Aetiology
It is caused by fowl/ turkey/ pigeon virus belonging to the genus Avipox. The virus grow in the chorioallantoic membrane of embryonated chicken egg forming pocks which can be stained and seen using a light microscope. The virus may also be observed in cytoplasm of the affected bird. They form intracytoplasmic inclusion body known as Bollinger bodies. Fowl pox virus is the largest known virus and can be seen using light microscope.
Host
It can be seen in fowl, turkey, pigeons etc. infection can occur in susceptible host of all ages.
Transmission
It occurs through mechanical transmission of the virus to the injured or lacerated skin. Biting insects can also infect birds.
Clinical signs and post-mortem lesions
Incubation period varied from 4-10 days in chicken and turkey.
The disease may occur in two or more forms: cutaneous, diphtheritic or both.
The cutaneous or dry form is characterized by appearance of cutaneous nodular lesions on the comb, wattle, eyelids and other non-feathered part of the body.
Diphtheritic or wet form is characterized by diphtheritic yellow lesion in mucous membrane of the mouth, esophagus and trachea.
The cutaneous form however is the most common form seen in most disease outbreaks. Sometimes, the disease may show mild reduction in the rate of weight gain or lack of vigor. Fowl pox can cause reduction in laying bird. The cutaneous lesion vary in appearance. There is a papule which progress to vesicle which result in pustle and finally to scab stage. After about 2 weeks, the scab will fall off and the lesion is healed which may or may not leave a scar on the bird.
In the diphtheritic form, most lesions are formed in the mouth but others are present in the larynx, trachea and esophagus. These lesions can give rise to inappetence and difficulty in breathing. Lesions in the nares give rise to nasal discharge while those in conjunctiva give rise to ocular discharge and rarely, it may result to blindness. Mortality in chicken is always low while it may be high in turkey.
Diagnosis
This is based on clinical sign and post-mortem lesions.
Histological examination of the skin section for fowl pox lesion will reveal intracytoplasmic inclusion body or Bollinger bodies. Materials can be scraped from the lesions and smear made on glass slide and using appropriate stain and the virus can be seen using a light microscope.
Also, ground up scab inoculated into the chorioallantoic membrane of 9-12 days old embryonated egg has produced a characteristic pock lesion. Ground up scab may also be inoculated into a known susceptible chicken by scarification of comb and wattle. Characteristic pock lesion should be present at 5-7 days post inoculation.
Control
There is no satisfactory treatment. However, some workers remove the scab and dress the wound surface with a tincture of iodine.
Heavy pox can be prevented by vaccination with live vaccine. Two main routes are employed: wing web and thigh muscle. In wing web method, a bifurcated needle with a vaccine in central route is used to introduce the vaccine. In the thigh method, the feathers are removed and the vaccine brushed into the follicles.
Birds can be vaccinated at 3 weeks of age but preferable age is 6 weeks. Turkeys should not be vaccinated by wing web because they sleep with head under their wings which could lead to head lesion.
Vaccination of chicken is done using chicken pox vaccine or pigeon pox vaccine. Pigeon should not be vaccinated by fowl pox vaccine.