The wetland areas in Jæren, in Rogaland county in south-western Norway (Fig. 1), are important habitats for migratory birds during the autumn, winter and spring roosts (7). Jæren also has Norway's highest density of poultry farms, which entails a risk for outbreaks of HPAI in poultry farms (8). It was thus not a surprise that the first detection of HPAI in Norway happened in Rogaland. However, the timing of HPAI virus introduction to Norway was more unforeseen, since the influx of potentially infected birds from central Europe during the spring migration was considered to pose the greatest risk to Norway (8).
The population of Pink-footed geese that breed in the Arctic archipelago of Svalbard, migrate through Norway in the autumn, from mid-September to late October (9). The majority of the birds stage in Trøndelag county, where they are subject to regular hunting. After the staging period, they continue their migration southward, crossing inland to the Oslofjord and further to Denmark. Some birds, however, migrate along the western coast of Norway, and some stage in the wetlands of Jæren in Rogaland (Fig. 1). Most birds belonging to this population winter in the Netherlands and Belgium (9).
Observations from neck banded birds indicate regular short distance return migrations from other North Sea countries to Southern Norway (10). In line with this, recent results from a goose fitted with a GPS-logger indicate that pink-footed geese may regularly move from Denmark to south-western Norway, especially in mild winters (Jesper Madsen, personal communication in Molværsmyr et al. (2020)). Pink-footed geese breeding in Iceland or eastern Greenland winter in the United Kingdom (10). However, a small number of pink-footed geese from these populations are occasionally found in Southern Norway (7, 11).
A seroprevalence of 47% to avian influenza A virus was found in pink-footed geese sampled during the spring roost in Central Norway (Svalbard breeding population) between 2016–2018 (12). Of the seropositive birds, 3% (12/427) of the pink-footed geese had been exposed to the subtypes H5 and/or H7 (12). Considering that HPAI viruses found in nature have almost always contained the H5 or H7 hemagglutinin, one may hypothesize that HPAI has circulated among pink-footed geese visiting Norway in previous years. The retrospective detection of four positive Eurasian wigeons, eight days before the first pink-footed goose, indicates that HPAI virus has been circulating in the Jæren area for some time, and also emphasizes the uncertainty about which species may have introduced the virus to Norway.
From the analyses of data deriving from counts of geese in Jæren, as well as neck band observations and ordinary recoveries of ringed birds (10), some potential introduction routes of the HPAI virus to Norway are discussed below.
Wild birds migrating from Russia to the south-west during autumn have been proposed as a potential pathway for the introduction of HPAI virus to Europe (13), but it is difficult to assess whether this may also apply to Norway because of the lack of data on bird migration from Russia to Norway.
Similarly, we cannot rule out that geese from Iceland brought the virus to Norway during autumn migration. However, HPAI virus has not been detected in Iceland and this route of introduction is considered less likely.
A northward return migration of geese from their wintering sites in areas extending from Denmark to Belgium, to sites in Southern Norway, has recently been documented (10). Thus geese and other bird species may visit this region of Norway anytime during the winter season. Given that these birds have been in areas with outbreaks of avian flu during the autumn of 2020, such as the west coast of Denmark and the Netherlands (4), they may have introduced HPAI virus to Southern Norway.
The Barnacle goose, like the pink-footed goose, also migrates through Norway and breeds on Svalbard. It was the species with the highest number of reported cases of HPAI virus of subtype H5N8 in Europe during the autumn of 2020 (4). Therefore, based on data by Molværsmyr et al. (10), we believe that HPAI virus of subtype H5N8 was most probably introduced by pink-footed geese and/or barnacle geese migrating to Norway from common night roosts along the west coast of Denmark, or from the Netherlands. However, more detailed studies of migratory routes of wild birds species, bird genetics (14) and molecular characterization of the H5N8 virus, are needed to answer this question. This is also illustrated by the four wigeons shot in mid-November, as it leaves an unanswered question of where and how these ducks were infected.
Already on November 19th 2020, prior to the first detection of HPAI, a request for increased vigilance to report sick or dead wild birds had been communicated to the public and bird watchers, through the national media and on the websites of the NFSA and NVI.
As an immediate response to the detection of HPAI in Norway, the NFSA introduced a high-risk area in line with Commission Implementing Decision (EU) 2018/1136 of August 10th 2018 on risk mitigation and reinforced biosecurity measures and early detection systems in relation to the risks posed by wild birds for the transmission of highly pathogenic avian influenza viruses to poultry. This included a housing order for poultry and birds kept in captivity in the coastal municipalities from Rogaland county in the southwest to the Swedish border in the southeast. As a result of multiple detections of HPAI in two counties in the two weeks following the initial detection (Table 1), the high-risk area was extended to Southern Norway south of Nordland county on December 7th 2020 (15). Poultry farmers were also informed of the risk from wild birds and encouraged to strengthen their biosecurity measures, and to immediately submit birds for AI analysis in the event of increased mortality, decrease in egg-production or decrease in intake of water and feed in their flocks.
On December 14th 2020, the NFSA issued regulation for an immediate ban of waterfowl hunting in Southern Norway until further notice (16). The main reason for introducing this ban was to avoid unnecessary disturbance to the birds and to prevent potentially infected waterfowl from moving out of infected areas. Hunters generally shoot healthy birds, but occasionally they may shoot sub-clinically infected birds or those early in the incubation period. Handling and processing these infected birds may pose a potential risk to poultry in areas with a high density of farms (Fig. 1).