Mycoplasma gallisepticum in Chickens: Respiratory Mycoplasmosis

Etiology and Taxonomy

Mycoplasma gallisepticum (MG) is a cell wall deficient bacterium belonging to the class Mollicutes, family Mycoplasmataceae [1]. The absence of a peptidoglycan layer renders MG intrinsically resistant to beta lactam antimicrobials and confers pleomorphic morphology [2]. The MG genome is among the smallest known for self replicating bacteria, encoding a minimal set of metabolic pathways and a suite of virulence factors that facilitate host colonization and immune evasion [1]. Key adhesins include GapA, CrmA, PlpA, and Hlp3, which mediate cytoadherence to chicken respiratory epithelial cells [1, 3]. Variable surface lipoproteins, particularly the VlhA family, undergo phase and antigenic variation to evade host antibody responses [1]. A recently characterized TatD nuclease contributes to immune modulation and nutrient acquisition [1]. The organism can be propagated in specialized mycoplasma broth and agar media, but growth is slow (3 to 21 days) compared to conventional bacteria, requiring enriched media such as Frey's medium [2].

Epidemiology and Transmission

The question of how is mycoplasma spread in chickens is central to understanding MG epidemiology. MG is transmitted both horizontally and vertically [4, 29]. Horizontal transmission occurs via direct contact, aerosolized respiratory droplets, and contaminated fomites [1, 29]. Birds housed in close confinement, as in commercial layer and broiler operations, experience rapid flock-level dissemination [5]. Vertical (egg borne) transmission is a major route of perpetuation; infected breeder hens can transmit MG to progeny through the egg, establishing infection in subsequent generations [4, 27, 29]. The rate of egg transmission varies with strain virulence and stage of infection [4]. Once introduced, MG can persist in a flock indefinitely, with carrier birds serving as reservoirs [6, 33].

Seroprevalence surveys demonstrate widespread MG exposure globally. In the Ga East district of Ghana, 29.5% of chickens tested seropositive [5]. In Duhok Governorate, Iraq, overall seroprevalence in broilers reached 52.48%, with higher titers in younger birds [28]. In Bangladesh, MG was detected in commercial flocks with variable rates [7]. Broilers (short lifespan) may show lower prevalence than layers due to reduced time for exposure [5, 28]. Molecular detection by PCR from tracheal swabs in Iraqi broilers confirmed MG in 85.9% of sampled birds, indicating high infection pressure [32].

Host factors influencing transmission include age, immune status, and concurrent infections [8, 34]. Coinfection with infectious bronchitis virus enhances MG pathogenicity and shedding [9, 5]. Secondary bacterial invaders, particularly Escherichia coli, exacerbate respiratory disease [34].

Pathogenesis and Immune Dysregulation

MG initiates infection by adhering to ciliated respiratory epithelium via GapA and CrmA [1, 3]. Adhesion is followed by ciliostasis, epithelial cell death, and desquamation, disrupting mucociliary clearance [33]. The organism then penetrates the mucosa and invades deeper tissues, including air sacs and lungs [10, 33]. Chronic infection is marked by persistent colonization despite a robust host immune response, a phenomenon driven by immune dysregulation [1, 33].

Studies show that unvaccinated, MG infected chickens exhibit elevated transcription of interferon gamma (IFN gamma), interleukin 17 (IL 17), RANTES (CCLi4), and CXCL 14 in the tracheal mucosa, along with downregulation of IL 2 [33]. This pattern suggests a shift toward a Th1/Th17 dominated response and suppression of Th2 and regulatory pathways [33]. Infiltration of B cells, CD3+ and CD4+ T cells, and macrophages occurs, yet CD8+ cytotoxic T cells are conspicuously absent, indicating impaired cytotoxic clearance [33]. In vaccinated birds (e.g., with attenuated ts 304 strain), IFN gamma transcription is upregulated while IL 6, IL 2, RANTES, and CXCL 14 are lower, correlating with reduced lesion severity [33].

Matrix metalloproteinase 7 (MMP7) has been identified as a molecular determinant of resistance. Overexpression of MMP7 in avian type II alveolar epithelial cells and macrophages inhibits MG adhesion and modulates inflammatory responses, while MMP7 inhibition enhances infection [11]. Locally adapted chicken breeds, such as Tianlu Partridge, show higher MMP7 expression and relative resistance to MG compared to susceptible commercial lines like Jingfen Layers [11].

Long non coding RNA Lnc90386 acts as a competing endogenous RNA, sponging miR 33 5p to regulate JNK signaling. MG infection upregulates miR 33 5p, which normally represses apoptosis and inflammation by targeting JNK1; Lnc90386 counteracts this repression, promoting inflammatory and apoptotic responses [26]. Glycyrrhizic acid, a plant derived compound, suppresses MG induced inflammation and apoptosis by inhibiting the p38 and JUN arms of the MAPK pathway and downregulating MMP2/MMP9, while also reducing virulence gene expression (pMGA1.2, GapA) [12].

Clinical Signs

Chronic respiratory disease (CRD) is the hallmark of MG infection in chickens [1, 2]. Clinical signs develop after an incubation period of 6 to 21 days and include:

• Nasal discharge (serous to mucopurulent)
• Sneezing and coughing
• Tracheal rales
• Dyspnea
• Conjunctivitis
• Reduced feed intake and weight gain
• Decreased egg production in layers
• Increased mortality (typically 5-20%, higher in complicated cases)

[1, 13, 2, 32].

The disease is often more severe in colder weather, under poor ventilation, or when concurrent viral or bacterial infections are present [8, 34]. In some outbreaks, salpingitis (oviduct inflammation) has been attributed to MG, leading to egg peritonitis and reproductive failure [14]. Arthritis and synovitis are occasionally observed, though less common than with Mycoplasma synoviae [1].

Pathology

Gross pathological lesions are primarily confined to the respiratory tract. In acute cases, the tracheal mucosa is hyperemic, thickened, and may contain mucoid exudate [13, 33]. Air sacs show cloudiness, thickening, and fibrinous exudate (airsacculitis) [10, 13, 15]. Pneumonia and lung consolidation can occur, particularly in severe infections [10, 12, 22]. In chronic cases, caseous plugs may form in the trachea and bronchi [13].

Histopathologically, the tracheal mucosa exhibits epithelial hyperplasia, loss of cilia, and mononuclear cell infiltration (lymphocytes, plasma cells, macrophages) [33]. The lamina propria is thickened and edematous [33]. Air sac lesions consist of fibrin deposition, heterophilic infiltration, and granulomatous inflammation progressing to fibrosis [10]. In the lungs, interalveolar septa are thickened with inflammatory cells, and bronchial associated lymphoid tissue is hyperplastic [12, 22].

How Is Mycoplasma Spread in Chickens: Transmission Dynamics

Beyond the routes described in epidemiology, several factors influence transmission efficiency. The organism can survive for several hours to days on fomites such as feeders, drinkers, and egg flats [29]. Windborne dust and feathers from infected flocks can carry MG over short distances [1, 29]. Egg transmission is particularly insidious because it introduces MG into hatcheries, where day old chicks become infected before placement [4, 27]. Breeder vaccination with bacterins or live vaccines can reduce, but not eliminate, vertical transmission [27, 15]. Medicated feed containing tylosin or other anti mycoplasmal drugs can suppress shedding but may not eradicate infection from a flock [24, 35]. The use of antimicrobials after live vaccination can reduce the duration of protective immunity, potentially allowing re emergence of infection [35].

Diagnostics

Accurate diagnosis of MG infection requires a combination of culture, serology, and molecular methods.

Culture: Isolation of MG from tracheal swabs, choanal clefts, or air sac lesions remains the gold standard despite taking 7-21 days [2]. Colonies on Frey's agar display a characteristic "fried egg" appearance. Mycoplasma broth is used for enrichment. Sensitivity and specificity of culture are high, but the procedure is labor intensive and slow [2].

Serology: Commercial enzyme linked immunosorbent assays (ELISAs) are widely used for flock level screening [5, 28]. The rapid serum agglutination (RSA) test is a simple, inexpensive field test, though it can yield false positives with cross reacting antibodies [25]. Hemagglutination inhibition (HI) is more specific and is often used as a confirmatory test [25]. Antibody detection is valuable for monitoring vaccine responses and identifying recent infections, but serology cannot differentiate vaccinated from naturally infected birds unless DIVA (differentiating infected from vaccinated animals) strategies are employed.

Polymerase Chain Reaction (PCR): Direct PCR targeting the 16S rRNA gene or mgc2 gene is rapid, highly sensitive, and specific [2, 16, 32]. In a comparative study, direct PCR detected MG in 32.77% of tracheal swabs versus 30.5% by culture, with 100% sensitivity and 96.8% specificity [2]. Real time quantitative PCR allows quantification of bacterial load [10, 17, 22]. Multiplex PCR can simultaneously differentiate MG from M. synoviae [16].

Immunohistochemistry (IHC): IHC using monoclonal antibodies can localize MG antigen in tissue sections, aiding retrospective diagnosis [16].

The following table summarizes diagnostic methods:

[2, 16, 25, 32]

Treatment

Antimicrobial therapy for MG is complicated by the organism's lack of a cell wall (intrinsic resistance to beta lactams) and increasing acquired resistance to commonly used drugs [1, 18]. The following classes are employed:

Macrolides: Tylosin, tilmicosin, and other macrolides are widely used. Tilmicosin exhibits concentration dependent killing; a dose >7.5 mg/kg achieves mycoplasmacidal activity in lung tissue, with an AUC/MIC target of 6950 h for a 3 log10 reduction [17]. Tylosin can reduce vertical transmission but its use post vaccination may shorten protective immunity [23, 35].

Fluoroquinolones: Enrofloxacin and danofloxacin are effective, but resistance has emerged. Danofloxacin clinical breakpoint in infected lung tissue is 1 μg/mL; rational dosing requires consideration of MIC and effects on lung microbiota [18]. Enrofloxacin treatment can reduce bacterial load but may not achieve complete clearance without resistance development [19].

Tetracyclines: Doxycycline and oxytetracycline are commonly administered via drinking water. Population pharmacokinetic modeling for doxycycline indicates that the registered dose (20 mg/kg/24h) achieves the pharmacodynamic target (%fT > MIC ≥ 80%) for isolates with MIC ≤0.5 mg/L [31]. Co administration with N acetylcysteine does not significantly alter doxycycline pharmacokinetics [31].

Pleuromutilins: Novel derivatives such as p furoylamphenmulin show high in vitro activity (MIC 0.00195 μg/mL) against MG. PK/PD modeling suggests a dose of 62.64 mg/kg intramuscularly once daily for three days achieves a 3 log10 CFU reduction in lung [10].

Other antibiotics: Tiamulin and valnemulin are also used. Enrofloxacin persistence of MG after treatment without resistance development has been reported [19]. Antimicrobial susceptibility testing should guide therapy, as resistance to tetracyclines and macrolides is increasingly documented [1]. Alternative approaches include use of plant derived compounds. Glycyrrhizic acid, a licorice derivative, inhibits MG virulence gene expression and MAPK mediated inflammation [12]. Thyme oil combined with tilmicosin improved clinical signs and pathological lesions in experimental infections [13]. Probiotics such as Bacillus subtilis KC1 and Lactobacillus salivarius modulate gut microbiota and reduce MG induced lung injury via the gut lung axis [22, 34].

Control and Vaccination

Control of MG relies on biosecurity, management of bird density and ventilation, eradication strategies in breeder flocks, and vaccination [1].

Biosecurity: All in/all out management, cleaning and disinfection of facilities, control of fomite movement, and isolation of new or returning birds are critical to prevent introduction [1]. For established infections, eradication can be achieved by depopulation of positive flocks, followed by thorough cleaning, disinfection, and downtime [29].

Vaccination: Both live attenuated and inactivated (bacterin) vaccines are available. Live F strain vaccine is widely used in layers. It colonizes the respiratory tract, induces local and systemic immunity, and can displace wild type MG when used strategically [6, 15]. However, it retains some virulence and can cause disease in turkeys [1]. The novel live attenuated ts 304 strain provides protection for at least 57 weeks and induces long term mucosal immunity, reducing tracheal pathology and shedding [30, 33, 35]. Inactivated bacterins, often administered in two doses, reduce clinical signs and egg production losses but do not prevent colonization as effectively as live vaccines [15]. Combination protocols (live priming followed by bacterin boost) have shown statistically significant improvements in reducing air sac lesions and challenge strain colonization compared to live vaccine alone [15].

Vaccine development: Current research focuses on improving efficacy through recombinant and epitope based vaccines. A multi epitope peptide vaccine (MEPV) derived from cytoadherence proteins (GapA, CrmA, PlpA, Hlp3) was expressed in Nicotiana benthamiana and shown to be immunogenic in chickens, inducing neutralizing IgY antibodies [3]. Recombinant adenovirus vectors expressing both infectious bronchitis virus and MG antigens have been evaluated experimentally [9]. Plant based production offers advantages in safety and scalability [3].

Immunosuppression and vaccine efficacy: Immunosuppressive conditions (e.g., cyclosporin A treatment or coinfection with immunosuppressive viruses) can impair vaccine induced protection, leading to higher lesion scores and increased shedding [8, 30]. Therefore, managing concurrent immunosuppressive diseases is essential for vaccine success.

Antimicrobial growth promoters and withdrawal: The prophylactic use of antibiotics after vaccination can reduce the duration of protective immunity. Tylosin treatment initiated 6 weeks after ts 304 vaccination led to waning serum antibodies and loss of protection against air sac lesions by 22 weeks post vaccination [35]. This underscores the need to limit antimicrobial use after live vaccination.

The following flow diagram summarizes the decision process for MG control in a commercial layer flock:

flowchart TD
    A[Flock MG status?], > B{Serology + PCR}
    B, >|Negative| C[Maintain biosecurity]
    C, > D[Vaccination optional; consider risk]
    D, > E[Monitor periodically]
    B, >|Positive| F{Clinical signs?}
    F, >|No| G[Assess prevalence]
    G, > H[Low prevalence: cull positives?]
    G, > I[High prevalence: vaccinate all]
    F, >|Yes| J[Confirm MG by PCR/culture]
    J, > K[Antimicrobial therapy based on MIC]
    K, > L[Follow with vaccination if chronic]
    L, > M[Reduce stress, improve ventilation]
    H, > N[Re-test after 4 weeks]
    I, > O[Choose live or killed vaccine]
    O, > P[Monitor efficacy via PCR and lesions]
    P, > Q[If breakthrough: adjust vaccine protocol]

Public Health Significance

MG is not considered a zoonotic pathogen. There is no evidence of human infection or foodborne transmission. However, economic impacts on poultry production have indirect public health implications through food security and economic sustainability.

Cross References

See also related articles on this portal:

• How Mycoplasma gallisepticum Spreads in Chicken Flocks: Transmission Dynamics and Control
• Mycoplasma gallisepticum Infection in Chickens: Diagnosis and Management
• Avian Mycoplasmosis: Mycoplasma gallisepticum and Other Species, Vaccination and Control in Poultry
• Mycoplasma gallisepticum Vaccine in Poultry: Protocols and Efficacy
• Ornithobacterium rhinotracheale (ORT): A Comprehensive Guide to Respiratory Disease in Poultry

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