Fowl Pox (Avian Diphtheria)
Fowl Pox (Avian Diphtheria) also known as Avipox and variola gallinarum in animals.
Fowl Pox (Avian Diphtheria) is a slowly spreading viral disease of chicken, characterized by development of discrete nodular proliferative skin lesions on the non- feathered part of the body (cutaneous form) or fibrino-necrotic and proliferative lesions in the mucous membrane of upper respiratory, mouth and oesophagus (Diphtheritic form).
Etiology
- Fowl Pox (Avian Diphtheria) caused by Avipox virus of family pox viridae, a single linear double stranded DNA with hairpin loop at each end.
- The virus is inactivated by 1% caustic potash and resistant to chloroform and ether.
- The virus can survive in dried scab for months or even years.
Epidemiology
- Mortality rate: Cutanous form- low and Diphtheritic form- high.
- Avipox virus infection of canaries and finches can significantly cause high rate of mortality than those of chicken.
Transmission
- Mechanical transmission of virus to injured or lacerated skin.
- Inhalation or ingestion of virus.
Clinical signs
- Incubation period: 4-10 days in chicken, turkey and pigeon, and about 4 days in canaries.
Cutaneous form (Dry pox)
- Characterized by appearance of nodular lesions on the comb, wattle, eyelids and non feathered area of the body.
- Cutaneous eye lesions will interfere with birds ability to reach food and water leads to starvation and death.
Diphtheritic form (Wet pox)
- Cankers or diphtheritic yellowish lesion occur on the mucous membrane of the mouth, oesophagus or trachea with accompanying coryza like mild or severe respiratory signs similar to ILT.
- Lesions in the corner of the mouth, on the tongue, throat and upper part of the throat interfere with eating, drinking and breathing.
- Emaciation, poor weight gain.
- Unthriftiness and reduced egg production.
Necropsy Finding
- Characteristic cutaneous form of lesion- epithelial hyperplasia (scab in head, feet and legs).
- Diphtheritic form- white opaque nodule or yellowish patches develop on the mucous membrane of the mouth, oesophagus, tongue and upper trachea.
- Histopathology-intracytoplasmic eosinophilic inclusion bodies (borrel bodies) on the affected tissue.
Sample collection
- Live birds-Scab in 10% glycerol saline and paired sample.
- Dead birds- trachea and oseophagus.
Diagnosis
- Based on clinical signs and lesions.
- Isolation of virus in embryonated egg by chorioallantoic route of inoculation (10-12 days).
- Isolation of virus in cell culture.
- Molecular diagnosis of viral antigen by PCR and Restriction fragment length polymorphism (RFLP) is used to identify the strain of virus.
- Antibody detection by AGID, VNT and ELISA.
Differential diagnosis
- Infectious laryngiotracheitis
Prevention and control
- Fowl pox vaccine is commonly applied to chicken by the wing web method at 6-8 weeks and also to be administered as early as 4 week old. The pullet birds are vaccinated 2 months before egg production is expected to start.
- Vaccination should not be done when birds in laying.
- Live virus vaccines used for immunization of birds against pox.
- Vaccines of fowl pox and pigeon pox virus origin is routinely used for vaccination of chicken and turkeys.
- Turkeys are vaccinated at 2-3 months old; breeders should be revaccinated before egg production. Site: midway of thigh stick or wing web method.
- Fowl pox vaccine should not to be used in pigeons.
- Recombinant fowl pox vaccine is also available for vaccination.
