Of the numerous tumors of unknown etiology in poultry, dermal squamous cell carcinoma (avian keratoacanthoma), multicentric histiocytosis, and adenocarcinomas are the most common but appear to be of limited economic importance.
Avian keratoacanthomas can occur at relatively high frequencies in some broiler flocks. Carcasses with extensive lesions are condemned at slaughter, whereas less affected birds undergo trimming. Condemnations of whole carcasses represent a significant economic loss. Typically, the lesions are seen during processing after the skin has been defeathered as crater-like ulcers with raised margins within feather tracts. In live chickens, the ulcers usually contain a mixture of keratin, cell debris, and bacteria. Histologically, lesions appear as proliferative expansion of feather follicle epithelium. An etiologic agent has not yet been identified, and the true neoplastic nature of the lesion has not been confirmed. Transmissibility of this tumor has neither been demonstrated nor excluded.
Multicentric histiocytosis, a condition of young broiler chickens, is characterized by both splenomegaly and hepatomegaly. Miliary (0.5–5 mm), white to yellow nodules can be seen in the spleen, liver, and kidneys. Microscopically, nodules of spindle-shaped cells diffusely expand periarteriolar lymphoid sheaths. These histiocytic cells contain elongated oval, fusiform, or more bizarrely configured nuclei. No definitive etiologic agent has been identified. A somewhat similar condition with lesions, termed “histiocytic sarcomatosis,” has been described in meat-type chickens experimentally infected with subgroup J avian leukosis virus.
Adenocarcinomas of the ovary or oviduct are relatively common incidental tumors in mature chickens. These neoplasms often are characterized by multiple miliary implant tumors on the mesentery and other visceral surfaces, frequently accompanied by ascites. These tumors are not known to be virus-induced or to be transmissible.