'Bayesian thinking' helps in disease control

Thomas Bayes wrote:

The probability of any event is the ratio between the value at which an expectation depending on the happening of the event ought to be computed, and the value of the thing expected upon its happening.

Put in simpler terms, Bayesian thinking is about conditional probability, said Peter Davies with the Department of Veterinary Population Medicine at the University of Minnesota's College of Veterinary Medicine during a presentation. Even though Bayes, who was an English theologian and statistician, lived 300 years ago, his theories are useful in studying modern biosecurity and disease control.

Davies explained that prior probability (belief or opinion) exists, but with new information, the probability (belief or opinion) is revised. He used this basis to discuss Porcine Epidemic Diarrhea virus (PEDv) and how biosecurity measures have changed as a result of knowing more about PEDv.

The Food & Agriculture Organization describes biosecurity as protection of health through avoidance of disease; and implementation of measures that reduce the risk of the introduction and spread of disease agents. Biosecurity requires the adoption of attitudes and behaviors by people to reduce risk in all activities, Davies said.

"It's difficult to 'know' sources of entry," Davies said. "Producers have to consider what might happen, what can happen, and what does happen."

Farm Inputs and Outputs
Variables can be difficult to control, considering the number of farm inputs and outputs, Davies said. The variable inputs include air, water, feed, people, supplies, equipment and pests, in addition to the animals themselves (semen, boars and gilts). When the inputs become part of the farm processes, waste, garbage and reclaim are outputs. Breeding stock and commercial pigs are outputs, too, along with the processes involved in selling animals.

Controlled access is the key, said Davies. From a national perspective, animals, products and some inputs (like vaccines and feed ingredients) can be controlled. From a farm perspective, animals, people, inputs, the barn environment and fomites (objects that may be contaminated with infectious organisms and serve in their transmission) can be controlled to a certain extent.

It can be done, and has been done with the exclusion of mange, lice, brucellosis, pseudorabies and other disease agents from herds, regions and/or countries, Davies pointed out.

Success Determinants
Davies said there are many factors that can impact the success - or failure - of disease control and eradication.

"Characteristics of the agent or disease itself are important, including routes of exit and transmission, the host's range and the agent's survival outside the host, and infectivity," Davies said. "The characteristics of the host, including immunity and infectious period, as well as characteristics of the environment - like area and animal density and biosecurity and management - also are important success factors.

"We know biosecurity is important, but we need clearly described procedures, and people who know those procedures," Davies added. "Compliance is the key." He said every farm needs a "biosecurity culture," and everyone on the farm needs to take ownership of biosecurity by understanding why it's important.

Even with high standards and carefully followed management practices, diseases like PEDv may not be prevented.

"PEDv changed the equation nationally, and refinement requires real knowledge," Davies said. That means "Bayesian thinking" will continue to be important as the industry gains more knowledge about diseases, disease agents, and the protocols that will help producers manage them effectively.