Researchers are investigating the consequences of PRRS on pig fetuses
By Jackie Clark
Researchers in Western Canada are working to better understand the reproductive mechanisms and consequences of porcine reproductive and respiratory syndrome (PRRS). Dr. John Harding’s lab group at the University of Saskatchewan is the only research team in North America investigating the reproductive aspect of the disease.
“Most people focus more on the respiratory disease,” Carolina Malgarin told Farms.com. She received her doctor of veterinary medicine designation from the Universidade Federal do Rio Grande do Sul in Brazil in 2015, and is now a PhD candidate in the department of large animal clinical sciences at the University of Saskatchewan. She is researching the mechanisms of death in pig fetuses infected with PRRS.
Sow gestation lasts about 114 days.
“We don’t know exactly why or what the mechanisms are, but before the 70th day of gestation the fetuses are not as susceptible to be infected,” Malgarin explained. When a sow is infected after that time “the virus is able to cross the endometrium and the placenta and infect the fetuses.”
The sow usually does not show many clinical signs, other than potentially flu-like symptoms, however, there are often reproductive consequences such as abortion or stillborn piglets.
Fetuses, and therefore piglets, can also be “congenitally infected. They will be weak fetuses, they’ll have a lower resistance to other infections, they’re more susceptible to secondary bacterial infections especially, so that will increase the mortality rate pre-weaning,” Malgarin explained. “Our job now is asking what happens after infection” of the fetuses.
Stillborn and aborted fetuses don’t seem to have legions on their hearts, lungs and brains, as scientists might expect from fetuses that die from a disease.
“In most pathologies, you see legions, either grossly or microscopically,” Malgarin added.
To get a better understanding of the mechanisms of death in pig fetuses infected with PRRS, the researchers infected pregnant sows with the virus, and euthanized them at five different time points to collect fetal samples, she explained. Scientists infected the sows nasally and intramuscularly at day 85 of gestation, and then euthanized groups of pigs two, five, eight, 12 and 14 days after infection.
“The most important finding for us was how fast the infection” spread from the sow to the fetus, Malgarin said. “The placenta was already infected after two days.”
PRRS reached the umbilical cord and serum of fetuses after five days, by eight days the infection reached the amniotic fluid and thymus. The thymus runs along both sides of the trachea from the larynx to the heart.
“Fetal death related to PRRS started at 12 days post-infection, and it’s also the day that the infection in the womb peaks,” she added. The progression of PRRS “was much quicker than we were expecting. … We can clearly see the progression of the virus from the infected mother to the endometrium, the placenta, the fetal umbilical cord, the serum and then the internal organs.”
This experiment was “the first step in understanding this pathway,” Malgarin said. In the next steps of this work, she is looking at fetal pig metabolites 21 days after infection, and also investigating cell death and low oxygenation in fetal organs.
Here, too, the researchers were surprised by some of the initial findings.
The scientists expected that the thymus was the main site for viral replication of PRRS in fetuses and, therefore, that it would be the most damaged. However, the thymus was less impacted by cell death and low oxygenation and “the heart was very affected, much more than we were expecting,” Malgarin explained. That finding indicates that the fetuses infected with PRRS experience cardiac stress.
These discoveries impact our understanding of how PRRS leads to abortion and stillborn fetuses.
“We are in the stage of making sense and connecting the molecular and theoretical results to the clinical side of the disease. We’re still figuring a lot of that out,” Malgarin said.
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