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Sow prolapse syndrome is not one disease with one proven cause. It is an industry term for a group of pelvic-organ events—most often vaginal or cervical prolapse, uterine prolapse, and rectal prolapse—that can share risk factors but do not have identical mechanisms.
The 13 causes traditionally proposed for sow prolapse include feed contaminants, mineral and vitamin status, disease and straining, constipation, genetics, parity, housing, farrowing intervention, tail docking, and litter characteristics. Some are biologically plausible; others have population-level associations but limited proof. The strongest newer evidence supports partly heritable susceptibility, while parity, reproductive timing, season, genetics, body condition, microbiome differences, and environmental conditions may modify risk.
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Important: A visible prolapse requires prompt assessment. Uterine prolapse is an emergency, and rectal, vaginal, or cervical tissue can swell, dry, become contaminated, lose its blood supply, or die. Contact the herd veterinarian immediately, particularly when tissue is dark, bleeding, damaged, non-reducible, or accompanied by weakness, fever, continued straining, dystocia, retained membranes, or urinary or gastrointestinal signs.
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Classifying the event is more useful than immediately trying to assign one of the 13 causes. The timing, appearance, urgency, and likely mechanism differ substantially between vaginal or cervical, uterine, and rectal prolapse.
| Type | Typical timing or appearance | Why it matters |
|---|---|---|
| Vaginal or cervical prolapse | Often occurs during the final third of gestation or around farrowing. Early tissue may appear only when the sow lies down and retract when she stands. | Persistent exposure causes swelling and trauma and may obstruct or complicate farrowing. |
| Uterine prolapse | Usually occurs immediately or within several hours after parturition, when the cervix is open and uterine tone is reduced. | This is an emergency because traction on the uterus can cause severe congestion, hemorrhage, shock, infection, and tissue death. |
| Rectal prolapse | Exposed rectal tissue is commonly associated with repeated straining, or tenesmus, rather than only with late pregnancy. | Enteritis, diarrhea, parasites, urinary disease, dystocia, and anorectal disease must be investigated. Exposed tissue can darken, harden, ulcerate, and become necrotic. |
More than one organ may be involved, and a farm may use the word prolapse for events that are clinically different. Record the type separately. Combining all events into one rate can hide the actual problem.
How to interpret the 13 proposed causes
Most proposed causes fit into one or more of three broad pathways:
- Weakening or relaxation of support: changes in pelvic tissues, uterine tone, connective tissue, or neuromuscular function may make prolapse more likely.
- Increased pressure or straining: coughing, gastrointestinal disease, constipation, diarrhea, urinary disease, dystocia, abdominal distension, or repeated contractions may push tissue outward.
- Predisposition plus trigger: genetic susceptibility, parity, body condition, reproductive timing, season, housing, and diet may create risk, while farrowing or straining provides the immediate trigger.
The original industry list was presented in approximate order of perceived importance, not as a proven ranking. The evidence should therefore be described carefully: recognized causes of rectal straining have a stronger direct association than many proposed causes of vaginal or uterine prolapse.
1. Mycotoxins and estrogenic phytoestrogens
Zearalenone is an estrogenic mycotoxin capable of causing reproductive abnormalities in swine. It is therefore biologically plausible that contaminated feed could contribute to reproductive-tract edema, altered reproductive function, or a prolapse cluster in a particular herd. Estrogenic compounds associated with plants such as clover, alfalfa, and soybeans have also been proposed.
That plausibility does not make zearalenone the explanation for every outbreak. Feed testing during the Smithfield investigation did not identify zearalenone as the reason for the increase in prolapse in the affected region. A positive result should also not be interpreted as proof that the toxin caused an individual sow’s prolapse; concentration, exposure duration, feed intake, sample quality, other toxins, and the clinical pattern all matter.
When feed contamination is a reasonable concern, collect representative samples from the relevant batch, ingredients, bins, and delivery points and have them interpreted through the herd veterinarian or a qualified feed laboratory. USDA’s Federal Grain Inspection Service maintains performance-verified rapid-test categories for zearalenone and other mycotoxins, but not every test is appropriate for every feed matrix or decision. A zearalenone feed test kit may be a screening option, not a substitute for representative sampling, confirmatory analysis, or veterinary interpretation.
Commercial disclosure: Any shopping or service reference in this article is secondary to veterinary diagnosis. Test availability, validation, feed-matrix suitability, and commercial-program eligibility should be verified before purchase.
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Calcium is involved in smooth-muscle contraction, uterine tone, and normal neuromuscular function. Hypocalcemia, an unfavorable calcium-to-phosphorus relationship, or changes in the mineral formulation have therefore been proposed as contributors, especially around late gestation and farrowing.
This is a reasonable nutritional differential, but sow-specific evidence is mixed. In one six-month field trial, removing phytase from part of a system’s sow farms did not improve prolapse incidence. That result does not prove mineral status is irrelevant, but it does show why changing one mineral-related formulation should not be treated as a universal prevention strategy. Genomic research has also identified calcium-homeostasis pathways as biologically interesting without establishing hypocalcemia as the primary cause in an affected sow.
Review the complete ration, premix inclusion, ingredient analysis, feed mixing, water intake, and clinical findings with a swine veterinarian and nutritionist. Avoid adding calcium, phosphorus, vitamin D, or other products without a ration-based reason.
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3. Disease pressure and increased abdominal pressure
Coughing, respiratory disease, huddling, piling, and other conditions that raise intra-abdominal pressure could theoretically promote prolapse. However, the large Smithfield experience did not find respiratory distress and piling to be consistently associated with prolapse, and some heavily affected farms had little obvious disease.
Disease should still be investigated, but the absence of obvious respiratory disease does not rule out prolapse risk. For rectal prolapse, the connection is more direct: persistent tenesmus from enteritis, colitis, diarrhea, parasites, urinary obstruction, cystitis, dystocia, or anorectal disease can force rectal tissue outward. A herd investigation should look for coughing and respiratory signs while also examining fecal consistency, urinary behavior, abdominal discomfort, and farrowing problems.
4. Abdominal or digestive disturbances
Rapid changes in feed intake, automated or ad-lib feeding, altered particle size, alternative ingredients, fermentation, gas production, diarrhea, and other digestive changes have all been proposed. They may increase abdominal pressure, change gut motility, alter fecal consistency, or increase straining.
For vaginal and uterine prolapse, these links remain largely observational or mechanistic. For rectal prolapse, digestive disease is a more actionable lead. Look for recent ingredient or feed-form changes, diarrhea or unusually dry feces, increased mortality or treatment rates, signs of enteritis, and changes that coincide with the first cases. Compare feed and health records rather than assuming that the most recent feed change is necessarily responsible.
5. High-density diets, especially starch or lysine
The original industry discussion noted associations between high-starch or high-lysine diets and increased rectal prolapse, but the mechanism was unclear. Possible explanations include fermentation and gas, altered intestinal motility, fecal consistency, feeding level, ingredient composition, or interactions with fiber and water intake.
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Starch and lysine should therefore be treated as ration-review signals, not as isolated causes. A high nutrient density may be appropriate for one group and poorly suited to another depending on body condition, intake, temperature, fiber profile, and stage of gestation. The correct question is not simply whether the diet contains “too much” starch or lysine, but whether the entire formulation and feeding program match the sows’ needs and the observed fecal and reproductive pattern.
6. Vitamin deficiency
Early Iowa State/AASV field work observed lower vitamin E levels in prolapsed sows than in controls, although the significance of that finding was uncertain. Vitamin D status was also proposed by analogy with pelvic-floor weakness described in humans.
There is not enough evidence to claim that vitamin E or vitamin D supplementation prevents sow prolapse. A more useful response is to review premix inclusion, ingredient quality, feed storage, oxidation risk, laboratory results, and the complete ration. Blind supplementation can create new mineral or vitamin imbalances and may distract from constipation, disease, water, flooring, or genetic factors that are more relevant to the herd.
7. Genetic susceptibility
Genetics now has stronger support than it did when the original 13-cause list was published. A 2023 genomic study estimated moderate heritability for susceptibility to vaginal or uterine prolapse in the population studied—about 0.35 in its across-parity analysis—and identified associated regions on several chromosomes.
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Genetics becomes more informative when events are recorded accurately and compared within relevant groups. Record genetic line or supplier, parity, farm, season, and event type. Discuss culling and breeding decisions with the production veterinarian and breeding program rather than making a decision from one isolated case.
8. Constipation and inadequate water or fiber
Constipation is one of the more actionable proposed risk factors because straining can directly increase pressure on pelvic and rectal tissues. The original field report associated constipation with prolapsed sows and identified inadequate water and low-fiber diets as possible contributors.
Audit drinker function and flow, water access, competition at the drinker, ambient heat, fecal consistency, recent feed changes, and the type and fermentability of dietary fiber. Fiber is not a single intervention: its effect depends on source, particle size, fermentability, energy density, inclusion rate, and timing. A swine nutrition consultant can help review a gestating-sow ration when constipation, fiber, mineral balance, or diet density is suspected. Any change should be made with the herd veterinarian and nutritionist, not by adding an unselected fiber or supplement product.
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9. Parity and loss of tissue tone
Repeated pregnancies and farrowings may reduce uterine, connective-tissue, or pelvic-muscle tone. This provides a plausible explanation for higher risk in older sows. More recent research has found parity-related differences in some populations, including increased risk in higher-parity animals, although the pattern varies by dataset and genetic group.
Parity is best viewed as a risk modifier. The original field list included it, but the observed increase occurred across parities, so age alone could not explain the problem. Compare first-, middle-, and high-parity animals using the same denominator and separate vaginal, uterine, and rectal events.
10. Housing, flooring, posture, and exercise
Limited exercise, crate housing, slippery flooring, excessive floor slope toward the rear, tethering, and posture have all been proposed as contributors. Slippery floors can make rising and lying down difficult, while rearward slope or prolonged posture may alter pressure on pelvic support structures. A swine-disease reference also describes greater occurrence in older, heavier late-gestation sows and associates risk with slippery floors and rearward slope.
Housing type alone does not explain the syndrome. In the Smithfield experience, prolapse increased even after some farms moved to group housing. Review the details of the environment instead: traction, slope, pen or crate design, stocking density, ability to rise and lie normally, access to water, temperature, and opportunities for movement. Compare affected and unaffected areas of the same farm when possible.
11. Oxytocin and assisted farrowing
Excessive oxytocin use and manual assistance were proposed because powerful uterine contractions, dystocia, or manipulation could theoretically contribute to tissue injury or prolapse. In the Smithfield cases, however, most prolapsed sows had not received oxytocin or assistance before prolapsing. That weakens the claim that routine use is the dominant cause of the syndrome.
In individual cases, inappropriate timing, dosing, or forceful manipulation can still create reproductive risks. Oxytocin should be used under a veterinary protocol, with attention to cervical opening, fetal position, dystocia, farrowing progress, and the possibility that contractions are not the correct solution.
12. Excessively short tail docking
Short tail docking can damage nerves near the anal sphincter and is a recognized biological mechanism for rectal prolapse in growing pigs. Its relevance to adult-sow pelvic-organ prolapse is much less certain.
In the Smithfield investigation, measured tail length in prolapsed sows was not associated with prolapse. Do not transfer evidence about tail-related rectal problems in young pigs directly to vaginal or uterine prolapse in adult sows. Tail length may be worth recording when investigating rectal cases, but it should not be treated as the universal explanation for sow prolapse.
13. Larger litters or larger piglets
Large litters or large fetuses may increase uterine and abdominal pressure, prolong farrowing, or increase the chance of dystocia. The original report proposed increasing litter size as a possible industry-wide contributor during a period of rising prolificacy, while acknowledging that more study was needed.
More recent reviews have also examined reproductive timing, litter outcomes, gestation length, and stillborn-piglet measures as associated factors in some populations. The relationship is not deterministic. High prolificacy may interact with maternal anatomy, fetal size, parity, uterine tone, farrowing duration, and management. A large litter is a context clue, not proof of causation.
What newer research adds
Heritability, population differences, and season
The most important development beyond the original list is evidence that susceptibility is partly heritable. A 2024 analysis of more than 75,000 purebred and nearly 19,000 crossbred sow records reported average incidence rates of 1.81% in purebreds and 3.93% in crossbreds. Those figures describe the populations and records in that study; they are not a universal expected rate for every farm.
The same work included parity and region in its analysis and found seasonal variation. This reinforces the need to compare farms and genetic groups carefully. A farm with more cases may have a different parity distribution, event-reporting system, climate, litter profile, or denominator rather than a single distinct cause.
Endocrine and microbiome signals
Late-gestation endocrine changes and vaginal microbiota have been examined as possible indicators of risk. A 2024 fecal-microbiota study described dysbiosis as a possible component of disease and reproductive dysfunction. These findings are promising research signals, not validated on-farm diagnostic tests or proven preventive treatments.
Microbiome differences should not lead to routine probiotic, antimicrobial, or diet claims without clinical evidence. They may eventually help explain why apparently similar sows respond differently to the same environment, but they currently supplement—not replace—basic herd investigation.
Climate and environmental conditions
A North American analysis published in 2026 examined prolapse incidence from 2019 through 2024 alongside climatic variables. Its abstract reported possible associations with environmental conditions, including soil moisture, while emphasizing that the pathophysiology of swine prolapse remains poorly understood.
This is an emerging association, not a simple weather-based cause. Climate may act indirectly through heat stress, intake, water use, flooring conditions, fecal consistency, or management changes. Use environmental patterns to generate questions and compare records, not to stop investigating feed, disease, genetics, and farrowing management.
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Call the veterinarian promptly for any visible tissue. Do not wait to see whether a uterine prolapse resolves on its own.
- Uterine prolapse: treat as an emergency immediately after birth. The exposed uterus can become congested, torn, contaminated, hemorrhagic, or necrotic, and the sow can deteriorate rapidly.
- Rectal prolapse: protect the tissue from further trauma and seek veterinary treatment, especially when it is dark, dry, ulcerated, bleeding, hard, or cannot be replaced. Find the cause of the straining.
- Vaginal or cervical prolapse: intermittent tissue may become persistent and swollen. It can interfere with farrowing and should be assessed before the problem becomes traumatic or non-reducible.
Veterinary treatment depends on the organ involved and the viability of the tissue. References describe cleaning or lavage, lubrication, reduction when tissue is viable, temporary retention sutures for selected vaginal or rectal cases, anesthesia or epidural techniques to reduce straining, and resection or amputation when tissue is necrotic or cannot be replaced.
The correct procedure varies with tissue condition, prolapse type, local law, drug rules, food-animal withdrawal requirements, and the veterinarian’s protocol. Do not cut tissue, force a reduction, apply harsh chemicals, or use consumer products as a substitute for veterinary care. If the veterinarian directs you to protect exposed tissue while help is coming, keep it clean, moist, and shielded from additional trauma without delaying examination.
Practical herd-investigation checklist
When cases cluster, investigate the event as a pattern rather than searching for one culprit. A veterinary feed testing laboratory or diagnostic service may be appropriate for commercial herds with multiple cases or a credible feed-exposure history, but representative sampling and veterinary interpretation are essential.
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- Record timing: note date relative to service, gestational age, farrowing, and weaning. Record whether the prolapse was intermittent or persistent.
- Record sow factors: document parity, genetic line or supplier, body condition, litter size, known piglet size, farrowing duration, dystocia, stillborn piglets, and retained membranes.
- Review feed: preserve batch information and examine ingredient changes, grind size, starch and lysine density, fiber profile, premix inclusion, storage conditions, mold risk, and possible mycotoxin exposure.
- Check water and feces: test drinker function and flow, access, temperature, fecal consistency, constipation, diarrhea, and recent changes in feed or water intake.
- Look for straining diseases: investigate enteritis, colitis, parasites, urinary obstruction, cystitis, respiratory disease, anorectal disease, and other causes of repeated abdominal pressure.
- Inspect the environment: assess flooring traction, slope, crate or pen design, stocking density, posture, movement, ability to rise and lie normally, and heat conditions.
- Review farrowing records: check oxytocin timing and dose, manual assistance, dystocia, retained membranes, treatments, and whether intervention preceded the event.
- Compare meaningful groups: calculate incidence by farm, barn, season, parity, genetic group, feed batch, and event type. Use the number of relevant sows or farrowings as the denominator instead of comparing raw case counts.
- Preserve suitable samples: submit feed, clinical, fecal, or other diagnostic samples through the herd veterinarian. An isolated rapid test or one abnormal laboratory result rarely identifies the cause by itself.
Prevention: prioritize controllable risks without overclaiming
Because the syndrome is multifactorial, prevention is usually a layered program rather than a single additive or management change.
- Maintain accurate event records and use them in genetic and culling decisions where appropriate.
- Keep feed stored properly, investigate ingredient changes, and test when the clinical pattern and exposure history justify it.
- Ensure reliable water access and review constipation, diarrhea, and fiber with a swine nutritionist.
- Correct obvious flooring hazards such as poor traction, damaging slope, or design features that prevent normal movement.
- Use veterinary protocols for oxytocin, manual assistance, dystocia, and treatment of gastrointestinal, urinary, respiratory, and reproductive disease.
- Review heat, season, body condition, parity structure, litter characteristics, and farm-specific patterns together rather than treating any one factor as decisive.
Do not assume that a vitamin injection, mineral adjustment, added fiber, housing change, or feed test will prevent every case. A useful intervention is one that addresses a documented herd risk and is followed by a properly measured change in incidence.
Evidence-strength guide
Use precise language when discussing the 13 causes:
| Evidence wording | Appropriate examples |
|---|---|
| Established association | Enteritis, diarrhea, parasites, urinary disease, dystocia, and anorectal disease causing persistent straining, particularly in rectal prolapse. |
| Supported risk factor | Partly heritable susceptibility and some parity, genetic-group, regional, seasonal, or population-level effects. |
| Plausible contributor or hypothesis | Mycotoxins in a particular herd, hypocalcemia, vitamin status, diet density, housing details, oxytocin, adult-sow tail length, and litter size. |
The central conclusion is not that all 13 causes are equally likely. It is that no single item has been proven to cause sow prolapse in all herds. The most productive investigation combines accurate classification, immediate veterinary care, good records, targeted testing, and comparison of the herd’s own patterns.
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Frequently Asked Questions
Does a positive zearalenone feed test prove that zearalenone caused the prolapse?
No. A positive result must be interpreted in the context of the concentration, feed matrix, sample representativeness, exposure duration, clinical signs, other toxins, and the herd pattern. It supports further investigation but does not prove causation in an individual sow.
Is sow prolapse inherited?
Susceptibility appears to be partly heritable. Genomic studies have estimated moderate heritability in studied populations and identified associated genomic regions, but prolapse is not established as a simple single-gene disorder. Environment, parity, season, diet, disease, and farrowing factors also matter.
Can I wait to call the veterinarian if the prolapse retracts?
No. Intermittent vaginal or cervical tissue can become swollen or traumatized, and a uterine prolapse is an emergency even when it initially appears viable. Prompt veterinary assessment is the safest course for any visible prolapse.
What should a farm compare when cases increase?
Compare event type and incidence by farm, barn, season, parity, genetic group, feed batch, body condition, litter characteristics, farrowing duration, disease signs, water access, fecal consistency, flooring, and farrowing interventions. Use the number of relevant sows or farrowings as the denominator.
The Bottom Line
Bottom line: Sow prolapse syndrome is a multifactorial group of vaginal, uterine, and rectal events—not a single proven disease. Treat every visible prolapse promptly, classify the organ involved, and investigate straining, constipation, water, feed quality, nutrition, flooring, farrowing management, parity, genetics, season, and disease together. Genetic susceptibility is increasingly supported, but the cause of a herd cluster usually cannot be established from one observation or one test.
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