Time to reboot disease surveillance

US pork production has gone through dramatic changes over recent decades, but one thing that hasn’t changed is the challenge that infectious disease presents.

Swine veterinarians, researchers and producers have learned much about disease pathogens, transmission, and biosecurity strategies, as well as gained access to improved vaccines and diagnostics, yet industry losses to endemic diseases continue.¹

“This represents an opportunity for improvement,” said Jeff Zimmerman, DVM, Iowa State University, during his Howard Dunne Lecture presented at the 2017 American Association of Swine Veterinarians’ annual meeting.

“Understanding a pathogen’s pathogenesis, immunity, genetic diversity and diagnostics is only one part of the picture. We need to understand how a pathogen acts within populations.”

Sizing up the population

Over the decades, pig numbers within the US have held relatively stable. However, farm numbers have been on a slow, steady decline since the 1900s, which means today’s production sites house more pigs than in the past. Zimmerman cited the example that in 1980 an average herd size in Iowa totaled 250 hogs versus 1,500 in 2002, a trend that has continued.

Simply put, increasing hog numbers within a population leads to larger and more frequent disease outbreaks, as well as making disease control via herd immunity more difficult. Influenza, porcine reproductive and respiratory syndrome, Salmonella and other pathogens become endemic as herd size increases, Zimmerman pointed out. Also worth noting, swine veterinarians began reporting “production diseases,” or disease complexes, caused by multiple infections and non-infectious factors.

Larger herd sizes have triggered other production characteristics that have affected pathogen behavior, such as rapid population turnover. This is readily considered in the breeding herd where 40% of the sow herd is replaced annually. But the finishing barn will have an annual turnover of 250% as pigs grow and ship to market.

“Population turnover and continually introducing new, immunologically susceptible animals promotes pathogen circulation,” Zimmerman said.

There’s also the subpopulation impact. To enhance management, pigs today are segregated by age, production stage and/or function. But as a consequence, it can create subpopulations with different disease/immune status on the same production site. The idea of subpopulations is nothing new; Scott Dee, DVM, was teaching us about subpopulations in the mid-1990s.

A significant and somewhat recent management scheme involves moving large numbers of weaned pigs across state lines from farrowing sites to finishing units, mainly in the Midwest.  Zimmerman pointed out that in 1990, fewer than 5 million pigs moved across state lines, and by 2015 that grew to nearly 50 million.

“This process connects distant farms and provides an efficient mechanism to rapidly disseminate pathogens,” he added.

So, what’s next?

“Pork production is not going to return to the 60-sow herds of our great-grandparents and the surveillance methods they used,” Zimmerman said. “Rather, disease-surveillance strategies must catch up to a progressive pork industry and help address the risks it faces.”

That may include the need for ongoing, on-farm surveillance. He proposed that a viable system would need to address the following:

• Provide a return on investment. Data should accommodate production and business planning.
• Be easy to implement from sample collection to data analysis.
• Offer value. People using the system must believe that it has value.
• Provide timely and accurate data.
• Be flexible. It should be useful for more than one purpose; provide data on a range of targets.

The two greatest barriers that Zimmerman foresees include easy, efficient on-farm sample collection and simple, yet statistically valid, sample-size and sampling-location guidelines.

“It can become expensive and time consuming to routinely conduct on-farm collection,” Zimmerman said. “There are other options, but my thoughts turn to oral fluids.”

Because oral fluids provide an aggregate sample — the accumulation of fluid from all pigs that chewed on the rope – it provides an accurate, inexpensive, easy and more diagnostically sensitive option than individual pig samples.

Adding to the challenge is the long-held convention of sampling 30 random animals, to achieve a 95% probability of detecting at least one positive in a population with a ≥ 10% prevalence. “The calculation relies on assumptions that were true of small herds in the 1980s but often not true for today’s large, complex swine farms,” Zimmerman said. “In particular, it relies on the assumption that all pigs have the same chance of acquiring disease and that the results are independent.”

Based on recent research, those assumptions are no longer true, and more details on oral-fluid testing protocols will be forthcoming.

“Our highly productive industry has an Achilles’ heel that leaves us highly vulnerable to infectious diseases,” Zimmerman concluded. “We swine veterinarians need to find ways to make the industry more agile and devise solutions that neutralize the power of infectious disease.”

¹ Zimmerman J. Swine Medicine in the 21^st Century: Immovable Object Meets Unstoppable Force.  Howard Dunne Memorial Lecture, the 48^th American Association of Swine Veterinarians’ Annual Meeting. 2017;14-19.