Very virulent infectious bursal disease virus (vvIBDV) in vaccinated broiler flock: Course of the disease, identification and characterisation of isolated strain* Hochpathogenes Infektiöses Bursitis Virus (vvIBDV) in geimpften Broilerherden: Verlauf der Erkrankung, Identifizierung und Charakterisierung des isolierten Stammes

Infectious bursal disease (IBD) is an acute, highly contagious viral disease of young chickens. It is most often found in highly concentrated poultry producing areas throughout the world. IBD is caused by a virus that is classified as a member of the genus Avibirnavirus of the family Birnaviridae (Leong et al., 2000), which is characterised by a bisegmented dsRNA genome (Müller et al., 1979, Kibenge et al., 1988). There are many different strains of the IBD virus (IBDV), which vary considerably in their pathogenicity and global distribution. Since 1988, outbreaks of IBD caused by very virulent (vv) IBDV strains occurred in broilers in several European countries, Asia, Africa and recently in Central and South America (Chettle et al., 1989; van den Berg et al., 1991; Eterradossi et al., 1992; Nunoya et al., 1992; Di Fabio et al., 1999; Zierenberg et al., 2000). These strains are antigenically similar to the classic virulent strains but can be distinguished by RT-PCR combined with restriction enzyme analysis (REA) or by genome sequencing (Eterradossi et al., 1997; 1999, Zierenberg et al., 2000, 2001). In contrast, “variant strains” with a different antigenic profile were described in the USA. Vaccination failure was incriminated to be responsible for the emergence of antigenic variation (Rosenberger and Cloud, 1986). In general, IBDV is resistant to many disinfectants and environmental factors, and remains infectious for at least four months in the poultry house environment. Because of the resistant nature of IBDV, once a poultry house becomes contaminated, the disease tends to recur in subsequent flocks (Lukert and Saif, 1997). Hygienic measures alone are ineffective and vaccination is essential. Several vaccines are available. When they are given correctly, good immunity and protection can be achieved (van den Berg and Meulemans, 1991). Beside the proper application the major problem with the live vaccination of young chickens with maternally derived antibodies (MDA) is determining the proper time of vaccination, through monitoring of the antibody level in a breeder flock or its progeny. The present paper describes an outbreak of IBD in vaccinated broiler flocks. Material and Methods