Infection of the chicken with a virulent or avirulent strain of Mycoplasma gallisepticum alone and together with Newcastle disease virus or E. coli or both

The influence of the virulence of M. gallisepticum (Mg) was studied in multiple infections of chickens involving Mg, Newcastle disease virus (NDV) and E. coli. Separate groups of 3-week-old chickens were inoculated supra-conjuctivally with a virulent and an avirulent strain of Mg alone and in combination with the La Sota strain of NDV, the 01 serotype of E. coli, and both NDV and E. coli. In addition, chickens were inoculated with NDV alone, E. coli alone, and with NDV and E. coli; one group was left uninfected.Clinical signs, lesions, recovery of the pathogens and the serological response were observed for all groups for 3 weeks after infection and for those involving Mg alone and Mg together with NDV for 18 weeks.No clinical signs were seen in any of the birds; in each infected group some showed mild lesions of the trachea and air sacs without any marked difference among the groups.Although the numbers of birds examined were small, the virulent strain of Mg was more readily recovered than the avirulent, from the respiratory tract, in the first 3 weeks following multiple infections. However, the virulence of Mg had no influence on the recovery of the other pathogens.For the first 6 weeks after infection there was a direct relationship between the virulence of the Mg and the proportion of birds with agglutinins to the Mg rapid serum agglutination (RSA) test in single or multiple infections; multiple infections enhanced the antibody response to both virulent and avirulent mycoplasma. For NDV, multiple infections, particularly involving E. coli, enhanced the peak titre of haemagglutination-inhibition antibodies and accelerated their appearance; the virulence of the Mg had no apparent effect on this.Neither the mycoplasma nor NDV were detected in the trachea by immunofluorescence.