Pathogenicity of Czech Isolates of Infectious Bursal Disease Virus

Juranová, R. , Nguyen Thi Nga, L. Kulíková, V. Jurajda: Pathogenicity of Czech Isolates of Infectious Bursal Disease Virus. Acta Vet. Brno, 2001, 70: 425-431. An epidemics of infectious bursal disease (IBD) affected chicken farms in the Czech Republic in the mid nineties. Characteristics of six strains of infectious 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 on 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. Domestic chickens, infectious bursitis, infectious bursal disease virus, pathogenicity, immunosuppression, IBDV isolates Epidemics of infectious bursal disease (IBD), affecting in the eighties and nineties most of the countries with advanced poultry industry, extended from Slovakia to Czech poultry farms in the mid nineties (Pospí‰ i lová 1997). Outbreaks of IBD caused by virulent strains were reported from European countries (Box 19889; Chet t le et al.1989; Van den Berg et al. 1991), western Africa (Cissé et al. 1989; Eterradossi et al. 1999), the United States (Rosenberg and Claud 1986; Saif et al. 1987; Sharma et al. 1989, Ismail et al. 1990), and Japan (Nuoga et al. 1992; Tsukamoto et al. 1992). In addition to the standard IBDV strains, domestic chickens can be infected by local variant strains causing bursal atrophy without signs of inflammation and damage to thymus (Rosenberg 1987); the infections are accompanied by severe suppression of humoral and cell-mediated immunity (Sharma et al. 1989). Immunisation with vaccines prepared of a standard strain does not protect against challenge with variant strains (Craf t et al. 1990). The first outbreaks of acute bursitis in the Czech Republic were diagnosed in broiler chicken operations, flocks of hybrid laying pullets, and sporadically on small farms. Since outbreaks were reported also from vaccinated flocks, the question whether variant strains of IBDV circulate in the Czech chicken population has been raised. ACTA VET. BRNO 2001, 70: 425–431 Address for correspondence: MVDr. RÛÏena Juranová, CSc. Klinika chorob drÛbeÏe a ptactva FVL VFU Brno, Palackého 1-3 612 42 Brno, Czech Republic Phone: + 420 5 4156 2366 E-mail: [email protected] http://www.vfu.cz/acta-vet/actavet.htm