The avian flu virus spreading through American dairy cows is likely traced to a single spillover event. Scientists believe the virus jumped from wild birds to livestock in the Texas Panhandle late last year. This spring, the virus known as H5N1 had traveled hundreds of miles or more, appearing on farms in Idaho, North Carolina and Michigan.
The virus did not travel those distances on its own. Instead, it hitched a ride with its hosts, the cows, and moved to new states as cattle were transported from the epicenter of the outbreak to farms across the country.
The transport of live animals is essential for industrial livestock farming, which has become increasingly specialized. Many facilities focus on just one step in the production process – producing new young, for example, or fattening adults for slaughter – and then send the animals on.
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The exact number of chickens, cows and pigs transported in trucks, ships, planes and trains in the United States is difficult to determine because there is no universal national system to track their movements.
But estimates from official sources and animal activists give an idea of the scale: In 2022, some 21 million cattle and 62 million pigs were shipped to states for breeding or feeding, according to the Ministry of Agriculture; these figures do not include poultry, intra-state movements or journeys to slaughter. That same year, more than 500,000 young dairy calves, some just a few days old, were shipped from just six states, according to the nonprofit Animal Welfare Institute. Some traveled more than 1,500 miles.
“The movement can contribute to long-distance transport of pathogens and outbreaks and make outbreak management challenging,” said Colleen Webb, a livestock epidemiology expert at Colorado State University.
Many livestock pathogens, including bird flu, are zoonotic, meaning they can jump from animals to humans. Larger, longer-lasting outbreaks involving livestock can increase the likelihood of people coming into contact with infected animals or contaminated food products and create more opportunities for pathogens to evolve.
Since March, bird flu has been confirmed in 51 dairy farms in nine states, and at least one dairy worker has been infected. In an effort to curb the outbreak, the USDA last month began requiring influenza A testing for lactating cows crossing state lines.
“But that only addresses a very small part of the problem,” said Ann Linder, associate director of the animal law and policy program at Harvard Law School.
The United States places few restrictions on the transportation of farm animals, which poses an often overlooked threat to human and animal health, experts say. Livestock movement presents what Linder called “a perfect mix of factors that can facilitate disease transmission.”
Shipping fever
Every step in the transport process offers opportunities for pathogens to spread.
Trucks and boarding facilities can cram animals from multiple farms into small, poorly ventilated spaces. In a randomized trial, researchers found that 12% of chickens slaughtered on farms harbored Campylobacter bacteria, a common cause of food poisoning. After transport, the bacterium was found in 56% of the birds.
Transport conditions can also take a physical toll. Animals can be exposed to extreme heat and cold, transported hundreds of miles without a break and deprived of food, water and veterinary care, experts say. There is virtually no data on how many people become ill or die during trips.
Such stressful conditions “jeopardize the animal’s health and well-being and also weaken their immune system, which obviously increases the risk of disease transmission,” says Ben Williamson of Compassion in World Farming, an animal welfare nonprofit.
Numerous studies suggest that transportation can suppress cows’ immune systems, leaving them vulnerable to bovine respiratory disease, also known as “shipping fever.”
During their travels, farm animals can also leave behind pathogens. In one study, scientists found that disease-causing bacteria, including some that were resistant to antibiotics, flowed from moving poultry trucks into the cars behind them. The trucks “just spread these antibiotic-resistant bacteria,” said Ana Rule, a bioaerosol expert at the Johns Hopkins University Bloomberg School of Public Health and an author of the study.
Contaminated transport vehicles are also known to spread pathogens long after the infected animals have disembarked and may play a role in the dairy cow outbreak, officials say.
Infected animals can then cause outbreaks at their destinations, including livestock auctions, which often attract animals that are too old, sick or too small for the commercial food supply. Such auctions “would be a great place for H5N1 to move from cattle to pigs,” Linder said.
Pigs are particularly worrying. They can be infected by multiple strains of flu at the same time, allowing different strains to exchange genetic material and create new versions of the virus.
The global trade in live pigs has fueled the evolution of swine flu, sending pigs with one flu virus to parts of the world where different flu viruses are circulating. Harmful new forms of Streptococcus suis, bacteria that can make both pigs and humans sick, have emerged through a similar process.
The global pig trade “increases the diversity of pathogenic strains around the world,” says Gemma Murray, an evolutionary geneticist at University College London, who conducted the streptococcus study.
Gaps and loopholes in the law
The Department of Agriculture has the authority to restrict the movement of livestock between states, but in practice there are few barriers to cross-country transportation. “I think for the most part, the USDA wants to make that life cycle journey as seamless as possible,” Linder said.
Under a federal law first passed in 1873, livestock transported for more than 28 consecutive hours must be unloaded for at least five hours for food, water and rest. But critics say the 150-year-old law is more lax than regulations in comparable countries and is rarely enforced. The Animal Welfare Institute found only 12 federal investigations into possible violations in the past 15 years.
The law also exempts shipments by water or air. Compassion in World Farming has documented the use of “cowtainers” to transport calves from Hawaii to the continental United States, on boat trips that can last five days or more.
Livestock traveling between states must carry a certificate of veterinary inspection issued by the state agriculture department or a licensed veterinarian certifying that the animals are healthy. But those visual inspections would not find infected but asymptomatic animals, which likely played a role in the spread of bird flu to new dairy herds.
Some states have their own disease testing requirements. For example, Utah requires some cattle to test negative or be vaccinated against brucellosis, a bacterial infection, while Maryland requires chickens to test negative for pullorum disease and typhoid fever.
But most routine disease surveillance takes place at the end of the supply chain. “There are inspectors at the slaughter plants who inspect the carcasses as they come in for signs of disease,” Webb said.
When inspectors identify sick animals, experts can conduct epidemiological research to determine where the animal came from. But these studies are not always successful.
Many countries in Europe now have mandatory livestock identification and tracking systems, which record the movements of individual animals throughout their lives. “It’s a no-brainer in the modern world, where we are so connected,” says Dr. Dirk Pfeiffer, a veterinary public health researcher at the City University of Hong Kong.
While a handful of states, including Michigan, have created similar systems, there are none nationally. A USDA spokesperson defended the U.S. system in an email, noting that the U.S. livestock industry is far larger than that of any European country.
A national tracking system would have allowed officials to quickly trace the paths of dairy cows infected with bird flu, identify affected farms and perhaps contain the outbreak, scientists said.
“The faster you have the data on where infectious animals may be, the faster you can do your checks,” Webb said. “When you’re trying to control an outbreak, it’s really a race against time.”
Animal welfare advocates are pushing for the introduction of new regulations for the transportation of livestock. One bill, proposed by Sen. Cory Booker, D-N.J., would reduce the 28-hour law to eight hours and require stricter administration. Rep. Dina Titus, D-Nev., plans to introduce a new bill that would strengthen enforcement and require compliance with international transportation standards.
“Consumers and Americans should care about the way farmed animals are transported because they are sentient beings that can suffer,” said Dena Jones of the Animal Welfare Institute. “But also because their well-being affects the safety of our food and our health.”
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