H5N1 Bird Flu in Dairy Cattle Explained

The emergence of Highly Pathogenic Avian Influenza (HPAI), specifically the H5N1 strain, in dairy cattle marks a significant biological shift that has captured the attention of virologists and agricultural officials worldwide. While bird flu has historically decimated poultry populations, its crossover into bovine hosts—animals previously thought to be largely resistant—raises critical questions about food safety and viral mutation.

The Unprecedented Jump to Mammals

In late March 2024, the United States Department of Agriculture (USDA) confirmed a surprising development: dairy cows in Texas and Kansas tested positive for H5N1. This was the first time this specific virus was detected in cattle.

For years, H5N1 has been circulating in wild birds and poultry. However, the virus belongs to the clade 2.3.4.4b, a genetic family that has shown an increased ability to infect mammals. While sporadic cases have occurred in foxes, seals, and raccoons, the spread through dairy herds represents a different type of threat because of the close contact between these animals and human workers.

By May 2024, the outbreak had expanded significantly. The USDA confirmed cases in herds across nine states, including Idaho, Michigan, New Mexico, North Carolina, Ohio, South Dakota, and Colorado. This geographical spread suggests that the virus moves efficiently, likely through the transport of asymptomatic cattle or contaminated equipment.

Symptoms to Watch in Cattle

Unlike poultry, which often die quickly from the virus, dairy cows appear to be more resilient. The mortality rate in cattle is low, but the morbidity (illness) rate creates economic strain for farmers. Symptoms reported by veterinarians include:

  • Drastic drop in milk production: Infected herds see a sharp decline in yield.
  • Abnormal milk appearance: The milk often becomes thick and discolored, resembling colostrum.
  • Reduced appetite and lethargy: Cows eat less and show signs of fatigue.
  • Fever and dehydration: These are common immune responses to the viral load.

Interestingly, the virus appears to concentrate heavily in the mammary glands and milk of infected cows, rather than just in the respiratory tract. This biological detail suggests that the milking process itself may play a role in transmission.

Transmission Mechanics: How It Spreads

Scientists are currently investigating exactly how the virus moves between cows. Initial infections likely originated from wild birds. Dead migratory birds were found on several properties where herds later tested positive. However, the subsequent spread between herds points to cow-to-cow transmission.

The leading theory involves “mechanical transmission.” Because the virus is present in high concentrations in milk, milking equipment that is not thoroughly sanitized between cows could transfer the virus from an infected udder to a healthy one. Furthermore, the movement of cattle between states for breeding or feeding purposes has accelerated the spread.

To combat this, the USDA issued a Federal Order in late April 2024. This mandate requires lactating dairy cows to test negative for Influenza A viruses before they can be moved across state lines. This is a containment strategy designed to isolate the virus to currently affected regions.

Is the Food Supply Safe?

The most immediate concern for the average consumer involves the safety of milk and beef products. The Food and Drug Administration (FDA) and the USDA have provided extensive data to address these fears.

Pasteurized Milk

The commercial milk supply remains safe. This is due to pasteurization, a process that heats milk to a specific temperature for a set period to kill harmful bacteria and viruses.

In April 2024, the FDA conducted a national survey of retail dairy products. While they did find fragments of viral genetic material (RNA) in roughly 20% of samples, follow-up testing confirmed that this material was biologically inactive. The pasteurization process had successfully destroyed the virus, leaving behind only harmless genetic remnants. You cannot get sick from these fragments.

Raw Milk Risks

The advice regarding unpasteurized milk is vastly different. The FDA and CDC strongly advise against consuming raw milk. Because the virus sheds at high levels in the milk of infected cows, drinking raw milk poses a theoretically high risk of infection. While raw milk proponents argue for its benefits, the presence of H5N1 adds a severe pathogen to the list of risks, which already includes Salmonella and E. coli.

Beef Safety

The USDA’s Food Safety and Inspection Service (FSIS) conducted testing on ground beef sold in retail stores in affected states. All samples tested negative for the virus. Furthermore, standard cooking practices act as a safeguard. Cooking ground beef to an internal temperature of 160°F ensures that any potential virus is neutralized. Additionally, the U.S. food safety system mandates that animals showing signs of illness are diverted from the food supply chain before processing.

Human Health Risks and Worker Safety

The Centers for Disease Control and Prevention (CDC) currently assesses the risk to the general public as low. However, people who work directly with infected animals face a higher level of exposure.

There have been confirmed cases of human infection linked to this dairy outbreak. The first case involved a dairy worker in Texas who developed conjunctivitis (pink eye). The worker reported no other respiratory symptoms and recovered after treatment with antivirals. A second case was later identified in Michigan.

These cases highlight that the virus can jump from cows to humans, likely through direct contact with infected milk or fluids splashing into the eyes or nose.

Protective Measures for Workers

The CDC has updated its recommendations for farm workers. They advise the use of Personal Protective Equipment (PPE) when working with sick animals or raw milk. This includes:

  • Safety goggles or face shields to protect mucous membranes.
  • Waterproof aprons and boots.
  • NIOSH-approved respirators (like N95 masks) in enclosed milking parlors.
  • Disposable gloves.

Scientific Monitoring and Mutation

The primary fear among virologists is that H5N1 will mutate in a way that allows efficient human-to-human transmission. When a virus infects a mammal like a pig or a cow, it enters an environment more biologically similar to a human than a bird is. This gives the virus “practice” at adapting to mammalian cells.

Genomic sequencing of the virus found in the Texas worker showed a minor change in the PB2 gene, which is associated with adaptation to mammalian hosts. However, scientists note that this specific marker has been seen before and does not yet indicate that the virus can spread easily between people.

Surveillance remains the most effective tool. The CDC continues to monitor emergency room data and flu testing results in regions with infected herds to spot any unusual spikes in human illness.

Frequently Asked Questions

Can I get bird flu from drinking milk? No, provided you drink pasteurized milk. The heat treatment used in commercial processing kills the H5N1 virus. Avoid raw (unpasteurized) milk.

Is it safe to eat beef? Yes. USDA testing has not found the virus in retail ground beef. Always cook ground beef to 160°F and steaks to 145°F to ensure food safety.

Does H5N1 kill dairy cows? Generally, no. Unlike poultry, which suffer high mortality rates, dairy cows usually recover with supportive care. The main impact is a temporary loss of milk production.

Can my pets get sick from this? Cats living on dairy farms have died after consuming raw colostrum or milk from infected cows. If you live near a farm, keep pets away from farm animals and do not feed them raw milk.

Is there a vaccine for cows? Not currently. While poultry vaccines exist, there is no approved H5N1 vaccine for cattle in the United States. Research is underway to develop one if the outbreak persists or worsens.