Fowl-pox is a highly infectious disease of poultry, especially chickens and turkeys. Pheasants, ducks, geese and guineafowls also suffer from an infection similar to fowl- pox.
Epidemiology: The disease is caused by the fowl-pox virus which belongs to the genus Avipoxvirus in the family Poxviridae, has two distinct forms-
(i) the skin or 'cutaneous' type showing presence of wart like eruptions on the unfeathered portions of the body, particularly the legs, comb, wattles and eyelids; and
(ii) the 'diphtheritic' or throat type which has membranous pathches in the mouth and throat.
The fowl-pox virus is very resistant to cold and heat and remains viable in the scabs for a considerable time. In the epithelial cells of fowl-pox lesions minute, rounded or oval 'inclusion bodies' knows as 'Bollinger bodies' are present. Each one of these bodies is capable of inducing typical fowl-pox. The inclusion bodies are constituted by hundreds of extremely small definite entities known as 'Borrel granules' believed to represent the actual virus particles of the disease.
Clinical signs: Lesions commonly appear on the unfeathered parts of the head and legs. Generalized skin infection may occur in severe cases. Occasionally, the oral or the nasal cavity and sinuses may be affected. Exposure of the cracked skin is followed in about 5 to 7 days by raised yellowish lesions. These grow in size, may coalesce and eventually form black wart-like nodules. Lesions around the nares and eyelids block nasal passages and cause closure of the eyes. The conjunctival sac may be affected, resulting in ocular discharge. After intravenous inoculation of the virus, lesions are found in the pancreas, salivary glands and liver. Infection on the mouth, so-called 'wet pox', takes the form of caseous patches adhering grimly to mucosa of the pharynx, larynx and trachea and is accompanied by severe respiratory distress.The disease particularly affects chicks and causes a considerably higher mortality.
Pox infection in older birds results in unthriftiness, reduction in egg production and loss in weight. Such birds act as source of infection for healthy birds. Usually the course of the disease is chronic with symptoms of inappetence and dullness.
Diagnosis: It is diagnosed by slow spread of the disease and the typical appearance of the pox scabs. Microscopic examination of fresh scrapings from the thickened, affected areas of the skin reveals typical inclusion bodies. Infected tissue suspension when applied to denuded feather follicles of susceptible birds will produce typical pox lesions in 5 to 7 days. Demonstration of eosinophilic cytoplasmic inclusion bodies in the cutaneous or diphtheritic lesions is diagnostic of fowl-pox. Other tests include electron microscopy, immuno fluorescence , virus neutralization , passive haemagglutination and immuno-diffusion.
Treatment, prevention and control: Local application of tincture of iodine or solution of potassium permanganate over the lesions may prove ameliorative. The disease is controlled by using a vaccine consisting of freeze-dried emulsion of the infected membranes of chick embryo on which the fowl-pox virus vaccine strain has been grown. Fowl-pox vaccine is used for turkeys as well. It may be applied to young birds of 8 to 10 weeks of age by the wingweb technique or by brushing the virus into 4 to 5 feather follicles on the thigh. The latter is rarely practiced now a days. Vaccination may cause some reaction. As a consequence, laying birds may cease laying and younger chicken carrying the blackhead organisms may show an acute flare-up of the disease. Pigeon-pox virus vaccine is preferred for chicks in some areas since the reactions following this vaccination are mild. A successful fowl-pox vaccination is indicated by the development, within 7 to 10 days, of a red or yellow swelling and scab formation in the follicles. Pigeon-pox vaccine causes no scab formation, but a red pappular swelling of the follicles results within 8 to 12 days. Failure of either vaccine to produce a reaction indicates possible lack of potency of the vaccine or of faulty technique