Vaccination of chickens with nonpathogenic Mycoplasma gallisepticum as a means for displacement of pathogenic strains.

Attempts to solve the problem of Mycoplasma gallisepticum (Mg) infection of poultry by a combination of eradication and antibiotic treatment have at best met with only partial success. As a result of the continuing economic burden of the disease, there has been a renewed interest in vaccination as a tool in the control of Mg. A particularly pressing problem exists in the commercial egg industry, where the occurrence of MG infection of layer hens at the onset of egg production leads to a marked depression in productivity. Vaccination with the F strain of Mg has been demonstrated to efficacious in the alleviation of this problem, and the procedure is widely employed in the USA. Under field conditions of vaccination the F strain was found to be virtually nonpathogenic, although challenge experiments show that it retains some pathogenicity. The strain is carried in the trachea of vaccinated layers for as long as one year, but only spreads slowly. A specific serological response to Mg is produced in response to vaccination. To some extent, this response is dose dependent, but it is quantitatively less than that produced by virulent strains introduced by the same route. It has been suggested that the continued presence of a nonvirulent Mg in the upper respiratory tract, and the concomitant local immunological response, may prevent infection by field strains of Mg. In fact, judicious vaccination procedures appear to lead to the displacement of pathogenic Mg by the vaccine strain. By these means, it may be possible to eradicate Mg from flocks while maintaining production.