Avian pox or fowl pox is a pox-like disease of poultry, ornamental and wild birds caused by members of the genus Avipoxvirus (subfamily Chordopoxvirinae). Avipoxviruses are enveloped viruses with a linear, double-stranded DNA genome with a size of 300kbp. They are distinguished from other poxviruses by their brick-like shape (330x280x200nm). The viral envelope consists of an outer envelope and an inner envelope with irregularly arranged surface tubules incorporating viral and cellular lipoproteins.
Avian poxviruses (avipoxviruses) and diseases have been recorded in a variety of avian species worldwide. They are named after the host animal of their initial discovery, e.g. Fowlpox virus (chicken pox virus), Pigeonpox virus (pigeon pox virus), Sparrowpox virus (sparrow pox virus) etc. This does not mean that they can only infect the avian species whose name they bear. On the contrary, host specificity is not very high, the individual representatives also pass to heterologous hosts. For example, fowlpox virus has also been detected in sparrows. The canarypox virus differs immunologically from the other representatives, which is also reflected in the course of the disease. Environmental factors, immune status and route of infection play an important role in avipox virus infection. The duration of the disease is three to four weeks, unless complications occur. In general, a mortality rate of 50 % can be assumed for commercial poultry, pigeons and parrots. In canaries or other finches this rate is 100 %. In principle, birds of all ages are susceptible to infection with avipoxviruses. The infection is seasonal, mainly in the cooler seasons. A secular rhythm can often be identified, i.e. there may be phases between clustered infection events in which smallpox infections occur only sporadically. While sporadic outbreaks tend to occur in wild populations, smallpox infections in domesticated species are often regionally enzootic. The greatest risk here is the introduction of the pathogen through the purchase of poultry. If no suitable quarantine period is observed, virus can be transmitted directly from infected to healthy animals via the smallest skin and mucous membrane injuries. Indirect spread via pathogens in the inanimate environment - in feed, in drinking water, on contaminated objects, in dust, adhering to aerosols - is possible. Indirect virus transmission can also occur via arthropods, especially the red feather mite(Dermanyssus gallinae), but also via other mites, flies and feather mites. In heterologous hosts, transmission usually occurs primarily via insect bites or mites. Thus, avian pox can be transmitted to poultry birds via feather parasites from diseased wild birds, e.g. from sparrows. Avipox viruses are labile against fat solvents, ether, chloroform and other disinfectants; in skin flakes and feather dust, smallpox viruses are infectious for several months.
Infection may take the form of local infection or generalization. In local infection, only minor local lesions occur and the disease heals. Generalization, on the other hand, is characterized by a cyclic course of the disease; primary viral replication at the entry site is followed by a weak viremia. During this phase, the viruses reach the primary affine organs (liver, bone marrow), where further viral replication occurs, resulting in secondary viremia. The viruses then reach the target organs skin and mucosa. The incubation period is between 4 and 14 days.
In commercial poultry, a distinction is made between three forms of appearance:
(a) the skin form: it is characterized by characteristic skin lesions with crusting, especially on unfeathered parts of the body such as the rack, cloacal opening, comb, ear, wattles, beak region and eyelid. Rarely - in severe cases - skin lesions are also found on feathered areas. Initially, small reddish patches develop into yellowish nodules with a rough surface (papular stage). Large papules with a fissured surface then develop, which subsequently form brown-black crusts. Bloody pustules and papules are the starting point for secondary infections. Smallpox crusts heal after 3 to 4 weeks without scarring.
b) The mucous membrane form (diphtheroid form): It is characterized by proliferative changes and yellowish membranous coatings in the beak cavity, on the tongue, in the palate of the pharyngeal cavity and in the larynx. The mucosa is first severely reddened, then yellowish white; secondary bacterial infections may follow. Feed intake is severely impaired, resulting in reduced laying performance and retarded growth.
c) the mixed form (combination of skin and mucous membrane form): this is the most common form in commercial poultry.
In ornamental birds (e.g. parrots), wild birds, birds of prey and carrier pigeons, all forms occur as in commercial poultry. For pigeons, finches and other wild birds, a tumorous form is additionally described, in which benign skin tumours occur after a mildly progressing skin form. In pigeons, so-called "blood ulcers" (symmetrically occurring bilateral cherry-sized formations) may occur. In addition to the forms described above, canaries and other ornamental birds also show a peracute (septicaemic-toxic) form, in which only non-specific general symptoms without smallpox exanthema are observed (risk of infection!). In the lethal pulmonary form, the animals develop life-threatening respiratory distress due to bronchopneumonia, which leads to death after only a few days. The mortality can be 50 % in commercial poultry, pigeons and parrots, and up to 100 % in canaries and finches.
Differential diagnoses include:
in skin florescences:
- Papillomavirus infections,
- bacterial skin diseases
- fungal skin infection (Trichophyton sp.)
- the cutaneous form of Marek's disease (Marek's disease)
- mite infestation ( Knemidocoptes sp. infestation)
- biotin or vitamin A deficiency
in case of mucosal deformations:
- Infectious laryngotracheitis (ILT, causative agent: Gallid herpesvirus 1)
- Psittacosis/ornithosis (pathogen: Chlamydia psittaci)
- Coryza contagiosa gallinarum (contagious chicken cold, pathogen: Haemophilus paragallinarum)
- Pharyngeal trichomoniasis
- Infectious bronchitis (pathogen: coronavirus)
- Fungal diseases (candidiasis, aspergillosis)
Control focuses on prophylactic vaccination with live attenuated vaccines and improvements in husbandry conditions, especially hygiene conditions. Pet birds should be vaccinated, at least in larger flocks, for protection against canary pox. The vaccine can be applied via water, the follicle method (application of the vaccine to feather follicles) or the wing-web method (insertion of the vaccine needle into the wing skin).Special treatment of sick birds is not possible. Diseased individual birds are to be isolated for the protection of the other birds. In commercial poultry, permanent removal of diseased birds is preferable. Newly purchased birds should be kept isolated from the other birds under observation for some time. The houses, aviaries and (house) utensils must be cleaned and disinfected after the culling of infected birds. A waiting period between culling and re-housing is recommended due to the long survival time of the pathogens in the environment.