How pathogens move between farms on people, boots, and vehicles

Direct and indirect transmission

Disease can spread when infected pigs are moved onto a farm. That's direct transmission. It can also spread indirectly, on the people, vehicles, equipment, and supplies that travel between sites. A biosecurity plan tries to control both. Because feed trucks, livestock trailers, service crews, and supply deliveries visit many farms, the indirect routes get a lot of attention.

Boots and vehicles in cold weather: a Minnesota experiment

Researchers at the University of Minnesota built a field model of a common winter sequence (Dee et al., Canadian Journal of Veterinary Research, 2002):

1. A vehicle picks up contamination on an infected site.
2. During a truck wash, the driver's boots are contaminated and then touch the cab floor mat.
3. The vehicle drives to a simulated farm, and the driver walks into the anteroom.
4. Meltwater from the boots touches containers headed into the animal area.

PRRS virus genetic material was detected at several points in 8 of 10 replicates and in none of the controls. Infectious virus was recovered in 5 of 10 replicates. Samples from the containers infected pigs in a follow-up test. In a separate part of the study, infectious virus survived for up to 4 hours on surfaces held just below freezing (−2°C).

The authors were open about the model's limits. The carriers were made from snow and inoculated with a dose of virus that may not reflect field conditions. Study staff deliberately stepped on the contaminated carriers during the wash, which would be unlikely in real life. And the study didn't test whether pigs exposed to the containers themselves would become infected. It shows a plausible route, not how often that route occurs.

Livestock trailers and the people around them

During the first months of PEDV in the US, researchers sampled 575 livestock trailers at six harvest plants in the Midwest in June 2013 (Lowe et al., Emerging Infectious Diseases, 2014). Of those, 6.6% were already contaminated when they arrived. Of the trailers that arrived clean, 5.2% became contaminated during unloading. That was more likely when plant staff entered the trailer (odds ratio 4.15) or when the trailer was unloaded right after a contaminated one. The authors noted that every driver stepped into the plant at least once. They suggested keeping drivers on their trucks during unloading as a simple measure that could make a big difference.

Feed-truck cabs

Truck cabs come up repeatedly. In a US investigation of PDCoV outbreaks, PEDV genetic material was found on a feed truck's floor, pedals, steering wheel, and dashboard (Elijah et al., 2022). In Vietnam, six of seven vehicle samples positive for African swine fever virus genetic material came from feed-truck cabs (Gebhardt et al., 2022). In both cases the authors cautioned that PCR detection doesn't show whether virus was infectious. In the US study, they found no evidence that feed delivery caused the outbreaks. Their broader takeaway was that people and vehicles can act as carriers. Read more in our feed-delivery biosecurity summary.

Shared vehicles connect farms: evidence from Great Britain

A network study used every recorded pig movement in Great Britain from April 2012 to March 2014. It modeled direct animal movements and shared haulage vehicles as two separate layers of contact (Porphyre et al., Transboundary and Emerging Diseases, 2020). The authors found that "sharing haulage vehicles increases the number of contacts between farms by >50%" and called it "an important driver of disease transmission." This was a model of livestock haulage in the British system, not of feed trucks. Still, it shows how shared vehicles can quietly connect farms that never trade pigs directly.

US and international context

• The Minnesota and PEDV trailer studies took place in the US Midwest. The haulage network study covers Great Britain, and the ASF example comes from Vietnam.
• Disease pressure differs a great deal by country. ASF has never been detected in the US, while it has been widespread in parts of Asia and Europe (USDA).
• Rules and transport systems also differ, so results from one country don't always carry over directly to another.

What this means in practice

The common thread is contact. Every time a person or vehicle that has been to another site crosses into the farm's controlled areas, there's a chance to carry something in. Industry guidance focuses on reducing those crossings: keeping deliveries away from animal buildings, keeping drivers in or at their trucks, cleaning and drying vehicles, and recording who and what comes onto the site (Secure Pork Supply). Your veterinarian and your site's biosecurity plan should guide the details.