Sow mortality has been a major rising concern for the swine industry, gathering attention from producers, researchers and industry experts. The trend of increased mortality is being observed globally, posing a serious production and welfare issue for the global swine industry. The economic losses attached to sow mortality are also substantial. Such losses can be attributed to the opportunity cost of weaned pig losses, loss of culled sows and sow replacement in the production system. A rise in sow mortality can drastically affect on-farm employee morale and well-being.

There are a multitude of factors that interact and vary over time that can lead to increased sow mortality, including nutritional imbalances, changes in management practices, environment, infectious and non-infectious stressors, and reproductive complications, among others.

One such contributor to the rise in sow mortality is pelvic organ prolapse, characterized by the loss of support from tissues and muscles of the pelvic floor, which leads to a drop of pelvic organs from their normal position, resulting in protruding of pelvic organs, including the rectum, urethra, bladder, cervix, uterus or vagina. Typically, vaginal, uterine and rectal prolapse are the most prevailing types encountered in sows, either in combination or individually.

An industry-wide survey reported the role of multiple factors that can contribute to increased risk of sow POP, and genetics was thought to be one. However, there have been contrasting reports in previous years concerning the role of genetics in a sow's susceptibility to POP.

A recent study by Topigs Norsvin utilized pedigree-based records and reported 22% of phenotypic differences in susceptibility of sows to POP was due to genetics. Our study used the same dataset but advanced further to evaluate and understand the role of genetics in a sow's susceptibility to POP using genomic information. This study was conducted as part of an internship experience jointly supported by Topigs Norsvin and the Pig Survivability Grant to Iowa State University from the National Pork Board.

This study was conducted using farrowing and cull data collected on 30,429 purebred sows from two multiplier farms located in the Midwest of the U.S., between 2012 and 2022. Of these, genotype on 48,075 genetic markers across the genome was available for 14,186 sows. The overall incidence of POP among dead and culled sows was 7.1% and was defined as the presence of vaginal or uterine prolapse, since it was hard to distinguish between the two conditions on-farm. The statistical genetic analyses were performed for both across parities and by-parity to estimate the heritability of susceptibility to POP and genetic correlations of susceptibility to POP between parities. In addition, a so-called genome-wide association study was conducted to identify genomic regions associated with susceptibility to POP, followed by functional genomic analyses of these regions to identify potential genes and biological processes associated with susceptibility to POP.

Susceptibility to POP was confirmed to have a substantial genetic basis in this population and in these herds. This indicates that it can be selected against increase the survival of sows. It was also confirmed that susceptibility to POP has a similar genetic basis in different parities. Genome-wide association studies revealed six regions across the genome that were associated with susceptibility to POP, but together, these explained only 9% of the genetic differences between sows. This means that there are many other genomic regions that contain genes that are associated with susceptibility to POP but that have small effects. Functional analyses further provided detailed insights into several biological processes that are associated with susceptibility to POP, including the role of collagen, the estrogen receptor gene, and glycoproteins that are important for calcium/phosphorus homeostasis.

Although there is no silver bullet, susceptibility to POP can be reduced by genetic selection because it has a substantial heritability, at least in some populations and herds (implemented by Topigs Norsvin in Jan. 2021 based on the initial pedigree-based results). Thus, using POP data from such herds, breeding companies can select to reduce susceptibility to POP of their lines using similar procedures as they use to select for, e.g., litter size. In addition, with additional research, the knowledge of the biological pathways that were discovered to affect susceptibility to POP can be used to develop targeted management recommendations or interventions to reduce the incidence of POP in herds across the globe.

References:

Bhatia, V., Stevens, T., Derks, M., Dunkelberger, J., Knol, E., Ross, J., & Dekkers, J. (2023). Identification of the genetic basis of sow pelvic organ prolapse. Frontiers In Genetics, 14. doi: 10.3389/fgene.2023.1154713