Pathogenicity of Czech isolates of infectious bursal disease virus

An epidemics of infectious bursal disease (IBD) affected chicken farms in the Czech Republic in the mid nineties. Characteristics of six strains of infections bursal disease virus (IBDV), isolated from broiler chickens, lying hybrid pullets, and chickens kept on small farms, were compared using standard methods of assessment of lymphatic tissue (cloacal bursa, thymus, spleen) index and morphological lesions indicative of immunosuppression, and haematological tests. The results of the investigations confirmed the presence of pathogenic IBDV strains in the local chicken population; four of the isolates induced in susceptible chickens clinical IBD manifested by inflammatory and degenerative lesions in cloacal bursa, thymus and spleen, Irrespective of the severity of the clinical disease, all the isolates induced significant (p < 0.05) bursal atrophy oil postinfection days 3 to 6 and a serious damage to the two central lymphatic organs. The effect of infection on haematopoietic tissues Was manifested by shifts in the leukogram and in percentages of immature and mature lymphocytes. Infection by some of the isolates resulted in early predominance of heterophils and an increase in the number of mature lymphocytes. Increased number of lymphocytes and a higher percentage of immature lymphocytes Were found in chickens recovered from the infection. Although the isolates differed from each other in the indexes of pathogenicity, cloacal bursa, and thymus, they showed similar immunosuppressive activity in regard to the primary lymphatic tissues. The IBDV isolates can be classified as standard strains differing in virulence. None of the isolates showed characteristics of a variant IBDV strains.