Page 375 - Avian Virology: Current Research and Future Trends

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Figure 13.1 Smear of a cutaneous pox lesion stained with Gimenez Figure 13.2 Fowlpox virus in a negatively stained preparation.
stain: a red-stained inclusion body with elementary bodies. Some
elementary bodies are dispersed free.
of Diptera have been reported as vectors of avian pox virus. The
poultry red mite (Dermanyssus gallinae) also has been implicated
in the spread of FWPV. The introduction of mosquito vector and
avian pox viruses to the Hawaiian Islands is considered to be a
primary contributing factor to population declines and extinc-
tions of native forest birds.
In recent years the epizootiology of fowlpox in many areas
has changed because of an increasing concentration of poultry
in large complexes, retention of layer flocks for a second cycle of
production, and maintenance of multiple age birds. The disease
is usually more common during summer months when the mos-
quito population is high. However, in large poultry operations,
especially in multiple age complexes, fowlpox may occur during
any time of the year.
Figure 13.3 Three fowlpox virus particles in ultrathin sections from Since avian pox viruses can withstand extreme environmental
a fowlpox-virus-infected lesion.
conditions and remain viable in dried scabs for extended periods,
the disease continues to occur as long as susceptible birds are
susceptible birds. For example, mechanical transmission of tur- available. The housing of a large number of birds in close quarters
keypox virus from infected toms to turkey hens through artificial facilitates transmission. Since the disease spreads slowly, the virus
insemination has been reported. Insects also serve as mechanical may circulate in the susceptible population for a considerable
vectors of the virus, resulting in ocular infection. In addition, oral time. This is a common occurrence where multiple age chicken
and respiratory infections may occur by exposure to aerosols flocks are maintained.
present in contaminated environments especially in concentrated
housing. In this regard, inhalation of virus-laden dust that may Clinical signs
contain particles of feathers, skin, or scabs provides an important The signs of poxvirus infections in birds vary depending upon
route for virus exposure to susceptible birds. The virus may reach the susceptibility of the host, virulence of the virus, distribution
the laryngeal region via the lacrimal duct to cause infection of the of the lesions, and other complicating factors. For example, vac-
upper respiratory tract (Eleazer et al., 1983). Cells of the mucosa cine strains of FWPV produce a localized lesion that persist for a
of the upper respiratory tract and mouth appear to be highly short period while virulent strains of FWPV cause both primary
susceptible to the virus as initiation of infection may occur in the and secondary lesions which may persist for several weeks (Fig.
absence of apparent trauma or injury. 13.4). Significant decrease in flock performance is observed in
Mosquitoes can infect a number of different birds after a single such cases. The morbidity rate of FWPV in chickens and turkeys
feeding on a bird infected with avian pox virus. Eleven species varies from a few birds being infected to involvement of the entire