CAMPHUYSEN CJ, KELDER L, ZUHORN C & FOUCHIER R (2023) Avian influenza panzootic leads to mass strandings of Northern Gannets Morus bassanus in the Netherlands, April-October 2022. LIMOSA 96 (2): 49-59.
In April 2022, reports were received of an unprecedented
stranding of dead Northern Gannets along the Dutch
North Sea coast, especially on the Wadden Sea islands.
Some 221 individuals washed ashore within one month,
where around 22 dead birds would have been normal,
considering an average of 0.06 ± 0.07 stranded gannets
per km coastline from systematic beached bird surveys
(1980-present; or 10x background levels in April 2022).
In addition, 15 birds were found within the Wadden
Sea, where 6 would have been normal (0.02 ± 0.09 km-1,
or 2.4x background levels). Numbers along the North
Sea coast decreased again in May (still 6x background
levels) and the incident was therefore not investigated,
but it was established that the stranded birds were
mostly adults, with a high body mass and that they
were (subcutaneous) very fat, with no indications for
an unnatural death such as oiling or entanglements.
Around the North Sea, the first indications for what
would turn out to be a major avian influenza panzootic
all over the Northern Hemisphere affecting colonial
seabirds, became apparent in May 2022: high mortality
of breeding gannets on nests, large numbers of corpses
of gannets afloat under several of the major breeding
colonies and widespread in the North Sea basin. Within
the Netherlands, stranding frequencies increased
again in June (17x background levels), soared to a peak
of 2.19 gannets km-1 in July (63x background levels),
and subsequently declined somewhat in autumn (Fig.
2). Strandings occurred all over the Dutch coastline
(Fig. 3), with at least 2837 casualties in total, as an
extrapolation based on monthly densities between
April and October (236, 116, 430, 1131, 465, 247, and 212
respectively), where about 170 dead birds in total would
have been normal given the long-term dataset (1980-
2021; Tab. 1). The data suggest that adults suffered
disproportionally during the current panzootic, with
immature birds (2nd to 5th plumage types) thus far
relatively being spared. Between Apr-Oct 2022, 89.5%
of all gannets found dead were mature birds (n= 344),
while only 60.1% were adults in that period over 1980-
2021 (n= 759, X21=95.1, P< 0.001). In the spring, during
the first strandings (Apr-May), the difference was not
(88.7% adult over 1980-2021, n= 221, 91.7% in 2022, n=
60, X21= 0.18, n.s.), but in that season adults normally
always outnumbered immatures. Both during the peak
in summer (May-June, 63.1% versus 94.7% adult), and
in early fall (Aug-Oct, 44.8% versus 85.4% adult), the
age distribution was significantly different (X21= 31.2,
P< 0.001 and 7.2, P< 0.01 respectively; Fig. 4). Against a
background of long-term survey data, the significance
of the mortality inflicted in Northern Gannets could
clearly be appreciated. The results suggest that birds
were infected whilst underway towards their breeding
grounds (in April), but also that many birds returned to
the south, or at least abandoned the breeding grounds
early, in mid-summer. The April-October 2022 event
can be seen as an exceptional outlier in the long-term
database (Fig. 5), in terms of overall densities, age
composition and apparent cause of death. Of 19 tested
individuals sampled on the Wadden Sea island Vlieland
(June-July), 18 individuals (95%) tested positive for
avian influenza, of which at least 16 cases involved the
H5 subtype. Of these, five were characterised as HPAI
H5N1, and these were genetically closely related to
viruses found in gannets sampled in England, Scotland,
Wales, and Sweden in 2022. Individual gannets sampled
in Wieringen (1x July, 1x August, 1x September), also
tested positive for the H5 subtype, but the virus found
in the gannet sampled in August was genetically closer
to that found in Dutch geese than to the virus found in
other gannets sampled in Europe.
The panzootic of highly pathogenic avian influenza
virus (HPAIV) H5N1 in summer 2022 posed an enormous
challenge to colonial (pelagic) seabird populations.
The emergence of H5N1 infections in the marine
environment signalled a significant change in the
dynamics of the virus, with unprecedented mortalities
affecting dozens of seabird species in seabird colonies
throughout the North Atlantic. High densities of H5N1
susceptible birds (at roosts, foraging grounds or, now,
in colonies) are at risk to become infected and to
subsequently infect conspecifics. Well-known routes of
transmission are through virus excreted in faeces, or,
for scavengers and predators, following consumption
of infected birds. High pathogenic H5N1 viruses are
exceptional, because they can also replicate in and
be shed from the respiratory tract of infected birds. If
the virus can survive sufficiently long under abiotic
conditions, for example in freshwater basins, the
frequent virus outbreaks in wetlands affecting swans,
geese and various ducks are easily understood. Pelagic
seabirds were apparently not at risk, or not thus far, until
the most recent outbreaks in the marine environment.
Current pressing questions therefore include: how
persistent is avian influenza in salt water and how and
where come piscivorous, strictly marine, Northern
Gannets in contact with the virus? It is therefore critical
to understand the incidence and mortality risks of
HPAIV H5N1 for seabirds, including the scale and timing
of inflicted damage.
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