Why is mycoplasmosis rampant in poultry farms? Focus on predisposing factors and pathogenesis mechanisms
Dr. Majid Hamid Al-Saegh/ Australia
First: Who are the two agents?
MG = Mycoplasma gallisepticum: A cell-wallless bacterium that infects the respiratory system and causes chronic respiratory disease. It is characterized by its ability to adhere to the tracheal epithelium and cause cilia disruption. BioMed Central
MS = Mycoplasma synoviae: It often causes subclinical respiratory infections, may progress to infectious synovitis, and in laying hens, it leads to eggshell apex abnormality (EAA). MSD Veterinary Manual SpringerLink.
Important note: In chicken flocks, the picture is often one of a mixed infection with respiratory viruses (such as IBV = infectious bronchitis virus, and NDV = Newcastle disease virus) and environmental factors, according to a 2025 review of respiratory diseases. This “mixture” is what makes MG/MS appear as a severe secondary infection. BioMed Central
Second: Mechanisms of Pathogenesis—What does the bacterium do inside the bird?
1) Mechanisms of Mycoplasma gallisepticum (MG)
Adhesion (Cytadherence): The surface proteins GapA and CrmA are essential for adhesion to the ciliated epithelium; their presence is associated with a higher ability to colonize the trachea and spread to the air sacs. Loss of their expression is a cause of impaired pathogenesis. BioMed Central
Disruption of the cilia and mucus barrier: After adhesion, MG causes ciliostasis and loss of epithelial integrity, opening the door to secondary infections. BioMed Central
Activation of inflammatory immunity via TLR2/NF-κB: Transcriptional studies in the trachea show an increase in inflammatory mediators (such as TNF-α, IL-1β, and IFN-γ) mediated by the TLR2 and NF-κB pathways; this explains the cough, thick mucus, and mucosal changes. BioMed CentralSpringerLink
Antigenic/Phase Variation: The pMGA family of surface proteins mutates to evade immunity, prolonging infection and making eradication difficult. ScienceDirect
Practical Summary: MG initiates adhesion (GapA/CrmA) → breaks down cilia and mucus defenses → triggers chronic mucosal inflammation via TLR2/NF-κB → prolongs its survival by altering pMGA, setting the stage for other pathogens, if present.
2) Mechanisms of Mycoplasma synoviae (MS)
Adhesion and translocation: The major adhesion factor, VlhA, binds to ligand-bound receptors on host cells; its translocation via recombination of the vlhA gene enables MS to escape immune cells. PLOSBioMed Central
Disease trends:
Subclinical respiratory disease: with a predisposition to airsacculitis when viruses such as IBV/NDV are present. MSD Veterinary Manual
Articular/tendon: Colonization of synovial membranes causes synovitis, lameness, and production losses. MSD Veterinary Manual
EAA in laying hens: Infection of the egg shell gland leads to thinning and asymmetry of the shell at the apex and increased breakage. Experimental and field evidence. SpringerLink
Additional pathogens are being characterized: Novel adhesion/immunological proteins (e.g., EF-Ts) are under study (2024), which could explain the variability in serotypes in the field. BioMed Central
Third: Predisposing Factors—Why Does the Problem “Explode” in the Field?
Environmental and Building-Level Factors (Common in Iraq)
Dust (Particulate Matter, PM): Particulate matter carries pollutants and microbes and triggers respiratory inflammation; high PM concentrations are associated with exacerbation of respiratory syndromes in poultry. ScienceDirect
Ammonia (NH₃): Impairs ciliary function and local immunity; it is recommended to maintain it <25 ppm inside the house through ventilation and litter management. CAES
Heat and Heat Stress: Decreases humoral and cellular immunity and increases intestinal permeability and inflammation, facilitating the exacerbation of MG/MS; the effect is documented in recent reviews and meta-analyses. ScienceDirectFrontiers
Local Climatic Conditions: Iraq is prone to heat waves, dust storms, and drought; these conditions make it difficult to regulate PM and NH₃ and increase bird stress. SIPRIweatheringrisk.org.
Factors at the herd and management level
Co-infections: IBV/NDV concurrently with MG/MS increases severity and lesions (a “secondary” pattern). BioMed Centralvetjournal.ankara.edu.tr
Multi-age and uncontrolled movement protocols: Multi-age layer farms facilitate the circulation of MS, and the movement of people/equipment spreads infection between farms. International standards rely on a source-free approach plus biological barriers to mitigate this. WOAH+1
Source (vertical): A recent discovery from the University of Baghdad (2025) in hatching egg yolks revealed the presence of MG in 16.1% and MS in 4.8% by PCR—implying that vertical transmission is present locally and predisposes to early infection. Iraqi Journal of Veterinary Medicine
Conclusion: Elevated PM/NH₃/temperature + presence of IBV/NDV + poor mobility management = fertile ground for MG/MS to develop into a severe form.
Fourth: “Primary or Secondary?”—Read this in light of pathogenesis and predisposition.
Primary: When MG or MS is confirmed by qPCR = quantitative PCR in the absence of other pathogens, and the lesions are compatible (such as arthritis in MS or purely respiratory lesions in MG). MSD Veterinary Manual
Secondary/Opportunistic: When there is a recent viral infection or a poor environment (PM/NH₃/heat), which weakens the ciliary barrier and immunity and allows mycoplasmas to multiply and prolong the disease (as explained in the 2025 review). BioMed Central
Fifth: What do I do now? (Focused plan for farms in Iraq)
Control environmental factors:
Maintain NH₃ < 25 ppm through ventilation and litter management; reduce dust (PM) through controlled humidification/extractors/proper feeding. CAESScienceDirect
Reduce heat stress: Controlled evaporative cooling, cold water, appropriate density, and anti-heat stress feeding programs according to the 2022–2023 revisions. ScienceDirectFrontiers
Break transmission pathways: MG/MS-free hatchery (ELISA = enzyme immunoassay and qPCR on eggs periodically) + a “clean path” for worker and vehicle entry + no sharing of tools between farms. WOAHIraqi Journal of Veterinary Medicine
Smartly diagnose at symptomatic stage: qPCR for MG/MS with a respiratory virus panel (IBV/NDV) to determine primary or secondary infection. Correlate results with the pathotype (joint/EAA with MS, upper respiratory with MG). MSD Veterinary Manual.
Vaccinate wisely and monitor the effect:
MS: MS-H vaccine reduces shedding/horizontal transmission in the field (4-year field study, 2024). Europe PMC
After vaccination, use DIVA methods to differentiate vaccine strains from the field (e.g., dual-qPCR to differentiate MS1/MS-H, published 2024). Frontiers
Review IBV/NDV programs: Consider timing of respiratory vaccines to reduce the “window of vulnerability” that facilitates MG/MS amplification. A 2025 review confirms the role of viral involvement in the severity of respiratory syndromes. BioMed Central.
Sixth:Why does this matter specifically to Iraq?
High temperatures and increasing dust storms exacerbate PM/heat stress and affect indoor air quality—this increases the risk of MG/MS transitioning from mild infections to more severe forms. SIPRIweatheringrisk.org
Local vertical transmission has been demonstrated (positive PCR in hatching eggs during 2022–2023 and published in 2025), necessitating stricter screening of mothers and eggs. Iraqi Journal of Veterinary Medicine Iraqi Academic Scientific Journals
