Ileitis (Lawsonia intracellularis)

Ileitis, formally known as porcine proliferative enteropathy (PPE), is an enteric disease of swine caused by the obligate intracellular bacterium Lawsonia intracellularis. The disease affects the ileum and sometimes the jejunum and colon, producing characteristic proliferative thickening of the intestinal mucosa that impairs nutrient absorption and causes diarrhea, reduced growth rates, and in the hemorrhagic form, sudden death. Ileitis is one of the most economically significant enteric diseases in global swine production, with seroprevalence surveys indicating that L. intracellularis infection is essentially ubiquitous in commercial pig herds worldwide.

Lawsonia intracellularis is a gram-negative, curved to vibrioid, obligate intracellular bacterium that cannot be cultivated on conventional bacteriological media. The organism was first identified as the causative agent of proliferative enteropathy in the 1990s, though the disease syndrome had been recognized under various names for decades, including porcine intestinal adenomatosis, proliferative hemorrhagic enteropathy, necrotic enteritis, and regional ileitis. The requirement for intracellular growth within intestinal epithelial cells made L. intracellularis exceptionally difficult to study and delayed the development of diagnostic tools and vaccines. The successful cultivation of the organism in rat intestinal epithelial cell lines (McCoy cells and IEC-18 cells) was a major breakthrough that enabled vaccine development and improved diagnostic capabilities.

The disease manifests in several clinically distinct forms that represent a continuum of the same underlying pathological process. The chronic form, porcine intestinal adenomatosis (PIA), is the most common clinical presentation and affects primarily growing pigs between 6 and 20 weeks of age, producing diarrhea, reduced feed efficiency, and depressed growth rates. The acute hemorrhagic form, proliferative hemorrhagic enteropathy (PHE), typically affects older pigs, replacement gilts, and young breeding animals, causing sudden-onset bloody diarrhea, pale skin, weakness, and acute mortality from massive intestinal hemorrhage. Subclinical infection, in which pigs are infected and shedding the organism without showing overt clinical signs, may be the most economically significant form because it affects large numbers of pigs with growth depression that is often unrecognized without specific diagnostic investigation.

The pharmaceutical approach to ileitis encompasses preventive vaccination with modified-live or inactivated L. intracellularis vaccines and therapeutic or metaphylactic antimicrobial administration for treatment and control of clinical and subclinical disease. The interplay between vaccination, antimicrobial use, and management practices determines the success of ileitis control programs in individual herds. The obligate intracellular nature of the organism, which limits the effectiveness of many conventional antibacterial agents, and the widespread distribution of infection across virtually all commercial swine populations make ileitis one of the more complex enteric diseases to manage in modern pork production.

The pathogenesis of porcine proliferative enteropathy begins with oral ingestion of L. intracellularis organisms shed in the feces of infected pigs. The bacteria survive passage through the acidic stomach environment and colonize the ileal mucosa, where they invade immature crypt epithelial cells (enterocytes) through a mechanism involving host cell-directed endocytosis. Once inside the enterocyte, L. intracellularis escapes the endosomal compartment and replicates freely within the cytoplasm, intimately associated with the host cell's mitochondria. The intracellular location protects the organism from extracellular immune mechanisms and limits the effectiveness of antimicrobials that do not achieve adequate intracellular concentrations.

The hallmark pathological feature of L. intracellularis infection is proliferative hyperplasia of immature intestinal crypt epithelial cells. Infected crypt cells fail to undergo normal differentiation and maturation as they migrate toward the villous tip, instead continuing to divide and accumulate, producing the characteristic thickened, corrugated intestinal mucosa visible grossly and the adenomatous proliferation of immature epithelial cells observed histologically. The replacement of mature, absorptive villous epithelium with immature, non-functional proliferative cells disrupts normal nutrient absorption, producing the malabsorption, diarrhea, and growth depression associated with clinical ileitis. The mechanism by which L. intracellularis disrupts host cell differentiation pathways is an active area of research, with evidence suggesting interference with cell cycle regulation and apoptotic mechanisms.

Porcine intestinal adenomatosis (PIA), the chronic proliferative form, is the most frequently diagnosed clinical presentation. Affected pigs, typically 6-20 weeks of age, develop mild to moderate diarrhea with loose, paste-like feces that may contain mucus. Feed intake may be reduced or normal, but feed conversion efficiency is impaired, and average daily gain is depressed. In group-housed pigs, the primary observable effect is increased variation in body weight within the group, with affected pigs falling behind their uninfected pen mates. The diarrhea is generally non-hemorrhagic, and mortality from uncomplicated PIA is low, though secondary complications including necrotic enteritis (when anaerobic bacteria colonize the damaged mucosa) can increase morbidity and mortality.

Proliferative hemorrhagic enteropathy (PHE) represents the acute, potentially fatal form of the disease and is characterized by sudden onset of hemorrhagic diarrhea, pallor, weakness, and death. PHE occurs most commonly in older animals, including replacement gilts, young boars, and growing-finishing pigs approaching market weight. The pathogenesis of the hemorrhagic form involves massive hemorrhage from the proliferative intestinal mucosa, with the intestinal lumen becoming distended with blood and blood clots. The reason that some pigs develop hemorrhagic disease while most develop the chronic proliferative form is not fully understood but may relate to the host immune response, the infecting dose, concurrent stressors, or the virulence characteristics of the specific L. intracellularis strain involved.

Subclinical infection represents the most insidious form of ileitis and arguably causes the greatest aggregate economic impact across the swine industry. Subclinically infected pigs harbor L. intracellularis in their intestinal epithelium and shed the organism in their feces, contributing to environmental contamination and transmission to pen mates, but do not develop overt clinical signs. Despite the absence of visible illness, subclinical infection impairs intestinal absorptive function sufficiently to reduce feed efficiency and daily gain by measurable amounts, typically estimated at 2-6% depression in average daily gain and 3-6% impairment of feed conversion. Across large populations of pigs, these subclinical losses represent a substantial economic burden that is often unrecognized without prospective diagnostic monitoring and performance analysis.

Two fundamentally different vaccine technologies are available for the prevention of porcine proliferative enteropathy caused by L. intracellularis: a modified-live oral vaccine and an inactivated injectable bacterin. These products differ in their mechanism of immunity induction, route of administration, timing of use relative to antimicrobial treatments, and practical application logistics. Product selection depends on the herd's disease epidemiology, management system, concurrent antimicrobial programs, and veterinary recommendations.

Enterisol Ileitis (Boehringer Ingelheim) is a modified-live, avirulent L. intracellularis vaccine administered orally as a single dose to pigs 3 weeks of age or older. The vaccine strain colonizes the intestinal mucosa and replicates intracellularly to a limited extent, stimulating both mucosal and systemic immune responses that closely mimic the immunity produced by natural infection. The oral route of administration is a significant practical advantage, as the vaccine can be delivered through the drinking water system, avoiding the labor of individual pig injection. Immunity develops within approximately 3 weeks after vaccination and provides protection throughout the growing-finishing period when clinical ileitis risk is highest.

The critical management consideration for Enterisol Ileitis is the requirement to avoid concurrent antimicrobial exposure that could kill the live vaccine organisms before they colonize the gut and stimulate immunity. Antimicrobials with activity against L. intracellularis, including tylosin, chlortetracycline, tiamulin, lincomycin, and carbadox, must be withdrawn from the feed and water for a minimum period before and after vaccination, typically 3 days before through 3 days after oral vaccine administration. Failure to observe this antimicrobial-free window is one of the most common causes of vaccine failure and may result in unprotected pigs entering the disease-risk period. This interaction between live vaccine and antimicrobials requires careful coordination between vaccination scheduling and medication programs.

Porcilis Ileitis (Merck Animal Health) is an inactivated (killed) L. intracellularis bacterin administered by intramuscular injection as a single dose to pigs 3 weeks of age or older. As a killed vaccine, Porcilis Ileitis does not require an antimicrobial withdrawal window and can be used concurrently with in-feed or in-water antimicrobial programs without interference. This compatibility with antimicrobial use is a significant practical advantage in production systems where continuous or pulse antimicrobial medications are integral to the health program. The injectable route requires individual pig handling but provides certainty of dose delivery to each vaccinated animal.

Vaccination timing should be calibrated to ensure that protective immunity is established before the expected disease window on each farm. Since ileitis typically manifests clinically between 6 and 20 weeks of age, and immunity requires approximately 3 weeks to develop after vaccination, administration at 3-5 weeks of age positions the immunity onset before the primary risk period. However, farms experiencing disease later in the growing-finishing period may adjust timing accordingly. Diagnostic tools including fecal PCR, serological profiling, and production data analysis help determine the typical age of infection onset on each farm, enabling precise vaccination timing decisions.

Sow vaccination with inactivated L. intracellularis products has been evaluated as a strategy to boost colostral antibody transfer and delay the age of first infection in piglets. While maternal antibodies can provide early-life protection, their interference with active immunization and their relatively short duration limit the independent utility of sow vaccination as a sole prevention strategy. Some production systems use sow vaccination in combination with piglet vaccination to provide layered protection, though the cost-benefit analysis of this dual approach must be evaluated on a farm-by-farm basis.

Antimicrobial therapy plays a dual role in ileitis management, serving both as a treatment for clinically affected pigs and as a metaphylactic or strategic medication approach to suppress L. intracellularis infection during high-risk periods. The obligate intracellular location of L. intracellularis means that effective antimicrobials must achieve adequate concentrations within the cytoplasm of intestinal epithelial cells to reach the organism. This requirement narrows the range of effective antimicrobial agents compared to extracellular bacterial infections and influences drug selection decisions.

Tylosin (Tylan) is one of the most widely used antimicrobials for ileitis control in swine and has demonstrated consistent efficacy against L. intracellularis in both clinical trials and field use. Tylosin is a macrolide antibiotic that achieves good intracellular concentrations through ion-trapping in the acidic intracellular environment, making it well-suited for targeting intracellular pathogens. Tylosin phosphate or tylosin tartrate is administered in feed at therapeutic levels (40-100 g per ton of feed, depending on the label claim and indication) during the period of L. intracellularis challenge. Strategic or pulse-dose tylosin programs, in which the drug is provided during defined high-risk windows rather than continuously, are commonly used to balance disease control with antimicrobial stewardship objectives. In-feed tylosin use requires a Veterinary Feed Directive.

Tiamulin (Denagard) is a pleuromutilin antibiotic that similarly achieves effective intracellular concentrations and has demonstrated good efficacy against L. intracellularis. Tiamulin can be administered in feed, in water, or by injection, providing flexibility for different management situations. The water-soluble formulation is particularly useful for acute outbreak situations where rapid antimicrobial delivery to affected and at-risk pigs is needed. Tiamulin is also effective against other swine enteric pathogens including Brachyspira hyodysenteriae (swine dysentery) and Mycoplasma hyopneumoniae, providing ancillary benefits in herds dealing with multiple enteric or respiratory challenges.

Chlortetracycline (CTC), either alone or in combination with other antimicrobials, has been used for ileitis control in swine, though its efficacy against L. intracellularis is considered less consistent than that of tylosin or tiamulin. The tetracycline class achieves moderate intracellular concentrations and can suppress L. intracellularis infection when used at therapeutic feed levels during the risk period. Chlortetracycline and oxytetracycline combinations with other antimicrobials in approved feed medication programs can provide broader-spectrum enteric disease coverage in herds facing multiple pathogen challenges. In-feed use of chlortetracycline requires compliance with VFD regulations.

Lincomycin, a lincosamide antibiotic, represents an additional option for L. intracellularis control, with good intracellular penetration and demonstrated activity against the organism. Lincomycin can be administered in feed or water and is sometimes used in combination with spectinomycin (Linco-Spectin) for broader enteric pathogen coverage. The choice among available antimicrobials should be guided by the farm's specific disease situation, concurrent pathogen challenges, antimicrobial stewardship considerations, regulatory requirements, and cost-effectiveness analysis. Rotation among antimicrobial classes between production groups or seasons may help manage resistance risk, though clinically significant antimicrobial resistance in L. intracellularis has not been extensively documented due to the difficulty of conventional susceptibility testing for this obligate intracellular organism.

Accurate diagnosis of L. intracellularis infection and ileitis is essential for making informed decisions about vaccination, antimicrobial use, and management interventions. The obligate intracellular nature of the organism and its inability to grow on conventional culture media have historically made diagnosis challenging, but advances in molecular diagnostics and serological testing have greatly improved the ability to detect and monitor L. intracellularis infection in swine herds.

Fecal PCR (polymerase chain reaction) testing is the most widely used diagnostic tool for detecting active L. intracellularis infection. PCR assays amplify specific gene sequences of L. intracellularis DNA from fecal samples, providing highly sensitive and specific detection of shedding pigs. Quantitative PCR (qPCR) provides not only a positive or negative result but also estimates the quantity of organisms being shed, which correlates broadly with the severity of intestinal infection. Fecal PCR can be performed on individual pig samples or on pooled pen-level samples for herd screening. The timing of sample collection is important, as fecal shedding begins approximately 1-2 weeks after oral exposure, peaks at 2-4 weeks, and then declines as the immune response clears the infection, creating a defined shedding window that sampling must target.

Serological testing using ELISA (enzyme-linked immunosorbent assay) or immunoperoxidase monolayer assay (IPMA) detects antibodies against L. intracellularis in serum. Seroconversion typically occurs 2-3 weeks after infection and persists for several months, providing a longer detection window than fecal PCR. Cross-sectional serological profiling of pigs at multiple ages within a herd reveals the timing of natural infection onset and the dynamics of herd immunity. A serological profile showing seroconversion at 12-14 weeks of age, for example, indicates that natural infection is occurring in the mid-nursery to early-grower phase and that vaccination should be timed to provide immunity before this age. Serial serological monitoring also allows assessment of vaccine program effectiveness, as vaccinated pigs should develop antibodies before the expected natural infection window.

Postmortem examination of clinically affected or recently dead pigs provides definitive diagnosis through the combination of characteristic gross and histological lesions. Gross findings in proliferative ileitis include thickening and corrugation of the ileal mucosa, which may extend to the jejunum and proximal colon. In PHE, the intestinal lumen contains blood and fibrin clots, and the mucosa appears hemorrhagic. Histological examination reveals the pathognomonic adenomatous hyperplasia of immature crypt epithelial cells, and Warthin-Starry silver staining or immunohistochemistry can demonstrate L. intracellularis organisms within the apical cytoplasm of infected cells. These histological findings are considered the gold standard for definitive diagnosis.

Production data analysis serves as an indirect diagnostic tool for detecting subclinical ileitis. Monitoring close-out performance data, including average daily gain, feed conversion, mortality, and coefficient of variation for body weight, can reveal patterns consistent with subclinical L. intracellularis infection. Herds experiencing unexplained growth depression during the growing-finishing phase, particularly when associated with increased body weight variation and mild diarrhea without an identified cause, should be investigated for subclinical ileitis through targeted fecal PCR and serological testing. The economic analysis of production data before and after implementing vaccination or antimicrobial intervention programs quantifies the return on investment of ileitis control measures.

Both modified-live and inactivated L. intracellularis vaccines have been evaluated for safety in field and laboratory studies, with generally favorable profiles consistent with other swine vaccines. The nature and frequency of adverse reactions differ between the two vaccine types due to their fundamentally different compositions and routes of administration. Understanding the expected side effect profiles supports proper vaccine selection and management of post-vaccination events.

Enterisol Ileitis, the modified-live oral vaccine, produces minimal observable adverse effects in most vaccinated pigs. Because the vaccine is administered orally through the water system, injection site reactions are absent. Transient mild diarrhea has been reported in some pigs following vaccination, reflecting the limited intestinal colonization and immune stimulation produced by the attenuated vaccine strain. This mild enteric response is generally self-limiting and resolves within a few days without treatment. Feed intake and growth rate are not significantly affected in most vaccinated pigs. The live vaccine organisms are shed in the feces of vaccinated pigs for a limited period after vaccination, which is important for biosecurity considerations in multi-age or multi-site production systems.

Porcilis Ileitis, the inactivated injectable vaccine, produces local and systemic reactions characteristic of adjuvanted killed bacterial vaccines. Injection site reactions, including transient swelling, firmness, and soreness at the intramuscular injection site, occur in a proportion of vaccinated pigs and typically resolve within 1-3 weeks. The adjuvant component of the vaccine is responsible for most local reactions, as it creates a depot of antigen and immune stimulant that enhances the immune response but also causes local inflammation. Systemic reactions including mild fever, lethargy, and reduced appetite for 24-48 hours following vaccination may occur but are generally self-limiting.

Anaphylactic reactions to either vaccine type are rare but possible, as with any biological product. For Porcilis Ileitis, the risk of anaphylaxis is consistent with that of other injectable killed vaccines in swine. Epinephrine should be available during vaccination events for emergency treatment. For Enterisol Ileitis, systemic anaphylactic reactions from oral administration are extremely uncommon due to the mucosal route of antigen delivery, which favors tolerogenic rather than sensitizing immune responses.

Reproductive safety considerations are relevant for herds considering sow vaccination. Enterisol Ileitis is not labeled for use in breeding animals, and the modified-live nature of the vaccine raises theoretical concerns about reversion to virulence or adverse reproductive effects, though clinical evidence of reproductive problems from the vaccine is lacking. Porcilis Ileitis, as an inactivated product, does not carry the same theoretical risks and has been evaluated in breeding animals. Specific product label recommendations regarding use in pregnant or breeding animals should be consulted and followed.

Withdrawal time compliance for products used in ileitis prevention and treatment is essential for ensuring food safety when treated pigs enter the pork supply chain. The withdrawal requirements vary significantly among the different products used in ileitis management, from vaccines with minimal or zero withdrawal to antimicrobials with specific tissue depletion periods that must be strictly observed.

Enterisol Ileitis carries a zero-day slaughter withdrawal, reflecting the modified-live nature of the vaccine and the absence of pharmacological residues that would persist in tissues. The attenuated L. intracellularis vaccine strain is cleared by the pig's immune system and does not accumulate as a persistent residue. This zero-day withdrawal allows maximum flexibility in pig marketing timing relative to vaccination. Porcilis Ileitis similarly carries a short withdrawal period (typically 21 days), as established during the product licensing process. Producers should verify the specific withdrawal for each vaccine product on its label.

Antimicrobial withdrawal times represent the primary food safety compliance requirement in ileitis management programs. Tylosin phosphate in feed carries a zero-day withdrawal for swine, while tylosin tartrate formulations may have different withdrawal requirements depending on the specific product and inclusion level. Tiamulin carries withdrawal periods that vary by formulation and dose, with injectable formulations typically requiring longer withdrawal than in-feed or in-water administration. Chlortetracycline products carry established withdrawal periods that must be observed. The specific withdrawal for each antimicrobial product, formulation, and route of administration must be verified on the product label, as generalizations across products within the same antimicrobial class can lead to errors.

VFD compliance for in-feed antimicrobials used in ileitis control adds a documentation layer to withdrawal time management. The Veterinary Feed Directive must specify the drug, indication, duration of use, and withdrawal period, and the producing feed mill and the producer must maintain VFD records for the required retention period. Electronic and paper-based record systems that integrate VFD documentation with treatment and withdrawal tracking provide the most reliable compliance assurance. Pork Quality Assurance Plus (PQA Plus) protocols reinforce withdrawal compliance through producer education, site assessment, and record-keeping verification.

Residue monitoring through the USDA FSIS National Residue Program includes surveillance for antimicrobials commonly used in swine production, including macrolides and tetracyclines. Positive residue findings in market hogs can result in carcass condemnation, producer notification, intensified sampling, and regulatory follow-up. Maintaining accurate treatment records with calculated withdrawal expiration dates for each group of pigs treated protects producers against inadvertent marketing of animals with unexpired withdrawal periods and demonstrates responsible medication use practices.

The epidemiology of L. intracellularis infection in commercial swine herds is shaped by the organism's environmental persistence, the fecal-oral transmission route, the relationship between herd immunity and disease expression, and the management factors that influence exposure timing and infectious dose. Understanding these epidemiological dynamics is essential for designing effective ileitis control programs that integrate vaccination, antimicrobial use, and management practices.

Environmental survival of L. intracellularis outside the host is surprisingly robust for an obligate intracellular organism. Studies have demonstrated that L. intracellularis can survive in feces and environmental conditions for at least 2 weeks at moderate temperatures, and the organism's viability is extended under cool, moist conditions typical of many swine housing environments. This environmental persistence facilitates continuous transmission within endemically infected herds, particularly in continuous-flow production systems where infectious material from older pigs contaminates the environment before younger, susceptible pigs arrive. The organism is susceptible to common disinfectants when used at appropriate concentrations with adequate contact time, but thorough cleaning to remove organic material is a prerequisite for effective disinfection.

Transmission dynamics within herds follow predictable patterns related to pig age, immune status, and management transitions. In endemically infected herds, pigs typically encounter L. intracellularis during the late nursery or early grower phase (8-16 weeks of age) as maternal antibody protection wanes. The timing of first exposure determines the clinical presentation: early exposure when some maternal immunity remains may result in subclinical infection and development of active immunity, while later exposure of immunologically naive pigs can produce more severe clinical disease. The goal of vaccination programs is to establish active immunity before natural exposure occurs, regardless of maternal antibody status.

All-in/all-out (AIAO) production flow is one of the most effective management tools for reducing L. intracellularis transmission pressure. AIAO management at the room or barn level, combined with thorough cleaning and disinfection between groups, breaks the chain of continuous transmission that sustains high environmental contamination in continuous-flow systems. Herds converting from continuous flow to AIAO management frequently experience significant reductions in ileitis prevalence and severity, often enabling reduced antimicrobial use. The synergy between AIAO management and vaccination enhances the effectiveness of both strategies.

Biosecurity measures to prevent introduction of L. intracellularis into naive herds include controlling animal movements, implementing quarantine and acclimation procedures for incoming breeding stock, managing human and equipment traffic between facilities, and maintaining pest control programs to limit mechanical vector transmission by rodents, birds, and insects. Given the near-universal prevalence of L. intracellularis in commercial swine populations, maintaining a truly L. intracellularis-negative herd is difficult and may not be a practical goal for most production systems. Instead, the management objective is typically to control the timing and magnitude of infection to minimize clinical and economic impact.

Multi-site production systems face particular epidemiological challenges related to the mixing of pigs from multiple sow farms at nursery or wean-to-finish sites. Pigs from different sources may have different L. intracellularis exposure histories and immune statuses, creating heterogeneous populations where some pigs are immune while others are susceptible. This immunological heterogeneity can result in rolling infection waves as naive pigs are exposed by immune carrier pigs shedding the organism. Standardizing vaccination protocols across all source sow farms and ensuring consistent timing of pig movement relative to vaccination coverage helps reduce the impact of source-farm variability on ileitis control at the receiving site.

The economic impact of porcine proliferative enteropathy extends across all phases of swine production but concentrates in the growing-finishing phase where the disease most commonly manifests. Quantifying these losses is essential for justifying the investment in vaccination and antimicrobial prevention programs and for evaluating the cost-effectiveness of different control strategies. Economic analyses consistently rank ileitis among the top three most costly enteric diseases in global swine production.

Direct economic losses from clinical ileitis include mortality (particularly from PHE), treatment costs for clinically affected pigs, increased feed costs from impaired feed conversion, extended days to market weight, and reduced carcass value from lighter or less uniform market hogs. PHE-related deaths in replacement gilt populations carry particularly high economic impact because of the genetic value and development investment lost with each gilt. Subclinical losses, though more difficult to quantify precisely, are estimated to represent the majority of total ileitis-related economic impact because they affect a much larger proportion of pigs than does overt clinical disease.

Controlled field trials evaluating vaccination against L. intracellularis have consistently demonstrated measurable improvements in production performance. Vaccinated pigs show improved average daily gain, better feed conversion efficiency, reduced mortality, more uniform market weights, and fewer days to reach target market weight compared to unvaccinated controls in herds with endemic ileitis. The magnitude of improvement varies with the severity of disease challenge on each farm, with the greatest benefits observed in herds experiencing moderate to severe clinical ileitis and the most modest improvements in herds with mild subclinical infection. Return on investment calculations for ileitis vaccination typically show favorable economics, with the cost of vaccination recovered multiple times through improved production performance.

Integrated control program design combines vaccination, strategic antimicrobial use, and management practices into a comprehensive approach tailored to each herd's specific situation. The first step in program design is diagnostic characterization of the farm's ileitis status, including the age of infection onset, the clinical versus subclinical disease ratio, and the severity of production impact. This diagnostic baseline informs the selection and timing of interventions. Vaccination provides the foundation of prevention, while strategic antimicrobial medication during periods of highest risk provides additional suppression of disease expression. Management improvements including AIAO flow, enhanced sanitation, and optimized stocking density reduce environmental pathogen load and stress-related immunosuppression.

Program monitoring and continuous improvement require ongoing data collection and analysis. Serial fecal PCR testing and serological profiling confirm that vaccination is producing the intended immune response and that infection timing has shifted favorably. Close-out production data analysis tracks trends in daily gain, feed conversion, mortality, and weight variation over time, quantifying the ongoing benefit of the control program. Periodic reassessment of the program, including diagnostic evaluation of any emerging disease events and cost-benefit analysis of the intervention components, ensures that the ileitis control strategy evolves with the herd's changing disease status and production objectives.