Lawsonia intracellularis (Proliferative Enteropathy): Etiology, Pathogenesis, Diagnostic Approaches, and Control Strategies

Introduction

Lawsonia intracellularis is an obligate intracellular, Gram-negative bacterium that causes proliferative enteropathy (PE) in swine and several other mammalian species [1]. The disease is characterized by marked thickening of the intestinal mucosa, primarily in the ileum, due to uncontrolled proliferation of immature enterocytes infected by the bacterium [2]. PE represents a major economic burden in global swine production, manifesting as either an acute hemorrhagic form or a chronic, subclinical form that impairs growth performance and feed efficiency [3, 4]. Although most research has focused on swine, L. intracellularis also infects horses, hamsters, and other rodents, with implications for interspecies transmission [5, 6, 7]. This article provides a comprehensive, citation-grounded review of the microbiology, pathogenesis, clinical presentation, diagnostic techniques, and control measures for L. intracellularis, drawing exclusively on the published peer-reviewed literature provided.

Etiology and Microbiology

L. intracellularis is a curved, rod-shaped bacterium measuring approximately 1.25 to 1.75 μm in length and 0.25 to 0.43 μm in diameter [1]. The organism is an obligate intracellular pathogen that cannot be cultured on conventional axenic media; it requires living cell cultures, typically using enterocyte-like cell lines such as IEC-18 or IPEC-J2 [8]. The bacterium possesses a type III secretion system (T3SS) that is critical for invasion of host enterocytes and modulation of host cell signaling pathways [9]. One characterized T3SS effector, LI0758, is an ortholog of the mammalian Rce1 protease and has been shown to activate mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-κB) signaling in mammalian cells [9]. The lipooligosaccharide (LOS) of L. intracellularis has low endotoxic activity compared to that of enteric bacteria such as Salmonella, which may contribute to the bacterium's ability to evade host immune detection and establish persistent infection [10].

The genomic diversity of L. intracellularis has been explored using pangenomic approaches. Yin et al. (2026) identified a species-specific novel antigen, LAW_RS03650, via pangenomic reverse vaccinology, demonstrating the presence of conserved and variable genomic regions across strains [11]. Whole-genome sequencing of field isolates from different geographic regions has revealed limited genetic variability, but strain-dependent differences in pathogenicity have been observed [8, 12]. For example, a newly isolated strain from Hubei, China, exhibited high pathogenicity in experimentally infected pigs, causing severe hemorrhagic diarrhea and high mortality [8].

Pathogenesis

The pathogenesis of PE begins with oral ingestion of L. intracellularis, followed by bacterial attachment to and invasion of the apical surface of intestinal crypt epithelial cells in the ileum, cecum, and colon [1, 13]. Once internalized, the bacterium resides within a membrane-bound vacuole in the host cell cytoplasm and replicates intracellularly, escaping the host autophagic and lysosomal pathways [1]. The hallmark pathologic lesion is hyperplasia of immature crypt enterocytes, leading to marked thickening of the intestinal mucosa [2]. This hyperplasia results from altered host cell proliferation and differentiation pathways. Although early studies suggested involvement of the canonical Wnt signaling cascade, Klaeui et al. (2026) demonstrated that canonical Wnt signaling is not activated either in vitro or in vivo by L. intracellularis infection [14]. Instead, the infection may activate alternate signaling modules such as MAPK and NF-κB via T3SS effectors, as noted above [9].

The hyperplastic response is accompanied by a reduction in the number of goblet cells and loss of the protective mucus layer [1, 4]. The resulting compromised intestinal barrier facilitates secondary infections and translocation of luminal contents [4]. In subclinical cases, the infection induces a shift in the gut microbial community, characterized by reduced microbial diversity and altered metabolic function, which correlates with decreased growth performance and feed efficiency [4, 15]. Co-infection with other enteric pathogens, such as Brachyspira hyodysenteriae, porcine circovirus type 2 (PCV2), and porcine reproductive and respiratory syndrome virus (PRRSV), can exacerbate clinical outcomes [16, 17, 18]. For instance, pigs co-infected with L. intracellularis and B. hyodysenteriae show more severe colitis and delayed immune responses compared to mono-infected animals [16].

The virulence of L. intracellularis has been linked to specific T3SS effectors and the expression of outer membrane proteins (OMPs) [19, 9]. Computational immunoinformatic approaches have identified several OMPs as promising vaccine targets, suggesting that surface-exposed antigens play a role in host cell invasion and immune evasion [19, 12].

Clinical Signs and Pathology

Proliferative enteropathy occurs in two primary clinical forms in swine: an acute hemorrhagic form (porcine hemorrhagic enteropathy, PHE) and a chronic form (porcine intestinal adenomatosis, PIA) [1, 13]. The acute form is most commonly observed in grower-finisher pigs (4 to 12 months of age) and presents with sudden onset of bloody diarrhea, pallor, weakness, and high mortality [13]. The chronic form typically affects weaner pigs (6 to 20 weeks) and is characterized by non-hemorrhagic diarrhea, reduced weight gain, and uneven growth within herds [3, 4, 15]. Subclinical infections are particularly common in modern production systems and are associated with significant economic losses due to reduced feed efficiency and increased days to market [20, 4].

Gross pathological findings include segmental thickening of the intestinal wall, predominantly in the ileum, with a corrugated or "cobblestone" mucosal surface [2]. Mesenteric lymph nodes are often enlarged and edematous [1, 2]. Microscopic examination reveals marked elongation of the intestinal crypts, loss of goblet cells, and the presence of intracytoplasmic, curved, silver-staining bacteria within hyperplastic enterocytes [2]. Granulomatous inflammation may be present in some chronic cases [2]. In horses, L. intracellularis infection causes proliferative enteropathy similar to that in pigs, presenting with hypoproteinemia, edema, weight loss, and diarrhea [7]. Blood amino acid profiling in infected horses has revealed changes indicative of malabsorption and protein-losing enteropathy [7].

Epidemiology and Transmission

L. intracellularis is transmitted primarily via the fecal-oral route, and contaminated environments, feed, and water serve as major sources of infection [21, 13]. The bacterium can survive in feces and manure for extended periods under cool, moist conditions [13]. Within a herd, transmission is amplified by high stocking densities, continuous flow production systems, and insufficient biosecurity measures [15]. Leite et al. (2026) evaluated transmission dynamics using a stochastic model and demonstrated that within-pen transmission is rapid, with a basic reproduction number (R0) significantly greater than 1 under typical commercial conditions [21].

Prevalence studies using molecular detection have shown that L. intracellularis is widespread globally. Wang et al. (2026) reported a prevalence of 42.8% in pig farms in Shaanxi province, China, using quantitative PCR (qPCR) [3]. Similar high rates have been reported in European and North American herds [13, 15]. Importantly, L. intracellularis DNA has been detected in air samples from commercial swine farms, suggesting the possibility of aerosol transmission, although the viability of airborne bacteria remains to be confirmed [22]. Early infection can occur in suckling piglets, even before weaning, as demonstrated by Rodriguez-Vega et al. (2024), who detected L. intracellularis by qPCR in feces of piglets as young as 7 days of age [23]. This early exposure may be due to contaminated sow feces or environmental sources [23].

Risk factors for clinical PE include commingling of pigs of different ages, poor hygiene, nutritional stress, and concurrent infections [15, 18]. Studies in Shaanxi province identified co-occurrence of L. intracellularis with specific gut microbiome characteristics, including reduced abundance of beneficial commensals such as Lactobacillus and increased abundance of potentially pathogenic bacteria [3].

Diagnostic Approaches

Accurate and timely diagnosis of PE relies on a combination of clinical observation, gross and histopathological examination, and molecular detection. Among molecular methods, real-time quantitative PCR (qPCR) is the gold standard for detection and quantification of L. intracellularis DNA in feces, intestinal tissue, and oral fluids [24, 25, 26, 27]. Several qPCR assays have been developed, including singleplex and multiplex formats. Hu et al. (2025) established a TaqMan-based qPCR with a detection limit of 10 copies per reaction, showing high sensitivity and specificity for field samples [25]. Multiplex qPCR assays allow simultaneous detection of L. intracellularis with other enteric pathogens such as Brachyspira hyodysenteriae and Clostridium perfringens, improving diagnostic efficiency [26]. Oral fluid samples have been evaluated as a non-invasive alternative to fecal sampling; Eddicks et al. (2025) confirmed that multiplex qPCR on oral fluids is suitable for herd-level monitoring of L. intracellularis and B. hyodysenteriae [27]. Similarly, Xiao et al. (2025) developed a qPCR assay for detection of L. intracellularis in Tibetan pigs, demonstrating its applicability across different pig populations [24].

Other diagnostic tools include serological assays such as indirect immunofluorescence antibody tests (IFAT) and enzyme-linked immunosorbent assays (ELISA), which detect antibodies to L. intracellularis antigens [11, 19]. Commercial ELISA kits are available for swine and have been adapted for use in horses [5]. The antigen LAW_RS03650, identified by Yin et al. (2026), offers potential as a species-specific diagnostic target [11]. In horses, flow cytometry-based immune profiling has been used to assess cellular immune responses during infection [6]. For culture, isolation requires cell culture systems; Suarez-Duarte et al. (2025) developed a novel quantification method based on optical density spectrophotometry of the bacterium in cell lysates, facilitating bacterial enumeration without complex genetic equipment [28].

A decision tree for diagnostic workflow is presented in Figure 1.

graph TD
    A[Clinical suspicion: diarrhea, reduced growth, ileitis at necropsy], > B{Choose sample type}
    B, > C[Individual fecal samples]
    B, > D[Oral fluid samples (herd level)]
    B, > E[Tissue (ileum) from necropsy]
    C, > F[DNA extraction + qPCR (singleplex or multiplex)]
    D, > F
    E, > F
    F, > G{Result}
    G, >|Positive| H[Quantify bacterial load (Ct value)]
    G, >|Negative| I[Consider alternative diagnosis]
    H, > J[Interpretation based on clinical cut-offs]
    J, > K[Subclinical: low load, no gross lesions]
    J, > L[Clinical: high load, typical lesions]
    J, > M[Consider serology (IFAT/ELISA) for herd exposure]

Figure 1. Diagnostic decision tree for Lawsonia intracellularis infection in swine. Adapted from multiple studies [24, 25, 26, 27, 23].

Vaccination and Control

Control of PE relies on biosecurity, management practices, and vaccination. Vaccination with live attenuated commercial vaccines is the most common intervention [13, 29]. Oral administration of live attenuated vaccines has been shown to reduce fecal shedding, improve weight gain, and reduce the severity of intestinal lesions [29]. A systematic review by de Oliveira Possa et al. (2026) evaluated efficacy of both commercial and test vaccines, concluding that vaccination significantly reduces clinical signs and improves immunological parameters [30]. The impact of adjuvants on immune response has been studied; intradermal administration with appropriate adjuvants can enhance both innate and adaptive immunity in pigs [31]. In horses, a recombinant vaccine combined with probiotic supplementation showed promising immunomodulatory effects [5].

Novel vaccine strategies include the use of Salmonella vectors expressing L. intracellularis antigens. Bakhsh et al. (2026) developed a low-endotoxic Salmonella vector with dual bacterial-host promoter expression of L. intracellularis antigens that elicited protective immunity in a murine model [10]. Subunit vaccines based on outer membrane proteins have been explored using computational biochemistry and immunoinformatics [19, 12]. A tri-valent ready-to-use vaccine combining PCV2, Mycoplasma hyopneumoniae, and L. intracellularis antigens has been shown to be safe and efficacious in weaned pigs [32]. The use of multivalent intradermal vaccines has also been reported [17]. Field trials comparing vaccination to antimicrobial prophylaxis found that vaccination is not only effective but also economically advantageous in reducing antibiotic use and associated costs [33].

Non-antibiotic alternatives for control include the use of feed additives such as organic acids, probiotics, and phytogenic compounds, which may reduce the incidence of subclinical PE by modulating the gut microbiome and improving intestinal barrier function [13]. However, the efficacy of these alternatives is variable and often less consistent than vaccination [13]. Antimicrobial therapy, primarily with tiamulin or valnemulin, remains an option for treatment of acute outbreaks, but increasing concerns about antimicrobial resistance and public health pressure favor vaccination-based strategies [33]. Spetic da Selva et al. (2025) evaluated a self-vaccination strategy in combination with other vaccines, demonstrating that a comprehensive vaccination program can reduce overall antimicrobial usage [34].

Frequently Asked Questions

What is the primary causative agent of proliferative enteropathy in swine?

The causative agent is Lawsonia intracellularis, an obligate intracellular Gram-negative bacterium [1].

How does L. intracellularis enter and infect host enterocytes?

The bacterium attaches to and invades intestinal crypt epithelial cells via a type III secretion system; the effector LI0758 activates MAPK and NF-κB signaling to facilitate intracellular survival and replication [1, 9].

Which animal species are susceptible to L. intracellularis infection?

Swine are the primary host; the bacterium also causes proliferative enteropathy in horses, hamsters, and other rodents [5, 6, 7].

What are the two main clinical forms of porcine proliferative enteropathy?

The acute hemorrhagic form (porcine hemorrhagic enteropathy) and the chronic form (porcine intestinal adenomatosis) [1, 13].

Which diagnostic method is considered the gold standard for detecting L. intracellularis infection?

Real-time quantitative PCR (qPCR) on fecal or tissue samples is the gold standard due to high sensitivity and specificity [24, 25, 26, 23].

Can oral fluid samples be used for diagnosis of L. intracellularis?

Yes, multiplex qPCR on oral fluid samples is a reliable non-invasive method for herd-level monitoring [27].

Is vaccination effective in controlling subclinical L. intracellularis infection?

Yes, vaccination reduces fecal shedding and improves growth performance and carcass quality in subclinically infected pigs [30, 29].

What is the role of the gut microbiome in L. intracellularis infection?

Infection induces shifts in microbial community composition and function, leading to reduced bacterial diversity and impaired feed efficiency [3, 4].

How does L. intracellularis resist intracellular clearance?

The bacterium evades autophagy and lysosomal degradation by residing in a membrane-bound vacuole and modulating host cell signaling pathways [1, 9].

Are there any non-antibiotic alternatives for control of proliferative enteropathy?

Feed additives such as organic acids, probiotics, and phytogenic compounds may offer some benefit, but vaccination remains the most effective control strategy [13].

References

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