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Diagnostic Guidance: Fowl Pox

By Mallory Pfeifer
Fowl pox is caused by a DNA virus (fowl pox virus).  Many species of birds have their own specific pox virus.
Infection can occur in two different forms: dry and wet. Dry pox occurs through mechanical transmission of the virus to injured or lacerated skin and can occur at any age. Biting insects, such as mosquitos, serve as mechanical vectors often resulting in ocular infection. Wet pox causes diphtheritic lesions and often has a higher mortality rate. The lacrimal duct is a frequent transmission route, as the virus can reach the laryngeal region after entry.
Dry pox ranges from mild to severe depending on infection rate. The characteristic lesion of the cutaneous form is epithelial hyperplasia involving the epidermis and underlying feather follicles, with formation of nodules that first appear as small white foci that often rapidly increase in size and become dark in color (Figure 1). Adjoining lesions may coalesce and become rough and induce secondary infections or interfere with vision or breathing (Figure 2).
Wet pox occurs as elevated white opaque nodules or yellowish patches on the mucous membranes of the mouth (Figure 3), esophagus, tongue, glottis, or upper trachea (Figure 4).  Nodules rapidly increase in size and often coalesce to become a yellow cheesy, necrotic, or diphtheritic membrane.
Vaccination can be performed in the face of an outbreak or as a preventative measure. The vaccine comes as a dehydrated cake that contains 1,000 doses that must be rehydrated with an accompanying diluent and administered via a stab inoculation with a two-prong applicator in the wing web. The stab inoculation site should develop a small bump or scab (“vaccine take”) in about 7-10 days and should resolve in about 14 days.  In turkeys it is often recommended to administer the vaccine via a “thigh-stick” to avoid spreading the vaccine from the wing web to the head because turkeys may rest with their head tucked under their wing.  Vaccination is usually done in 4 to 6-week-old birds, but if early exposure occurs, vaccination of younger birds can be done by wing-web with a one-needle applicator.  A booster dose is recommended at 6 weeks of age.