H5N1 Bird Flu Spread in Mammals and Livestock

The highly pathogenic avian influenza, commonly known as H5N1 or bird flu, has historically been a threat primarily to poultry and wild birds. However, the virus has recently demonstrated an alarming ability to jump species barriers. The 2024 outbreak in United States dairy cattle marks a significant biological shift for the virus. This development has triggered investigations by the USDA, FDA, and CDC to understand how the virus spreads among livestock and what risks it poses to the human population.

The Outbreak in Dairy Cattle

In March 2024, the USDA confirmed a surprising development: H5N1 had infected dairy herds in Texas and Kansas. This was the first time this specific virus strain was detected in cows. Since that initial discovery, the virus has spread to herds in over nine states, including Michigan, Idaho, New Mexico, and Ohio.

This transmission event is distinct because cows were previously not considered a primary host for avian influenza. The symptoms in cattle differ significantly from those in birds. While the virus is often 100% fatal in chickens, cows generally survive the infection.

Symptoms observed in infected dairy cattle include:

  • Reduced Milk Production: A sudden, sharp drop in the amount of milk produced.
  • Altered Milk Consistency: The milk appears thick and discolored, often described as yellow or colostrum-like.
  • Lethargy and Appetite Loss: Cows eat significantly less and show signs of fatigue.
  • Fever: Affected animals often present with elevated temperatures.

The virus appears to concentrate heavily in the udder and milk of the cows. Scientific analysis suggests that the spread between cows is likely mechanical rather than respiratory. Milking equipment that is not thoroughly sanitized between cows may be moving the virus from one animal to the next.

Human Cases and Occupational Risk

The jump to mammals raises immediate concerns about human spillover. In April 2024, a dairy worker in Texas tested positive for H5N1 after exposure to infected cattle. This was the second confirmed human case of H5N1 in the United States (the first was in 2022 in Colorado involving poultry).

The clinical presentation in the Texas worker was mild. The primary symptom was conjunctivitis, or “pink eye,” likely caused by a splash of contaminated fluid or touching the eye with contaminated hands. The patient did not experience severe respiratory distress and recovered fully after treatment with oseltamivir (generic Tamiflu).

While this case was mild, it highlights a critical transmission route. The CDC currently assesses the risk to the general public as “low.” However, the risk is elevated for people with prolonged, unprotected exposure to infected animals or raw animal products.

Food Safety: Milk and Beef

Consumers are understandably worried about the safety of the food supply. Federal agencies have moved quickly to test commercial products and issue guidance.

Milk Safety

The FDA has conducted extensive testing on the commercial milk supply. They have confirmed that standard pasteurization is effective at inactivating the H5N1 virus.

  • Commercial Milk: Safe to drink. The heating process during pasteurization kills the virus. Even when viral fragments are detected in PCR tests, they are biologically inactive and cannot cause infection.
  • Raw Milk: The FDA and CDC strongly advise against consuming unpasteurized raw milk. High viral loads have been found in the raw milk of infected cows. Cats living on dairy farms that drank raw colostrum or milk from infected cows have died, suffering from severe systemic symptoms including blindness and neurological damage.

Beef Safety

The USDA’s Food Safety and Inspection Service (FSIS) conducted tests on ground beef sold in retail outlets in states with dairy herd outbreaks.

  • Testing Results: All samples tested negative for the virus.
  • Cooking Guidelines: The USDA continues to recommend cooking ground beef to an internal temperature of 160°F (71°C) to kill bacteria and viruses.

Spread to Other Mammals

The ability of the virus to infect cows suggests it is adapting. The H5N1 virus (specifically clade 2.3.4.4b) has been detected in a wide range of mammals across the globe over the last two years. This is not limited to farm animals.

Recent mammalian infections include:

  • Alpacas: In May 2024, the USDA confirmed H5N1 in alpacas on a farm in Idaho. This farm also had poultry that tested positive.
  • Marine Mammals: Thousands of sea lions and elephant seals in South America have died from the virus.
  • Scavengers: Red foxes, skunks, and raccoons often contract the virus after eating infected wild birds.
  • Domestic Pets: Barn cats have shown high susceptibility and mortality rates when exposed to infected dairy operations.

Every time the virus replicates in a new mammal host, it has a chance to mutate. The fear among virologists is that the virus could acquire mutations that allow it to bind more easily to human receptors or spread efficiently through the air between mammals.

Regulatory Response and Prevention

To contain the spread, the U.S. government issued a Federal Order in late April 2024. This order mandates that all lactating dairy cattle must test negative for Influenza A viruses before they can be moved across state lines. This was a direct response to evidence that the virus was spreading via the transport of asymptomatic or mildly symptomatic cows.

Prevention strategies currently in place include:

  • Enhanced Biosecurity: Limiting visitors to farms and preventing wild birds from accessing cattle feed and water sources.
  • PPE for Workers: The CDC recommends that workers in direct contact with livestock wear safety goggles, N95 respirators, gloves, and protective clothing.
  • Vaccine Preparedness: While there is currently no H5N1 vaccine distributed for humans or cows, the CDC has developed two candidate vaccine viruses (CVVs) that are well-matched to the current strain. Manufacturers could use these to produce a vaccine if a human pandemic were to occur.

Frequently Asked Questions

Is it safe to drink milk right now? Yes, provided it is pasteurized. Commercial milk sold in supermarkets undergoes pasteurization, a heating process that the FDA has confirmed kills the H5N1 virus. You should avoid raw, unpasteurized milk.

Can my pets get H5N1? Yes. Cats are particularly susceptible, especially if they hunt wild birds or consume raw milk from infected cows. Dogs can also be infected, though it appears less common. Keep pets away from dead birds and avoid feeding them raw meat or milk of unknown origin.

What are the symptoms of bird flu in humans? In the rare cases where humans catch H5N1, symptoms can range from mild eye redness (conjunctivitis) and upper respiratory symptoms to severe pneumonia. Fever, cough, sore throat, and muscle aches are common flu-like indicators.

Does cooking meat kill the virus? Yes. Properly cooking poultry, beef, and eggs kills the H5N1 virus. For poultry and eggs, cook to an internal temperature of 165°F. For ground beef, cook to 160°F. Steak and roasts should be cooked to at least 145°F.

How is the virus spreading between cows? Current evidence points to mechanical transmission. This means the virus is likely spread via contaminated milking machines, worker’s hands, or clothing moving from one cow to another, rather than the cows coughing on each other.