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Avian pox is a mild to severe, slow developing disease of birds caused by an avipoxvirus, and three common strains have been identified. The three strains are fowl pox virus, pigeon pox virus and canary pox virus. The strains vary in their virulence and have the ability to infect other avian species. However, many of the strains are group specific. Approximately sixty species of birds from 20 families have been diagnosed with avian pox. The strain seen in wild turkeys is the fowl pox virus.
Avian pox lesions (wart-like growths) occur on the unfeathered parts of the bird's body and, in some cases, the mouth, larynx, and/or trachea.
Avian pox has been observed in a variety of avian hosts worldwide. The disease is most common in the temperate (warm and humid) parts of the world and is usually observed in relation to seasonal mosquito cycles. Avian pox has been diagnosed in upland game birds, songbirds (mourning doves and finches), marine birds, pet birds (canaries and parrots), chickens, turkeys, occasionally raptors and rarely in waterfowl. In Michigan, it has been diagnosed in 22 avian species: the brown-headed cowbird, American goldfinch, ruffed grouse, mourning dove, broad-winged hawk, common raven, wild turkey, trumpeter swan, bald eagle, house sparrow, American robin, European starling, northern cardinal, black-capped chickadee, peregrine falcon, house finch, Canada goose, red-tailed hawk, chipping sparrow, song sparrow, sandhill crane and American crow. Avian pox has been identified in birds since earliest history, but it is still currently considered to be an emerging viral disease.
Transmission of the avian pox virus can occur in a number of ways. The disease can be spread via mechanical vectors, primarily by species of mosquitoes (at least 10). Transmission occurs when the mosquito feeds on an infected bird that has a viremia (pox virus circulating in the blood) present or on virus-laden secretions from a pox lesion and then feeds on an uninfected bird. Mosquitoes can harbor and transmit the virus for a month or longer after feeding on an infected bird. Experimentally, stable flies have shown the capability of being able to transmit the pox virus.
Avian pox can also be transmitted by direct contact between infected and susceptible birds. The virus is transmitted through abraded or broken skin or the conjunctiva (mucous membrane covering the anterior surface of the eyeball). Indirect transmission of the pox virus can also occur via ingestion when food and water sources, feeders, perches, cages, or clothing are contaminated with virus-containing scabs shed from the lesions of an infected bird. The pox virus is highly resistant to drying and may survive months to years in the dried scabs. Indirect transmission can also occur via inhalation of pox virus infected dander, feather debris and air-borne particles.
Mosquitoes are probably responsible for transmission within local areas, while wild birds are responsible for outbreaks over greater distances.
Clinical signs observed with avian pox are weakness, emaciation, difficulty in swallowing and breathing, vision problems, a reduction in egg production, soiled facial feathers, conjunctivitis, edema of the eyelids and the presence of the characteristic wart-like growths on the unfeathered portions of the skin and/or formation of a diphtheritic membrane on the upper portion of the digestive tract.
Avian pox occurs in 2 forms, cutaneous (dry) and diphtheritic (wet). The cutaneous form is the most commonly observed and is a self-limiting infection with the lesions regressing and forming scars. Initially, this form of pox appears as a small white, pink, or yellow vesicle (blister) on unfeathered parts of the skin (feet, legs, base of the beak, eye margins and head). The vesicle is a result of the separation of the surface layer of the skin with the formation of pockets of watery fluid rich in multiplying virus. The vesicles become nodules as they increase in size, coalesce, and burst. Lymph from the cells congeals and scabs are formed. The surface of the nodules become rough and dry and the color changes to dark brown or black. The size and number of nodules present depends on the stage and severity of the infection. Bacteria may gain access causing secondary infection and resulting in a purulent discharge (pus) and necrosis. Eventually, the scab falls off and a scar forms at the site. It takes 2 to 4 weeks for complete healing of the affected areas on the skin providing the lesions aren't too extensive thereby preventing the bird from feeding.
The diphtheritic form involves the mouth, throat, trachea, and lungs and consists of yellow or white, moderately raised, moist cheese-like necrotic areas. A diphtheritic membrane forms and may restrict air intake and result in labored breathing and possible suffocation.
Histologically, intracytoplasmic inclusion bodies (Bollinger bodies) are present in the infected skin and respiratory tract mucosa. In the diphtheritic form of the disease, nodular hyperplasia (increase in the number of cells) of the mucosa is observed.
A presumptive diagnosis of avian pox can be made due to the gross lesions on the body. Confirmation of avian pox is accomplished by microscopic examination for the characteristic Bollinger bodies. Virus isolation by transmission of the organism via egg inoculation, serological results and polymerase chain reaction can also be a means of confirming the disease.
There is no known treatment for avian pox in wild birds. In captive situations, there are a variety of treatments that have been used along with supportive care to treat the pox lesions and to prevent secondary infections in various avian species. These treatments consist of removing skin lesions and utilizing sodium bicarbonate or Lugol's solution of iodine washes, removing the diphtheritic membrane from the mouth and throat and swabbing the area with Lugol's solution of iodine, bathing the eyes with a 1-2% saline solution, and raising the environmental temperature. In all cases, providing assistance for recovery may spread the infection to other parts of the skin or to other birds.
The best control for avian pox in captive (turkeys, pheasants, threatened and endangered species) and domestic (chickens and turkeys) birds is vaccination with a modified live vaccine. In wild birds, vaccination is not a feasible method of control.
Avian pox is a highly contagious disease and there are three primary control methods that can be used if infected birds are present. Artificial feeding, which can concentrate birds, should be stopped. Eliminating standing water will control the primary vector, the mosquito. Infected birds should be isolated or culled to remove the source of the virus. Feeders, waterers, birdbaths, and cages should be decontaminated with a 10% bleach solution.
There is no evidence that the avian pox virus can infect humans and therefore it is not a public health concern.
In some populations of birds (wild turkey, bald eagle, and albatross), avian pox may be a significant mortality factor.