H5N1 ravages 1,000+ U.S. dairy herds as Canada stays virus-free. Raw milk risks, $400M losses, and why biosecurity gaps are fueling the crisis.
EXECUTIVE SUMMARY: The H5N1 avian flu has infected over 1,000 U.S. dairy herds across 18 states since March 2024, with California losing 9.2% of milk production ($400M) in 2024 alone. The virus spreads via contaminated raw milk, equipment, and personnel, with 41 human cases linked to dairy exposure. Despite aggressive testing and movement restrictions, biosecurity failures-like shared vehicles and lax sanitation-drive transmission. Canada’s rigorous surveillance and protocols have kept its dairy herds virus-free, highlighting preventable gaps. Federal agencies confirm pasteurized dairy and cooked beef remain safe, but the outbreak underscores the urgent need for industry-wide biosecurity reforms.
KEY TAKEAWAYS:
- Milk = main transmitter: Infected cows shed high viral loads in raw milk, driving herd-to-herd spread.
- Biosecurity breakdowns: 62% of Michigan farms shared uncleaned vehicles; movement of sick cattle worsened outbreaks.
- $400M milk loss: California’s production plummeted to 20-year lows, with national costs still rising.
- Human risk: 41 mild dairy worker cases (mostly eye infections) but no human-to-human spread.
- Canada’s clean slate: 2,954 negative tests prove proactive surveillance works-zero cases despite U.S. chaos.
The highly pathogenic avian influenza (HPAI) H5N1 virus has now infected 1,034 U.S. livestock premises across 18 states, primarily devastating dairy operations while making concerning jumps to swine and alpacas. As of April 28, 2025, the virus continues its relentless spread through America’s dairy sector, with California bearing the brunt of a staggering 765 affected herds. Meanwhile, Canadian dairy cattle remain virus-free despite extensive surveillance, starkly contrasting North American animal health status just one year after the outbreak began.
The unprecedented mammalian outbreak, which first emerged in Texas dairy cattle in March 2024, has transformed from isolated cases into a full-blown crisis threatening the economic viability of affected operations. Federal authorities have implemented aggressive testing and movement restrictions, but biosecurity failures continue driving transmission through contaminated milk, shared equipment, and personnel movement between farms.
For dairy producers, the key message is clear. This virus isn’t going away anytime soon, and protecting your operation requires rigorous, consistent biosecurity practices that many farms have failed to implement correctly.
California Dairy Crisis Goes from Bad to Worse
California’s massive dairy industry has been devastated by this outbreak, with nearly 70% of the state’s dairy operations affected. Other heavily impacted states include Idaho (65 infected herds), Colorado (64), Michigan (31), and Texas (27).
The virus responsible is primarily H5N1 clade 2.3.4.4b, genotype B3.13, which originated in wild birds before making the unprecedented jump to cattle. A second viral genotype (D1.1) was detected in Nevada and Arizona herds in early 2025, indicating the virus continues to evolve within its new mammalian hosts.
What started as a mystery illness in Texas dairy herds quickly morphed into a national crisis. The first official H5N1 detection came on March 25, 2024, when USDA confirmed cases in Texas. By early April 2024, the virus had leaped to 12 operations across five states.
The Evolving Federal Response
The regulatory response has evolved as the situation deteriorated:
- April 29, 2024: First Federal Order implemented mandatory testing for interstate movement of lactating dairy cattle
- October 2024: First H5N1 detection in swine confirmed in an Oregon backyard farm
- November 2024: USDA expanded to bulk tank testing to contain the spread
- December 2024: A new federal order required mandatory nationwide raw milk testing
- January 2025: A second viral genotype (D1.1) detected alongside the original strain
The National Milk Testing Strategy (NMTS) now provides systematic surveillance through bulk tank sampling, helping detect new cases early and track the outbreak’s evolution. Since April 2024, U.S. laboratories have performed over 204,300 PCR tests for H5N1 in livestock samples from all 48 contiguous states.
What Infected Cows Look Like
Unlike in poultry, where H5N1 is often fatal, infected dairy cows typically show milder symptoms. The most obvious sign is a sudden drop in milk production, with affected cows producing thick, discolored, or colostrum-like milk or stopping lactation completely.
Other symptoms include reduced feed consumption, lethargy, dehydration, fever, and abnormal feces described as tacky or loose. Respiratory signs may occur but aren’t usually prominent.
Most infected cows recover with supportive care. Mortality and culling rates directly attributed to H5N1 infection have remained relatively low, averaging 2% or less in affected herds, but the economic damage from production losses can be substantial.
The Milk Connection: Primary Transmission Route
The most critical finding is that infected cows shed extremely high virus concentrations in their milk. This makes raw milk the primary vehicle for transmission within and between farms.
Any object, person, or animal coming into contact with contaminated raw milk can spread the virus. The practice of feeding untreated waste milk to calves or farm cats has been directly linked to infections in these animals, cats often suffering severe neurological symptoms and death.
Indirect transmission via fomites (contaminated objects) and personnel represents another significant risk. The virus spreads through shared equipment like milk trucks, feed vehicles, and manure handling equipment, especially when not adequately cleaned between farm visits.
Biosecurity Failures Drive Continued Spread
Investigations on affected farms have revealed alarming gaps between recommended biosecurity practices and actual implementation. Approximately 62% of affected Michigan farms reported sharing vehicles between operations without proper cleaning.
Personnel frequently moved between locations without changing clothing or disinfecting footwear. Some farms continued moving cattle even after clinical signs appeared, helping the virus jump state lines.
Environmental contamination also plays a role. Infected cattle shed the virus in feces, leading to contamination of manure slurry and wastewater that can spread the virus if not properly treated before land application.
Breaking Transmission: What Works
Enhanced biosecurity remains the most effective tool for preventing H5N1 introduction and spread. USDA offers financial assistance for implementing improved measures, but compliance has been inconsistent.
Critical biosecurity steps include:
- Limiting farm access to essential personnel only
- Requiring clean, dedicated clothing and footwear for anyone entering animal areas
- Never feeding raw milk or colostrum to calves or other animals
- Treating all waste milk through pasteurization or acidification before disposal
- Minimizing cattle movements and isolating new arrivals for 30 days
- Thoroughly cleaning and disinfecting shared equipment
Farms that have successfully implemented these measures have demonstrated significantly lower risk of infection, even when located in heavily affected regions.
Movement Restrictions and Testing Requirements
Under federal orders, lactating dairy cows must test negative for H5N1 via PCR on individual milk samples within 7 days before crossing state lines.
Animals that test positive for H5N1 cannot move interstate for 30 days following the positive test date. Affected premises are placed under state quarantine until they complete disease response protocols.
The USDA also strongly recommends minimizing non-essential movements of cattle whenever possible. Specific guidance for livestock exhibitions includes documenting farm origins, isolation protocols for sick animals, and post-event quarantine for returning animals.
Human Health: Farm Worker Infections Climbing
The CDC has confirmed 70 human cases of H5 influenza since the broader outbreaks began, with 41 explicitly linked to dairy cow exposure. Most cases in farm workers have been mild, often involving conjunctivitis (eye infection), though severe illness has occurred rarely.
The first case of likely cow-to-human transmission occurred in a Texas dairy worker in late March 2024. Most dairy-related human cases were reported from California (36), Michigan (2), Colorado (1), Nevada (1), and Texas (1).
No human-to-human transmission has been detected, and CDC assesses the risk to the general public as low. However, farm workers, veterinarians, and others with direct animal contact face moderate-to-high risk and should use appropriate PPE, including eye protection and N95 respirators.
What About My Food?
Pasteurization effectively inactivates the H5N1 virus. Extensive FDA testing of 464 retail pasteurized dairy products found no viable virus in any sample, confirming that properly processed dairy products remain safe.
While sensitive PCR testing detected fragments of viral RNA in some pasteurized samples, additional testing confirmed no infectious virus was present. This distinction between detecting viral fragments and actual live virus required careful public communication.
USDA expresses confidence in the safety of the meat supply. Their inspection process removes visibly sick animals from the food chain, and cooking effectively kills the virus. Testing of retail ground beef found no viral particles.
The Canadian Contrast: Zero Cases
In stark contrast to the American situation, no cases of HPAI H5N1 have been detected in Canadian dairy cattle as of late April 2025, despite active monitoring by Animal Health Canada and its partners.
As of April 2, 2025, Canadian Food Inspection Agency laboratories tested 2,954 samples of raw milk from processing plants across all Canadian provinces, with every sample testing negative.
This difference highlights how wild bird migration patterns, cross-border controls, industry structures, and biosecurity practices influence disease emergence and spread. Canadian authorities continue surveillance efforts despite having no confirmed cases.
The Bottom Line
The H5N1 outbreak represents an unprecedented challenge for the U.S. dairy industry. Controlling it requires a multi-pronged approach: implementing strict biosecurity, maintaining robust surveillance, researching transmission dynamics, and developing effective vaccines.
Field trials for candidate H5N1 vaccines were underway in early 2025, potentially adding another tool to the control arsenal and understanding the whole picture- including subclinical infections and environmental persistence- which requires further investigation.
For U.S. dairy producers, this crisis demands uncompromising attention to biosecurity protocols that many farms have failed to implement correctly. For Canadian producers, maintaining protection means staying vigilant despite the current absence of cases. This virus has fundamentally changed dairy farm management across North America, with consequences that will likely be felt for years.
Learn more:
- H5N1 Crisis One Year Later: What Dairy Farmers Need to Know
Get the full story on how H5N1 evolved, spread, and forced new biosecurity rules across U.S. dairies-including economic impacts and key lessons learned. - How Canada Keeps Its Dairy Cows Free from Bird Flu
Dive into Canada’s proactive biosecurity, surveillance, and farm management strategies that have kept H5N1 out of its national dairy herd. - H5N1 Crisis Hits 1000 U.S. Dairy Herds: California Epicenter and Nevada’s New Viral Threat Demand Immediate Action
Explore the latest on the U.S. outbreak’s epicenter, the emergence of new viral strains, and what urgent actions producers should take now.
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