Mycoplasma gallisepticum infection
Etiology
Mycoplasma gallisepticum (MG) infection is typically associated with slowly progressive chronic respiratory disease in chickens and infectious sinusitis in turkeys. MG is a eubacteria, a small self-replicating prokaryote lacking a cell wall. MG is the most pathogenic and economically significant of the mycoplasma pathogens known to affect poultry. Numerous strains of MG exist and there is wide variability in the relative virulence of these strains. Synergism between MG and other infectious agents, such as Newcastle disease and infectious bronchitis, is common.
Host range
Mycoplasma gallisepticum is primarily a respiratory disease affecting chickens and turkeys in commercial production; however ducks, geese, pheasants, peafowl, quail, and some wild avian species are also susceptible to the infection. MG has recently been implicated in natural outbreaks of conjunctivitis in free-ranging house finches (Carpodacus mexicanus) in the Eastern United States. In finches, the infection causes conjunctivitis and periorbital swelling. Avian mycoplasma infections appear to be limited to avian species and pose no threat to humans.
Epidemiology
Mycoplasma gallisepticum is spread primarily via horizontal transmission, through both direct contact with infected birds and indirect contact with contaminated equipment, environment, and personnel. MG can survive outside the host (e.g. in feces, feathers, and litter) for several days. Aerosol spread also occurs. The infection enters a susceptible bird through the upper respiratory tract and/or conjunctiva. Exposure of susceptible birds to clinically or subclinically infected birds can play a significant role in the spread of this disease within a flock. Vertical transmission can also occur in eggs laid by hens that are in the acute phase of the disease, and at a lower level during chronic infections. The disease tends to be most severe in younger birds and outbreaks frequently occur in winter.
MG is distributed worldwide. In the United States, most breeder flocks are free from MG; however, infections in growing turkeys and broilers continue to occur. MG infection is endemic in many large multiple-age commercial egg production complexes. The infection may enter a commercial poultry flock via a number of routes including exposure to carrier birds in backyard flocks, exposure to free-ranging wild species, and poor biosecurity practices.
Clinical Signs
MG has a long and variable incubation period, typically ranging from 6-21 days. The clinical signs are highly variable and depend on the host species, age, virulence of the strain, the presence of concurrent infections, as well as any environmental or physiological stress (e.g onset of egg production).
In chickens, the disease may produce no clinical signs or it may produce nasal discharge, coughing, and rales. Conjunctivitis, periorbital edema, and eyelid edema may also occur. Other more general signs may include inappetence, weight loss, and a drop in egg production. The highest morbidity and mortality usually occurs in the presence of concurrent infections and environmental stress.
In turkeys, MG is sometimes associated with more severe clinical signs than typically observed in chickens. Signs may begin with nasal discharge and a foamy ocular discharge, followed by swelling of the infraorbital sinuses. Swelling may become so severe that the birds can no longer open their eyes. As the disease progresses, coughing, rales, and dyspnea may occur. Generalized depression, inappetence, weight loss, and drop in egg production often occur. In 12-16 week old commercial turkeys, torticollis and opisthotonus has been documented.
Although morbidity due to MG is high, mortality is generally low, except in cases complicated with concurrent infections or environmental stress. However, broilers and market turkeys may suffer high condemnation due to airsacculitis and, in layer flocks, the overall production rate may be compromised.
Post-mortem Lesions
On post-mortem examination, lesions may be found throughout the upper and lower respiratory tract. Catarrhal exudate may be present in the nasal passages, infraorbital sinuses, trachea, and bronchi. Caseous exudate may be seen in the air sacs. Other findings may include fibrinous or fibrinopurulent perihepatitis and pericarditis, conjunctivitis, corneal opacities, and periocular edema. Sinusitis is most frequently observed in turkeys but may be present in chickens and other avian species as well.
Differential Diagnosis
It is necessary to differentiate MG from other respiratory diseases of poultry. Pulmonary and air sac lesions may be confused with colibacillosis.
In turkeys, sinusitis can also be caused by avian influenza, Mycoplasma synoviae, aspergillosis, pasteurellosis, chlamydiosis, cryptosporidiosis, Newcastle disease, and turkey rhinotracheitis. Fowl cholera (Pasteurella multocida) is a frequent complication in turkeys and may be accompanied by fibrinous pneumonia.
In chickens, MG must be differentiated from respiratory diseases including Newcastle disease, infectious coryza (Avibacterium paragallinarum), infectious bronchitis, avian influenza, chlamydiosis, Fowl Cholera (Pasteurella multocida) or respiratory cryptosporidiosis.
Diagnosis
Isolation and identification of the MG organism is the gold standard for diagnosis. It is commonly performed by culturing swabs collected from tracheal, sinus, or air sac exudates, or from choanal swabs or tissue samples collected from the turbinates or lungs. In the acute stages of infection, collect and submit samples from 10-20 live birds. In chronic infections, collect samples from 30-100 birds in order to improve the chances of recovering the organism. Mycoplasma broth should be inoculated directly from the bird (via a swab of exudate or tissue sample) and incubated immediately. If the sample must be shipped, store the sample on cold pack for less than 24 hours and use an overnight carrier. In the laboratory, growing cultures are plated out on mycoplasma agar. Suspect colonies may be identified by immunofluorescence tests. Isolation and identification of Mycoplasmas requires specialized media and reagents that are often not commercially available. Detection of MG DNA by polymerase chain reaction (PCR) is becoming a common diagnostic test. A commercial PCR test for MG is available and PCR is performed if it is urgent to determine the flock status.
Serologic testing is used for flock monitoring programs and to aid in diagnosis when infection is suspected. Serology tests of sera for antibodies against MG may include serum plate agglutination (sometimes used for rapid initial screening), enzyme-linked immunosorbent assay (ELISA), and hemagglutination-inhibition (HI). Because of the rapid turn around time, serum plate agglutination is often the standard initial screening test. However, because sera from birds with M. synoviae may cross-react, MG is confirmed with the HI or ELISA test.
Prevention and Control
The best way to prevent Mycoplasma gallisepticum infections is to start with clean stock, maintain good biosecurity, and if possible, practice all-in all-out management. The use of antimicrobial therapy may be used to reduce morbidity and mortality, losses at processing, or egg transmission. Mycoplasma gallisepticum live vaccines are highly effective to control the infection. Bacterins and a recombinant fowlpox-MG vaccine are also available.
Selected references
- Charlton, B. R. (ed). 2006. Avian Disease Manual, 6th ed. American Association of Avian Pathologists (AAAP), 953 College Station Road, Athens, Georgia 30602-4875.
- Glisson, J.R. 1998. Bacterial respiratory disease of poultry. Poult Sci Aug;77(8):1139-42. Review.
- Kleven, S.H. 2008. Mycoplasmosis. In A Laboratory Manual for the Isolation and Identification of Avian Pathogens, 5th edition. L. Dufour-Zavala Louise et al. (ed.) OmniPress, Inc., Madison, Wisconsin.
- Lancaster, J.E. and J. Fabricant. 1988. The history of Avian medicine in the United States. IX Events in the History of Avian Mycoplasmosis 1905-70. Avian Dis 32:607-623.
- Levisohn, S. and S.H. Kleven. 2000. Avian Mycoplasmosis (Mycoplasma gallisepticum). Rev Sci Tech Aug;19(2):425-42. Review.
- Ley, D.H. 2008. Mycoplasma gallisepticum Infection. In Diseases of Poultry, 12th ed. Y.M. Saif. et al. (ed.). Blackwell Publishing, Ames, Iowa.
- World Organization for Animal Health (OIE) website. 2008. www.oie.int
Thank you to the following individuals for reviewing this material:
Naola Ferguson-Noel
Jaime Ruiz
Jose Bruzual
