Despite the wealth of knowledge surrounding risk factors for porcine epidemic diarrhea and porcine reproductive and respiratory syndrome, the relationship between extreme weather events and PED and PRRS outbreaks has not been thoroughly investigated.

“These weather events may have indirect effects with these farms … some of those effects such as disruption of movements, feed, dead animal removal, how often do these actions get affected by extreme weather events?” says Igor Paploski. “Can maintenance crews actually reach the farms after these events … they may not be able to get to farms. Their houses may have been damaged. Also, biosecurity concerns with a flooded farm, how well do our biosecurity practices hold during those events, and distressed animals may have immune systems which may facilitate diseases.”

Do extreme weather events have an impact on farms breaking with new cases of PRRS or PED? That was the question Paploski set out to answer in a recent study he conducted and presented on during the North American PRRS Symposium in Chicago.

After obtaining data on both PED and PRRS outbreaks at the farm level from the Morrison Swine Health Monitoring Project, the researchers divided the study in two periods, based on the occurrence of those diseases. “Prior to 2015, we called that ‘the epidemic period for both PED and PRRS’ and after 2015, we call that the ‘endemic period for both PRRS and PED,’” Paploski says.

Next, county-level extreme weather event data from the National Oceanic and Atmospheric Administration website was obtained. This included data on the occurrence of floods, rain and wind at a county level. While the team did not have access to weather events at a farm level, they tried to control for several aspects such as farm type, farm size and season.

Analysis was conducted using a 1:4 unmatched logistic regression – one farm in a given week in which an outbreak occurred (case) to four farms in weeks in which an outbreak had not occurred (controls). They then checked if the frequency of exposure to weather events was different between cases and controls.

For each weather event, the researchers ran 10 different models with each model including a different lag of time between exposure and outbreak occurrence. Analyses were run on two periods of time based on trends of PED incidence epidemic (2014-2015) and endemic (2016-2019) periods.

The team found that certain extreme weather events were associated with the occurrence of PED, even if they are distally located in the causal chain. During the endemic period, farms located in counties exposed to floods had three to four times higher odds of having a PED outbreak in the four to nine weeks prior to the flood compared to the controls. They also saw increased odds of PED occurrence after heavy rain and high wind during the endemic period.

However, associations between weather events and PRRS were less clear. The PRRS cases only had increased odds of being exposed to high wind in 2014-2019 and 2016-2019. Also, PED and PRRS outbreaks also did not occur homogenously over the study years.

Overall, the researchers found the associations between outbreaks and weather events tended to be weak and with wide confidence intervals, but associations could be detected between PED and the reporting of floods and high wind. PED and PRRS occurrence were not associated with tornado reports.

“We found evidence of association between extreme weather events, mostly flood and PED occurrence. For PRRS, those signals were harder to detect,” Paploski says. “We believe that swine companies may benefit from developing biosecurity protocols that account for these extreme weather events, perhaps develop some sort of emergency plan when these weather events occur.”