Chicken Infectious Anaemia (Blue Wing Disease) is a viral disease of chickens caused by the Chicken Anaemia Virus (CAV). It is caused by a Gyrovirus of the family Circoviridae and was first isolated in Japan in 1979, but is now known to occur worldwide. Clinical disease occurs in chickens but infection has been detected in turkeys and quail3.
It can infect chickens of any age but clinical disease is generally seen in the first two weeks of life1, with birds becoming more resistant as they get older. However, concurrent infections with immunosuppressive agents such as Infectious Bursal Disease or Marek’s Disease viruses may compromise the development of age resistance3. Transmission is both vertical and horizontal with chicks hatched from infected eggs being viremic and spreading virus to uninfected chicks via faeces and feathers3. Infected roosters may transmit virus via semen3. Infected older birds will undergo seroconversion and become immune3. Seroconverted breeders will pass protective antibodies to their chicks which will not, therefore, develop clinical disease.
The virus primarily affects the immune and haemopoietic systems, particularly invading the thymus. This results in decreased T-lymphocytes and red blood cells in circulation.
Clinical signs of illness4 include pallor, depression and poor growth.. There may be a significant and sudden rise in mortality usually at about 2 weeks of age. Subcutaneous haemorhages may be seen on the wings and hock joints causing a bluish discolouration (hence the name Blue Wing Disease). Hematocrits may be in the 5-15% range (normal 27-36%). Immunosuppression may lead to opportunistic infections such as gangrenous dermatitis and mycotic pneumonia. Infected adult birds will show no clinical signs.
Post mortem lesions4 include overall pallor, thymic and bursal atrophy, pale bone marrow and discoloured liver and kidneys. Haemorhages in the proventriculus may be seen. Signs of gangrenous dermatitis or pneumonia may be present. Histological examination of thymus or bursa of Fabricius will show depletion of lymphocytes.
Diagnosis is based on post mortem findings and serological or PCR testing1,3,4. CAV monitoring is carried out in Jamaica using an ELISA test from IDEXX Laboratories.
There is no treatment. Secondary bacterial infections may be treated with the appropriate antibiotics.
Prevention strategies include vaccination and general hygiene. Live attenuated vaccines are used for either injection or drinking water administration for breeder flocks prior to the start of laying. This is usually done between 12 and 15 weeks of age to allow time for seroconversion2. Direct use of infected material (litter or tissue homogenates from infected birds) to infect flocks prior to breeding, again allowing time for seroconversion, has also been used. Serological testing may be used to ascertain seroconversion.
In Jamaica, broiler breeder and layer pullets are routinely vaccinated against the Chicken Anemia Virus and thus it has not been a significant problem for the poultry industry. Nevertheless, it is one of the diseases that must be considered when dealing with cases of mortality in young growing chickens.
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