The pathological and immunological effects of two infectious bursal disease (Gumboro disease) vaccines containing attenuated virus were evaluated under experimental conditions. A total of 200 conventional broiler chicks were divided into three batches, two of which were administered an experimental dose of Gumboro disease vaccine via the ocular route, at 14 days of age, using an ‘intermediate’ strain for one group and an ‘intermediate plus’ strain for the other. At different post-inoculation intervals, the chicks from the three batches were weighed and slaughtered and their bursa of Fabricius were removed to be weighed and subjected to histological examination. This allowed a lesion score and a ratio between the weight of the bursa of Fabricius and body weight to be calculated for each group of chicks. The immunosuppressive potential of the two vaccines was also estimated by evaluating the chicks’ serological response to a Newcastle disease vaccine administered by eye-drop two weeks following vaccination against Gumboro disease. The results showed that the two vaccine strains were not of equal pathogenicity and that one was potentially immunosuppressive. The ‘intermediate plus’ strain proved to be more virulent than the ‘intermediate’ strain, as it caused greater atrophy and more severe lesions of the bursa of Fabricius. By exerting a more suppressive effect on the serological response of the chicks, the ‘intermediate plus’ vaccine also proved to be immunosuppressive. The three weeks of observation were not sufficient to ascertain whether the lesions caused by the two vaccines were temporary or whether the immunosuppressive state induced by the ‘intermediate plus’ strain was reversible.
 
Keywords
Atrophy – Bursa of Fabricius – Gumboro disease – Immunosuppression – Infectious bursal disease – Pathogenicity – Safety – Vaccination.