by The bird flu virus spreading through American dairy cows can probably be traced back to one consequential event. Late last year, scientists believe, the virus jumped from wild birds to cattle in the Texas panhandle. By this spring, the virus, known as H5N1, had traveled hundreds of miles or more, showing up on farms in Idaho, North Carolina and Michigan.
The virus did not cross those distances on its own. Instead, he hitched a ride with his hosts, the cows, moving into new states and transporting cattle from the epicenter of the outbreak to farms across the country.
The transport of live animals is essential to industrial animal agriculture, which is becoming increasingly specialized. Many facilities focus on one step in the production process — producing new young, for example, or fattening adults for slaughter — and then sending the animals on.
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Determining the exact number of chickens, cows and pigs being transported on trucks, ships, planes and trains within the United States is difficult because there is no universal national system to track their movement.
But estimates from official sources and animal advocates give a sense of the scale: In 2022, about 21 million cattle and 62 million pigs were shipped into states for breeding or feeding, according to the Department of Agriculture; these figures do not include poultry, movement within the same state or trips to slaughter. That same year, more than 500,000 young dairy calves, some just a few days old, were shipped from six states, according to the Animal Welfare Institute, a nonprofit group. Some traveled more than 1,500 miles.
“The movement can increase the long-distance transport of pathogens and make outbreaks, and outbreak management, challenging,” said Colleen Webb, an expert in livestock epidemiology at Colorado State University.
Many livestock pathogens, including bird flu, are zoonotic, meaning they can jump from animals to humans. Larger, longer-lasting livestock outbreaks can increase the chance of people coming into contact with infected animals or contaminated food products and create more opportunities for pathogens to emerge.
Since March, bird flu has been confirmed in 51 dairy herds in nine states, with at least one dairy worker infected. Last month, in an effort to contain the outbreak, the USDA began mandating influenza A testing for dairy cows crossing state lines.
“But that’s only addressing 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 imposes few restrictions on the transportation of farm animals, which often pose a threat to animal and human health, experts said. Livestock movement provides what Linder calls “the perfect mix of factors that can facilitate disease transmission.”
Shipping Fever
Each step in the transport process provides opportunities for pathogens to spread.
Trucks and holding facilities can cram animals from multiple farms into small, poorly ventilated spaces. In one randomized study, researchers found that 12% of chickens killed on farms had Campylobacter bacteria, a common cause of food poisoning. After transport, the bacteria were found on 56% of the birds.
The conditions of transportation can also take a physical toll. Animals could be subjected to extreme heat and cold, dragged for hundreds of miles without rest and without food, water and veterinary care, experts said. There is little data on how many get sick or die from the trips.
Such stressful conditions “endanger the animal’s health and welfare and also weaken their immune system, which of course increases the risk of disease transmission,” said Ben Williamson of Compassion in World Farming, an animal welfare nonprofit.
Numerous studies indicate that transportation can suppress cows’ immune systems, making them vulnerable to bovine respiratory disease, often called “shipping fever.”
When they travel, farm animals can also leave pathogens behind. In one study, scientists found that disease-causing bacteria, including some resistant to antibiotics, flowed from moving poultry trucks and into the cars behind them. The trucks “were just spreading these antibiotic-resistant bacteria,” said Ana Rule, an expert on bioaerosols at the Johns Hopkins 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 are on board and may have played a role in the dairy cow outbreak, officials said.
Infected animals can then spark outbreaks at their destinations, including livestock auctions, which 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 of particular concern. They can be infected by multiple strains of the flu at the same time, which allows different strains to exchange genetic material and create novel versions of the virus.
The global trade in live pigs has fueled the evolution of swine flu, by sending pigs carrying one flu virus to parts of the world where different flu viruses are circulating. New harmful forms of Streptococcus suis, bacteria that can sicken both pigs and humans, have evolved through a similar process.
The global pig trade is “increasing the diversity of pathogenic strains around the world,” said Gemma Murray, an evolutionary geneticist at University College London, who conducted the strep research.
Gaps and Loop Poles
The Department of Agriculture has the authority to restrict the interstate movement of livestock, but in reality there are few barriers to cross-country transportation. “I think the USDA, for the most part, 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 laxer 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 water or air shipments. Compassion in World Farming has documented the use of “cowtainers” to transport calves from Hawaii to the continental United States, on boat trips of five days or longer.
Livestock traveling between states must have a certificate of veterinary inspection, issued by the state department of agriculture or an approved veterinarian, certifying that the animals are healthy. But those visual inspections would only catch infected but asymptomatic animals, which probably played a role in the spread of bird flu to new dairy herds.
Some states have their own disease testing requirements. Utah, for example, requires some cattle to test negative or be vaccinated for brucellosis, a bacterial infection, and Maryland requires chickens to test negative for distemper and typhoid.
But most routine disease surveillance happens at the end of the supply chain. “The slaughterhouses have inspectors inspecting the carcasses as they come through for signs of disease,” Webb said.
When inspectors identify sick animals, experts can conduct epidemiological investigations to find out where the animal came from. But these investigations are not always successful.
Many countries in Europe now have mandatory livestock identification and tracking systems, which log individual animals throughout their lives. “It’s a no-brainer in the modern world, where we’re so connected,” said 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, none are on the national level. A USDA spokesperson defended the American system in an email, noting that the US livestock industry is far larger than any European nation.
A national tracking system could have allowed officials to quickly trace the paths of dairy cows infected with bird flu, identify affected farms and possibly contain the outbreak, the scientists said .
“The sooner you have the data on where infectious animals might be, the faster you can implement your controls,” Webb said. “When you’re trying to control an outbreak, it’s really a race against time.”
Animal welfare advocates are calling for new livestock transport regulations to be put in place. One bill, proposed by Sen. Cory Booker, D-N.J., would reduce the 28-hour law to eight hours, and require stricter record-keeping. Rep. Dina Titus, D-Nev., plans to introduce another bill that would strengthen enforcement and require compliance with international standards of behavior.
“Consumers and Americans should care about the way farm animals are treated because they are sentient beings, capable of suffering,” 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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