IBD genotype A3B1 dominates in Europe, A2dB1b rising within the Center East
12 August 2024
3 minute learn
[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.
