Whirling Disease — A Threat to B.C.’s Fish and Freshwater Ecosystems

Invasive species can have devastating impacts on B.C.’s freshwater ecosystems, threatening fish populations, altering habitats, and compromising water quality. One of the most concerning emerging threats is whirling disease, a microscopic parasite that affects juvenile salmonid fish, including trout and whitefish.

For National Invasive Species Awareness Week 2025, the B.C. Wildlife Federation is bringing attention to whirling disease, a devastating invasive threat to B.C.’s freshwater ecosystems. This article explores the disease’s impact, how it spreads, and what can be done to prevent it from taking hold in the province’s waters.

What is Whirling Disease?

Whirling disease is a serious parasitic infection caused by Myxobolus cerebralis, a parasite that targets the cartilage near the spine of juvenile fish, leading to neurological damage and skeletal deformities in the skull or spine. This results in abnormal tail-chasing behavior, a distinctive “whirling” motion, and/or a blackened tail.

According to the Invasive Species Council of BC, the disease is caused when a small freshwater worm (Tubifex tubifex)—common in river and stream systems—becomes infected with a microscopic parasite. The parasite then spreads to salmonid fish, leading to whirling disease.

While harmless to humans, it can be lethal to young rainbow, steelhead, and cutthroat trout, leading to population declines and disrupting freshwater ecosystems. Infected fish have a 90% mortality rate, and currently, no treatment options are available.

How it Spreads

The parasite spreads through spores that persist in water, sediment, and host species for years, making eradication nearly impossible once introduced to a water body.

Contaminated boats, gear, trailers, and even mud carried by pets and people can introduce the parasite to new locations. Additionally, fish stocking and natural fish movement contribute to its spread.

The disease was first identified in B.C. in Yoho National Park in 2023 and has now been detected in Kootenay Lake as well.

Elsewhere in Canada, Alberta has been battling whirling disease for several years, prompting extensive monitoring and management efforts. In the United States, the disease has had a devastating impact on wild trout populations, particularly in Colorado, Montana, and Idaho, where fisheries and conservation programs have worked for decades to mitigate its effects.

In late 2024, three trout samples from Kootenay Lake’s southern arm tested positive for Myxobolus cerebralis, the parasite responsible for whirling disease. This parasite primarily affects juvenile fish and can severely impact freshwater ecosystems. Its presence in Kootenay Lake—one of B.C.’s premier freshwater fisheries—poses potential risks to the iconic Gerrard rainbow trout and kokanee populations. — Central Kootenay Invasive Species Society (CKISS)

Fish infected with whirling disease. Photo by Alberta Environment and Parks via the Central Kootenay Invasive Species Society.

Prevention is the Only Solution

Because there is no treatment for whirling disease, preventing its spread is critical. All water users should follow strict decontamination practices:

Clean – Remove mud, plants, and debris from boats, trailers, and gear. Use hot water (over 60°C) or a high-pressure wash station
Drain – Empty all water from boats, live wells, and equipment before leaving a water body. Never transport live fish or water
Dry – Let all gear, boats, and pets dry completely for at least 48 hours before entering a new water body
Decontaminate – Use diluted bleach or approved disinfectants to clean gear*
Report Suspected Cases – If you see fish showing signs of whirling disease, report it immediately (see below)
Stay Informed & Educate Others – Follow updates from informed organizations such as the Province of British Columbia, the B.C. Wildlife Federation and the Invasive Species Council of BC and share prevention tips with others

*The Decontamination Protocol is particularly important for those who are carrying out water-based activities within the Columbia River Watershed, as there are additional steps they should take before moving their boat and/or equipment between waterbodies.

The Columbia River Watershed has been identified as a high-risk area for whirling disease due to the connectivity to upstream areas where whirling disease has been detected in both Kootenay Lake and Yoho National Park.

How to Report Whirling Disease in British Columbia

Whirling disease is a reportable disease in Canada, meaning any suspected cases must be reported to the Province of BC and/or the Canadian Food Inspection Agency (CFIA). Report suspected whirling disease to the appropriate authorities:

Province of British Columbia

Email: WhirlingDisease@gov.bc.ca

Canadian Food Inspection Agency (CFIA)

Phone: 1 (587) 230-2200
Local CFIA Animal Health Office: Contact your nearest office for guidance.

The CFIA collaborates with Fisheries and Oceans Canada (DFO), Parks Canada, and provincial governments to control the spread of whirling disease. DFO regulates live aquatic organisms, while Parks Canada enforces disease restrictions in national parks. Provincial governments set guidelines to prevent whirling disease and offer resources on identification and prevention.

Despite these efforts, limited funding for aquatic invasive species prevention in B.C. remains a challenge. Increased investment in surveillance, monitoring, and public education is essential to protect B.C.’s waterways from this devastating disease.

Whirling disease impacts— Affects the brain and spinal cord of juvenile salmonid fish, causes erratic “whirling” swimming behavior, and often leads to death. Photo by Alberta Environment and Parks via CKISS.

Report Whirling Disease! – A blackened tail is another potential symptom of the disease. Photo by Alberta Environment and Parks via CKISS.

Learn More About Whirling Disease

Protecting BC’s Fish & Waterways: What Anglers & Boaters Need to Know About Whirling Disease

Learn about the threat whirling disease poses in BC, and what you can do to help protect our fish and freshwater ecosystems with the Central Kootenay Invasive Species Society, Columbia Shuswap Invasive Species Society, Province of B.C., and B.C. Wildlife Federation.

Existing Webinar — Whirling Disease in Western Canada

Whirling disease is a growing threat to aquatic life in British Columbia. In this webinar, Dr. Zoé Stacey, DVM, Veterinary Inspector with the National Aquatic Animal Health Program (CFIA), provides an overview of the disease, its history in Canada, and current watershed disease declarations. She discusses the implications for aquatic stakeholders, including anglers, conservation groups, and industry, and shares prevention strategies to help protect B.C.’s waterways.