Oral fluids offer practical on-farm disease surveillance

Monitoring the spread of a pathogen is one part of disease surveillance. But early identification is another part — arguably with growing importance as millions of pigs move across state lines each week to larger production sites.

The threat of transboundary or exotic diseases surfacing in the US only raises the stakes.

“How do we prepare to respond efficiently, effectively and successfully?” asked Jeff Zimmerman, DVM, Iowa State University, at this year’s American Association of Swine Veterinarians’ annual meeting.¹

He emphasized the need for routine on-farm disease surveillance. Cost, efficiency, sampling method and sample size have been significant obstacles, but oral-fluid (rope) sampling may offer a solution. While oral-fluid testing has provided a solid diagnostic option for group-housed pigs, validated sample-size formulas for surveillance purposes have not been available.

Consequently, Iowa State University researchers have been working to fill that void, and Zimmerman outlined the recommendations for sample-size formulas and guidelines.

Begin at the barn level

Design the sampling plan — the number, location and frequency of sample collection — with the objective of 1) disease detection or 2) monitoring trends over time.  For disease detection, a good goal is to collect one or more samples from each barn or airspace.  This is because animals are housed by age and stage, with minimal mixing, so barns on the same site may have different infection status.

“Sampling individual barns provides flexibility to tailor surveillance for farms ranging widely in size and complexity,” Zimmerman said. “Also, sampling multiple barns on a site is a powerful approach to detect infection or prove that a population is truly negative.”

Identify the sample number

Collect two to six samples per barn or airspace, Zimmerman noted. A larger sample size may increase detection probability but may not be practical, affordable or necessary to meet the objective.

“Let the purpose of the sampling and the budget drive this decision,” he advised.

Sample size is not determined by the number of pens in a barn. If a barn has many pens, then samples will likely come from separate pens; if there are few pens, more than one sample per pen will be collected. The key is a fixed-spatial approach — space samples equally over the barn’s length — which is further explained in the next step.

Establish collection locations

Once sample numbers are set, evenly distribute the collection ropes over the space that the pigs occupy. For example, if you want to collect two samples, hang each rope at approximately one-third of the barn length from each end.

“Spatial sampling works because it reflects reality,” Zimmerman said. “Infectious disease has a spatial component. This is why neighboring pens are likely to share the same disease status.”

Sampling the same pens over time provides perspective of pathogen shedding as well as immune responses, he added.

Outline frequency

Depending on your surveillance goals, plan to collect samples at regular intervals — every 1 to 4 weeks. “This frequency will provide a clear picture of pathogen circulation or a strong assurance that the barn is negative,” Zimmerman said.

He emphasized that fewer samples collected at frequent intervals is more valuable than more samples collected less frequently.

The Iowa State researchers will release more oral-fluid surveillance-sampling details and disease-detection probabilities in the near future.

Added benefits

Beyond the need to control present infectious diseases as well as protect the US swine herd from transboundary and exotic diseases, individual producers also can benefit from regular on-farm surveillance. That’s especially true if surveillance data and on-farm production data are combined.

As Zimmerman outlined, it would allow swine veterinarians to:

• Identify the circulation of specific pathogens.
• Quantify the pathogens’ effects on pig health and productivity.
• Target interventions to the correct pathogen and population.
• Time the intervention to maximize its effect.

“Utilizing aggregate samples, such as oral fluids, also will make regional control programs more practical, affordable and successful,” Zimmerman concluded.

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.