Bird Flu in Cattle: Assessing the Risk to Humans

The detection of highly pathogenic avian influenza (HPAI) H5N1 in United States dairy cattle marks a significant and unexpected shift in viral behavior. Since the initial confirmation by the USDA in March 2024, public health officials and virologists have closely monitored the situation to determine if this spillover event signals a greater threat to human health. While the general public risk remains low, the adaptation of a bird virus to mammals requires careful scrutiny.

The Shift from Birds to Bovine

For decades, scientists have tracked H5N1 primarily as a threat to poultry and wild birds. The virus is lethal to chickens and turkeys, often requiring the depopulation of entire flocks to stop the spread. However, the spring of 2024 brought a new variable: dairy cows in Texas and Kansas tested positive for the virus.

This was not a simple case of a cow encountering a sick bird. The virus established itself within the cattle population. Genomic sequencing revealed that the virus had likely been circulating among cows undetected for months before the official announcement. This jump is concerning because pigs and cattle can sometimes act as “mixing vessels” where viruses mutate, potentially becoming more transmissible to humans.

How the Virus Spreads Among Cattle

Unlike flu transmission in humans, which is largely airborne (coughing and sneezing), the spread of H5N1 in dairy herds appears to be mechanical. The virus concentrates heavily in the udders and milk of infected cows.

• Milking Equipment: The primary vector appears to be milking claws and cups that are shared between cows.
• High Viral Load: Raw milk from infected cows contains massive amounts of live virus.
• Movement: The movement of cattle between states for breeding or milk production facilitated the spread from Texas to states like Michigan, Idaho, Ohio, and North Carolina.

Assessing the Danger to Humans

The Centers for Disease Control and Prevention (CDC) maintains that the current risk to the general public is low. However, “low” does not mean “zero.” The risk profile is distinctly different for two groups: the general public and dairy farm workers.

Risks to Farm Workers

The individuals at the highest risk are those working directly with infected animals. Since the outbreak began in cattle, there have been confirmed cases of human infection in the United States linked specifically to dairy exposure.

For example, a dairy worker in Texas developed conjunctivitis (pink eye) after close contact with infected cows. Subsequent cases in Michigan involved similar eye infections and mild respiratory symptoms. In these instances, the transmission likely occurred through direct contact with infected milk or unpasteurized droplets hitting the eyes or mouth.

The CDC has issued strict guidelines for farm workers, recommending the use of PPE (Personal Protective Equipment) such as safety goggles, masks, and gloves. However, compliance can be difficult in hot, demanding dairy environments.

The Pandemic Potential

The ultimate fear is that H5N1 will gain the ability to spread efficiently from human to human. Currently, there is no evidence of sustained human-to-human transmission. The virus receptors in human upper respiratory tracts are different from those in birds.

For a pandemic to occur, the virus would need to undergo specific genetic mutations that allow it to bind easily to human receptors and survive airborne transmission. Scientists are constantly sequencing samples from cows and humans to watch for these specific markers. As of mid-2024, the virus has not acquired these dangerous mutations.

Food Safety: Milk and Beef

A major concern for the average consumer is the safety of the food supply. When the outbreak was first announced, questions immediately arose regarding the safety of milk on grocery store shelves.

The Safety of Commercial Milk

The Food and Drug Administration (FDA) conducted extensive testing on commercial milk products. Their findings were conclusive: Pasteurization kills the virus.

Pasteurization is the standard process of heating milk to a specific temperature for a set time to kill harmful bacteria and viruses.

• PCR Testing: The FDA did find genetic fragments of the H5N1 virus in roughly 20% of retail milk samples tested during the initial survey.
• Viability: Crucially, these fragments were “dead.” Follow-up egg inoculation tests showed that zero live, infectious virus remained in the pasteurized milk.

This means the commercial milk supply is safe. However, officials strictly warn against consuming raw (unpasteurized) milk. Because the virus concentrates in the milk, drinking raw milk from an infected herd poses a severe health risk.

Beef Safety

The USDA Food Safety and Inspection Service (FSIS) also tested ground beef retail samples in affected states. The results showed no virus particles in the meat supply. Furthermore, standard cooking practices act as a safeguard. Cooking meat to an internal temperature of 145°F (for steaks/roasts) or 160°F (for ground beef) destroys the flu virus and other pathogens like E. coli and Salmonella.

Government Response and Monitoring

To contain the spread, the USDA issued a Federal Order in April 2024. This mandate requires lactating dairy cows to test negative for Influenza A before they can be moved across state lines. This was a massive regulatory step intended to halt the geographic expansion of the virus.

Additionally, the government allocated funding to support testing and compensate farmers for lost milk production. Infected cows typically recover after a few weeks, but they experience a sharp drop in milk production, causing economic strain on the industry.

Surveillance systems are now more aggressive. Wastewater monitoring has been deployed in some areas to detect spikes in Influenza A that fall outside the normal human flu season, providing an early warning system for potential outbreaks in both humans and animals.

Frequently Asked Questions

Can I get bird flu from drinking milk? You cannot get bird flu from drinking pasteurized milk found in grocery stores. The heating process kills the virus. However, you should avoid raw, unpasteurized milk, as it may contain live virus.

What are the symptoms of bird flu in humans? Recent cases linked to cattle exposure presented mainly as conjunctivitis (redness and irritation of the eyes). Other possible symptoms include fever, cough, sore throat, runny nose, muscle aches, and fatigue.

Is it safe to eat beef? Yes. There have been no reported cases of transmission through eating cooked beef. Always cook ground beef to an internal temperature of 160°F to ensure all bacteria and viruses are destroyed.

How many humans have been infected by cows? As of the mid-2024 data, confirmed cases remain very low (fewer than a dozen directly linked to the cattle outbreak), and all experienced mild symptoms. The CDC updates these numbers regularly as monitoring continues.

Does the seasonal flu shot protect against H5N1? No, the standard seasonal flu vaccine is designed for H1N1 and H3N2 strains. It does not provide protection against H5N1. However, the U.S. government maintains a stockpile of H5N1 vaccines that could be distributed if a pandemic risk emerged.