Over the past year, while North America was struggling with its worst avian flu outbreak in history, parts of the EU – including the UK – were dealing with outbreaks of avian flu as well. The appearance of HPAI H5N8 in Germany, the Netherlands, the UK, and Italy made the biggest headlines, but several `lesser’ LPAI & HPAI H7 outbreaks also occurred.
Until the emergence of H7N9 in China in 2013 – and the resultant infections (and in some cases, deaths) of hundreds of people – H7 viruses as a group were considered to pose a fairly low threat to public health.
So far - except for China’s H7N9 virus - nearly all reported H7 infections have been mild, often producing little more than conjunctivitis (see ECDC Update & Assessment: Human Infection By Avian H7N7 In Italy).
H7 viruses are considered a serious threat to the poultry industry, and have caused hundred of millions of dollars in losses over the years.
There are two broad categories of avian influenza; LPAI (Low Pathogenic Avian Influenza) and HPAI (Highly Pathogenic Avian Influenza).
- LPAI viruses are quite common in wild birds, cause little illness, and only rarely death. They are not considered to be a serious threat to public health, although H5 & H7 strains have the potential to mutate into HPAI strains.
- HPAI viruses are more dangerous, can produce high morbidity and mortality in wild birds and poultry, and can sometimes infect humans with serious result. The number of HPAI viruses that scientists have been tracking has increased markedly over the past 3 years, and now includes H5N1, H5N2, H5N6, H5N8,H10N8, and others.
Before the middle of the last decade, there was no uniform requirement to report or track LPAI infections in poultry. That changed in 2006 when the OIE made reporting of LPAI H5 & H7 viruses mandatory. The concern with these LPAI H5 and H7 viruses is that when they are not controlled - they have the potential to mutate into highly pathogenic strains.
HPAI viruses have been generated in the laboratory by repeated passage of LPAI viruses through chickens (cite FAO) but exactly how and why this occurs naturally is poorly understood (see JVI Emergence of a Highly Pathogenic Avian Influenza Virus from a Low Pathogenic Progenitor).
While a serious concern, in practice this has been a fairly rare phenomenon, and according to Defra:
In the last 10 years, there have been only four such documented events of mutation from LPAI to HPAI occurring all within chicken layers in Europe: UK in 2008, Spain in 2009 (SCoFCAH, 2010), Italy in 2013 and UK in 2015; all involved H7N7.
The `UK in 2015’ reference is to the Lancashire H7N7 outbreak of last July (see Defra: Lancashire Avian Flu Confirmed As HPAI H7N7) .Today we have published a 37-page Epi report by the UK’s Animal & Plant Health Agency - and while there are still some areas of uncertainty - it provides us the most detailed look to date at that event.
PDF, 1.84MB, 37 pages
1. Executive Summary
Description of premises: The Infected Premises (IP - designated as AIV 2015/02) is owned by a family-run business based in the administrative territory of Lancashire, England. The company owns seven linked premises, two of which rear pullets to supply the five other commercial laying premises, including the IP, which produce table eggs for human consumption.
Description of the virus: HPAI H7N7 was confirmed as the outbreak’s causative agent on 13 July 2015. Laboratory results indicate that the incident is likely to have resulted from an incursion of LPAI virus that mutated to HPAI virus within the IP. The H7N7 virus is closely related to contemporaneously circulating strains in wild birds and poultry in Northern Europe. However the virus has probably derived from genetic reassortment in nature of two or more progenitor strains. This is a different strain to that seen in the LPAI outbreak in broiler breeders in Hampshire, England (AIV 2015/01 - February, 2015).
Source and spread windows: The most likely time that LPAI infection is estimated to have entered the IP is between the end of May 2015 and mid-June 2015. The high risk spread window for LPAI virus opened on 19/06/2015,, The mutation event from LPAI to HPAI is likely to have taken place at the end of June 2015 (most likely on 29 or 30 June 2015), with the spread window for HPAI virus extending until the completion of statutory preliminary cleansing and disinfection of the infected premises on 16 July 2015 following the sanitary slaughter of the birds.
Hypothesis for the source: There is uncertainty as to the most likely source of LPAI infection for the IP. However, the available evidence suggests that the source was the wildfowl present on the ponds on the premises, followed by an incursion into one group of free-range birds as a result of indirect contact, with subsequent spread to other epidemiological groups. In the case of the HPAI infection the evidence strongly suggests an initial mutation event in one of the sheds.
Evidence base for the source: This assessment of the source is based on the evidence that (i) no poultry were brought on to the premises in the source window, (ii) there are no relevant industry related national or international source tracings, (iii) the presence of wild waterfowl close to the first shed which could have been infected, (iv) production records, and (v) strong laboratory evidence based on genetic analysis of the virus which indicate a recent introduction from wild birds to domestic poultry.
Assessment of potential spread: Following extensive investigations, no evidence of avian influenza virus infection has been found in other domestic poultry premises in the country. At this time the outbreak appears to have arisen as the result of an LPAI to HPAI mutation event on the IP and to be limited to the single IP.
14. Remaining uncertainty
- The source of the progenitor LPAI virus.
- The route of introduction onto the IP.
- The earliest likely date that LPAI was introduced onto the IP.
- The precise date that the mutation event from LPAI to HPAI occurred.
There is a continually present, global risk of further outbreaks of avian influenza as a result of the ongoing presence of AI viruses within the wild bird population. There is ongoing AI surveillance (both active and passive) in the UK aimed at early detection of such an incursion
15. Concluding remarks
The most likely source of infection is the introduction of LPAI virus from wildfowl present on the ponds on the infected premises. Genetic analysis of the HPAI virus identified on the holding and other epidemiological/laboratory data gathered from all groups on site indicate that this originated as the result of a mutation from LPAI virus that occurred within the IP. This mutation has made this a complex investigation.
Investigation of tracings from other premises identified as potential sources, via tracings of personnel and vehicle movement, have revealed no other premises that could have been the origin of the HPAI infection on this premises. Investigations of similar spread tracings have not revealed any spread of HPAI virus from the IP to other premises.
Although our investigations suggest that the most likely route of introduction of virus onto this infected premises was direct or indirect contact with wild waterfowl, an incursion such as this remains a low likelihood event.
National Emergency Epidemiology Group
28 August 2015
While we will no doubt be concentrating primarily on the spread of HPAI H5 viruses this fall and winter, the experience with China’s H7N9 virus shows that H5 viruses are not the only ones worthy of our attention, and reminds us how quickly the status quo can change.