Can hot weather stop the spread of bird flu in US dairy cattle?

A Michigan dairy farm worker has become the second confirmed case of a person infected by the influenza A(H5) virus in the US, federal authorities confirmed on May 22.

According to the Michigan Department of Health and Human Services, the worker had been regularly exposed to infected livestock and tested positive for influenza A(H5) – of which H5N1 is a subtype – and experienced mild symptoms but have since recovered.

This is the second confirmed case of a human infection with bird flu in US history. The first case was reported in March when a Texas dairy farmworker was diagnosed after being in close contact with infected cows. The worker’s only symptom was eye redness (conjunctivitis).

The Centers for Disease Control and Prevention (CDC) has maintained that the risk to the public remains low, though those who have been exposed to infected animals are at a higher risk.

Why is this important?

Since the start of the outbreak in Spring, dairy farmers have been told to bolster their biosecurity,​ including PPE wearing and isolating herds from potential sources of infection, such as water troughs or ponds.

Experts have warned that taking preventative measures is key to ensuring the virus does not acquire dangerous mutations that would increase its transmissibility to humans.

WHO’s chief scientist warned​ that previous cases of bird flu in humans globally have had high mortality rates while vaccination efforts haven’t advanced sufficiently at this point. According to CDC, human infections with HPAI A(H5N1) virus have been reported in 23 countries since 1997, resulting in severe pneumonia and death in about 50% of cases.

Prior to this year, there were no reported cases of cattle contracting HPAI, or highly pathogenic avian influenza. While the virus that infects dairy cows isn’t deadly, it causes lactating cows to produce thick, colostrum-like milk and dry off, causing around 20% drop in milk production in affected farms.

The first infections were reported in Texas and Kansas in March, and the virus has since spread to 9 states and 52 herds.

Dairy cattle outbreak was 'bubbling away' undetected for months 

Experts continue to look into how the initial infection occurred whilst building their knowledge of how the virus spreads between species, including among cows and from cows to other farm animals and to humans.

Speaking during a National Milk Producers Federation (NMPF) webinar on biosecurity, Dr. Tavis Anderson, a researcher at the National Animal Disease Center, said that experts have now identified when the initial infection occurred.

The virus was transmitted from wild birds into dairy cattle in a ‘single spillover event’ – i.e. a one-off transmission, rather than multiple transmissions happening in different parts of the country all at the same time.

“That was approximately at the end of December, give or take a couple of weeks,” he explained. “Which means there was relatively limited local transmission.”

He added that the genotype linked to the outbreak in dairy cows had ‘spilled over’ more than 100 times over the last 4 years across 20 different species, all of which were dead-end hosts that did not transmit it further.

The outbreak in cattle however was different and only surfaced after the virus had ‘bubbled away’ for a couple of months, ‘and then it then got into that time and place where it was able to spread relatively rapidly and be noticed by veterinarians in 2024.’

Asymptomatic cows are likely spreading the virus interstate

Dr. Anderson added that the virus was likely being transmitted interstate by cattle not yet showing clinical symptoms when seemingly healthy herds were moved from one location to another.

He explained that a team of researchers created a heat map of movement whilst also tracking how the virus had evolved genetically between those locations.

“What you start looking at with Texas…, those initial cases are the ones that then moved out into Kansas, to Michigan, and then into New Mexico,” Dr. Anderson explained. “There was additional movement then from some of these other locations to different regions.

“The basic point here is that animals are moved and the virus goes with them.”

Are US dairy farmers doing enough to prevent the spread of H5N1?

Since the start of the outbreak, dairy farmers have been advised to bolster their biosecurity practices, including by keeping visitor logs, increasing cleaning of vehicles and workwear, adopting personal protective equipment (PPE) when working with infected animals, and separating symptomatic animals from the rest of their herd. CDC is also monitoring people with known exposure to infected animals for symptoms.

On April 24, a federal order​ was issued mandating the testing of lactating cows prior to interstate movement. However, farmers are legally required to test ‘at least’ 30 animals from assembled lots, meaning that if more than 30 animals are being moved interstate in a single lot, it would be at the farmer’s discretion whether more than 30 animals are tested.

Meanwhile, USDA has agreed to reimburse farmers​ for testing at designated laboratories and for purchasing personal protective equipment and state agencies have been told to prioritize supplies to hand out to producers.

DairyReporter found out that only a limited number of farms have taken advantage of the free PPE that states are providing. 

A spokesperson for the New Mexico Department of Health told us that PPE is available at its Roosevelt and Curry County public health offices and the Curry Country Cooperative Extension Service. However, only two dairies have requested and received PPE, including disposable gloves and gowns as well as face shields, N-95 masks and safety goggles.

A spokesperson for the Michigan Department of Health and Human Services told us they’ve had just one request for PPE so far but added that many farms already own PPE supplies. “If any farm is in need of more PPE, MDHHS and MDARD are coordinating a one-time provision to meet this need and protect workers,” the spokesperson said. “Farms can reach out to MDARD to request a one-time provision of PPE. MDARD will relay these requests to MDHHS which will determine local, regional, state or federal resources to fill requests.”

Farmers are also facing practical challenges in adhering to PPE requirements, particularly those that work in high-temperature environments, such as milking parlors.

But with the summer arriving, there’s hope that warmer temperatures may also hamper the spread of the virus.

Sunshine can help – but humans still have a part to play

Similarly to other influenza A viruses, they tend to spread more rapidly during colder months.

“When the temperatures start to rise, virus doesn't like to live as well,” Dr. Danelle Bickett-Weddle, a veterinarian, said responding to a DairyReporter enquiry during an NMPF webinar. “So I'm hopeful that maybe we get some of that environmental kill-effect too from our good old sunshine. But humans have got to manage heat stress and all that.”

Asked how dairy farmworkers can stay comfortable during hot weather while wearing PPE, Bickett-Weddle said that staff should take regular breaks, stay hydrated and use fans and other cooling equipment where possible.

“If people are wearing goggles in a parlor, they're deciding to wear something to protect their mouth. It's weird, but you do get used to it. I know that people struggle,” she added.

“If you've got facial hair, it's harder to stay protected, but frequent breaks, drinking lots of water, keeping the fans moving, keeping people cooled is going to be really essential on those farms that are dealing with cows that are infected to keep our humans safe.”

Is milk still safe to drink?

Commercial milk is safe for consumption according to the US Food and Drug Administration (FDA) as pasteurization inactivates the virus. On May 10, FDA announced that all 297 samples of retail dairy products it tested did not harbor any viable viral particles.

However, consumers have been warned to avoid eating raw dairy products, since infected cows – including asymptomatic carriers – can shed large amounts of the virus in milk.

While proponents of raw dairy have argued that evidence of a higher risk of transmission from raw milk consumption is inconclusive, veterinarians found that transmission from cows to domestic cats through milk is possible (more on this here​), raising questions about transmissibility through raw milk consumption.

FDA also holds a longstanding view that raw milk is unsafe for human consumption.