The virus can be transmitted through direct contact with infected animals via rashes, pustular fluids, secretions from the eye and nose. Direct transmission of the virus may also occur via arthropods, in particular the red mite (Dermanyssus gallinae), but also through other mites, flies and bird ectoparasites. Additionally, dust, aerosols, watering stations and feed help spread the infection. Furthermore, the virus can also be transmitted through wild birds. Thus, Fowlpox can be transmitted from sparrows to poultry via ectoparasites. Avipox viruses are susceptible to degreasing agents, ether, chloroform and other disinfectants. Pox viruses can remain infectious for several months in skin scales and feather dust.
The infection can be local or general in nature. Only small, localised lesions appear during a local infection and the disease will subside. A general infection, on the other hand, has a cyclical progression: primary virus reproduction on the entry point is followed by mild viremia. During this stage, the virus makes its way to the primary organs (liver, bone marrow) where it reproduces again, resulting into a second viremia. Then, the virus reaches its target organs, the skin and mucous membranes. The incubation period is between 4 to 14 days.
The following three forms occur in commercially kept poultry:
- the cutaneous (dry) form features characteristic skin lesions that form scabs, especially on unfeathered skin, such as feet, cloaca, comb, ear, wattles, around the beak and the eyelids. In, severe, rare cases, lesions can also be found on feathered skin. The first visible symptom will be small red spots, which gradually develop into yellowish wart-like growths with a rough surface (papular stage). This is followed by the development of extensive papules with crusty surfaces, which then form brown-black scabs (see Image 1). Bloody pustules and papules are the sources of secondary infections. The pox scabs will heal after three to four weeks without leaving scars.
- the diphtheritic (wet) form is characterised by proliferative changes and yellowish, membranous growths forming in the beak, on the tongue, the palate, and the throat. The mucosa appears very red at first and later yellowish-white. This can be followed by bacterial secondary infections. Eating is heavily affected, which will result in poorer egg-laying and delayed growth.
- the mixed form (= combination of cutaneous and wet form) is the most common form in poultry.
All these forms can also be found in exotic birds (e.g. parrots), wild birds and raptors, as well as carrier pigeons. Additionally, pigeons feature so-called blood blisters (cherry-sized blisters appearing on both sides symmetrically). There is an additional tumorous form for pigeons and finches. Canaries exhibit a peracute (=septicaemic-toxic) form, in addition to the forms described above, which only shows general symptoms with infected animals dying after three days, as well as the fatal lung form (agonal respiration) and the non-lethal, but highly dangerous, asymptomatic form (risk of infection!).
The mortality rate in poultry, pigeons and parrots can be around 50 %. In canaries and finches it is up to a 100 %.
Skin lesion diseases such as papilloma virus infections, bacterial skin conditions, injuries, the cutaneous form of Marek’s Disease and knemidocoptaiasis may also come into consideration from a differential diagnostic perspective; psittacosis/ornithosis, coryza, bronchitis and fungal infections should there be deformations of the mucous membrane ILT.
Combating Avian Pox
The fight against Avian pox focuses on preventative vaccination with attenuated live vaccines and the improvement of farm conditions, primarily hygiene standards. The specific treatment of infected birds is not possible. Individual affected animals must be isolated to protect other animals. In poultry, infected animals should preferably be removed permanently. Stalls and aviaries must be cleaned.