Formula sheds light on disease risk

Understanding the transmission route of a disease pathogen, how long it’s infective, its survivability in the environment and routes of infection all build the foundation for creating effective biosecurity protocols.

Infected animals are the most direct path to introducing diseases into a herd. But as herd health status improves, higher-risk practices are eliminated and other pathogen-exposure routes assume greater relative importance, such as indirect transmission, according to Anna Romagosa, DVM, with PIC Europe.¹

While exposure routes vary depending on the pathogen, the more common ones include:

• Airborne transmission
• Animal, feed and mortality transports
• People, including workers, visitors and maintenance staff
• Equipment
• Slurry management
• Pests

When implementing biosecurity procedures, it’s not only important to assess the chance of a disease being brought in through certain routes but also to consider how often the risk occurs.

In her paper submitted to the 2017 American Association of Swine Veterinarians’ meeting, Romagosa offered a predicted-probability formula, provided by Professor Jeroen Dewulf, University of Ghent, Belgium, to assess the combined risk of transmission multiplied by the frequency:

P = 1-(1-P)^N

P = the chance of disease transmission per risk occurrence

N = the number of times this risk occurs

For example: If you assume that the chance of a particular route to actually transmit the disease is only one chance in 1,000, but you know that this route occurs 52 times a year (weekly, for example), the chance of disease transmission at the end of the year is 5%.

5.0% =1-(1-0.001) ⁵²

That is already a significant risk, Romagosa noted.

Baseline and low-risk outcomes

Romagosa cited a recent Swedish study that looked at the disease-introduction risk on two different farm scenarios. The infectious agents were Brachyspira hyodysenteriae, causing swine dysentery, and Mycoplasma hyopneumoniae, causing swine enzootic pneumonia. The production examples included a 1,600-head finishing herd and a 484-sow farrow-to-finish, single-site farm. The most common between-farm contacts were considered, including live animals, animal-transport vehicles, mortality collectors and veterinarian visits.

Biosecurity measures included quarantine for new-animal introductions, preventing livestock truckers and mortality collectors from entering buildings and providing protective clothing and boots for all visitors.

The measures were then combined into two scenarios for each herd — baseline and low-risk — used to reflect differences of between-farm contacts. The baseline scenario included the predicted yearly contacts in a typical farm for each type. In the low-risk scenario for the farrow-to-finish herd, the number and sources for live-animal introductions were reduced.

The results of the yearly risk model for introduction of M. hyopneumoniae and B. hyodysenteriae are illustrated in the accompanying table:

Lewerin, et. al, 2015

¹Romagosa A, et al. Applied Review of Evidence-based Biosecurity. Biosecurity Seminar Proceedings of the 48^th American Association of Swine Veterinarians’ Annual Meeting. 2017;5-10.