Fowlpox is a slow-spreading viral infection of chickens and turkeys characterized by proliferative lesions in the skin (cutaneous form) that progress to thick scabs and by lesions in the upper GI and respiratory tracts (diphtheritic form). It is seen worldwide.

Etiology and Epidemiology:

The large DNA virus (an avipoxvirus, family Poxviridae) is highly resistant and may survive for several years in dried scabs. Recently, molecular analyses of vaccine and field strains of fowlpox viruses have shown some significant differences,generally it is minor. The virus is present in large numbers in the lesions and is usually transmitted by contact through abrasions of the skin,Skin lesions (scabs) sheddings, Mosquitos and other biting insects ( fastest way of spread of disease). Some affected birds may become carriers, and the disease may be reactivated by stress (eg, moulting) or by immunosuppression due to other infections. The disease tends to persist for extended periods in multiple-age poultry complexes.

Clinical Findings:

Only a few birds develop lesions at one time which are prominent in some birds and may significantly decrease flock performance. In cutaneous form has nodular lesions on various parts of the unfeathered skin of chickens and on the head and upper neck of turkeys.Some times lesions of feathered skin may also be seen. In some cases, lesions are limited mainly 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 come together. Localization around the nostrils may cause nasal discharge. Cutaneous lesions on the eyelids may cause complete closure of one or both eyes.

Scab-like lesion, fowlpox, laying hen

Scab-like lesion, fowlpox, broiler breeder male

Scab-like lesion, fowlpox, turkey

In the diphtheritic type, lesions may be seen on the mucous membranes of the mouth, esophagus, pharynx, larynx, and trachea (wet-pox or fowl diphtheria). Caseous patches firmly adherent to the mucosa of the larynx and mouth or proliferating masses may be seen. Mouth lesions affect  feeding. Tracheal lesions cause difficulty in respiration and may simulate infectious laryngotracheitis in chickens. Laryngeal and tracheal lesions in chickens must be differentiated from those of laryngotracheitis.

Generally the course of the disease in a flock is prolonged. Extensive infection in a layer flock results in decreased egg production. Cutaneous infections alone can cause low mortality, such flocks generally return to normal production after recovery. Mortality is usually high in the generalized or diphtheritic form.


Cutaneous infections usually produce characteristic gross and microscopic lesions. When less lesion are there, it is often difficult to distinguish them from machenical abrasions. Microscopic examination of affected tissues stained with H&E reveals eosinophilic cytoplasmic inclusion bodies. Cytoplasmic inclusions are also detectable by fluorescent antibody and immunohistochemical methods. The elementary bodies in the inclusion bodies can be detected in smears from lesions stained by the Gimenez method. Viral particles with typical poxvirus morphology can be demonstrated by negative-staining electron microscopy as well as in ultrathin sections of the lesions. The virus can be isolated by inoculating chorioallantoic membrane of developing chicken embryos(9-12 days old is ideal), susceptible birds, or cell cultures of avian origin.

Field isolates and vaccine strains of fowlpox virus can be compared by restriction endonuclease analysis for closely related viral genomes. However,detailed genetic analysis reveals differences between vaccine strains and field strains responsible for outbreaks of fowlpox in previously vaccinated chicken flocks. While vaccine strains of fowlpox virus contain remnants of long terminal repeats of reticuloendotheliosis virus (REV), most field strains contain full-length REV in their genome.

Nucleic acid probes derived from cloned genomic fragments of fowlpox virus can be useful for differentiation of the diphtheritic form of fowlpox (involving the trachea) from infectious laryngotracheitis.

PCR can be used to amplify genomic DNA sequences of various sizes using specific primers. The complete sequence of the fowlpox virus genome has been identified recently.

Recently, 2 monoclonal antibodies that recognize different fowlpox virus antigens have been developed. These monoclonal antibodies(by immunoblotting),complete genome and PCR can be used for strain differentiation.

Prevention and Treatment:

The most widely used vaccines are attenuated fowlpox virus and pigeonpox virus isolates of high immunogenicity and low pathogenicity. In high-risk areas, vaccination with an attenuated vaccine of cell-culture origin in the first few weeks of life and revaccination at 12-16 wk is often sufficient. Health of birds, extent of exposure, and type of operation determine the timings of vaccinations. Because the infection spreads slowly, vaccination is often useful in limiting spread in affected flocks if administered when <20% of the birds have lesions. Because passive immunity may interfere with multiplication of vaccine virus, progeny from recently vaccinated or recently infected flocks should be vaccinated only after passive immunity has declined. Vaccinated birds should be examined 1 wk later for swelling and scab formation (“take”) at the site of vaccination to confirm potency and efficacy of vaccination. Revaccination with another serial lot of vaccine may be indicated.

Naturally infected or vaccinated birds develop humoral as well as cell-mediated immune responses. Humoral immune responses can be measured by ELISA or virus neutralization tests.

Reference :- Merck Veterinary Manual

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