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Fowlpox is a slow-spreading viral infection of
chickens and turkeys characterized by proliferative lesions in the skin
that progress to thick scabs (cutaneous form) and by lesions in the
upper GI and respiratory tracts (diphtheritic form). Virulent strains
may cause lesions in the internal organs (systemic form). Fowlpox is
seen worldwide.
Etiology and Epidemiology
The large DNA virus (an avipoxvirus in the Poxviridae
family) is resistant and may survive in the environment for extended
periods in dried scabs. Photolyase and A-type inclusion body protein
genes in the genome of fowlpox virus appear to protect the virus from
environmental insults. Field and vaccine strains have only minor
differences in their genomic profiles, although the strains can be
differentiated to some extent by restriction endonuclease analysis and
immunoblotting. Recently, molecular analyses of vaccine and field
strains of fowlpox viruses have shown some significant differences. The
virus is present in large numbers in the lesions and is usually
transmitted by contact through abrasions of the skin. Skin lesions
(scabs) shed from recovering birds in poultry houses can become a source
of aerosol infection. Mosquitoes and other biting insects may serve as
mechanical vectors. Transmission within flocks is rapid when mosquitoes
are plentiful. The disease tends to persist for extended periods in
multiple-age poultry complexes because of slow spread of the virus and
availability of susceptible birds.
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.
Diagnosis
Cutaneous infections usually produce characteristic
gross and microscopic lesions. When only small cutaneous lesions are
present, it is often difficult to distinguish them from abrasions caused
by fighting. 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 (using antibodies against fowlpox virus
antigens). 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, susceptible birds, or cell
cultures of avian origin. Chicken embryos (9–12 days old) from an SPF
flock are the preferred and convenient host for virus isolation.
Cytoplasmic inclusion bodies of avianpox virus infection
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The genomic profiles of field isolates and vaccine
strains of fowlpox virus can be compared by restriction fragment length
polymorphism. This method is useful to compare closely related DNA
genomes. However, because of the large size of the genome, minor
differences are difficult to detect by this method. Detailed genetic
analysis reveals differences between vaccine strains and field strains
responsible for outbreaks of fowlpox in previously vaccinated chicken
flocks. Whereas 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 also be used for diagnosis. This
procedure is especially 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. This procedure is useful when an
extremely small amount of viral DNA is present in the sample. PCR has
been used effectively to differentiate field and vaccine strains of the
virus, whether full-length REV is present in those strains that are
associated with outbreaks in vaccinated birds. DNA isolated from the
formalin-fixed tissue sections of birds that are histologically positive
for fowlpox can be used for PCR amplification of genomic fragments
using specific primers. Because most outbreaks of fowlpox in previously
vaccinated chickens are caused by strains with a genome that contains
full-length REV, use of REV envelope-specific primers to determine the
presence of full-length REV is helpful in such cases.
Two monoclonal antibodies that recognize different
fowlpox virus antigens have been developed. These monoclonal antibodies
are useful for strain differentiation by immunoblotting.
The complete nucleotide sequence of the fowlpox virus
genome has been determined. It is useful in comparing the sequences of
selected genes of other avian poxviruses.
Prevention and Treatment
Where fowlpox is prevalent, chickens and turkeys
should be vaccinated with a live-embryo or cell-culture-propagated virus
vaccine. 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 timing 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. 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. Absence of “take”
indicates lack of potency of vaccine, passive or acquired immunity, or
improper 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.
Zoonotic Risk
There is no zoonotic risk associated with fowlpox
virus. Avian poxviruses cause a productive infection in avian species
but a nonproductive infection in mammalian hosts. Consequently, avianpox
viruses have been used as vectors for expression of genes from
mammalian pathogens in the development of safe recombinant vaccines.
Last full review/revision July 2013 by Deoki N. Tripathy, DVM, MS, PhD, DACVM, DACPV
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