As they travel, farm animals can also leave pathogens behind. In one study, scientists found that disease-causing bacteria, including some resistant to antibiotics, were spilling out of moving poultry trucks and into the cars behind them. The trucks “were just spreading these antibiotic-resistant bacteria,” said Ana Rule, a bioaerosol expert at Johns Hopkins University’s Bloomberg School of Public Health and an author of the study.
Contaminated transport vehicles are known to spread pathogens long after infected animals have disembarked and could play a role in the dairy cow outbreak, officials said.
Infected animals can then spark outbreaks at their destinations, including livestock auctions, which often attract animals too old, sick or small for the commercial food supply. Such auctions “would be a great place for H5N1 to move from cattle to pigs,” Ms. Linder said.
Pigs are of particular concern. They can be infected with multiple types of flu at the same time, allowing different strains to exchange genetic material and giving rise to new versions of the virus.
The global trade in live pigs fueled the evolution of swine flu, sending pigs carrying one flu virus to parts of the world where different flu viruses circulate. Through a similar process, new harmful forms of Streptococcus suis have emerged, bacteria that can sicken both pigs and humans.
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 led the strep research.
Gaps and loopholes
The Department of Agriculture has the authority to restrict interstate movements of livestock, but in practice there are few barriers to cross-country transportation. “I think the USDA, for the most part, wants to make the life cycle journey as smooth as possible,” Ms. 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 rarely enforced. The Animal Welfare Institute has found only 12 federal investigations into potential 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 department of agriculture or an approved veterinarian, declaring the animals healthy. But those visual inspections would not have detected infected but asymptomatic animals, which likely played a role in the spread of avian influenza to new dairy herds.