[Editor's note: The following is from two presentations by Dr. Matteo Legnardi and colleagues, University of Padova, Italy, at the 2024 annual conference of the American Association of Avian Pathologists.

Reassortment in European strains

The infectious bursal disease virus (IBDV) features a remarkable genetic variability, which in turn may profoundly affect disease manifestation and antigenicity. The present work reports the result of molecular diagnostic activities conducted on samples from 9 Western European countries in 2020-2023. In total, 1,443 pooled bursal samples were collected from flocks representing different productive categories and vaccinated following different protocols. Vaccine and field strains were discriminated based on partial VP2 sequencing, then the latter were subjected to partial VP1 sequencing to achieve a full characterization.

Demonstrating a considerable infectious pressure, 348 samples tested positive for field strains (24.1%), while 599 were positive for vaccine strains (41.5%), 17 could not be sequenced (1.2%), and 479 were negative (33.2%). Although the sampling intensity varied significantly among the different countries, the obtained results offered valuable insights on which field IBDVs are currently present in the region.

Some IBDV types were found to circulate only nationally, such as a segregated clade of genotype A3B1 (17 strains, 4.9%) found solely in Italy and genotype A9B1 (18 strains, 5.2%) which was only detected in Portugal.

Most of the detected field strains (311, 89.3%) were identified as Northwestern European reassorted strains (genotype A3B1). One strain belonging to serotype 2 (genotype A0B1) and another with atypical VP2 features (provisionally classified as genotype AxB1) were detected in France. Despite being first described only in 2017, Northwestern European strains, originated by a reassortment event involving a very virulent and a classical attenuated virus, have rapidly become predominant in most of Western Europe.

The case of Portugal is worthy of particular attention: while only local A9B1 strains were detected at the beginning of the study period, they appear to have been largely displaced by reassorted strains following their entry in the country in 2021. These findings appear crucial to properly plan monitoring and control activities in the surveyed countries, especially considering the well-established immunosuppressive potential of Northwestern European reassorted strains. This should serve as a warning for other epidemiological contexts, as an increasing number of reassortment events are being reported in different parts of the world and the spread of Northwestern European reassorted strains appears not to be over.

Genotype A2dB1b in the Middle East

Among the most notable epidemiological events of the last decade is the emergence of genotype A2dB1b, also known as novel variant IBDV. First reported in 2015 in China, this genotype is thought to have originated from the spread and divergent evolution of antigenic variant IBDVs from North America to East Asia. Subsequently, it has been responsible for large-scale IBD epidemics in China, Malaysia, South Korea, and Japan. Infection by novel variant IBDVs consistently results in severe immunosuppression, but its subclinical course may hamper diagnosis, impact estimation and control efforts.

This study reports the recent detection of novel variant IBDVs in the Middle East, a region historically characterized by very virulent IBDV circulation. Following multiple identifications of A2dB1b strains in Egypt (earliest known detection in March 2023), molecular diagnostic activities also highlighted their presence in Jordan and Lebanon (earliest detection in December 2023) in flocks showing diminished performance and increased susceptibility to respiratory diseases. The sequenced strains from the three countries showed a high genetic identity between each other (above 98.7% and 99.1% identity at VP2 and VP1 level, respectively).

Since novel variant IBDV has never been seen outside of East Asia and now the Middle East, the mode of transmission between these two regions remains unexplained. Nonetheless, these preliminary results represent an epidemiological update of global concern.