HPAI H5N8 outbreaks in poultry farms have been reported in the Republic of Korea, Japan, China, Germany, United Kingdom and the Netherlands. EFSA has compiled a scientific report addressing the mode of entry of HPAI H5N8 into Europe and its potential further spread.
The first outbreak report in domestic ducks was in the Republic of Korea in January 2014. In Europe, the first affected holding was reported on the 4 November 2014 in the Mecklenburg-Vorpommern region of Germany in a turkey farm. HPAI H5N8 has subsequently been confirmed in a duck farm in East Riding of Yorkshire in the UK and in five poultry farms in the South Holland region of the Netherlands.
Both initial and subsequent cases have been analysed with the data available to determine if they are all as a result of primary introduction or other factors identified indicating secondary spread. The epidemiological situation and information available until 5 December 2014 was taken into account.
Migratory birds as one possible route of entry into Europe have been suggested based on the role of migratory birds in other avian influenza outbreaks. HPAI H5N8 has been detected in wild birds (common pochard (Aythya farina), tundra swan (Cygnus columbianus), great egret (Ardea alba), bean goose (Anser fabalis), Baikal teal (Anas formosa), coot (presumably Fulica atra), mallard (Anas platyrhynchos), white-fronted goose(Anser albifrons), common teal (Anas crecca) and spot-billed ducks (Anas poecilorhyncha) in Asia. There are no known direct bird migration routes from south east Asia to Western Europe. Although the movement of individual birds cannot be excluded, this is highly improbable. It has been hypothesised that long-distance transmission of HPAI viruses could occur as a result of cross-infection between different birds in north Eurasian breeding areas, where wild bird populations from different flyways overlap but this hypothesis needs further investigation.
Future analysis of HPAI H5N8 virus sequences from Asia and Europe will be valuable in exploring hypotheses on routes of spread. Furthermore the rates of virus evolution in different populations of both poultry and wild birds is uncertain. However historical data supports higher rates of virus evolution especially upon entry to domestic galliforme species.
Within the European wild bird populations, HPAI H5N8 has been confirmed so far in samples in one common teal in Germany and two faecal samples of Eurasian wigeons (Anas penelope) in the Netherlands. The teal was shot in Germany (Rügen island) and showed no indication that its health had been impaired. Field data and preliminary bird experiments indicate that HPAI H5N8 may have a relatively moderate pathogenicity for some wild bird species with limited mortality (<20%). Nevertheless, importantly in infected mallards HPAI H5N8 replicated efficiently and virus shedding was greater than HPAI H5N1 and at levels indicating it could be spread by contact.
If the virus is circulating in wild birds, the interface between wild birds and farmed poultry offers a pathway for introduction of the virus into poultry holdings. Spill-over events could also lead to the virus being transmitted from poultry to wild birds.
Investigations in the Netherlands using next-generation full genome sequencing with phylogenetic tree analysis suggested separate introductions into four holdings and one between-farm transmission. Close genetic homology among the viral genes of the HPAI H5N8 viruses detected in the United Kingdom, the Netherlands and Germany suggest they all share a common ancestor with the recent Japanese HPAI H5N8 viruses isolated from wild ducks, which is estimated to have occurred in June 2014. However reliable interpretation of the topology of the European and Japanese cluster cannot be made with these similar sequences. Phylogenetic analysis of other viral gene segments and importantly sequences from more viruses will help to resolve these relationships.
The outbreaks in poultry in Europe occurred in indoor facilities; therefore direct contact between wild water birds and the farmed birds in the affected holdings is unlikely. If the virus is circulating in wild waterbird populations contamination of environmental surfaces by faeces and contamination of standing waters through viral shedding might occur. However, data regarding the virus load in the environment due to viral shedding from wild birds are currently lacking. Studies however have demonstrated that at 4oC virus can survive for several weeks in water and may therefore be a clue to fomite transmission from contaminated environments into housed poultry holdings where biosecurity practices may not be robust.
Given the apparent presence of HPAI H5N8 within some wild bird populations in Europe and the occurrence of HPAI H5N8 infection in several poultry holdings, it is more plausible that indirect introduction of HPAI H5N8 to poultry holdings via humans, vehicles, equipment, fomites, live animals and/or animal-derived products contaminated with virus (for instance in faeces) of infected birds took place.
An appropriate biosecurity system is required to prevent virus entry into and virus leaving from a holding since non-avian bridge species such as mice, cats, foxes, rats, dogs and mustelids may act at least as mechanical vectors. The biosecurity systems should also take into account streams of fomites, waste products and water leaving the holding to contain the virus in affected holdings. Detailed epidemiological investigations of the affected European farms and a detailed assessment of all transmission routes that might transport HPAI viruses from south-east Asia to the EU should be continued in order to identify the risk of HPAI introduction into Europe and into European poultry holdings.
Assessing biosecurity procedures at farm and area level with a focus on segregation, cleaning and disinfection, and improving where necessary, is recommended in high risk areas. The probability of introduction and spread of the HPAI H5N8 via contaminated humans, vehicles, equipment, fomites, live animals and/or animal-derived products is dependent on several factors like the prevalence of the virus, stability of the virus under the conditions prevailing at the time and characteristics of the fomite (e.g. water content of the material).
Knowledge of the prevalence and pathogenesis of HPAI H5N8 infection in wild bird populations is required in order to better understand the risk of transmission to poultry, which is important in the design of risk management strategies. Given the apparent low pathogenicity of HPAI H5N8 for several wild bird species, focussed strategic and proportionate enhancement of active (targeted) and passive (scanning) surveillance of both living and dead wild birds in the high risk areas is recommended.