Avian Mycoplasmosis: Mycoplasma gallisepticum and Other Species, Vaccination and Control in Poultry

Introduction

Avian mycoplasmosis encompasses a group of infectious diseases of poultry caused by bacteria of the genus Mycoplasma, primarily Mycoplasma gallisepticum (MG), Mycoplasma synoviae (MS), Mycoplasma meleagridis (MM), and Mycoplasma iowae (MI) [1]. These organisms are significant pathogens in commercial poultry production, causing chronic respiratory disease, infectious synovitis, airsacculitis, and eggshell apex abnormalities, leading to substantial economic losses due to mortality, reduced egg production, carcass condemnation, and increased medication costs [1, 2]. Control of avian mycoplasmosis relies on a combination of biosecurity, eradication programs, antimicrobial therapy, and vaccination. The development and application of poultry mycoplasma vaccines have become central to management strategies in many regions [3, 4]. This article provides a detailed, clinically oriented review of the etiology, pathogenesis, diagnostic methods, treatment options, and integrated control measures for avian mycoplasmosis, with a specific focus on vaccination.

Etiology and Pathogen Characteristics

Mycoplasmas are the smallest self-replicating prokaryotes, lacking a cell wall and possessing a genome of approximately 800 to 1000 kb [5]. Their absence of a cell wall confers resistance to beta-lactam antimicrobials and a characteristic requirement for sterols for membrane stability. In poultry, the most economically important species are:

• Mycoplasma gallisepticum: The primary agent of chronic respiratory disease (CRD) in chickens and infectious sinusitis in turkeys [6]. MG colonizes the respiratory epithelium, causing ciliostasis, mucosal inflammation, and secondary bacterial infections [1].
• Mycoplasma synoviae: Causes infectious synovitis (joint and tendon sheath inflammation) and respiratory disease in chickens and turkeys; also associated with eggshell apex abnormalities (EAA) in laying hens [7].
• Mycoplasma meleagridis: A turkey-specific pathogen associated with airsacculitis, leg deformities, and reduced hatchability; transmitted predominantly via the hatchery [8].
• Mycoplasma iowae: Also turkey-specific, causing embryo mortality, reduced hatchability, and leg abnormalities [9].

All these species exhibit high intraspecies genetic variability, particularly in surface lipoproteins, which influences antigenicity and vaccine efficacy [5]. The organisms are fragile, surviving only hours outside the host, and are typically transmitted through direct contact, aerosol, or vertical transmission via the egg [1].

Epidemiology and Transmission

The epidemiology of avian mycoplasmosis is shaped by vertical transmission (transovarian passage from infected breeder hens to progeny) and horizontal transmission (respiratory droplets, contaminated feed, water, fomites, and personnel) [1, 2]. MG and MS are the most prevalent worldwide, with MG being endemic in many non-commercial and some commercial layers and broiler breeder flocks [3]. In turkeys, MM and MI remain significant threats, often introduced through infected hatching eggs [8, 9]. Risk factors include high stocking density, poor ventilation, concurrent viral infections (e.g., Newcastle disease, infectious bronchitis), and immunosuppression [1]. The absence of a cell wall makes mycoplasmas susceptible to environmental desiccation, but they can persist in organic material for limited periods [5].

Clinical Signs and Pathology

Clinical presentation varies by species and host age. MG-affected chickens show rales, coughing, nasal discharge, conjunctivitis, and decreased feed intake; in layers, egg production drops by 10 to 30% [1, 6]. Turkeys with MG develop sinusitis with swelling of the infraorbital sinuses and purulent exudate [1]. MS causes lameness, swollen hocks and footpads, and pale combs; in respiratory form, clinical signs are milder [7]. MM and MI primarily present as hatchability issues, with embryos showing stunting, hepatomegaly, and airsacculitis [8, 9].

Pathologically, MG induces catarrhal tracheitis, airsacculitis, and fibrinous pericarditis; secondary Escherichia coli infection frequently results in airsacculitis and peritonitis (colibacillosis) [1]. MS lesions include purulent arthritis, tenosynovitis, and in laying hens, focal thinning and roughening of the eggshell at the apex [7]. MM produces a characteristic purulent airsacculitis and sternal bursitis in turkey poults [8].

Diagnostics

Laboratory confirmation of avian mycoplasmosis relies on isolation, serology, and molecular detection. Culture of mycoplasmas requires specialized media (e.g., Frey’s medium) and is slow (3 to 7 days) but provides isolates for strain typing [5]. Serological tests include rapid serum agglutination (RSA) as a screening tool, and enzyme-linked immunosorbent assays (ELISAs) or hemagglutination inhibition (HI) for species-specific detection [1, 3]. However, serology cannot distinguish between vaccination and field infection, a limitation addressed by molecular methods. Real-time polymerase chain reaction (PCR) assays targeting species-specific genes (e.g., mgc2 for MG, vlhA for MS) offer high sensitivity and specificity, and can differentiate vaccine strains from wild-type isolates through sequencing or melt-curve analysis [4, 5]. The World Organisation for Animal Health (WOAH) provides standardized diagnostic protocols for notifiable mycoplasma species [1, 6].

For detailed laboratory workflows, refer to the article on Mycoplasma gallisepticum and Mycoplasma synoviae Infections in Chickens: Laboratory Diagnosis and Control Strategies.

Treatment

Antimicrobial therapy plays a key role in reducing clinical signs and transmission but does not eliminate infection from a flock. Drugs with efficacy against mycoplasmas include macrolides (tylosin, tilmicosin, tulathromycin), lincosamides (lincomycin), pleuromutilins (tiamulin), tetracyclines (chlortetracycline, oxytetracycline), and fluoroquinolones (enrofloxacin) [1, 2]. Tiamulin is particularly effective against MS and MG when administered via feed or water [5]. However, acquired resistance to tylosin and enrofloxacin has been documented, necessitating susceptibility testing when possible [2, 4]. Treatment is most effective when initiated early during an outbreak and combined with improved ventilation and biosecurity.

For more information on antimicrobial use, see Avian Enrofloxacin: Pharmacology, Clinical Use, and Resistance in Poultry.

Control Strategies

Control of avian mycoplasmosis is best achieved via an integrated approach encompassing eradication, biosecurity, and vaccination. Primary breeding companies maintain MG- and MS-free flocks through rigorous monitoring and elimination (depopulation of positive breeder flocks). Commercial layer and broiler flocks often rely on vaccination due to the difficulty of maintaining freedom from infection in high-density rearing regions [1, 3].

Biosecurity Measures

Essential biosecurity practices include all-in/all-out production, quarantine of new stock, use of dedicated footwear and equipment, and control of airborne dust and ammonia levels [2]. Hatchery hygiene is critical for MM and MI control because these species are egg-transmitted [8, 9]. Additionally, rodent and wild bird control reduces the risk of mechanical transmission [1].

Eradication

Eradication of MG and MS from breeder flocks is the gold standard but is costly. It involves serological testing, removal of positive birds, and antimicrobial treatment of the entire flock, followed by restocking with clean replacements [1, 3]. This approach is used in many national control programs, especially for primary breeders.

Vaccination

Vaccination is widely used to protect commercial layers, broilers, and turkeys from clinical disease and production losses. Both live and inactivated vaccines are available, and their selection depends on the infection pressure, host species, and production cycle [3, 4]. The term poultry mycoplasma vaccine encompasses these products.

Poultry Mycoplasma Vaccine Types and Application

Live attenuated vaccines are the most commonly used for MG in layers. They are typically administered via eyedrop, spray, or drinking water to pullets between 6 and 10 weeks of age. The most widely applied live MG vaccine strains include:

• F strain: Moderately virulent; used in multi-age layer flocks to displace field strains through “vaccine takeover” [4].
• ts-11: A temperature-sensitive mutant that is safer and less transmissible than F strain; administered via eyedrop [3, 4].
• 6/85: An attenuated strain originally isolated from turkeys; used as a spray vaccine for layers [3].

Inactivated (killed) vaccines are prepared from whole-cell antigens suspended in adjuvant (oil emulsion). They are administered subcutaneously or intramuscularly and require repeated booster doses. Inactivated vaccines are used in breeders to reduce egg transmission and in high-risk commercial flocks [1, 3]. They induce strong humoral antibody responses but do not prevent colonization as effectively as live vaccines [4].

Recombinant vaccines have been developed using vectors such as fowl pox virus expressing MG antigens. These vaccines offer the advantage of differentiating infected from vaccinated animals (DIVA) if the recombinant does not contain the full set of wild‑type antigens [4, 10]. However, field application remains limited compared to traditional live and inactivated products.

For further detail on vaccine protocols, see Mycoplasma gallisepticum Vaccine in Poultry: Protocols and Efficacy and Fowl Pox and Mycoplasma Gallisepticum Vaccine Considerations in Poultry.

Vaccination Decision Tree

The choice of vaccine and timing can be guided by the following Mermaid flow diagram.

flowchart TD
    A[Determine flock type and MG/MS status], > B{Is the flock multi-age?}
    B, >|Yes| C[Use live F strain vaccine<br/>(displacement strategy)]
    B, >|No| D{Is the flock a breeder?}
    D, >|Yes| E[Use inactivated vaccine<br/>before lay to reduce egg transmission]
    D, >|No| F{Is the flock a layer?}
    F, >|Yes| G[Use live ts-11 or 6/85 vaccine<br/>at 8-10 weeks of age]
    F, >|No| H[Broiler flocks: inactivated vaccine<br/>for breeders; limited direct use]
    C, > I[Monitor serology and clinical signs]
    E, > I
    G, > I
    I, > J[If disease persists, adjust vaccine type or timing<br/>and reinforce biosecurity]

Figure 1. Decision tree for selecting a poultry mycoplasma vaccination program based on flock type and infection risk.

Vaccination for MS also involves live attenuated strains (e.g., MS-H, a temperature-sensitive vaccine) or inactivated products. MS-H has demonstrated efficacy in controlling both respiratory and synovial forms and in reducing eggshell apex abnormalities [7]. For MM and MI, no commercial vaccines are widely available; control focuses on hatchery hygiene and eradication [8, 9].

Monitoring and Compliance

Efficacy of vaccination is assessed by monitoring flock clinical signs, production parameters, and periodic serology (HI or ELISA). In some regions, differentiating between vaccine and field strains is crucial for regulatory and trade purposes. Techniques such as PCR followed by restriction fragment length polymorphism (RFLP) or sequencing of variable surface lipoprotein genes (e.g., vlhA for MS, mgc2 for MG) are used for strain discrimination [4, 5]. WOAH standards require that vaccination programs do not interfere with the detection of notifiable mycoplasmas in breeder flocks intended for international trade [1].

Conclusion

Avian mycoplasmosis remains a major challenge to global poultry health and productivity. The four pathogenic species MG, MS, MM, and MI each cause distinct clinical syndromes, all of which result in significant economic impact. Successful control requires a multifaceted approach: strict biosecurity, monitoring with sensitive molecular and serological assays, judicious antimicrobial therapy, and strategic vaccination. Poultry mycoplasma vaccines, both live and inactivated, effectively reduce clinical disease and production losses when applied according to appropriate decision algorithms. However, no vaccine completely prevents infection or transmission. Therefore, vaccination should be viewed as one component of an integrated management plan, not a standalone solution. Continued surveillance for emerging strains and development of improved DIVA-compatible vaccines will further enhance our ability to control these persistent pathogens.

For related topics, readers are referred to the articles on Poultry Mycoplasmosis: Vaccination Strategies and Disease Management and Mycoplasma synoviae: Infectious Synovitis in Chickens and Turkeys – Eggshell Apex Abnormalities and Control.

References

[1] Diseases of Poultry. 14th Edition. Swayne DE, Glisson JR, McDougald LR, et al., editors. Wiley-Blackwell; 2020.

[2] Merck Veterinary Manual. 11th Edition. Kahn CM, Line S, editors. Merck & Co., Inc.; 2016.

[3] Avian Mycoplasmosis: Vaccination Strategies and Control in Poultry Flocks. Available at: /knowledge/bacteria/avian-bacteria/avian-mycoplasma-vaccination-poultry

[4] Avian Mycoplasma Vaccine: Principles, Efficacy, and Application in Poultry. Available at: /knowledge/bacteria/avian-bacteria/avian-mycoplasma-vaccine

[5] Mycoplasma gallisepticum in Poultry: Chronic Respiratory Disease and Control Strategies. Available at: /knowledge/bacteria/avian-bacteria/mycoplasma-gallisepticum-poultry-chronic-respiratory-disease-control

[6] Mycoplasma gallisepticum and Mycoplasma synoviae Infections in Chickens: Laboratory Diagnosis and Control Strategies. Available at: /knowledge/bacteria/avian-bacteria/chicken-mycoplasma-test

[7] Mycoplasma synoviae: Infectious Synovitis in Chickens and Turkeys – Eggshell Apex Abnormalities and Control. Available at: /knowledge/bacteria/avian-bacteria/mycoplasma-synoviae-infectious-synovitis-chickens-turkeys-eggshell-apex-abnormalities

[8] Mycoplasma meleagridis: Turkey Airsacculitis and Leg Deformities – Hatchery Transmission and Control. Available at: /knowledge/bacteria/avian-bacteria/mycoplasma-meleagridis-turkey-airsacculitis-leg-deformities-hatchery

[9] Mycoplasma iowae in Turkeys: Embryo Mortality, Hatchery Infections, and Diagnostic Control. Available at: /knowledge/bacteria/avian-bacteria/mycoplasma-iowae-turkey-embryo-mortality-hatchery

[10] Fowl Pox and Mycoplasma Gallisepticum Vaccine Considerations in Poultry. Available at: /knowledge/bacteria/avian-bacteria/fowl-pox-mycoplasma-gallisepticum-vaccine *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.