Section: Avian Bacteria

Mycoplasma gallisepticum Infection in Chickens: Diagnosis and Management

Etiology and Taxonomy

Mycoplasma gallisepticum is a cell wall deficient bacterium belonging to the class Mollicutes and the family Mycoplasmataceae. The organism is characterized by its small genome (approximately 1.0 Mb) and its absolute dependence on a host for nutritional support due to limited biosynthetic capacity. The absence of a peptidoglycan cell wall renders M. gallisepticum intrinsically resistant to beta-lactam antimicrobials and confers a pleomorphic morphology. The organism possesses a specialized terminal tip organelle that mediates adhesion to host respiratory epithelial cells, a critical step in pathogenesis. This adhesion is mediated by cytadhesin proteins such as GapA and CrmA, which facilitate intimate contact with the host cell membrane and subsequent ciliary stasis and loss.

Epidemiology and Transmission

Mycoplasma gallisepticum is the primary etiological agent of chronic respiratory disease (CRD) in chickens. The infection is distributed globally and represents a major economic burden to the poultry industry due to reduced egg production, increased feed conversion ratios, and elevated mortality from secondary bacterial infections. Transmission occurs both horizontally and vertically. Horizontal transmission is mediated by direct contact between infected and susceptible birds, aerosolized respiratory droplets, and fomite contamination including feed, water, and equipment. Vertical (transovarian) transmission is a hallmark of M. gallisepticum epidemiology, as infected breeder hens can pass the organism to progeny via the egg, establishing infection in subsequent generations. This vertical route complicates eradication efforts and necessitates rigorous monitoring of breeder flocks.

Clinical Signs and Pathogenesis

The incubation period for M. gallisepticum infection ranges from 6 to 21 days depending on the strain, dose, and host immune status. Clinical signs are often insidious in onset and may be exacerbated by concurrent viral infections (e.g., Newcastle disease virus, infectious bronchitis virus) or environmental stressors such as ammonia exposure and poor ventilation. Classic signs include serous to mucoid nasal discharge, conjunctivitis, sinusitis, tracheal rales, coughing, and sneezing. In laying hens, a marked decline in egg production (10 to 40 percent) is frequently observed. Infected birds may exhibit reduced feed intake and subsequent weight loss.

The pathogenesis of M. gallisepticum infection begins with colonization of the upper respiratory tract mucosa. The tip organelle mediates adherence to ciliated epithelial cells, leading to ciliostasis, ciliary loss, and epithelial desquamation. This disruption of the mucociliary escalator predisposes the lower respiratory tract to invasion by secondary bacterial pathogens, most commonly Escherichia coli, resulting in airsacculitis, pericarditis, and perihepatitis. The inflammatory response is characterized by infiltration of lymphocytes, plasma cells, and macrophages, leading to hyperplasia of mucosal-associated lymphoid tissue.

Pathology

Gross pathological findings in uncomplicated M. gallisepticum infection are often limited to catarrhal tracheitis and mild airsacculitis. In cases complicated by secondary E. coli infection, lesions become more pronounced and include fibrinous airsacculitis, pericarditis, and perihepatitis. The air sacs may appear thickened, opaque, and contain caseous exudate. Histopathological examination reveals mucosal hyperplasia, loss of cilia, and lymphoplasmacytic infiltration of the lamina propria. In chronic cases, lymphoid follicle formation is observed in the tracheal submucosa.

Diagnosis

Accurate diagnosis of M. gallisepticum infection requires a combination of clinical observation, serological testing, and molecular confirmation. The differential diagnosis should include other respiratory pathogens such as Mycoplasma synoviae, Avibacterium paragallinarum (infectious coryza), Ornithobacterium rhinotracheale, and respiratory viruses including avian influenza virus and infectious bronchitis virus. For a detailed comparison of mycoplasma species, refer to the article on Mycoplasma gallisepticum and Mycoplasma synoviae Infections in Chickens: Laboratory Diagnosis and Control Strategies.

Serological Methods

Serological screening is widely used for flock-level surveillance. The rapid serum agglutination (RSA) test is a simple, inexpensive method that detects antibodies against M. gallisepticum. However, the RSA test has limited specificity due to cross-reactivity with other mycoplasma species, particularly M. synoviae. The hemagglutination inhibition (HI) test offers improved specificity and is considered a confirmatory serological test. Enzyme-linked immunosorbent assays (ELISAs) are commercially available and provide quantitative antibody titers suitable for large-scale screening. Seroconversion typically occurs 7 to 14 days post-infection, and antibodies may persist for weeks to months.

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting the 16S rRNA gene or the mgc2 gene are the gold standard for direct detection of M. gallisepticum. Real-time PCR (qPCR) offers quantitative data and higher sensitivity compared to conventional PCR. Sample types include tracheal swabs, choanal cleft swabs, air sac exudate, and lung tissue. PCR is particularly valuable for detecting carrier birds with low-level infection and for confirming vertical transmission in day-old chicks. The high sensitivity of PCR necessitates strict adherence to contamination control protocols to avoid false-positive results.

Culture and Isolation

Mycoplasma gallisepticum can be isolated on specialized media such as Frey's medium or Hayflick's medium supplemented with 10 to 20 percent swine serum, yeast extract, and nicotinamide adenine dinucleotide (NAD). Colonies exhibit a characteristic fried-egg appearance after 3 to 10 days of incubation at 37 degrees Celsius in a humidified atmosphere with 5 to 10 percent carbon dioxide. Culture is labor-intensive and slow, and it is often superseded by PCR for routine diagnosis.

Diagnostic Workflow

The following Mermaid diagram illustrates a recommended diagnostic decision tree for a suspected mycoplasma chicken infection.

flowchart TD
    A[Clinical signs: nasal discharge, rales, sinusitis], > B[Collect tracheal or choanal swabs]
    B, > C{Initial screening}
    C, > D[RSA or ELISA serology]
    C, > E[PCR (mgc2 or 16S rRNA)]
    D, > F{Seropositive?}
    F, >|Yes| G[Confirm with HI test]
    F, >|No| H[Consider other etiologies]
    G, > I{Confirmed?}
    I, >|Yes| J[Positive for M. gallisepticum]
    I, >|No| K[Consider cross-reaction or other mycoplasma]
    E, > L{PCR positive?}
    L, >|Yes| J
    L, >|No| M[Consider culture or repeat sampling]
    J, > N[Implement control measures]

Treatment

Antimicrobial therapy for M. gallisepticum infection is complicated by the organism's lack of a cell wall, which renders beta-lactam antibiotics ineffective. Effective antimicrobial classes include macrolides (e.g., tylosin, tilmicosin), tetracyclines (e.g., oxytetracycline, chlortetracycline), fluoroquinolones (e.g., enrofloxacin), and pleuromutilins (e.g., tiamulin). Treatment is typically administered via drinking water or feed for 3 to 7 days. However, antimicrobial resistance has been reported for several of these agents, and susceptibility testing should be performed when possible. It is critical to note that antimicrobial therapy reduces clinical signs and shedding but does not eliminate the organism from a flock. Treated birds may remain persistently infected and serve as reservoirs for future outbreaks.

Control and Prevention

Control of M. gallisepticum infection relies on a multi-faceted approach including biosecurity, surveillance, and vaccination. Establishment of M. gallisepticum-free breeder flocks through rigorous testing and culling is the most effective long-term strategy. All-in-all-out management, strict isolation of replacement stock, and disinfection of equipment and facilities are essential biosecurity measures.

Vaccination is a valuable tool for reducing clinical disease and economic losses in endemic regions. Both live attenuated vaccines (e.g., ts-11, 6/85) and inactivated bacterins are available. Live vaccines are administered via eye drop or spray and confer protection by colonizing the respiratory tract and inducing a local immune response. Inactivated vaccines are administered parenterally and stimulate systemic humoral immunity. Vaccination does not prevent infection but reduces shedding and clinical severity. For a comprehensive discussion of vaccination protocols, refer to the article on Mycoplasma Infections in Poultry: Vaccination Strategies and Control Programs.

Eradication programs in commercial breeding stock rely on periodic serological testing of all birds, removal of seropositive individuals, and strict biosecurity. In multi-age layer operations, eradication is rarely feasible, and management focuses on vaccination, antimicrobial therapy during outbreaks, and maintaining optimal environmental conditions to minimize stress.

Differential Diagnosis

Respiratory disease in chickens has a broad differential etiology. Infectious coryza caused by Avibacterium paragallinarum presents with similar facial swelling and nasal discharge but typically lacks the pronounced tracheal rales and airsacculitis seen in M. gallisepticum infection. Mycoplasma synoviae can cause respiratory signs and synovitis, and it is differentiated by serological and molecular testing. Viral infections such as infectious bronchitis virus and Newcastle disease virus produce respiratory signs and can exacerbate M. gallisepticum pathology. For a detailed overview of respiratory bacterial infections, see the article on Bacterial Respiratory Infections in Chickens: Salmonellosis, Colibacillosis, and Necrotic Enteritis. Parasitic infections such as Syngamus trachea (gapeworm) should also be considered in birds with dyspnea and coughing; refer to Syngamus trachea (Gapeworm) Infection in Chickens: Diagnosis and Treatment for further details.

Conclusion

Mycoplasma gallisepticum remains a significant pathogen in commercial poultry production worldwide. Its ability to cause chronic respiratory disease, reduce egg production, and predispose birds to secondary bacterial infections results in substantial economic losses. Effective management requires a combination of accurate laboratory diagnosis using serology and PCR, strategic antimicrobial therapy, rigorous biosecurity, and vaccination in endemic settings. Eradication from breeder flocks through test-and-remove programs is the gold standard for long-term control.

References

  1. Ley, D. H. Mycoplasma gallisepticum infection. In: Swayne, D. E., editor. Diseases of Poultry. 14th ed. Wiley-Blackwell; 2020. p. 922-960.
  2. Kleven, S. H. Mycoplasmosis. In: Saif, Y. M., editor. Diseases of Poultry. 12th ed. Blackwell Publishing; 2008. p. 805-845.
  3. Ferguson-Noel, N. M., Williams, S. M. Mycoplasma gallisepticum. In: Swayne, D. E., editor. Diseases of Poultry. 13th ed. Wiley-Blackwell; 2013. p. 875-900.
  4. Bradbury, J. M. Avian mycoplasmosis. In: Jordan, F. T. W., Pattison, M., editors. Poultry Diseases. 5th ed. W.B. Saunders; 2001. p. 178-193.
  5. Stipkovits, L., Kempf, I. Mycoplasmoses in poultry. Revue Scientifique et Technique (International Office of Epizootics). 1996;15(4):1495-1525.
  6. Whithear, K. G. Control of avian mycoplasmoses by vaccination. Revue Scientifique et Technique (International Office of Epizootics). 1996;15(4):1527-1553.
  7. Feberwee, A., Mekkes, D. R., de Wit, J. J., Hartman, E. G., Pijpers, A. Comparison of culture, PCR, and different serologic tests for detection of Mycoplasma gallisepticum and Mycoplasma synoviae infections. Avian Diseases. 2005;49(2):260-264.
  8. Raviv, Z., Kleven, S. H. The development of diagnostic real-time TaqMan PCR for the detection of Mycoplasma gallisepticum. Avian Diseases. 2004;48(4):817-824.
  9. Grodio, J. L., Dhondt, K. V., O'Connell, P. H., Schat, K. A. Detection and quantification of Mycoplasma gallisepticum genome load in conjunctival samples of experimentally infected house finches (Haemorhous mexicanus) using real-time polymerase chain reaction. Avian Pathology. 2008;37(4):385-391.
  10. Nascimento, E. R., Pereira, V. L., Nascimento, M. G., Barreto, M. L. Avian mycoplasmosis: diagnosis and control. Brazilian Journal of Poultry Science. 2005;7(2):67-74.
  11. Kempf, I., Gesbert, F., Guittet, M., Bennejean, G. Efficacy of different antimicrobials against Mycoplasma gallisepticum infection in chickens. Avian Pathology. 1992;21(3):475-484.
  12. Jordan, F. T. W., Horrocks, B. K. The minimum inhibitory concentration of tilmicosin and tylosin for Mycoplasma gallisepticum and Mycoplasma synoviae and a comparison of their efficacy in the control of experimental Mycoplasma gallisepticum infection in chickens. Avian Pathology. 1996;25(1):67-78.
  13. Evans, J. D., Leigh, S. A., Branton, S. L., Collier, S. D., Pharr, G. T., Bearson, S. M. D. Effects of Mycoplasma gallisepticum vaccination on the respiratory tract transcriptome of commercial layer chickens. Avian Diseases. 2012;56(4):677-683.
  14. Burnham, M. R., Branton, S. L., Peebles, E. D., Lott, B. D., Gerard, P. D. Effects of F-strain Mycoplasma gallisepticum inoculation on serum constituents of commercial broiler chickens. Avian Diseases. 2002;46(4):875-880.
  15. Levisohn, S., Kleven, S. H. Avian mycoplasmosis (Mycoplasma gallisepticum). Revue Scientifique et Technique (International Office of Epizootics). 2000;19(2):425-442.

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.