Monitoring of the population of great tits at Wytham wood began in 1947 and is ongoing. In 2010 the first birds infected with the virus were seen, with characteristic signs of nodules on skin around the eyes and beak.
The Zoolological Society of London's Becki Lawson explained that the current infection is more severe in great tits which have particularly large nodules around the beak and eyes.
The large nodules would affect the birds’ vision and ability to feed, and make them vulnerable to predators.
Currently the infection is also affecting wild birds in Surrey, Kent, Sussex and Staffordshire. Sporadic cases of infected dunnocks and wood pigeons were reported from Staffordshire in 2005, but the nodules in many of the birds were not serious and some appeared to heal.
The RSPB is monitoring the spread of Avian pox in the UK and are asking people to report any infected birds seen in their gardens. A recording form is available from the RSPB website.
There are two types of infection caused by the avian pox virus. The wet diphtheric form involves the mucous membranes lining the oral cavity and trachea. The lesions are so closely adhered to the mucus membranes that if a lesion is removed a wet eroded area of epithelium is exposed.
The infected wild birds seen recently in Wytham Woods seem to have the dry form of the disease where lesions are seen on the skin.
The first sign of infection is a blister on an area of bare skin such as the face, legs and feet, the borders of the eyes and beak. As the blisters grow and multiply, they may burst discharging watery fluid loaded with virus particles.
The blisters form the nodules that we see in the photographs in the news reports. Nevertheless infected birds appear to move round and feed normally despite the nodules.
Eventually the nodules become dry and darken in colour to grey or brown, and can become infected by bacteria, when they form a scab. After the scab falls off, a lesion may heal and some infected birds may even recover from the virus infection.
However, live virus can persist for a long time in the sloughed-off dry scabs.
They disintegrate into smaller flakes and particles, which can be inhaled or contaminate the food and water ingested by birds.
The virus can also remain on surfaces such as bird tables, branches, twigs, and can be blown about with dust particles. So there are various routes by which virus in the fragments of scabs can infect birds.
The virus is also transmitted by mosquitoes when they inject saliva into an avian host; virus can also spread by transmission from infected to healthy birds. So the infection once established can persist for a long time in a community of wild birds.
There is much research investigating the avian pox virus, Avopoxivirus, which is most common in warm and temperate climates.
Avopoxivirus comprises a group of similar virus species that cause pox infections in various groups of birds. The avian virus group includes include fowlpox virus, canarypox virus, sparrowpox virus as well as other species that can infect different groups of birds.
Each virus 'species' infects a specific group of birds, but within a species there may be different strains or variants. Avian pox virus does not infect mammals or other vertebrate groups.
Many bird species are susceptible to avian pox, including song birds, upland game birds and raptors. Further afield, albatrosses, and bald eagle populations have suffered significant mortality linked to infection with avian pox.
Pet birds and poultry, such as chickens or turkeys, can be infected with the virus. Domestic birds can be vaccinated against the infection - but this is not an option for wild birds.
Molecular biologists are studying the DNA collected from the virus particles in lesions from sick birds in order to find out if the great tits in Wytham Wood and elsewhere are being infected by a strain of the virus new to the UK that may have originated in mainland Europe.
What can I do to help?
In the meantime we can only monitor the spread of this infection and ensure that garden bird feeding stations are kept clean and disinfected.
Feed enough food for a day – if food is left over for a day or two then reduce the amount of food to prevent accumulation of rotting food that could become contaminated by virus.
Bird baths need to be emptied daily, cleaned and filled with fresh water.
The RSPB is monitoring the spread of avian pox in the UK and are asking people to report any infected birds seen in their gardens.
More advice on how to help, and a recording form is available from the RSPB website.
So if you see any birds that have avian pox do download the form and record your sighting/s and return it by e-mail it to the RSPB.
You can also send your completed form by post to: Wildlife Enquiries, RSPB, The Lodge, Sandy, Beds, SG19 2DL
Its well worth exploring the RSPB website as there is a lot of good advice there about maintaining garden bird feeding stations in a hygienic condition.