Examining the spread of porcine coronaviruses, PRRS virus in growing pigs
By Alexis Berte, Kate Dion, Derald Holtamp, Daniel Linhares, Gustavo De-Sousa-E-Silva and Mafalda Pedro Mil-Homens
Biosecurity plays an important role in preventing the spread of pathogens between swine premises. Each risk event can potentially spread pathogens and cause economic loss.
Caretaker movements from one swine premises to another is an event that may spread porcine reproductive and respiratory syndrome virus (PRRSV) and porcine coronaviruses.
The role of caretaker movements in spreading pathogens from one premises to another is not well established in wean-to-market premises.
The objective of this study was to investigate if the movement of caretakers from one premises to another impacted the spread of porcine epidemic diarrhea virus (PEDV), porcine delta coronavirus (PDCoV), transmissible gastroenteritis virus (TGEV), and PRRSV.
Methods Eight oral fluids (OF) were collected every two weeks from equidistant locations (i.e., fixed spatial sampling) from 63 nursery groups from May 2020-June 2021.
To be eligible, the groups originated from sow farms that were Category II-vx or IV for PRRSV and negative for PEDV, PDCoV and TGEV.
The OF samples were pooled into groups of four and tested for PEDV, PDCoV, TGEV and PRRSV using reverse transcriptase polymerase chain reaction (PCR) to determine the group's status for each pathogen.
Timesheet Mobile (Freedom Telecare, Westborough, MA) was used to identify all caretaker entries into the study premises and movements between other premises three days prior.
This system allows caretakers to be tracked via their GPS location on their phones. It records when they enter and exit a specific swine premises. All of this information can then be downloaded and sorted using spreadsheets.
Figure 1. This image shows how oral fluids are collected using ropes that pigs chew on to saturate them. The oral fluids are then collected into tubes and sent into the diagnostic lab for testing.
The pathogen status of the other premises visited prior to entering the study premises was collected utilizing production system diagnostic data.
The number of caretaker entries to the study premises for each two-week period, and the visits to other premises visited in the three days before entering the study premises were evaluated as a ratio of the number of visits from positive premises over the total number of visits.
For the analysis, a logistic regression model per pathogen was performed to determine the association between the number of caretaker entries and the ratio of the number of visits from positive premises over the total number of visits and pathogen status in R package (version 4.1.1).
Results, take home message Out of the 63 groups, six tested positive for PRRSV, eight tested positive for PEDV, and 12 tested positive for PDCoV. Four groups for PEDV and three groups for PDCoV were removed from the analysis after their initial test was positive. No groups tested positive for TGEV.
The number of standard employee visits for the entire nursery period ranged from 39.1 to 141.9 for each two week period, with an average of 73.1 visits. Caretaker visits and the ratio of previous site visits were not statistically significant (p<0.05) in the logistic regression for any of the pathogens.
This result suggests that more frequent caretaker visits and visiting a higher ratio of positive sites is not associated with the introduction of virus into groups of pigs that were previously negative.
While this result was surprising, it does not imply that caretaker entry is not an important route by which virus may be transmitted from one premises to another. It does raise some interesting questions that warrant further investigation.
Periods other than three days for prior site visits may be evaluated, for example, to determine if the virus status of other premises visited in one or wo days, instead of three, are more closely associated with virus introduction.
Berte is a DVM candidate, Dion and and Mil-Homens are graduate students, Holtamp is a professor, Linhares is an associate professor, and De-Sousa-E-Silva is an assistant professor, all with Iowa State University College of Veterinary Medicine.
