The risks and consequences of a high pathogenicity avian influenza outbreak in Aotearoa New Zealand

In the last two years (2021–2023) there has been an unprecedented mortality of wild birds throughout the Northern Hemisphere due to avian influenza virus (WOAH 2023). During this period, over 400,000 dead birds have been recorded from over 2,600 separate mortality clusters in wild bird populations; however, the actual number of birds affected is estimated to be much higher (WOAH 2023). Deaths due to this subtype of the virus (clade 2.3.4.4b HPAI A(H5N1)) have been reported from almost every country in Europe and have also been found in wild birds in North America and Northern Africa (Bevins et al. 2022; Caliendo et al. 2022; WOAH 2023). Many bird species are affected, including a variety of seabirds, shorebirds, raptors, and other scavengers. The virus responsible is a subtype of avian influenza A (H5N1) that has been circulating in Asia since 2014. In 2022, the subtype of the virus has moved south through South America and South Africa (WOAH 2023) and as of November 2023 had been confirmed on South Georgia Island in the sub-Antarctic (SCAR 2023).

Avian influenza viruses are a complex group of viruses that are of importance to bird and human health. These viruses are divided into multiple subtypes (e.g. H5N1) which can be classified into two categories: low pathogenic (LPAI) and highly pathogenic (HPAI), where pathogenicity refers to the ability of the virus to produce disease in poultry. LPAI viruses usually cause subclinical infections or mild illnesses in poultry and other birds, which can be exacerbated by concurrent infections or poor husbandry, whereas HPAI can cause severe clinical signs and high mortality rates (WOAH 2023). Viruses evolve rapidly and this current strain of avian influenza is particularly concerning because it has high transmissibility and is much more likely to cause severe disease and death in some wild bird species (Chen et al. 2022; Abolnik et al. 2023; Tian et al. 2023). There is also good evidence of cross-species spread between bird species and also from birds to mammals, suspected to be mainly through the consumption of infected birds (Zhao et al. 2019; Leguia et al. 2023). Clusters of mortality are being seen in seabirds, birds of prey, and scavengers (Ramey et al. 2022; Lane et al. 2023; Reischak et al. 2023). This subtype of the virus has been documented as a cause of mortality in both terrestrial mammals such as cats, racoons, bears, foxes, and mountain lions, and in marine mammals such as harbour seals in the Northern Hemisphere and more recently sea lions and elephant seals in South America (Chestakova et al. 2023; Leguia et al. 2023; Tammiranta et al. 2023; Ulloa et al. 2023; WOAH 2023). Sporadic influenza A(H5N1) clade 2.3.4.4b virus detections in humans have also been reported but remain very rare and are mostly linked to recent contact with infected poultry. However, infections in humans can cause severe disease with a high mortality rate. The overall case fatality rate of human infections with all clades of A(H5N1) from 2002 to 2023 is ∼50% (Charostad et al. 2023). The human cases of the A(H5N1) clade 2.3.4.4b detected thus far are mostly linked to close contact with infected birds and contaminated environments (CDC 2023).

Geographic isolation and strict biosecurity laws are barriers to this disease entering Aotearoa New Zealand (Rawdon et al. 2010; Greening et al. 2020). Although our biosecurity laws prevent the importation of live birds, illegal smuggling does occur, and thus represents a potential route for entry (Holden 1997). Migratory shorebirds and seabirds are also possible vectors of the virus into the country (Battley et al. 2011; Boulinier 2023). The most likely route of entry of the subtype into New Zealand is through the movement of these wild birds. In the past, the major risk of wild bird mediated transmission has been thought to be through migratory waders, such as godwits and red knots, who move between the global hemispheres on an annual basis (Battley et al. 2011, 2012). However, the transmission of A(H5N1) into South Georgia Island increases the likelihood that the subtype will become established in Southern Ocean seabirds that move circum-polar through the sub-Antarctic and Antarctic colonies and then possibly into New Zealand from the south.

Although low pathogenic strains of avian influenza are already present in New Zealand's wild waterfowl populations and no recent introductions have been detected (Watts et al. 2016), if the A(H5N1) strain enters Aotearoa New Zealand the consequences for wild bird populations and poultry flocks may be severe. New Zealand and its sub-Antarctic islands are globally important for its diversity of seabirds and the number of endemic critically endangered species (Towns et al. 2012; Munro and Burg 2017). To conserve these species in the wake of a HPAI incursion will likely require intensive and expensive mitigation measures. The Ministry for Primary Industries (MPI) has primary responsibility for exotic disease surveillance, investigation of unusual mortality clusters, and the control of exotic disease incursions in animals and plants in Aotearoa New Zealand. MPI has a long-standing annual avian influenza targeted surveillance programme where wild birds, particularly ducks, are sampled at key field sites and samples are then screened back at their animal health laboratory for avian influenza viruses (Watts et al. 2016). Bird mortality events are also routinely reported and investigated by MPI's incursion investigators.

Veterinarians in New Zealand could be the first people to recognise wild birds as potentially affected by HPAI and anyone seeing wild birds is a vital part of the surveillance network. In particular, veterinary hospitals should be extremely careful with the admission of seabirds, waterfowl, or waders that show neurological signs such as ataxia, torticollis, or hemiparesis, as these may be the first cases of A(H5N1) subtype to enter New Zealand and should be reported to the Biosecurity New Zealand Pest and Disease Hotline [phone]). Our recommendation is that any veterinary staff handling these groups of birds should wear personal protective equipment (PPE) including a face mask, eye protection, disposable gowns, and gloves. They should ideally admit the birds into an isolation facility away from the hospital for assessment. MPI should be contacted immediately, and veterinarians should follow their advice with regards to body disposal, storage, or sampling. It is recommended that any birds from these high-risk groups (seabirds, waders, and waterfowl) showing neurological signs should be humanely euthanised and the bodies double-bagged and clearly labelled.

If you are in any doubt about unusual mortalities in wildlife, you should ring the Biosecurity New Zealand Pest and Disease Hotline for advice. MPI define a mortality cluster in wildlife for these purposes as the death of more than three of the same species at the same place. MPI collaborate with the Department of Conservation (DOC) in wildlife investigations, and veterinary pathology laboratories may be asked to be involved depending on where the mortality occurs. In conjunction with MPI's surveillance programmes, passive surveillance of wildlife mortality is also co-ordinated by DOC who contract Massey University's Wildbase Pathology service to undertake post-mortem investigation of the deaths of threatened native wildlife. A One Health approach is being taken for HPAI readiness, with a shared HPAI work plan across MPI, DOC and the Ministry of Health (MOH). While MPI would be the lead organisation in any incursion response, support of other government agencies, veterinarians, universities, wildlife experts and the community would be crucial. Early detection, reporting, containment, and biosecurity measures will be important in trying to manage the disease. Based on prior experience overseas, it is unlikely the subtype will be able to be eradicated once it enters the wild bird populations. The focus, for now, is on detecting the arrival of HPAI in Aotearoa as rapidly as possible to maximise the chances of limiting its impact. As with other exotic diseases, veterinarians are an integral part of front-line surveillance and it is vital to identify suspect cases and report them as rapidly as possible.