Pox viruses that infect birds all belong to the genus Avipoxvirus.
Avian pox viruses have been detected in hundreds of different species of birds in Europe, from songbirds to storks and from owls to pigeons. Some of the best described avipoxviruses include Pigeonpox virus, Quailpox virus, Canarypox virus, Sparrowpox virus and Fowlpox virus, but many more have not been fully characterized. Whilst some of these show species-specificity many can infect different species and induce cross-protective immunity to infection with the other types.
Different strains of avipox viruses vary in their ability to cause disease and clinical signs can vary between species.
The classic pox lesion, nodular, wart-like swellings in the skin, is caused by hyperplasia and hypetrophy (increased numbers and size) of the cells in the upper layer (epidermis) of the skin.
Some of the least pathogenic avipox viruses may only cause lesions locally at the site where the virus was introduced to the skin. In these cases, ‘dry pox’ may be seen and is characterized by raised white nodules that may coalesce over the course of a few weeks, becoming scabbed before the skin underneath heals. In some cases nodules around the eyes or beak may impair eating or increase the risk of predation but generally birds recover from this form of the virus.
In more severe cases, and possibly depending on the site of entry of the virus, lesions of ‘wet pox’ may develop; these include ulcerated plaques in the mouth, trachea (wind pipe) and oesophagus. The site of these lesions can mean that birds struggle to feed and swallow leading to loss of condition; secondary bacterial infection of the damaged tissues is also common.
Canaries are prone to developing a more serious septicemic form of the disease in which the virus spreads around the body, also affecting the lungs however, this form of the disease has rarely been reported in wild birds.
Amongst wild birds the main mode of transmission is thought to be via the bite of an insect that had previously fed on an infected individual. Direct contact with infected birds also plays a role with transmission occurring when the virus comes into contact with damaged skin or mucosal membranes. As the virus is relatively hardy it can survive in the environment; this means that feather dust from infected birds that comes into contact with damaged skin may also transmit infection. Certain conditions in which bird populations or stocking density are high, for example, at garden feeders, bird tables and in bird rearing facilities, are particularly conducive to virus transmission.
Cases of avian pox virus infection have been reported from across the world with the exception of in the Arctic and Antarctic. It is more common in warm areas with water sources that provide breeding grounds for the biting flies that serve as vectors for the virus. Species from remote Islands that have only recently been exposed to the virus (e.g. canaries) appear to be more likely to develop serious disease.
Eradication or control of disease in wild populations is generally very difficult to achieve. Furthermore, certain diseases may play an important role in population dynamics.
As feeding wild birds can impact on the health of local populations, it is advisable to observe some standard hygiene precautions when feeding birds in your garden.
- Clean and disinfect feeders and bird tables regularly with a product such as dilute household bleach (5% Sodium hypochlorite); rinse them thoroughly and allow to dry before putting out new food.
- Reposition feeders regularly to avoid dropped food and feces accumulating in one spot.
- Rinse out bird baths daily and allow to dry before re-filling.
- During an outbreak feeding should be reduced stopped during 2-4 weeks.
