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Chapter 19 - Avian Pox
Authors: Wallace Hansen.
Avian pox is the common name for a mild-to-severe, slow-developing disease of birds that is caused by a large virus belonging to the avipoxvirus group, a subgroup of pox-viruses. This group contains several similar virus strains; some strains have the ability to infect several groups or species of birds but others appear to be species-specific. Mosquitoes are common mechanical vectors or transmitters of this disease. Avian pox is transmitted when a mosquito feeds on an infected bird that has viremia or pox virus circulating in its blood, or when a mosquito feeds on virus-laden secretions seeping from a pox lesion and then feeds on another bird that is susceptible to that strain of virus. Contact with surfaces or exposure to air-borne particles contaminated with poxvirus can also result in infections when virus enters the body through abraded skin or the conjunctiva or the mucous membrane lining that covers the front part of the eyeball and inner surfaces of the eyelids of the eye.
The highly visible, wart-like lesions associated with the featherless areas of birds have facilitated recognition of avian pox since ancient times. Approximately 60 free-living bird species representing about 20 families have been reported with avian pox. However, the frequency of reports of this disease varies greatly among different species (Fig. 19.1). Avian pox has rarely been reported in wild waterfowl, and all North American cases have been relatively recent (Table.19.1). The first case was in a free-living green-winged teal in Alaska. Single occurrences have also been documented in a Canada goose in Ontario, Canada, a mallard duck in Wisconsin, a feral mute swan cygnet in New York, and a tundra swan in Maryland. Three cases in American goldeneye have been reported in Saskatchewan, Canada, and New York. Avian pox also appeared in Wisconsin among captive-reared trumpeter swans that were part of a reintroduction program. Zoological garden cases include common scoter in the Philadelphia Zoo and a Hawaiian goose in the Honolulu Zoo.
Avian pox in a bald eagle was first diagnosed in 1979 in Alaska and it was a lethal infection. Since then, additional bald eagles in Alaska and at other locations have been diagnosed with this disease (Fig. 19.2). The severity of infection resulted in several of these cases being lethal. Poxvirus infections have been reported in other raptors, most recently in the eastern screech owl and barred owl in Florida (Table 19.2).
On Midway Atoll, large numbers of colonial nesting birds, such as the Laysan albatross, have become infected with avian pox. Red-tailed tropicbirds on Midway Atoll previously had been affected by avian pox. The shift in predominant species infected relates to the dramatic shift in population densities for the two species over time (19631978). Mourning dove, finches, and other perching birds using backyard feeders (Fig. 19.3) frequently have been reported to have been struck by avian pox epizootics. Avian pox is suspected as a factor in the decline of forest bird populations in Hawaii and northern bobwhite quail in the southeastern United States, where it is also an important disease of wild turkey.
Table 19.1 Waterfowl in North America reported to have avian pox.
Avian pox occurs worldwide, but little is known about itsprevalence in wild bird populations. The increased frequency of reported cases of this highly visible disease and the involvement of new bird species during recent years suggests that avian pox is an emerging viral disease. Birds can become disease carriers and spread avian pox among local populations, such as between birdfeeding stations, and along migratory routes used by various bird species. Mosquitoes that feed on birds play the most important role for both disease transmission and long term disease maintenance. However, contamination of perches and other surfaces used by captive birds can perpetuate disease in captivity. Pox outbreaks are commonly reported at aviaries, rehabilitation centers, and other places where confinement provides close contact among birds. The disease can spread rapidly when avian pox is introduced into such facilities. Species that would not ordinarily have contact with avian pox virus in the wild often become infected in captivity if the strain of virus present is capable of infecting a broad spectrum of species. Common murres rescued from an oil spill in California developed pox virus lesions while they were in a rehabilitation center. Endangered avian species also have been infected during captive rearing. Return to top of page Seasonality
Although wild birds can be infected by pox virus year-round(Fig. 19.4), disease outbreaks have been associated with the environmental conditions, the emergence of vector populations, and the habits of the species affected. Environmental factors such as temperature, humidity, moisture, and protective cover all play a role in the occurrence of this disease by affecting virus survival outside of the bird host. Avian pox virus can withstand considerable dryness, thereby remaining infectious on surfaces or dust particles. Mosquitoes that feed on birds are the most consistent and efficient transmitters of this disease. Mosquito populations are controlled by breeding habitat and annual moisture.
The time of appearance and magnitude of vector populations varies from year to year, depending on annual weather conditions. This influences the appearance and severity of the disease in any given year. Only limited studies have been carried out to assess the relations between avian pox and insect vector populations. Studies on the Island of Hawaii disclose a close relation between the prevalence of poxvirus infections in forest birds and seasonal mosquito cycles. The lowest prevalence of pox virus infection in California quail in Oregon was reported in the dry summer months and the highest was reported during the wetter fall and winter months. In Florida, reports of avian pox in wild turkey correspond to the late summer and early fall mosquito season. On Sand Island of the Midway Atoll, avian pox was first reported in September l963 in the nestlings of the red-tailed tropicbird. In March and April of the late l970s, this disease was found in nestling Laysan albatross on Sand Island. This is an example of disease seasonality influenced by dramatic shifts in predominant species populations.
Birdfeeding stations have been the source of numerous poxvirus outbreaks in the continental United States (Fig.19.3). Contact transmission of the virus through infected surfaces and close association of birds using those feeders is the likely means of transmission during cooler periods of the year when mosquitoes are not a factor, and birdfeeders provide additional sources of infection when mosquitoes are present.
Birds with wart-like nodules on one or more of the featherless areas of the body, including the feet, legs, base of the beak, and eye margin should be considered suspect cases of avian pox (Fig. 19.5). The birds may appear weak and emaciated if the lesions are extensive enough to interfere with their feeding. Some birds may show signs of labored breathing if their air passages are partially blocked. Although the course of this disease can be prolonged, birds with extensive lesions are known to completely recover if they are able to feed.
Avian pox has two disease forms. The most common form is cutaneous and it consists of warty nodules that develop on the featherless parts of the bird. This form of the disease is usually self-limiting; the lesions regress and leave minor scars. However, these nodules can become enlarged and clustered, thus causing sight and breathing impairment and feeding difficulty (Figs. 19.6A and B). Secondary bacterial and other infections are common with this form of the disease, and these infections can contribute to bird mortality. In some birds, feeding habits result in the large warty nodules becoming abraded and then infected by bacterial and fungal infections (Figs. 19.6C and D).
The internal form of disease is referred to as wet pox and it is primarily a problem of young chickens and turkeys. This diphtheritic form appears as moist, necrotic lesions on the mucus membranes of the mouth and upper digestive and respiratory tracts (Fig. 19.7), and it has occasionally been reported in wild birds (Fig. 19.8). This form of avian pox probably occurs more frequently in wild birds than it is reported because it is less observable than the cutaneous form. Also, the more severe consequences of wet pox undoubtedly causes greater morbidity and mortality, thereby leading to removal of infected birds by predators and scavengers.
A presumptive diagnosis of avian pox can be made from the gross appearance of the wart-like growths that appear on body surfaces. However, these observations must be confirmed by examining lesions microscopically for characteristic cellular inclusion bodies. Avian pox is confirmed by virus isolation and serological identification. Submit the whole bird or the affected body part (for example, the feet or head) to a disease diagnostic laboratory that has virus isolation capabilities. Immediately freeze samples that must be held for more than 24 hours before shipment. Virus isolation can be attempted from a live bird by collecting samples from the affected area. However, consult with the diagnostic laboratory staff before collecting samples.
The fundamental principle for controlling avian pox is to interrupt virus transmission. The difficulty in applying control procedures is related to the type of transmission taking place, the mobility of the infected birds, and the size of the affected area. The more confined a population at risk, the more effective the control procedures will be. Therefore, prevention is the first method for controlling this disease. Vector control (primarily mosquitoes) in and around the disease area should be considered first. Identifying and eliminating vector breeding and resting sites together with controlling adult mosquito populations are most desirable. Removing heavily infected animals is also helpful because it diminishes the source of virus for vector populations. This also reduces the opportunity for contact transmission between infected and noninfected birds.
Special vigilance of captive birds is needed, especially when threatened and endangered species are involved. Because poxvirus is resistant to drying, disease transmission by contaminated dust, food, perches, cages, and clothing can pose a continuing source of problems. Therefore, these items need to be decontaminated with disinfectant, such as a 5 percent bleach solution, before they are disposed of or reused.
The poultry industry uses modified live vaccines to prevent avian pox, but their safety and effectiveness in wild birds have not been determined. In addition, strain differences in the virus, host response to those different strains, and logistical problems of a vaccination program further complicate using vaccines for wild birds. The greatest potential use of vaccination is for protecting captive-breeding populations of threatened and endangered species and for providing immunity in birds that are to be released into areas where pox is a problem.
Avian poxvirus is part of a larger family of poxviruses that includes the human disease known as variola or smallpox. However, there is no evidence that avipoxviruses can infect humans.
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