Here is an excerpt from The Merck Manual about avian pox that is pretty informative:
Clinical Findings
The cutaneous form of fowlpox is characterized by nodular lesions on various parts of the unfeathered skin of chickens and on the head and upper neck of turkeys. Generalized lesions of feathered skin may also be seen. In some cases, lesions are limited chiefly to the feet and legs. The lesion is initially a raised, blanched, nodular area that enlarges, becomes yellowish, and progresses to a thick, dark scab. Multiple lesions usually develop and often coalesce. Lesions in various stages of development may be found on the same bird. Localization around the nostrils may cause nasal discharge. Cutaneous lesions on the eyelids may cause complete closure of one or both eyes. Only a few birds develop cutaneous lesions at one time. Lesions are prominent in some birds and may significantly decrease flock performance.
In the diphtheritic form of fowlpox, lesions develop on the mucous membranes of the mouth, esophagus, pharynx, larynx, and trachea (wetpox or fowl diphtheria). Occasionally, lesions are seen almost exclusively in one or more of these sites. Caseous patches firmly adherent to the mucosa of the larynx and mouth or proliferative masses may develop. Mouth lesions interfere with feeding. Tracheal lesions cause difficulty in respiration. Laryngeal and tracheal lesions in chickens must be differentiated from those of infectious laryngotracheitis (see
Infectious Laryngotracheitis), which is caused by a herpesvirus. In cases of systemic infection caused by virulent fowlpox virus strains, lesions may be seen in internal organs. More than one form of the disease, ie, cutaneous, diphtheritic, and/or systemic, may be seen in a single bird.
Often, the course of the disease in a flock is protracted. Extensive infection in a layer flock results in decreased egg production. Cutaneous infections alone ordinarily cause low or moderate mortality, and these flocks generally return to normal production after recovery. Mortality is usually high in diphtheritic or systemic infections.