Signalment (JPC# 2070431): Chicken, breed and age unspecified

HISTORY: This bird had multiple raised proliferative lesions over its body.

HISTOPATHOLOGIC DESCRIPTION: Feathered skin: Multifocally, the epidermal and follicular epithelium are hyperplastic up to ten times normal thickness, characterized by a markedly thickened stratum spinosum (acanthosis) with long rete ridges, and multifocally overlain by a thick serocellular crust. Within the stratum spinosum, the majority of keratinocytes are markedly swollen with increased clear space (ballooning degeneration) and contain a 15-30µm eosinophilic intracytoplasmic viral inclusion body (Bollinger body) that often peripheralizes the nucleus. Multifocally there are large areas of coagulative necrosis within the epidermis that extend into the dermis and are characterized by loss of differential staining with retention of tissue architecture, admixed with foci of lytic necrosis characterized by partial to full thickness epithelial loss (erosion and ulceration) with replacement by karyorrhectic and cellular debris. Within the areas of necrosis and infiltrating into the dermis and subcutis are numerous viable and degenerate heterophils with macrophages, lymphocytes, and plasma cells admixed with abundant fibrin, hemorrhage, edema, and few colonies of 1x1µm cocci. Similar inflammatory cells infiltrate into and surround portions of the panniculus carnosus where the muscle fibers have the following changes: are pale, swollen with a vacuolated sarcoplasm, or are brightly eosinophilic, shrunken, loss of cross striations, and a pyknotic nuclei. Small caliber blood vessels within the superficial dermis are lined by markedly hypertrophied endothelium.

MORPHOLOGIC DIAGNOSIS: Feathered skin and subcutis: Dermatitis, necrotizing and proliferative, subacute, multifocal, marked, with ballooning degeneration and eosinophilic intracytoplasmic viral inclusion bodies, breed unspecified, chicken.

ETIOLOGICAL DIAGNOSIS: Avipoxviral dermatitis

CAUSE: Avian poxvirus

CONDITION: Fowl pox

GENERAL:

Poxviruses: Largest animal virus (300-400 nm); double stranded brick-shaped DNA virus; cytoplasmic DNA synthesis and packaging within infectious virus particle are characteristic of pox virus
Avian poxvirus: Genus Avipoxvirus; family Poxviridae

Common, slow spreading viral disease of domestic, pet and wild birds; reported in 232 species. Transmission via inoculation from direct contact with injured skin, through mechanical transfer (including cannibalism and artificial insemination), mosquitoes, blood-sucking arthropods, and inhalation of aerosolized feathers and scabs containing virus
Two forms (can manifest as one or both):
- Cutaneous form (dry pox): Most common form
- Diphtheritic form (wet pox): Lesions in respiratory, GI, nasal cavity and conjunctiva

PATHOGENESIS:

Virus enters host cells by fusion with plasma membrane and endosomal uptake
Poxviruses encode several proteins that affect host cell: e.g. vaccinia virus growth factor (VGF) uses cell receptors for epidermal growth factor (EGF) to stimulate proliferation of keratinocytes
- Increasing metabolism of keratinocytes surrounding infection causes greater susceptibility of those cells
Other proteins inhibit complement-mediated cell lysis and inhibit inflammatory response to retard immune clearance and analogues of tumor necrosis factor and superoxide dismutase

TYPICAL CLINICAL FINDINGS:

Gradual onset that may go undetected until cutaneous lesions are numerous
Turkey infection is generally more chronic than chickens
Canaries can have systemic infection with high mortality, up to 100% in susceptible flocks

Cutaneous form:

Mild reduction in weight gain
Decreased egg production
Lack of flock vigor
Mortality is low

Diphtheritic form:

Lesions in upper respiratory tract or GI tract may lead to dyspnea or inappetence
Lesions in nasal cavity or conjunctiva lead to nasal or ocular discharge
Mortality due to suffocation, starvation, and dehydration

TYPICAL GROSS FINDINGS:

Cutaneous form:

Papular lesions predominantly on unfeathered skin of the head, neck, vent, legs, and feet
Papules, vesicles, pustules, crusts which progress to terminal reddish-brown to black scabs; may appear as horny growths
Pigmented skin will usually be discolored after infection due to epidermal damage

Diphtheritic form:

Raised, white/opaque to yellow plaques or nodules on mucous membranes; predominate in mouth but also in sinuses, nasal cavity, conjunctiva, pharynx, larynx, trachea, esophagus

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Eosinophilic intracytoplasmic inclusion bodies (Bollinger bodies) located within epithelial cells - pathognomonic
Epithelial hyperplasia with ballooning (hydropic) degeneration

ULTRASTRUCTURAL FINDINGS:

Elementary bodies (Borrel bodies) measure 250 x 354nm and are located within the inclusion bodies (Bollinger bodies)
Virus is brick shaped and consists of an electron-dense biconcave core with two lateral bodies within each concavity

ADDITIONAL DIAGNOSTIC TESTS:

Skin scraping can demonstrate squamous epithelium with ballooning degeneration and Bollinger body
ELISA
Virus Isolation
EM

DIFFERENTIAL DIAGNOSIS:

Cutaneous pox:

Trichophyton megninii and T. simii (I-F11): Dermatophytic fungus
Knemidokoptes gallinae: Mite in basal shafts of feathers
Knemidokoptes mutans (I-P12): Mite that lives within unfeathered skin
Bumblefoot

Diphtheritic pox:

Vitamin A deficiency (D-M09): Pustules on the mucosa of the mouth, pharynx, esophagus and crop in young birds, inflamed eyelids; squamous metaplasia of the nasal mucosa; keratinization of intestinal enterocytes, decreased goblet cells and blunting of villi (severe deficiency)
Infectious laryngotracheitis (P-V11): Alphaherpesvirinae (Gallid herpesvirus 1); mucohemorrhagic or caseous exudates of the trachea; intranuclear inclusion bodies in epithelial cells
Trichomonas gallinae: Raised, caseous lesions in the mouth, pharynx, esophagus, and crop
Capillaria annulata or contorta: Thickened, inflamed to sloughed mucosa
Candida albicans (D-F05): White-grey pseudomembranous patches on the mouth, pharynx, esophagus and crop; pseudohyphae and budding yeast
Aspergillosis (P-F06): Yellow-grey nodules in trachea, lungs and airsacs

COMPARATIVE PATHOLOGY:

Avian poxviruses:

Avipoxvirus is recognized in several domestic and wildlife avian species with similar lesions (e.g. turkeypox virus, falconpox virus, psittacinepox virus)
Some avipoxviruses may have oncogenic properties; Passeriformes and Columbiformes may be predisposed to tumor formation after surviving infection
Canarypox usually has a desquamative pneumonia
Wildlife and Zoo: North Island brown kiwi, penguins, pelicans, psittacines, ducks, raptors, galliformes, passerines.

REFERENCES:

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