Bird Flu H5N1: Mammal Transmission Risks

The H5N1 avian influenza virus has historically been a threat primarily to poultry and wild bird populations. However, the situation changed rapidly in 2024. Scientists are now closely monitoring a significant shift in the virus’s behavior as it spreads to unexpected mammal hosts, specifically dairy cattle and marine mammals. This jump between species raises valid concerns about the virus’s ability to adapt and potentially pose a higher pandemic risk to humans.

The Shift From Birds to Mammals

For years, H5N1 (specifically the clade 2.3.4.4b) circulated among wild birds, occasionally spilling over into scavenging mammals like foxes or bears that ate infected carcasses. The recent developments are different because they involve sustained transmission among mammals that congregate in large groups.

Mass mortality events in South America provided the first major warning signs. In late 2023 and early 2024, researchers in Argentina and Chile documented the deaths of roughly 17,000 elephant seal pups and thousands of sea lions. The density of these animal colonies likely facilitated the spread of the virus. This proved that H5N1 could move efficiently through mammal populations, rather than just affecting solitary animals.

The 2024 Dairy Cow Outbreak

In March 2024, the USDA confirmed a surprising development: H5N1 was detected in dairy cows in Texas and Kansas. This was the first time this influenza strain was found in cattle. The outbreak quickly expanded to herds in over nine states, including Michigan, Idaho, and New Mexico.

The symptoms in cows differ from those in birds. While birds often die quickly, cows generally experience:

• Decreased appetite and lethargy.
• A significant drop in milk production.
• Thickened or discolored milk.
• Fever and dehydration.

Most cows recover with supportive care, but the presence of the virus in milk samples suggests that the mammary glands are a primary target for infection. This finding prompted immediate action from federal agencies to secure the food supply and protect farm workers.

Human Transmission Risks

The primary fear regarding H5N1 is its potential to mutate into a form that spreads easily between humans. As the virus replicates inside mammals, it gains opportunities to evolve. Mammalian hosts act as “mixing vessels” where the virus can swap genetic material and adapt to mammalian receptors, which are more similar to human receptors than avian ones are.

The Texas Case

In April 2024, the CDC reported a human case of H5N1 in Texas. The patient was a dairy worker who had direct exposure to infected cattle. Unlike the severe respiratory failure often associated with older strains of bird flu, this patient presented with conjunctivitis (pink eye) as the only symptom.

This case is significant for two reasons:

1. Confirmation of Jump: It confirmed that the virus could jump from cow to human.
2. Mild Presentation: The mild nature of the infection suggests that, while transmission occurred, the virus has not yet adapted to cause severe disease or spread efficiently via the human respiratory tract.

Current CDC Assessment

Despite the jump to livestock, the CDC currently assesses the risk to the general public as low. Genetic sequencing of the virus found in the Texas patient did not show the specific mutations required for efficient human-to-human airborne transmission. However, people with prolonged, unprotected exposure to infected birds or livestock remain at higher risk.

Food Safety: Milk and Meat

The infection of dairy herds naturally leads to questions about the safety of grocery store products. The FDA and USDA have conducted extensive testing to verify the safety of the commercial milk supply.

Pasteurized vs. Raw Milk

The consensus among scientists and food safety officials is clear regarding milk consumption:

• Commercial Milk: Pasteurized milk is safe. Pasteurization involves heating milk to a specific temperature for a set time to kill bacteria and viruses. FDA testing of retail milk samples showed that while fragments of the viral genetic material were present, the live, infectious virus was not. The pasteurization process effectively inactivates H5N1.
• Raw Milk: Raw (unpasteurized) milk poses a significant risk. Because the virus concentrates heavily in the milk of infected cows, consuming raw milk could introduce a high viral load directly into the human body. The FDA strongly advises against consuming raw milk or raw milk cheese during this outbreak.

Beef Safety

The USDA Food Safety and Inspection Service (FSIS) tested ground beef samples from retail outlets in states with dairy outbreaks. All samples tested negative for the virus. Additionally, cooking meat to a safe internal temperature kills influenza viruses.

Surveillance and Biosecurity Measures

To prevent the scenario of a human pandemic, aggressive surveillance is required. The USDA issued a federal order in April 2024 requiring mandatory testing for lactating dairy cows before they are moved across state lines. This helps track the movement of the virus and isolate infected herds.

Farms are implementing stricter biosecurity protocols, including:

• Limiting Visitors: Restricting access to essential personnel only.
• PPE Requirements: Encouraging workers to wear goggles, masks, and gloves when handling sick animals.
• Sanitization: Disinfecting equipment and vehicles that move between farms.

Scientists continue to sequence the genome of the virus from every new positive case. They are looking for specific markers, such as changes in the PB2 gene, which helps the virus replicate in mammalian cells. Detecting these changes early allows health officials to update vaccine candidates and antiviral strategies.

Frequently Asked Questions

Can I get bird flu from eating eggs? It is highly unlikely if you follow standard food safety practices. The risk of infected poultry entering the food chain is low because flocks are culled upon detection. Furthermore, cooking eggs until the yolk and white are firm kills bacteria and viruses.

Is my pet cat or dog at risk? There have been confirmed cases of H5N1 in domestic cats, particularly those that lived on or near dairy farms and consumed raw colostrum or milk from infected cows. Barn cats dying was actually an early indicator of the dairy outbreak. To protect pets, keep them away from wild birds, livestock, and raw milk.

Is there a vaccine for humans? The US government maintains a stockpile of vaccines tailored to H5 influenza strains. While these are not available to the general public, manufacturers are already preparing bulk antigens that could be bottled and distributed if a human-to-human outbreak were to occur.

How does the virus spread between cows? While the exact mechanism is still being studied, scientists suspect mechanical transmission plays a large role. This includes milking equipment that is not fully sanitized between cows or workers carrying the virus on their clothing or hands from one animal to another.

What should I do if I see a dead bird? Do not touch it. State wildlife agencies and the USDA track these deaths. Contact your local state department of natural resources or veterinarian to report the sighting so they can decide if testing is necessary.