Inside Bird Flu: How Did the Virus Get into Dairy Cows? Is Milk and Cheese Safe?

Inside Bird Flu: What Dairy Cows Tell Us About a Growing Public Health Threat

This week on Thermometer, I sat down with my colleague Dr. Jon Epstein for our continuing series Inside Bird Flu to unpack one of the most urgent—and underreported—public health mysteries of 2025: how did H5N1, a virus traditionally seen in birds, jump into U.S. dairy cattle?

How Did Bird Flu Get into Dairy Cows?

The H5N1 virus has been a known threat in avian populations for years. What’s new—and deeply troubling—is its emergence in American dairy cattle. We still don’t know precisely how it made the jump. Possibilities include contaminated equipment, shared workers between poultry and dairy farms, or insufficient on-farm biosecurity practices. What’s clear is that this virus has now reached large industrial farms, creating fertile ground for viral spread and evolution.

Is Milk and Cheese Still Safe?

For now, yes—pasteurization remains a vital public health barrier. While H5N1 has been found in raw milk, pasteurized milk has been shown to neutralize the virus effectively. However, raw milk and unpasteurized cheeses carry unknown risk, and this is not the time to gamble on “natural” dairy.

This reinforces a larger truth: our food safety system works—when we support it.

Why This Matters for Pandemic Preparedness

We also dove into the broader implications. Every time this virus infects a mammal—whether a sea lion, cow, or human—it gets more opportunities to adapt for human-to-human transmission. We are already seeing signs of adaptation. But to monitor and contain these shifts, we need:

• Widespread testing on farms
• Transparent data sharing
• A fully staffed and empowered public health workforce

Unfortunately, as we highlight in this episode, massive cuts to federal public health programs—including the firing of FDA veterinarians working on H5N1—leave us blind at a critical moment.

A Public Health System Under Threat

What makes this situation even more frustrating is that our surveillance systems are underfunded and underutilized. Many dairy farms are reluctant to allow testing due to fears of economic fallout. This is exactly why public health exists—to correct for market failures and protect the common good. Yet, instead of investing in that infrastructure, we’re hollowing it out.

What You Can Do

• Choose pasteurized dairy—always.
• Contact your elected officials and voice support for restoring cuts to public health agencies, especially the FDA and USDA.
• Support science journalism and public communication efforts that keep you informed.

This isn’t just a cow story. It’s a preview of how the next pandemic could begin—and how much we depend on systems most people never see.

You can watch the full episode here:
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