Can Genetic Selection Lower Incidence of Uterine Prolapse in Pigs?
Sow mortality continues to baffle the pork industry. Although no one has uncovered the smoking gun, most would agree that it’s a multifactorial problem.
There are three main factors responsible for this increase in sow mortality across the industry and one of those is uterine prolapse, explains Jenelle Dunkelberger, a geneticist with Topigs Norsvin, who presented on the topic at the American Society of Animal Science Annual Meeting.
“I think it’s an issue a lot of people in academia and industry have wrestled with because the incidence of uterine prolapse seems to be much higher now than in the past, and we’re just not sure what's driving this increase,” Dunkelberger says. “It’s a really devastating animal welfare issue, as well as economic issue for the sow farm.”
In 2019, the Iowa Pork Industry Center (IPIC) at Iowa State University, with funding from the National Pork Board, conducted an industry-wide survey to help identify potential causative factors of sow pelvic organ prolapse (POP) to move toward developing and disseminating prevention strategies to help reduce POP incidence. Jason Ross, director of IPIC, led the team that surveyed 104 commercial sow farms, representing approximately 385,000 sows across the U.S.
The results showed, at the farm level, the most apparent relationships with increased POP incidence were farms using untreated water sources and farms whose management strategies included late gestation bump feeding, particularly when targeting thin sows. On an individual sow basis, sows with lower body condition or greater swelling and protrusion of the perineal region were more likely to prolapse.
Ross noted in the final report 10 genetic sources were represented across the sow farms in the study, although the study was not balanced for genetic source resulting in some genetic sources being represented on very few sow farms. He said further investigation is warranted as recent reports indicate a lack of heritability of POP risk.
Is There a Genetic Link?
Dunkelberger and her colleagues wanted to better understand the potential role of genetics on susceptibility to uterine prolapse, so they dug further.
“As a genetics company, we have access to a lot of individual sow data. That’s not trivial for a trait like prolapse,” Dunkelberger adds. “Even though it’s a devastating issue, it's still a low incidence trait. That means it is really challenging to obtain the number of records needed to perform a proper statistical analysis.”
When they began to investigate a potential genetic component for susceptibility to uterine prolapse, they were only able to gather information from one line because the incidence of uterine prolapse in the other line wasn’t high enough to create a data set.
“Noticeable differences between lines or breeds in the incidence rate, or average value for a trait, is usually the first indication that the trait might be influenced by genetics. This is what we observed for uterine prolapse, so we started looking into the data set and applying all the different strategies that we use when performing a genetic analysis. When we did this, we were able to identify a genetic component,” she says.
A Look at the Study
Dunkelberger and others used more than 16,000 different data records for analysis from a commercial multiplication farm that had been recording incidence of uterine prolapse for a number of years already.
“For every one of those data points, we had the full genotype information of the sow and a matching record to indicate why she was removed — either due to uterine prolapse, or due to another reason,” she says.
Dunkelberger developed the study to estimate the heritability of uterine prolapse and to validate estimated breeding values (EBVs) for uterine prolapse in an independent, related population. The 16,000 uterine prolapse records referenced above were used as the training population, and an independent data set of 4,000 records collected from purebred females at one of their nucleus locations was used as the validation population. Phenotypes were recorded at the sow level as removal due to uterine prolapse, or due to another reason.
Heritability estimates were derived from the training dataset by fitting year-season of insemination and parity at removal of the sow as fixed effects and animal as a random effect. Using the same model, validation was performed by regressing corrected offspring performance on sire EBV in the validation dataset.
A Surprising Discovery
Dunkelberger and colleagues discovered that the heritability of uterine prolapse was 22%, which is considered to be moderately heritable.
“For reference, the maximum heritability of a trait is 100%. If a trait is 100% heritable, it means that the trait is completely influenced by an individual’s DNA,” she says. “Meanwhile, 0% means that an individual’s DNA has absolutely no influence on the trait. A heritability of estimate of 22% falls between litter size (a little less heritable) and average daily gain (a little more heritable), to put it in perspective.”
These results also indicate uterine prolapse is mainly due to environmental factors. The higher incidence of uterine prolapse within the U.S. versus other geographic locations confirms the existence of a strong environmental component, Dunkelberger says. Genetic selection can be used to address genetic factors, but identifying and mitigating environmental triggers remains critical to reducing the incidence of uterine prolapse within the U.S. swine industry.
“The point that we have been trying to communicate to customers is that, with a heritability estimate of 22%, we have an opportunity to address uterine prolapse through our breeding program, and we've already made steps to do that. However, genetic selection alone is not enough to solve the issue of uterine prolapse,” she says. “If we can attack this problem with genetic solutions and non-genetic solutions, we're going to make a lot more headway in reducing the overall incidence of this issue.”
The preliminary results from the Topigs Norsvin data are quite convincing there is a partial genetic basis to susceptibility to uterine prolapse, says Jack Dekkers, distinguished professor of animal breeding and genetics at Iowa State University (ISU). Although he says it is possible this genetic basis is specific to the population and environment that these data were collected on.
“Nevertheless, information on the genes that control susceptibility to uterine prolapse in this data set will allow us to conduct more specific and powerful studies in other populations and environments,” Dekkers says. “In addition, knowledge on the biological mechanisms that control susceptibility to uterine prolapse could lead to management interventions that can be applied to other populations and environments to reduce its incidence across the industry.”
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