Mycoplasma hyopneumoniae and Enzootic Pneumonia in Pigs: Chronic Cough and Diagnosis

Introduction

Enzootic pneumonia (EP) is a widely prevalent, chronic respiratory disease of swine caused by Mycoplasma hyopneumoniae. The condition imposes substantial economic losses on pig production worldwide due to reduced growth rates, feed conversion inefficiency, increased medication costs, and heightened susceptibility to secondary bacterial infections [1, 2]. The hallmark clinical sign is a persistent, dry, non-productive cough that can affect a large proportion of the herd. This article provides a detailed, evidence-based overview of the etiology, epidemiology, clinical presentation, pathology, diagnostic approaches, treatment, and control of M. hyopneumoniae infection, with particular emphasis on the chronic cough and modern diagnostic strategies.

Etiology and Pathogen Characteristics

Mycoplasma hyopneumoniae is a small, cell wall-deficient bacterium belonging to the class Mollicutes. It is the primary etiological agent of enzootic pneumonia [3, 4, 5]. The organism adheres specifically to the ciliated epithelial cells of the porcine respiratory tract, colonizing the trachea, bronchi, bronchioles, and alveoli [6, 7]. Adhesion is mediated by surface proteins such as P97 and P46, the latter being a conserved antigen used in diagnostic applications [8].

Once attached, M. hyopneumoniae disrupts mucociliary clearance through cilostasis, ciliary loss, and epithelial cell exfoliation. The resulting impairment of the mucociliary escalator predisposes the host to secondary bacterial invaders, most commonly Pasteurella multocida, Actinobacillus pleuropneumoniae, and Bordetella bronchiseptica [6, 9]. The inflammatory response involves infiltration of lymphocytes, macrophages, and plasma cells around bronchi and blood vessels, leading to peribronchiolar lymphoid hyperplasia and progressive consolidation of lung tissue [10, 11].

Strain variation influences virulence and pathogenicity. Highly virulent strains induce more severe lung lesions and pro-inflammatory cytokine responses compared to low-virulence isolates [11, 12]. Experimental infection models using Bama miniature pigs have confirmed that typical clinical and pathological features of EP can be reproduced in alternative swine breeds, facilitating vaccine and treatment studies [13].

Epidemiology and Transmission

M. hyopneumoniae is endemic in most swine-producing regions. Transmission occurs horizontally via direct contact with infected pigs or aerosolized respiratory secretions, and vertically from sows to piglets [6, 14]. The organism can persist in infected herds for long periods due to chronic carrier states and incomplete immunity [15, 16].

Risk factors for EP include high stocking density, continuous flow production systems, poor ventilation, and commingling of pigs from different age groups [14, 17]. Herds with all-in/all-out management and optimized climatic conditions demonstrate lower disease prevalence [2]. Seroprevalence surveys provide insight into regional control progress. In Switzerland, a national control program based on depopulation strategies resulted in a very low sow seroprevalence of 0.98% at the farm level and 3.92% at the herd level, indicating effective progressive control [18]. Conversely, in high-density endemic areas, seroprevalence in fattening herds can exceed 80% [14].

Co-infections are common. M. hyopneumoniae infection frequently exacerbates viral respiratory diseases such as swine influenza and porcine reproductive and respiratory syndrome (PRRS), contributing to the porcine respiratory disease complex (PRDC) [6]. The lung microbiota undergoes significant dysbiosis during infection, with enrichment of Mycoplasma and a reduction in alpha diversity, as shown by 16S rRNA metataxonomic analyses [12].

Clinical Signs: The Chronic Cough

The most consistent clinical sign of EP is a chronic, intermittent, dry cough that is non-productive. Coughing is often elicited when pigs are forced to move, exposed to cold or damp conditions, or during feeding [19, 20]. The coughing index, defined as the number of cough events observed over a fixed time period (e.g., 5 minutes with a group of pigs), is a standardized tool for quantifying disease severity. Nathues et al. demonstrated that a high coughing index correlates strongly with a high seroprevalence of M. hyopneumoniae in fattening herds [19, 14].

Coughing typically appears 2 to 4 weeks after infection and may persist for several weeks or months. In subacute forms, the cough is less frequent but can worsen with stress. Chronic forms of EP lead to reduced average daily gain (ADG) and increased feed conversion ratio (FCR). Morris et al. showed that the presence of lung lesions at slaughter was a better predictor of weight gain reduction than cough observation alone, but cough remains a valuable herd-level indicator for early detection [20].

Other clinical signs include dyspnea, tachypnea, and mild fever in secondary bacterial infections. Mortality is generally low, but morbidity can approach 100% in naive herds [2].

Pathology and Immune Response

Macroscopically, EP produces characteristic plum-colored consolidation of the anteroventral lung lobes, affecting the apical, cardiac, and intermediate lobes. Lesions are sharply demarcated from normal tissue and represent areas of catarrhal bronchopneumonia. At slaughter, the prevalence of EP-like lesions has been reported at 30-40% in heavy pigs [21, 22]. Chronic pleuritis and lung adhesions are often concurrent, reflecting secondary bacterial involvement.

Histopathology reveals hyperplasia of peribronchiolar lymphoid tissue (cuffing), loss of cilia, and infiltration of mononuclear cells into the alveolar septa. Immunohistochemical labeling of cytokines such as IL-1, IL-6, and TNF-alpha is increased in lung lesions from naturally infected pigs [10].

The immune response to M. hyopneumoniae is predominantly cell-mediated, with T-cell lymphopenia in acute disease and compensatory B-cell activation in the chronic phase. Studies using entrikim (a combination of enrofloxacin, trimethoprim, and tilmicosin) showed that subacute EP reduces T-lymphocyte counts to 28.9±1.4%, with recovery to normal levels after treatment. Chronic disease was associated with a shift toward a T-helper (Th) bias (Tx/Tc ratio 2.28) and marked suppression of cellular immunity [23]. Humoral immunity is also affected: IgA and IgM levels initially decline, while IgG increases in chronic cases [23].

Diagnosis

Accurate diagnosis of EP requires integration of clinical signs, gross pathology, and laboratory tests. Because chronic cough can have multiple etiologies (including Mycoplasma hyorhinis, swine influenza virus, and A. pleuropneumoniae), laboratory confirmation is essential [9, 19].

Clinical Examination

A standardized coughing index is a practical herd-level tool. Nathues et al. reported mean coughing indices of 4.3 in case herds (high seroprevalence) versus 0.7 in low-seroprevalence control herds [19, 14]. However, clinical examination alone cannot differentiate EP from other respiratory diseases.

Molecular Detection

Real-time PCR (qPCR) is the most sensitive and specific method for detecting M. hyopneumoniae DNA in nasal swabs, bronchoalveolar lavage fluid (BALF), or lung tissue. PCR targets genes such as p36 or p46 and can quantify bacterial load [18, 24]. PCR testing is particularly useful for confirming active infection in coughing pigs and for surveillance in control programs. In Switzerland, PCR testing of nasal swabs from coughing animals is the primary surveillance tool [18].

Serology

Commercial enzyme-linked immunosorbent assays (ELISAs) detecting anti-M. hyopneumoniae IgG antibodies are widely used for herd-level profiling. Seroprevalence data help categorize herds as infected or non-infected and evaluate vaccination efficacy [18, 14]. However, serology cannot differentiate vaccinated from naturally infected animals, and antibody titers wane over time. The P46 recombinant protein has been cloned and expressed for use as a specific antigen in diagnostic assays [8].

Postmortem Evaluation

Slaughterhouse monitoring of lung lesions is a cost-effective method for benchmarking EP prevalence. The scoring system (e.g., 0-10 scale) correlates with production losses. Sarturi et al. found that lung lesion frequency increased over successive monitoring periods despite declining clinical signs, highlighting the chronic nature of the disease [25]. Lung tissue can be subjected to histopathology, immunohistochemistry (IHC), and PCR for definitive diagnosis.

Diagnostic Methods Comparison

Diagnostic Decision Workflow

The following Mermaid diagram illustrates a structured approach to diagnosing EP in fattening pigs.

flowchart TD
    A[Chronic cough in grower-finisher pigs], > B[Coughing index assessment]
    B, > C{Index > threshold?}
    C, Yes, > D[Collect nasal swabs from 5-10 coughing pigs]
    C, No, > E[Monitor other signs and serology]
    D, > F[Perform M. hyopneumoniae qPCR]
    F, > G{Positive?}
    G, Yes, > H[Clinical case of EP confirmed]
    G, No, > I[Consider other pathogens: PRRSV, SIV, Pasteurella]
    H, > J[Submit lung tissue at slaughter for lesion scoring]
    J, > K[Correlate PCR and lesion data for herd-level diagnosis]
    E, > L[Serological profiling: sample 20-30 pigs per group]
    L, > M{Seroprevalence high?}
    M, Yes, > D
    M, No, > N[Low risk of enzootic pneumonia]

Treatment

Antimicrobial therapy is used to reduce clinical severity and bacterial shedding, but it does not eliminate the pathogen from the respiratory tract. M. hyopneumoniae lacks a cell wall and is therefore intrinsically resistant to beta-lactam antibiotics, sulfonamides, and trimethoprim [26]. Active antimicrobials include tetracyclines, macrolides, lincosamides, pleuromutilins, amphenicols, aminoglycosides, and fluoroquinolones [26, 2].

Metaphylactic strategies using valnemulin, tilmicosin, or tulathromycin have been evaluated. Stingelin et al. reported that valnemulin treatment improved growth performance, reduced lung consolidation, and decreased bacterial load at slaughter compared to controls [24]. Similarly, tylvalosin administered in drinking water (2.5 mg/kg BW for 7 days at weaning and at transfer to finisher barn) significantly reduced EP-like lesions and improved ADG [27]. Combined formulations such as entrikim (enrofloxacin, trimethoprim, tilmicosin) showed 90.7% therapeutic efficacy in subacute cases and 74.4% in chronic cases, with beneficial effects on T-cell and humoral immunity [23].

Parenteral administration of tulathromycin has demonstrated efficacy against experimental M. hyopneumoniae pneumonia, reducing lung lesions and clinical signs [28]. However, acquired antimicrobial resistance has been reported, although no clinical breakpoints are officially defined [26].

Control and Prevention

A comprehensive control program integrates vaccination, management, and biosecurity.

Vaccination

Commercial bacterins reduce clinical signs, lung lesion severity, and medication use, but they do not prevent infection or colonization [29, 2]. Vaccination strategies vary: piglets are often vaccinated at weaning, and sows may be vaccinated to boost passive immunity. The main effects are improved ADG and FCR, though strain-specific efficacy can vary. Newer vaccine approaches, including subunit, DNA, and aerosol-delivered vaccines, are under investigation [2, 30].

Management Practices

Optimizing housing conditions and production flow is essential. All-in/all-out systems reduce pathogen build-up. Acclimatization of gilts via controlled exposure to M. hyopneumoniae (using live animals or vaccines) can stabilize herd immunity [14, 1]. Reducing stocking density, improving ventilation, and minimizing temperature fluctuations lower disease pressure.

Eradication

Herd-level eradication is possible through age-segregation (depopulation of fattening units) combined with medication and strict biosecurity. Total depopulation of affected fattening farms, coupled with partial depopulation in breeding farms, has been successfully implemented in Switzerland [18]. Re-infection risk remains, so continued surveillance via PCR and serology is necessary [15, 31].

Conclusion

Mycoplasma hyopneumoniae remains a formidable challenge in swine production, causing chronic cough and substantial economic losses. Diagnosis relies on a combination of clinical observation, PCR, serology, and slaughterhouse lung lesion monitoring. Treatment with effective antimicrobials can mitigate disease impact, but control ultimately depends on integrated management, vaccination, and, where feasible, eradication programs. The chronic, persistent nature of the cough and the bacterium's ability to modulate host immunity underscore the need for continuous vigilance and adaptive control strategies.

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