Abstract
Pigeon pox is a common virus disease of unvaccinated susceptible pigeons (Fig. I). The epitheliotropic DNA virus has a predelection for skin and mucous membranes with the formation of visible wart-like, "pock" lesions on the surface epithelium. Gross internal pox lesions do not appear in pigeons even though the virus produces a systemic reaction and occasionally results in a viremia with virus circulating in the blood stream.
The agent is classified in the genus, avipoxvirus, which includes: fowl, turkey, pigeon and canary viruses. Sparrow, duck and guinea pox may also be separate viruses within this group. They are all similar and appear to be related. This avipox grouping is one of five genera. Other pox viruses affect animals, man and fish, and are classified on the basis of related characteristics.
Pigeon pox is related to other pox viruses by its ability to multiply solely in the cytoplasm of the cells it invades. It has a deoxyribonucleic acid core which largely occurs as double stranded molecules covered with double membranes the outer membrane being composed of; lipoprotein. The brick-shaped virus particle is among the largest in size ranging from 200 to 300 nanometers in length. This is very small when compared to bacterial cells which range in size between 0.2 micron and 200 microns. For comparison, a nanometer is one billionth of a meter and a micron is one millionth part of a meter.
As a general rule, pox virus particles invade rapidly growing new epithelial cells. Here they replicate, producing introcytoplasmic inclusions called Bollinger bodies. These may be identified under a light microscope as distinctive eosinophilic particles in stained tissue sections.
The avian pox viruses are antigenically closely related to one another but are immunologically distinct from other animal pox viruses. From work that we have conducted, it is obvious that even within the so-called pigeon pox vaccine strains, there are distinct antigenic differences. Members of the overall pox group of viruses are reported to contain a nucleo-protein precipitating antigen which is common to all, but this may not be specific for only pox viruses. Hemagglutinins are also produced by the avian pox viruses which means that red blood cells can be agglutinated by them.
Serologically it is interesting that pigeon pox may or may not affect other species of birds and may or may not engender a high degree of tissue immunity. In chickens, pigeon pox produces visible epithelial infection with resulting immunity proportional to the number of feather follicles innoculated. In one study by Irons, crows, hawks, owls, ducks, guinea fowl and starlings were not found to be susceptible to pigeon pox, but the virus was shown to be infectious for English sparrows and related species.
Kossack studied a natural pigeon pox outbreak in mourning doves in which the virus was transmitted to ring doves. Burnett reported that in turkeys, pigeon pox virus may produce a severe reaction without producing immunity to turkey
. or fowl pox viruses. Also, according to Brandly and Dunlop, canary virus irnmunizes against pigeon virus to a high degree, and pigeon virus protects against itself and canary virus but does not give complete protection against turkey and fowl viruses. It has also been reported by Jansen that vaccination with pigeon pox does not protect against a jackdaw strain of canary pox virus. Jactot, in a separate report, indicated that pigeon pox immune fowl were immune to a strain of canary pox virus that he isolated. From these various research observations. one can appreciate the intricate antigenic relationships that exist, probably as a result of the lipoproteins that form the outer membranes of the virus. The common nucleoprotein antigen, in the core of the virus, probably has little to do with the specificity of each agent.
Transmission of the disease is largely the result of wound infection created by mosquitoes and other biting and sucking insects such as mites or pigeon louse flies. These parasites serve purely as mechanical carriers of the virus, which appears to localize on or in the proboscis of mosquitoes. When the mosquito bites the unfeathered portion of the skin, the virus is merely wiped off or pushed out, and thus, it gains entrance into damaged cells. Pox virus is in itself unable to penetrate the intact epithelium to produce infection.