA virus.
• Negative: for sites that have three main conditions, as no clinical signs or mortality due to ISA virus and no virus identification. This one could be the situation of a site that has been 90 days without any fish.
• Quarantine Area: is the area located between a positive site and 5 Km. around it.
• Surveillance Area: is for sites that are located more than 5 Km., but related to a suspicious or positive case.
• ISA Free: is for sites that do not fill in the definitions above.
The authorities also established biosecurity and disinfection measures and procedures for all the sea sites in order to minimize the possibility of spreading the agent. These measures include, reporting of the weekly mortality, harvest or sacrifice of the affected fish, adequate handling of mortalities, avoidance of any fish movements between sites, etc.
The Chilean authorities declared the ISA cases to the OIE and now Chile is officially recognized as positive for ISA virus causing ISA disease in Atlantic salmon. The fish health laboratories have implemented all the techniques necessaries to confirm the virus, which are available for all the Chilean salmon companies in order to detect new outbreaks at Chilean seawaters.