Veterinary Division - Animal Health Programs
Avian Mycoplasmosis (Mycoplasma gallisepticum)
OIE, Manual of Standard Diagnostic Tests and Vaccines 2000
This only a portion of this article. For the entire 21page article, click here.
SUMMARY Mycoplasma gallisepticum (MG) can cause significant economic loss in chickens and turkeys due to chronic respiratory disease, downgrading of carcasses in meat-type birds and loss of production in layers. Infection is diagnosed by demonstration of the organism or its DNA or by the detection of specific humoral antibodies. Other pathogenic mycoplasmas infecting poultry are M. synoviae in the chicken and turkey, and M. meleagridis and M. iowae in the turkey.
Identification of the agent: MG can be detected in two ways. It can be identified by immunological tests after isolation in mycoplasma media or the presence of its DNA can be demonstrated in field samples or cultures.
Samples for isolation consist of swabs of organs or tissues, exudates or diluted tissue homogenates. Culture may also be attempted from aspirates from the infraorbital sinuses or joint cavities and from yolk or embryos. The sample selected will be influenced by the clinical signs and by any lesions present. Broth and agar are used for isolation, but it is normally necessary to obtain mycoplasma colonies on agar before attempting identification. If there is difficulty, an initial passage of the material through mycoplasma-free chicken embryos or mycoplasma-free chickens may be helpful.
Although some basic biochemical tests can be helpful in preliminary classification of mycoplasma isolates, final identification must be by serological tests. The most satisfactory of these are the indirect fluorescent antibody test and the immunoperoxidase test.
DNA detection methods, mainly based on the polymerase chain reaction, have come into use in specialised laboratories. A commercial kit has been marketed in several countries and can be used to detect field strains and to distinguish them from the vaccine F strain.
Serological tests: Several serological tests have been used to detect MG antibodies, but specificity and sensitivity have been lacking to some degree in all of them. They are more satisfactory for flock screening than for testing individual birds.
The most commonly used are the rapid serum agglutination (RSA) test, the enzyme-linked immunosorbent assay (ELISA) and the haemagglutination inhibition (HI) test. In the RSA test, sera from individual birds are tested for agglutination using commercially produced stained MG antigen. Chicken or turkey sera that yield agglutination reactions with the antigen within 2 minutes should be heated at 56°C for 30 minutes and retested. Sera that still react, especially when diluted, are considered to be positive. These may then be confirmed by an ELISA or a HI test. Doubtful results should be checked by performing RSA tests with M. synoviae antigen, as infection with this organism sometimes causes cross-reactions. Several commercial MG antibody ELISA kits are available, including a blocking ELISA that uses a monoclonal antibody.
Requirements for vaccines and diagnostic biologicals: Although the preferred method of control is maintenance of MG-free flocks, both live and inactivated vaccines have been used. Vaccination should be considered only on multi-age sites where it is inevitable that flocks will become infected. The usual use of vaccination is to prevent egg production losses in commercial layers, although vaccines may also be used to reduce the level of egg transmission in breeding stock or as a tool for MG eradication on multi-age production sites. It is important for either live or killed vaccine to be administered before the flock is exposed to field infection with MG.
Available live vaccines are generally produced from the F strain of MG, and, more recently, strains ts-11 and 6/85, which are apathogenic strains with improved safety characteristics. Administration of the F strain by the intranasal or eyedrop route is preferred, but aerosol or drinking water administration may be used. The eyedrop method is recommended for ts-11, while a fine spray is recommended for 6/85. Pullets are generally vaccinated between 12 and 16 weeks of age. A single dose is sufficient and vaccinated birds remain permanent carriers. Long-term use of the F strain on a multi-age site results in displacement of the field strain with the vaccine strain. F strain displaces virulent wild-type MG strains more efficiently than 6/85 or ts-11, but ts-11 has been successfully used to eradicate F strain MG in multi-age commercial layers. Multi-age production sites are also known to test serologically negative for MG after long-term use of 6/85. The F strain is fully virulent for turkeys.
Bacterins consist of a concentrated suspension of MG organisms in an oil emulsion. These are ordinarily administered to growing pullets at 12-16 weeks of age. They are administered parenterally, usually subcutaneously in the neck. Although two doses are desirable, a single dose is usually given because of cost and labour considerations. Bacterins are effective in preventing egg-production losses and respiratory disease, but they do not prevent infection with wild-type MG.
Mycoplasma gallisepticum (MG) belongs to the class Mollicutes, order Mycoplasmatales, family Myco-plasmataceae. MG is particularly important in chickens and turkeys as a cause of respiratory disease and decreased production and can also cause upper respiratory disease in game birds. More recently it has been recognised as a cause of conjunctivitis in house finches in North America. Under certain circumstances, MG may be associated with acute respiratory disease in chickens and turkeys, especially in young birds, with the turkey being more susceptible. The severity of the disease is greatly affected by the degree of secondary infection with viruses such as Newcastle disease and infectious bronchitis, and/or bacteria such as Escherichia coli. A more chronic form of the disease may occur and can cause reduced egg production in breeders and layers. The infection is spread vertically through infected eggs and horizontally by close contact. Other methods of spread are less well documented. It should be noted that M. synoviae, M. meleagridis and M. iowae can also cause disease in poultry.
The presence of MG can be confirmed by isolating the organism in a cell-free medium or by detecting its DNA directly in infected tissues or swab samples. When results are equivocal, chicken embryos or chickens may be inoculated with suspect material. Serological tests are also widely used for diagnosis.