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. 2026 Jul 8;10:txag096. doi: 10.1093/tas/txag096

Evaluation of antibiotic usage during late gestation on pelvic organ prolapse incidence in sows

Jamie M Studer 1, Zoë E Kiefer 2, R Blythe Schultz 3, Daniel Boykin 4,3, Jason W Ross 5,✉
PMCID: PMC13435607  PMID: 42553710

Pelvic organ prolapse (POP) is a major concern throughout the swine industry, and despite widespread acknowledgement of the issue, underlying causes and strategies to mitigate POP remain largely unknown. Therefore, the goal of this study was to test the efficacy of bacitracin methylene disalicylate (BMD) administration as a potential strategy to reduce the incidence of POP on sow farms.

Keywords: bacitracin, mitigation, pelvic organ prolapse, sow

Abstract

Throughout the last decade, rising sow mortality rates have afflicted U.S. pork producers, as mortality rates have increased from an average of 9.0% in 2014 to 12.5% in 2024. Part of this increase is due to increased incidence of pelvic organ prolapse (POP), which accounted for 22.6% of sow mortality in 2023. Despite widespread acknowledgement of this issue, mitigation strategies are lacking. Antibiotics represent a potential strategy to reduce POP incidence, as a 2019 survey reported POP incidence was lower when feed contained antibiotics compared to periods without antibiotics. Therefore, this study examined the effect of bacitracin methylene disalicylate (BMD) supplementation on POP incidence in late-gestation sows across two commercial farms. Administration of BMD differed between farms, with farm A administering BMD through the water and farm B administering BMD through the feed. A total of 1,014 sows were on trial at farm A, with 522 in the control group and 492 in the BMD group. A total of 1,275 sows were on trial at farm B, with 709 in the control group and 566 in the BMD group. Sows were administered BMD for approximately two weeks prior to farrowing and perineal score (PS) evaluations were conducted to determine presumed POP risk. In brief, a PS of 1 (PS1) presumes low risk of POP while a PS of 3 (PS3) presumes high risk of POP determined by protrusion and swelling of the perineal region. Analysis of PS between treatments revealed that BMD administration did not affect PS at either farm (P ≥ 0.13). At farm A, 60.0% of control sows and 60.6% of BMD treated sows were scored PS1 while 2.9% of control sows and 5.4% of BMD treated sows were scored PS3. At farm B, 78.3% of control and 76.7% of BMD sows were scored PS1 while 2.8% of control and 3.1% of BMD sows were scored PS3. Further, BMD did not affect POP incidence at either farm (P ≥ 0.71), with 2.7% of control sows and 2.8% of BMD sows experiencing POP at farm A, and 2.8% of control sows and 3.2% of BMD sows experiencing POP at farm B. When evaluating litter characteristics, BMD treated sows had a lower number of stillborn piglets compared to control (P ≤ 0.05), although this observation is largely due to PS3 scored sows treated with BMD having lower stillborns compared to PS3 control sows. Although BMD administration did not reduce POP incidence in the current study, further investigation into the effect of BMD on incidence of stillborn piglets is warranted.

Introduction

Throughout the last decade, rising sow mortality rates have been a major concern for U.S. pork producers, as sow mortality has increased from an average of 9.0% in 2014 to a high of 15.8% in 2023 (Eckberg 2023). More recently, PigCHAMP benchmarking summaries of U.S. and Canadian farms reported a 12.5% sow mortality rate in 2024 (PigCHAMP 2024). Since 2014, a corresponding rise in pelvic organ prolapse (POP) has greatly contributed to the overall rise in sow mortality, as POP accounted for 22.6% of sow mortality in 2023 (Eckberg 2023). Additionally, a study published in 2023 evaluating a production system in the Midwest U.S. reported that approximately 28% of sow mortality was attributed to prolapses (Paiva et al. 2023). Despite being widely acknowledged as an epidemic sow welfare and production issue, the underlying causes of POP are largely unknown. Further, lack of knowledge on potential causative factors hinders the ability to develop mitigation strategies necessary to reduce the economic burden of POP on the swine industry.

Pelvic organ prolapse is defined as an anatomical disorder characterized by the protrusion of pelvic organs, including the vagina, uterus, or rectum, due to loss of support from pelvic floor muscles (Jelovsek et al. 2007). In sows, vaginal, rectal, and uterine prolapses are most common, and rectal prolapses can occur concurrently with vaginal and uterine prolapses (Supakorn et al. 2017). Vaginal and uterine prolapses typically occur in the period surrounding farrowing, and can occur prior to, during, or up to several days after parturition (Althouse et al. 2019). Additionally, the etiology and pathogenesis of POP in sows has not been well documented, although factors suspected to increase the risk of prolapse have been reported, including genetics, housing, genital tract trauma, parity, nutrition, and exposure to estrogenic mycotoxins (Althouse et al. 2019). Further, prolapse in sows may be partially caused by factors such as weakness of the pelvic diaphragm muscles along with the long and flaccid nature of the porcine uterus (Supakorn et al. 2019).

Antibiotics represent a potential avenue for prevention of this condition, as an industry wide survey conducted in 2019 reported that incidence of POP tended to be lower during periods when feed contained antibiotics compared to periods without antibiotics in the feed (Ross 2019). Further, sow farms have utilized administration of bacitracin methylene disalicylate (BMD) in attempt to reduce the incidence of POP in sow herds, although no studies have been published to substantiate the effectiveness of this approach. Given that POP incidence rises and falls throughout the year and expresses seasonality, it is not possible to determine the efficacy of BMD on mitigation of POP without a controlled experiment.

The active ingredient in BMD is bacitracin, a cyclic polypeptide produced by the bacteria Bacillus licheniformis and some strains of Bacillus subtilis, possessing potent antibacterial activity against mainly gram-positive and a few gram-negative organisms (Kyriakis et al. 1996; Lin and Yu 2020). The discovery of bacitracin dates back to 1945 and FDA approval of BMD for use in animal feed did not occur until 1969 (Singer and Johnson 2024). Bacitracin feed supplements contain bacitracin complexed with divalent cations such as zinc or complexed with methylene disalicylate, as with BMD (Kyriakis et al. 1996). Bacitracin methylene disalicylate is the reaction product of bacitracin and methylene salicylic acid, which are secondary metabolites of Bacillus licheniformis (Chen et al. 2021). The antimicrobial properties of bacitracin occur through inhibition of synthesis of bacterial cell wall components peptidoglycan and teichoic acids via binding of bacitracin to undecaprenyl pyrophosphate, effectively inhibiting proliferation of bacteria (Mascher et al. 2003; Singer and Johnson 2024). Due to its poor intestinal absorption, BMD primarily exerts local effects on gastrointestinal tract bacteria, while systemic effects are minimal because the small amount absorbed is rapidly eliminated (Droumev 1983).

In the swine industry, BMD has historically been utilized in growing and finishing diets to increase the rate of weight gain and improve feed efficiency (Kyriakis et al. 1996). The growth promoting effects of oral bacitracin may occur through promotion of enhanced digestion in the small intestine and prevention of colonization of pathogenic bacteria (Adetunji et al. 2024). Additionally, BMD is used as a narrow spectrum antibiotic for control of clostridial enteritis caused by Clostridium perfringens and swine dysentery caused by Brachyspira hyodysenteriae (Kyriakis et al. 1996). Previously, Baker et al. (2010) reported that Clostridium spp. are ubiquitous on swine farms and Kiefer et al. (2021b) reported that Clostridium perfringens was the 39th most abundant operational taxonomic unit (OTU) in the vaginal microbiota of commercial sows during late gestation.

Recent work has focused on investigating the vaginal (Kiefer et al. 2021a, 2021b) and fecal (Kiefer et al. 2024) microbiome in relation to prolapse risk in late gestation sows. These studies reported differences in species abundance and community composition of the vaginal microbiota between sows with presumed high risk for POP compared to sows with presumed low risk for POP (Kiefer et al. 2021a). Microbial taxa of interest in relation to POP were reported to include Porphyromonas, Anaerococcus, Streptococcus, Treponema, Actinobacillus, and Veillonella (Kiefer et al. 2021a, 2021b). Collectively, these findings suggest that microbial populations may be associated with physiological conditions that contribute to POP development, although the mechanisms underlying these relationships remain incompletely understood.

Numerous studies in swine, poultry, and rabbits have demonstrated that dietary BMD alters microbiota within the gastrointestinal tract (Hung et al. 2019; Johnson et al. 2019; Proctor and Phillips 2019; Lin and Yu 2020; Chen et al. 2021; Li et al. 2022), with positive effects reported including growth of beneficial bacteria and improvements in intestinal function (Chen et al. 2021; Li et al. 2022). Because alterations in both fecal and vaginal microbial communities have been associated with prolapse risk in sows (Kiefer et al. 2021a, 2021b, 2024), it is plausible that BMD-induced changes in the gastrointestinal microbiota could influence factors associated with POP susceptibility. While indirect effects on the reproductive tract microbiota are biologically plausible through alterations in host physiology or microbial interactions, the extent to which orally administered BMD influences the reproductive tract remains uncertain.

Additionally, BMD may have the potential to reduce POP occurrence through effects on oxidative stress, as the role of oxidative stress in POP pathogenesis has gained interest in recent years. Oxidative stress is reported to contribute to POP by disrupting collagen and elastin synthesis, potentially compromising the structural integrity of pelvic connective tissues (Marcu et al. 2020). Alterations in extracellular matrix turnover have also been implicated in POP, with imbalances between matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) promoting connective tissue weakening (Gabriel et al. 2006; Campeau et al. 2011; Wu et al. 2012). Supporting this concept, Harshman et al. (2024) reported that prolapsed sows exhibited lower circulating concentrations of collagen type I and TIMP-1, along with greater concentrations of MMP-1, compared with non-prolapsed sows. Previous research has demonstrated antioxidant properties of bacitracin, including the ability to scavenge superoxide radicals and reduce markers of oxidative stress (Settle et al. 2014; Duarte et al. 2023). Collectively, these findings suggest BMD supplementation may potentially influence physiological pathways associated with POP risk, including oxidative stress and connective tissue integrity.

Further, POP is associated with local signs of inflammation of the perineal region characterized by redness and swelling, phenotypically resembling a potential inflammatory response (Kiefer et al. 2021a). Studies in poultry have reported that BMD modifies the innate immune response via inhibition of processes involved in inducing inflammatory responses or recruitment of cytokines (Adetunji et al. 2024). Additionally, as an antibiotic, BMD may reduce the risk of experiencing POP by preventing proliferation of opportunistic bacteria within the reproductive tract, which may contribute to inflammation associated with POP. Therefore, the objective of the current study was to evaluate the effect of BMD administration on incidence of POP in late gestation sows. To do this, we tested the hypothesis that administration of BMD for approximately two weeks prior to farrowing would reduce the incidence of POP in late gestation sows.

Materials and methods

Animals, treatments, and experimental design

All experiments in this study were approved by the Iowa State University Institutional Animal Care and Use Committee and methods were conducted in accordance with relevant guidelines and regulations. This study utilized 2,289 sows housed on two commercial sow farms from the same production system. All animals were individually housed in gestation stalls during the trial. Parity of sows utilized at farm A ranged from 0 to 11, while parity of sows at farm B ranged from 0 to 9.

For eight consecutive weeks, sows within the same breed week were allocated into treated (BMD) and non-treated (control) groups during gestation week 14 to ensure consistent representation of each treatment across all breed weeks. At farm A, average gestation day of sows on the first day of BMD administration was 95.3 ± 2.4 days, and gestation day ranged between 92 and 99 days at the start of BMD treatment. At farm B, average gestation day of sows on the first day of BMD administration was 95.6 ± 2.1 days, and gestation day ranged between 92 and 98 days at the start of BMD treatment. Sows in the treated group were administered BMD until being moved into farrowing crates, and all sows were moved from the gestation barn to the farrowing room at approximately 112 days of gestation. Following conclusion of the trial period coinciding with movement of sows into farrowing crates, BMD was provided in lactation diets at the same rate to all sows at both farms.

Route of BMD administration differed between farms, with farm A utilizing delivery of BMD (BMD® soluble; Zoetis Inc., Kalamazoo, MI) through the water, while on farm mixing was utilized at farm B to facilitate delivery of BMD through the feed. Both farms utilized a trough system for feed and water, therefore, sows utilizing the same trough were in the same treatment group. At farm A, a Dosatron medicator was utilized to deliver BMD soluble through the water at a rate of 1.14 grams per sow per day to deliver 0.555 grams of active product per sow per day. A total of 1,014 sows were on trial at farm A, with 522 in the control group and 492 in the BMD treated group. At farm B, sows in the BMD treated group were individually administered five grams of BMD (BMD® 50; Zoetis Inc., Kalamazoo, MI) through on-farm mixing with feed to deliver 0.555 grams of active product per sow per day. A total of 1,275 sows were on trial at farm B, with 709 sows in the control group and 566 sows in the BMD treated group.

Furthermore, conducting research studies in commercial production environments requires balancing priorities with production staff and adjusting project plans in response to unexpected challenges in order to maintain scientific rigor and ensure high confidence in study outcomes. Therefore, any sows moved from BMD treated or control stalls during the trial were removed from the analysis, resulting in exclusion of 379 sows at farm A and 284 sows at farm B. Movement of sows within the gestation barn is common to keep sows with similar breed dates close together and to minimize empty stalls resulting from sow deaths, culling events, or failure to maintain pregnancy. In addition to these reasons, movement of sows due to farm management decisions that were inconsistent with the study design contributed to the high number of animals excluded from the trial. Importantly, these numbers are not included in the sample sizes reported above, maintaining a total of 2,289 sows utilized in subsequent analyses.

Utilization of perineal scoring system to determine presumed risk of experiencing POP

Sows were evaluated for prolapse risk by personnel from Iowa State University during the second week of BMD administration using a 3-point perineal scoring system as described in Kiefer et al. (2021a). At farm A, average gestation day of sows on the day of perineal scoring was 105.3 ± 2.4 days, and gestation day ranged between 102 and 109 on the day of perineal scoring. At farm B, average gestation day of sows on the day of perineal scoring was 104.6 ± 2.1 days, and gestation day ranged between 101 and 107 on the day of perineal scoring.

In brief, a perineal score (PS) of 1 (PS1) presumes little to no risk of prolapse and has none of the following: swelling, redness, and protrusion of the perineal region. By comparison, a PS of 3 (PS3) presumes high risk of prolapse and describes the perineal region of a sow with all of the following characteristics: severe swelling, redness, and protrusion of the perineal region. Due to the objective nature of the perineal scoring system, precautions were taken to minimize variation in assigning PS as follows: (1) sows were evaluated for PS only while laying down, (2) perineal scoring was conducted by the same individual at each farm, (3) scoring was conducted for each sow at one time point during the second week of BMD administration, and (4) the individual conducting the perineal scoring was blinded to the treatments.

Statistical analysis

Statistical analysis was conducted using Statistical Analysis Systems (SAS) version 9.4 (Cary, NC). Data from each farm were analyzed separately and outcomes evaluated included PS, POP outcome, litter data (total born, liveborn, stillborn, mummies) and wean-to-estrus interval length in days.

Chi-square analyses were performed for evaluation of the effect of treatment on PS and POP frequencies using the PROC-FREQ function in SAS. Litter characteristics and wean-to-estrus interval length were evaluated for the effect of treatment, PS, and the interaction between treatment and PS using the mixed procedure (PROC MIXED). Additionally, stillborn and mummified piglets were analyzed as proportions of total born piglets using generalized linear mixed models (PROC GLIMMIX) with a binomial distribution. Parity and breed week were classified as random effects within these models.

A separate analysis of the effect of BMD treatment on litter data by parity group was conducted using PROC MIXED, in which case the effects of treatment, parity group, and their interaction were evaluated with breed week classified as a random effect. Parities were grouped into early (parity 0–1), mid (parity 2–4), and late (parity 5+) classifications. Additionally, stillborn and mummified piglets were analyzed as proportions of total born piglets using PROC GLIMMIX as described above.

Data are represented as least squares means (LSMEANS) and maximum estimated standard error of the mean (SEM) is reported in tables for main effect comparisons. Data are considered significant if P ≤ 0.05.

Results

Effect of BMD treatment on perineal score

Evaluation of the effects of BMD treatment on PS revealed that BMD administration did not affect PS of sows at either farm. At farm A, PS were assigned to 91.0% (475/522) of CON sows and 93.9% (462/492) of BMD treated sows. Within sows assigned a PS, 60.0% (285/475), 37.1% (176/475), and 2.9% (14/475) of CON sows were assigned PS1, PS2, and PS3; while 60.6% (280/462), 34.0% (157/462), and 5.4% (25/462) of BMD treated sows were assigned PS1, PS2, and PS3, respectively (Fig. 1a; P = 0.13). At farm B, PS were assigned to 95.5% (677/709) of CON sows and 95.4% (540/566) of BMD treated sows. Within sows assigned a PS, 78.3% (530/677), 18.9% (128/677), and 2.8% (19/677) of CON sows were assigned PS1, PS2, and PS3; while 76.7% (414/540), 20.2% (109/540), and 3.1% (17/540) of BMD treated sows were assigned PS1, PS2, and PS3, respectively (Fig. 1b; P = 0.79).

Figure 1.

Graphs and data on effect of BMD treatment on perineal score with subfigures labeled a and b, illustrating percent of sows scored perineal score 1-3 between BMD treated and control groups.

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Effect of BMD on perineal score. Administration of bacitracin methylene disalicylate (BMD) via water (farm A) or feed (farm B) did not affect perineal score (PS) at either farm. In brief, a PS of 1 (PS1) presumes low risk of pelvic organ prolapse (POP) while a PS of 3 (PS3) presumes high risk of POP. (a) At farm A, 60.0% of control (CON) sows and 60.6% of BMD treated sows were assigned a PS1, while 2.9% of CON sows and 5.4% of BMD treated sows were assigned a PS3 (P = 0.13). (b) At farm B, 78.3% of CON sows and 76.7% of BMD treated sows were assigned a PS1, while 2.8% of CON sows and 3.1% of BMD treated sows were assigned a PS3 (P = 0.79).

Effect of BMD treatment on prolapse incidence

Overall, BMD treatment did not affect POP incidence at either farm (Fig. 2). At farm A, 2.8% of BMD treated sows (n = 492) and 2.7% of control sows (n = 522) experienced POP (P = 0.87). At farm B, 3.2% of BMD treated sows (n = 566) and 2.8% of control sows (n = 709) experienced POP (P = 0.71). In total and between both farms, 3.0% of BMD treated sows (n = 1,058) and 2.8% of control sows (n = 1,231) experienced POP. Additionally, POP incidence was higher in sows assigned higher PS between both farms, with a total of 2.1% of sows assigned PS1, 3.3% of sows assigned PS2, and 13.3% of sows assigned PS3 subsequently experiencing POP.

Figure 2.

Graphs and data on effect of BMD treatment on POP incidence illustrating percent of sows prolapsed between BMD treated and control groups.

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Effect of BMD on POP incidence. Administration of bacitracin methylene disalicylate (BMD) via water (farm A) or feed (farm B) did not affect incidence of pelvic organ prolapse (POP) in sows. At farm A, 2.7% of control (CON) sows (n = 522) and 2.8% of BMD treated sows (n = 492) experienced POP (P = 0.87). At farm B, 2.8% of CON sows (n = 709) and 3.2% of BMD treated sows (n = 566) experienced POP (P = 0.71).

Effect of BMD treatment and perineal score on litter characteristics and sow performance

Litter data, weaning dates, and breeding dates were extracted from farm management software and utilized for subsequent analyses. When evaluating the effect of BMD treatment on litter characteristics, BMD treated sows had a lower number of stillborn piglets compared to control sows at both farms (Table 1). At farm A, BMD treated sows had an average of 1.1 stillborn piglets compared to 1.4 for control sows (P = 0.05). At farm B, BMD treated sows had an average of 1.7 stillborn piglets compared to 2.3 for control sows (P = 0.04). No evidence for a difference between treatments was observed for mummified piglets at either farm (P ≥ 0.27). However, a difference in number of liveborn piglets between treatments was observed at farm B, with BMD treated sows having an average of 13.5 liveborn piglets compared to 12.5 liveborn piglets for control sows (P = 0.06). At farm A, no effect of BMD treatment was observed on number of liveborn piglets (P = 0.92).

Table 1.

Effect of bacitracin methylene disalicylate (BMD) on litter data and return to estrus by perineal score.

CON
 BMD
 P-value
Parameter1 PS1 PS2 PS3 PS1 PS2 PS3 SEM2 TRT3 PS4 TRT × PS5
Farm A6
 Total born 14.64 15.07 16.14 14.52 15.52 14.85 1.07 0.50 0.02 0.34
 Liveborn 12.97 13.55 12.82 12.84 13.68 12.67 1.09 0.92 0.03 0.90
 Stillborn 0.91a 0.88a 2.41b 0.94a 1.08a 1.21a 0.38 0.05 < 0.01 0.02
  Stillborn, % 8.15 7.84 16.42 9.36 9.08 9.96 3.18 0.30 0.04 0.11
 Mummies 0.76 0.64 0.90 0.73 0.76 0.97 0.36 0.72 0.58 0.72
  Mummies, % 5.16 4.63 6.68 5.90 5.22 6.36 2.81 0.79 0.54 0.95
 Wean-to-estrus interval 4.20 4.26 4.20 4.18 4.25 4.39 0.20 0.58 0.38 0.74
Farm B7
 Total born 15.18 15.67 14.34 15.34 15.74 15.45 0.95 0.36 0.22 0.77
 Liveborn 13.25 13.18 11.19 13.44 13.49 13.47 0.95 0.06 0.33 0.31
 Stillborn 1.58 2.22 2.99 1.57 1.94 1.54 0.55 0.04 < 0.01 0.15
  Stillborn, % 11.52a 14.75a 24.85b 12.20a 12.55a 10.30a 4.06 0.01 0.05 0.01
 Mummies 0.35 0.27 0.17 0.37 0.32 0.45 0.21 0.27 0.52 0.65
  Mummies, % 2.19 1.53 1.12 2.32 1.72 2.68 1.27 0.34 0.23 0.73
 Wean-to-estrus interval 4.43 4.51 4.54 4.28 4.40 4.14 0.29 0.13 0.44 0.78
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1

Data are reported as means in addition to as proportion of total born piglets for stillborn and mummified piglets.

2

Maximum value for the standard error of the mean (SEM) reported.

3

P-value for the effect of treatment (CON and BMD).

4

P-value for the effect of perineal score (low risk = PS1; moderate risk = PS2; high risk = PS3).

5

P-value for the treatment × perineal score interaction.

6

Farm A utilized administration of BMD via water. Control (CON) sows (n = 522); BMD treated sows (n = 492).

7

Farm B utilized administration of BMD via on-farm mixing in feed. CON sows (n = 709) BMD treated sows (n = 566).

a,b

Means within a row with differing superscript letters indicate significant differences for the TRT × PS interaction at P ≤ 0.05.

When evaluating the effects of the interaction between BMD treatment and PS on litter characteristics, no effects were observed on number of liveborn or mummified piglets at either farm (P ≥ 0.31). However, there was an interaction effect on stillborn piglets at farm A (P = 0.02), with PS3 BMD treated sows having 1.2 fewer stillborn piglets compared to PS3 control sows, although this interaction did not maintain significance when evaluating stillborn piglets as a proportion of total born (P = 0.11). At farm B, there was no evidence for an interaction between treatment and perineal score on number of stillborn piglets (P = 0.15). However, when evaluating stillborn piglets as a proportion of total born piglets, control sows assigned PS3 had a substantially higher proportion of stillborns compared to all other PS by treatment classifications (P < 0.05).

Effect of BMD treatment on wean-to-estrus interval length was evaluated for sows that returned to estrus within seven days of weaning. No differences in wean-to-estrus interval length were observed between treatments at either farm (P ≥ 0.13). Additionally, no effects of PS (P ≥ 0.38) or interactions between treatment and PS (P ≥ 0.74) were observed on length of wean-to-estrus interval at either farm.

A separate analysis was conducted to evaluate the effect of BMD treatment on litter characteristics by parity, with parities grouped into early (parity 0–1), mid (parity 2–4), and late (parity 5+) classifications (Table 2). At farm A, 26.6% (131/492), 36.6% (180/492), and 36.8% (181/492) of BMD treated sows were classified as early, mid, and late parity, respectively, while 31.8% (166/522), 33.0% (172/522), and 35.2% (184/522) of CON sows were classified as early, mid, and late parity, respectively. At farm B, 36.4% (206/566), 47.0% (266/566), and 16.6% (94/566) of BMD treated sows were classified as early, mid, and late parity, respectively, while 40.9% (290/709), 42.6% (302/709), and 16.5% (117/709) of CON sows were classified as early, mid, and late parity, respectively. This analysis revealed that although litter characteristics differ significantly between parity classifications, no interactions with BMD treatment were observed on litter characteristics between parity groups at either farm (P ≥ 0.05).

Table 2.

Effect of bacitracin methylene disalicylate (BMD) supplementation on litter data by parity group.

CON
 BMD
 P-value
Parameter1 Early Mid Late Early Mid Late SEM2 TRT3 Parity4 TRT × Parity5
Farm A6
 Total born 13.92 16.12 14.17 14.01 16.03 14.21 0.36 0.96 < 0.01 0.95
 Liveborn 12.63 14.28 11.85 12.74 14.04 11.78 0.55 0.78 < 0.01 0.85
 Stillborn 0.51 1.14 1.26 0.63 1.13 1.34 0.17 0.47 < 0.01 0.82
  Stillborn, % 3.14 6.50 8.35 3.98 7.03 9.44 0.91 0.21 < 0.01 0.94
 Mummies 0.76 0.67 1.02 0.63 0.85 1.08 0.23 0.68 < 0.01 0.35
  Mummies, % 5.32 3.35 5.97 3.83 4.84 6.79 0.89 0.67 0.01 0.16
Farm B7
 Total born 14.38 15.92 15.22 14.53 16.00 15.13 0.44 0.84 < 0.01 0.93
 Liveborn 12.49 13.70 12.26 13.19 13.67 12.50 0.69 0.20 < 0.01 0.30
 Stillborn 1.52 1.89 2.55 1.16 1.94 2.22 0.40 0.12 < 0.01 0.29
  Stillborn, % 9.56 11.65 16.19 7.91 11.26 14.98 1.64 0.27 < 0.01 0.81
 Mummies 0.35 0.34 0.39 0.24 0.45 0.42 0.11 0.84 0.11 0.09
  Mummies, % 2.05 1.95 2.15 1.58 2.44 2.45 0.52 0.74 0.37 0.29
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1

Data are reported as means in addition to as proportion of total born piglets for stillborn and mummified piglets.

2

Maximum value for the standard error of the mean (SEM) reported.

3

P-value for the effect of treatment (CON and BMD).

4

P-value for the effect of parity group (Early = Parity 0–1; Mid = Parity 2–4; Late = Parity 5+).

5

P-value for the treatment × parity interaction.

6

Farm A utilized administration of BMD via water. Control (CON) sows (n = 522); BMD treated sows (n = 492).

7

Farm B utilized administration of BMD via on-farm mixing in feed. CON sows (n = 709) BMD treated sows (n = 566).

Discussion

Despite continued research and widespread concern, POP related mortality continues to afflict the U.S. swine industry. To our knowledge, few studies have been conducted focused on development and testing of mitigation strategies to reduce the impact of POP on sow farms. Therefore, the objective of this study was to evaluate the effect of BMD administration on incidence of POP in late gestation sows on two commercial swine farms.

Data obtained from a survey of the U.S. swine industry indicated antibiotics may represent a potential strategy to mitigate POP related mortality, as POP incidence on sow farms tended to be lower during periods when antibiotics were present in the feed (Ross 2019). Despite the positive response regarding antibiotic supplementation and occurrence of POP obtained from this survey, BMD administration was not successful in reducing POP incidence in the current study. However, the incidence of POP observed may influence the ability to detect treatment effects. In the current study, 2.8% of sows on trial at farm A and 3.0% of sows at farm B experienced POP, therefore, the effects of BMD may have been limited under the challenge conditions present during this study. Consequently, we cannot exclude the possibility that BMD may have different effects in herds experiencing a greater POP challenge, although no evidence currently exists to suggest that BMD would be more effective under higher challenge conditions.

Since studies have reported differences in vaginal microbiota of sows that experienced POP compared to sows that did not (Kiefer et al. 2021a, 2021b), the rationale for utilizing BMD in the current study was to promote microbial alterations that support proliferation of beneficial microorganisms within the vaginal microflora. Although studies have demonstrated that feeding BMD induces alterations to microbiota within the gastrointestinal tract (Hung et al. 2019; Johnson et al. 2019; Proctor and Phillips 2019; Lin and Yu 2020; Chen et al. 2021; Li et al. 2022), is not known whether oral BMD administration can impact vaginal microbiota. Further, few differences in fecal microbiota have been reported between sows at varying risk for POP, suggesting the gastrointestinal tract microbiota is less associated with POP compared to the vaginal microflora (Kiefer et al. 2024).

Additionally, the current study did not evaluate the effects of BMD on composition of fecal or vaginal microflora and instead focused mainly on prolapse outcome. Therefore, it is not known whether BMD treatment induced alterations in any microbial communities in the current study. Furthermore, the absence of microbiological data limits our ability to assess whether the lack of treatment effects on POP incidence was due to an absence of microbial modulation or because changes in the microbiota were not associated with POP risk.

One outcome of this study was the lower number of stillborn piglets observed in BMD treated sows compared to control, although farm specific differences may have contributed to the observed response. This observation appears to be partially driven by the lower number of stillborn piglets in PS3 BMD treated sows compared to PS3 control sows, particularly at farm A. This may be explained by the higher incidence of prolapse in PS3 sows, as increased numbers of stillborn piglets have been reported in prolapsed compared to non-prolapsed sows (Ross 2019). Additionally, a previous study reported sows that experienced prolapse had a greater number of stillborn piglets in the previous litter compared to non-prolapsed sows (Harshman et al. 2024). These observations provide context for the association between elevated POP risk and stillbirth occurrence, however, the mechanisms underlying the reduction in stillborn piglets observed in the present study remain unclear and warrant further investigation.

Stillbirths have also been associated with anemia in sows, as Bhattarai et al. (2018) demonstrated an inverse relationship between sow hemoglobin levels and occurrence of stillborn piglets. Additionally, the relationship between anemia and oxidative stress has been suggested as a potential component contributing to development of POP in sows due to the role of hemoglobin in scavenging nitric oxide (Shen and Crenshaw 2022). Despite the recent interest in sow hemoglobin levels during late gestation, this parameter was not evaluated in the current study.

A previous study fed diets containing BMD at 250 g/ton to sows from late gestation (between gestation days 96 and 100) until weaning (approximately 20 days into lactation) and reported no effects on sow or litter performance in regard to piglet survival and weight at weaning compared to sows fed a control diet (Richert et al. 1993). However, the authors reported no information on litter characteristics such as total born, liveborn, or stillborn piglets, and therefore cannot corroborate the observation of lower stillborn piglets in BMD treated sows from the current study.

One consideration in respect to the current study is the differences in routes of BMD administration between farms, as limited information is available comparing the bioavailability or biological effects of BMD administered via water versus feed in gestating sows. However, differences in consumption patterns between the two delivery methods could influence the magnitude and timing of gastrointestinal exposure to BMD. Administration through drinking water may have resulted in a different intake pattern than feed-based administration, as water consumption can fluctuate among animals, potentially affecting the amount of BMD consumed. In contrast, feed administration may provide a more consistent delivery of the compound. However, since BMD is considered to have low oral bioavailability and minimal systemic absorption, substantial differences in systemic exposure between feed and water administration are not expected. Another consideration in regard to the current study is the timing of BMD administration, as it is possible that starting treatments two weeks prior to farrowing is not adequate and, if BMD has the mitigation capacity, administration for a longer period of time may be required in order for beneficial effects to be observed.

Another avenue to explore in regard to understanding and preventing POP in sows is integrity of muscles supporting the pelvic organs, as studies in humans have identified the role of molecular changes in connective tissues in the pathogenesis of POP (Marcu et al. 2020). Additionally, the role of genetics in susceptibility to POP in sows has gained interest in recent years, as estimates of heritability based on genomic relationships suggest that susceptibility to POP is moderately heritable and therefore may be selected against (Bhatia et al. 2023). Further, genome wide association analyses have been utilized to identify genomic regions associated with susceptibility to POP in sows, and further investigation into candidate genes of interest may provide valuable insight into this condition (Bhatia et al. 2023).

In conclusion, administration of BMD to late gestation sows did not affect POP incidence in the current study. Further research is needed to understand if antibiotics have the potential to mitigate sow prolapse. Additionally, future work should focus on identifying the underlying causative agents in order to pinpoint the antibiotic with potential to reduce POP in the swine industry.

Acknowledgements

The authors would like to acknowledge the industry partners facilitating this research project, especially the farm managers and animal caretakers working with us to complete this trial. Additionally, the authors acknowledge Stacie Matchan and Amanda Chipman for their assistance with perineal scoring. Finally, the authors acknowledge the undergraduate students, Jameson Bell, Alexis Berte, Brooke Bowen, and Dalton Line, for their contributions throughout the on-farm portion of this study.

Glossary

List of abbreviations

BMD

Bacitracin methylene disalicylate

ECM

Extracellular matrix

FDA

Food and Drug Administration

MMP

Matrix metalloproteinase

OTU

Operational taxonomic unit

POP

Pelvic organ prolapse

PS

Perineal score

TIMP

Tissue inhibitor of matrix metalloproteinase

Contributor Information

Jamie M Studer, Department of Animal Science, Iowa State University, Ames, IA 50011, United States.

Zoë E Kiefer, Department of Animal Science, Iowa State University, Ames, IA 50011, United States.

R Blythe Schultz, Department of Animal Science, Iowa State University, Ames, IA 50011, United States.

Daniel Boykin, Cactus Family Farms, Osceola, IA 50213, United States.

Jason W Ross, Department of Animal Science, Iowa State University, Ames, IA 50011, United States.

Funding

This project was supported in part by the National Pork Board and the Foundation for Food and Agriculture Research (Grant #18-147).

Conflicts of interest

None declared.

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