Dead wild swans in the county of Dorset in south-west England first confirmed with H5N1 on 10 January may hold the key to an unexplained outbreak of the disease in poultry on the other side of the country (East Anglia) two months previously.

In November 2007, H5N1 virus (the strain of highly pathogenic avian influenza, HPAI) was confirmed at Redgrave Park Farm on the Suffolk/Norfolk border resulting in eventual loss of some 100,000 poultry (mostly turkeys) across five farms in the run-up to Christmas. Infection was controlled and restrictions lifted less than one month ago but the source of the outbreak was never identified. All potential poultry industry sources including duck chicks from the Netherlands and contaminated vehicles from Germany and Czech Republic were eliminated. Genetic sequencing of the virus showed close matching with the strain identified in Czech Republic and Germany in summer 2007, according to Fred Landeg, acting Chief Vet.

Wild birds may have infected Redgrave poultry

Wild birds were the only reasonable alternative. On-going routine testing around the affected farms in East Anglia was intensified but no H5N1 infected birds were identified, leaving worried farmers and a huge question mark over original source of the infection

Three dead swans subsequently confirmed with H5N1 virus were found at Abbotsbury Swannery, an open reserve in Dorset on 27 December 2007. This was during routine testing by Veterinary Laboratories Agency (VLA) as part of the Department of Environment, Food and Rural Affairs (DEFRA) routine surveillance programme. The Abbotsbury Swannery is a reserve for free-flying swans and wild birds, and is part of an internationally important wetland.

Control and Monitoring areas were promptly established around the infected site. The wild bird Control area extends 24km to the south-east, and a larger Monitoring Area extends 32km. Selected shape of the control and monitoring areas is based on expert ornithological advice given to DEFRA

All captive birds including poultry within the area must be housed or otherwise isolated from contact with wild birds. There is restriction on the movement of poultry, hatching of eggs except under licence, as well as increased biosecurity measures for premises, people and vehicles. Movement of birds is restricted. All functions that involve gatherings of birds such as poultry shows and pigeon racing are banned, as is game bird shooting. Animal health officials are inspecting poultry flocks and farms in the affected area.

John Houston, general manager at Abbotsbury Tourism Ltd, told The Press Association that affected birds were found by a member of staff at Abbotsbury Swannery, and DEFRA notified as part of standard procedure. “Our main concern is the welfare of the swans, our staff and the general public,” he said. “We are working closely with DEFRA to ensure that this outbreak is contained and that the number of swans affected is limited. DEFRA said there were no plans to cull wild flocks as this may disperse [potentially infected] birds.

Acting chief vet, Mr Landeg said, “While this is obviously unwelcome news, we have always said that Britain is at a constant low level of risk of introduction of avian influenza. Our message to all bird keepers, particularly those in the area, is that they must be vigilant, report any signs of disease immediately, and practice the highest levels of biosecurity.”

A further six dead birds found on 31 December and 4 January were sent for testing at VLA, although John Houston was optimistic. “That [number of deaths] is still much lower than the usual mortality rate for January but we are still sending them off for testing. It is not unusual for birds to die in the winter of natural causes. In fact, there are less dying at the moment than usual because it is quite warm. Until I hear otherwise I'm going to assume the best.”

The first two birds had tested negative but the third was positive, creating concern and calls to government to start vaccinating free-range poultry.

Migratory birds not directly to blame

Ornithologists are playing a vital advisory role because the situation is not as simple as it seems. Many experts claim wild migratory birds as probable source of infection but Andre Farrar of Royal Society for Protection of Birds (RSPB) commented that these infections could be considered unusual since very few birds are on the move at this time of the year. “It is slightly odd that it’s happened at this time of year, Mr Farrar told The Guardian. “It’s not a migration period.” He went on to suggest the virus could have been brought into south-west England some time ago but infected the birds only recently.

Colin Butter, an avian flu expert at the Institute for Animal Health – also talking to The Guardian – claimed H5N1-infected swans in Dorset are consistent with the outbreak at Redgrave Park Farm in November 2007. DEFRA investigations into the Redgrave outbreaks suggest that diseased free-range turkeys had been in close contact with infected migratory wildfowl from a nearby lake. No proof has been established.  

Here pops up another question mark because the dead infected swans at Abbotsbury Swannery are part of a managed flock of mute swans (Cygnus olor). These mute swans are resident wild birds and largely sedentary: they must have been infected by migratory wildfowl species coming into the area. Whether or not they were the same migratory birds infecting Suffolk turkeys in November 2007, the disease appears to have been transmitted across southern England – probably infecting other flocks on the way.

H5N1 has not been picked up during testing in Suffolk and Norfolk before during and after the Redgrave outbreak in November 2007. Britain’s only other wild bird ever confirmed with H5N1 was a whooper swan at Cellardyke, Fife in Scotland in April 2006. It was eventually decided the bird had been infected in another country, died at sea and subsequently washed up on the coast of Scotland.

Two other species of wild swan are found in the UK. The whooper swan (Cygnus cygnus) and Bewick’s swan (Cygnus columbianus bewickii) are both winter migrants, entering the UK in large numbers from more northerly locations in October. A small number of each species is usually recorded each year at the Abbotsbury Swannery.

Around 8000 Bewick’s swans fly some 3500km from breeding grounds in western Siberia (Russia) to spend the winter in Britain. Some 9000 whooper swans migrate to Britain from their breeding grounds in Iceland every winter. DEFRA is testing all the swans in the flock of around one thousand at Abbotsbury Swannery.

Winter migrant swans generally mix and mingle freely with resident mute swans and are an obvious candidate for disease introduction and spread. Wild swans succumb rapidly to H5N1 so whilst it is a possibility, there is some doubt as to whether H5N1-infected birds could make it all the way from East Anglia to Dorset. There are all sorts of other possibilities with H5N1 passed across England by wild birds in relay.

Virus carried by wild bird relay?

A report by The Times claimed the ‘Abbotsbury’ strain – like the ‘Redgrave’ strain – closely matched the ‘Czech Republic’ strain affecting wild birds and poultry in summer 2007. The story ran under the misleading headline ‘Bird flu swans fly in from Europe’. The report failed to make clear that mute swans at Abbotsbury generally remain there and certainly do not fly in from Europe. According to the article, experts say the first dead bird was discovered at Abbotsbury as long ago as 27 December, which means that an infected bird most likely arrived during the cold weather on the European continent just before Christmas. They described mallard, teal, widgeon, pochard and gulls as birds that flee westwards to Britain during such cold weather. 

More logical and likely explanation is that first infected bird(s) flew into Britain in the autumn, causing the outbreak at Redgrave Park Farm in Suffolk during November. The virus then found its way on the same bird(s) or a relay of birds across southern England into Dorset. Chances are infection was deposited along the way and is therefore present in wild bird populations along the route taken.