Questions and Answers: 

Infectious Salmon Anemia

Infectious salmon anemia (ISA) is a serious viral 

fish disease that affects farmed Atlantic salmon in 

several areas of the world. The U.S. Department 

of Agriculture’s (USDA) Animal and Plant Health 

Inspection Service (APHIS) works with fish health 

partners to help protect U.S. aquaculture and wild 

fish populations from this disease. 

About ISA

Q. What is ISA?

A. ISA is a viral disease that causes severe anemia 

in infected fish. The fish may develop pale gills, 

raised scales, pinpoint areas of bleeding, a bloated 

abdomen, or bulging eyes, and may swim close to 

the water surface. The disease can also develop 

without infected fish showing any external signs of 

illness—the fish maintain a normal appetite and then 

suddenly die. 

Because the signs of ISA are not unique and can 

occur with other fish diseases, testing for virus 

presence is the only way to tell if a fish has ISA.

Q. Which fish are most susceptible to the 

ISA virus?

A. ISA has caused high death rates in Atlantic salmon 

under marine farmed and laboratory conditions, but 

the virus has not been associated with die-offs in 

wild fish or freshwater farmed populations. Several 

species of marine and freshwater fish can be infected 

with ISA without showing any apparent signs of 

disease. Susceptible fish under stress due to human 

handling or environmental conditions may be more 

prone to the disease. 

Q. Where was ISA first detected?

A. Outbreaks of ISA were first identified in Norway 

beginning in 1984. Since then, outbreaks in farmed 

Atlantic salmon have occurred in eastern Canada, 

Chile, the Faroe Islands, Maine, Norway, and 

Scotland.

Q. Can people get ISA?

A. No. Fish with ISA pose no human health threat. 

Q. Can you cure ISA?

A. There is no cure for ISA. However, in farmed 

Atlantic salmon, producers can manage the disease 

by conducting regular surveillance, coordinating 

the stocking of farms in shared waters, leaving sites 

empty of fish between production cycles (fallowing), 

cleaning and disinfecting net pens and equipment 

between uses, employing proper quarantine 

measures, following good fish husbandry practices, 

and promptly removing infected fish. Salmon 

farmers can help prevent ISA by purchasing eggs 

and brood stock that are proven to be free of the ISA 

virus. Together, these steps reduce the risk of ISA 

outbreaks in farmed Atlantic salmon.

Q. Is ISA present in the United States?

A. ISA was found in Maine in 2001. In 2002, Federal 

and State partners began a surveillance and control 

program to look for the disease and detect and 

prevent its spread in farmed Atlantic salmon. This 

program is highly successful—there have been no 

confirmed cases of ISA in the United States since 

early 2006. However, a non-disease-causing form of 

the ISA virus (ISAV HPR0) is periodically detected 

in wild and farmed Atlantic salmon in this region of 

Maine. 

ISA and the U.S. Pacific Northwest

Q. Is ISA found in the U.S. Pacific Northwest?

A. No. Neither the disease nor the virus has ever 

been confirmed in the U.S. Pacific Northwest. While 

some media and a recently published research 

finding (Kibenge et al. Virology Journal 2016 

ANIMAL AND PLANT HEALTH INSPECTION SERVICE JULY 2016 

VETERINARY SERVICES 
13:3) reported ISA virus detections in salmon from 

fish markets and wild populations in British Columbia, 

neither the Canadian Food Inspection Agency (CFIA)—

which oversees fish health in Canada—nor a World 

Organization for Animal Health (OIE) reference laboratory 

has confirmed the findings. 

Q. How did we address ISA concerns in the U.S. Pacific 

Northwest?

A. In response to concerns about the possible 

occurrence of ISA in the U.S. Pacific Northwest, the 

U.S. Aquatic Animal Health Task Force—which is led 

by APHIS and includes the National Marine Fisheries 

Service and the Fish and Wildlife Service—collaborated 

with Federal, State, and tribal agencies to develop and 

carry out an enhanced surveillance plan for ISA virus in 

the region. APHIS and its partners have completed the 

sampling and testing outlined by the plan with more than 

4,000 fish tested in 3 years. Sampled fish included wild 

Pacific salmon, herring, and trout from coastal regions of 

Alaska and Washington and farmed Atlantic salmon from 

Washington. All tests were negative for ISA virus. 

Q. What was the focus of the enhanced surveillance plan? 

A. The plan detailed how fish health managers would 

look for the ISA virus and which screening tests to use. It 

built on existing Federal, State, tribal, and industry health 

infrastructures and activities. 

Q. Are details of the U.S. Pacific Northwest sampling and 

testing efforts available? 

A. In 2014, a paper titled “U.S. Response to a Report 

of Infectious Salmon Anemia Virus in Western North 

America” was published. It provided an interim report on 

the surveillance. APHIS and its partners are now drafting 

a final report. Once completed, the agency will share the 

report with stakeholders. 

Q. Are the U.S. and Canadian surveillance efforts for ISA 

similar?

A. Both countries use OIE-recommended testing 

methods for detecting the ISA virus. APHIS is working 

with CFIA and Canada’s Department of Fisheries and 

Oceans to make sure our countries’ surveillance efforts 

align and follow OIE recommendations. 

Q. Why did we need an enhanced surveillance program? 

What were the benefits?

A. ISA is a serious disease of farmed Atlantic salmon. 

There is no scientific evidence that Pacific salmon are 

at risk, but the enhanced surveillance program would 

have helped to quickly detect the disease and contain 

or manage it if found. Early detection and control efforts 

better protect the health of farmed and wild fish and 

reduce the potential for impacts to the aquaculture 

industry.

Q. What are the potential economic impacts of an ISA 

outbreak?

A. When ISA occurred in Maine in 2001 and 2002, 

17 marine net-pen sites—containing about 1.5 million 

fish—were depopulated. The United States spent $8.6 

million combating ISA in Maine between 2001 and 2007, 

including the cost of indemnity to farmers for animals 

destroyed. If ISA were found in the Pacific Northwest 

(U.S. or Canadian) the economic impact would be 

significant.

Managing ISA

Q. What steps are fish producers taking to prevent and 

manage ISA?

A. There are several simple steps that fish producers use 

to help prevent ISA virus from infecting their facilities. 

These basic biosecurity steps include the following:

• Keeping equipment clean and disinfected and not 

sharing equipment between sites

• Not transferring fish between sites

• Quickly removing any potentially sick or dead fish 

• Farming only one age group of fish at a time

• Controlling access to their facilities 

• Obtaining eggs and brood stock that are tested free 

of the ISA virus

• Leaving sites empty of fish between production 

cycles (fallowing) 

• Working together within the region to coordinate 

these practices among sites

These same steps would also help manage the disease if 

it were found at a facility.

Testing for ISA

Q. How do you know if a fish has the ISA virus?

A. Testing is the only way to know if a fish has the ISA 

virus. ISA testing is a multistep process that uses OIErecommended methods.

Screening tests look for viral genetic material in fish 

tissues. If these tests show evidence of the ISA virus, 

laboratory officials conduct additional tests (e.g., genetic 

sequencing and virus isolation in cell cultures) to confirm 

infection. 
Q. Who conducts the testing for ISA?

A. Several U.S. laboratories screen samples for 

ISA virus, following APHIS-approved test protocols. 

However, APHIS’ National Veterinary Services 

Laboratories (NVSL) must confirm any presumptive 

positive cases. As the U.S. reference laboratory for 

animal diseases, NVSL has the expertise to make 

a final determination on whether the ISA virus is 

present.

More Information

Q. Where can I find more information about ISA and 

the ISA virus?

A. For more information about the ISA virus and 

ISA disease surveillance programs, go to: 

• APHIS’ Aquaculture Web page (https://www.

aphis.usda.gov/aphis/ourfocus/animalhealth/

animal-disease-information/aquaculture/

ct_aquaculture_index)

• CFIA’s 2012-2013 Status Update on 

the Surveillance of Wild and Enhanced 

Anadromous Salmonids in British Columbia 

(http://www.inspection.gc.ca/animals/aquaticanimals/diseases/reportable/isa/

wild-anadromous-salmonids/eng/

1410057185143/1410057186002)

• “U.S. Response to a Report of Infectious 

Salmon Anemia Virus in Western North 

America” Paper (http://dx.doi.org/10.1080/

03632415.2014.967348)

• Chapter 10.4 of the OIE Aquatic Animal 

Health Code, Infection with Infectious Salmon 

Anaemia Virus (http://www.oie.int/index.

php?id=171&L=0&htmfile=chapitre_isav.htm)

• Chapter 2.3.5 of the OIE Manual of Diagnostic 

Tests for Aquatic Animals, Infection with 

Infectious Salmon Anaemia Virus (http://www.

oie.int/index.php?id=2439&L=0&htmfile=

chapitre_isav.htm)

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