Chicken anaemia agent: a review.

Chicken anaemia agent (CAA) is a small, unclassified, icosahedral DNA virus with a single-stranded, circular genome. It seems to have a worldwide distribution. Only one serotype of CAA has been found, and all isolates investigated so far are pathogenic for young chicks. CAA causes a syndrome in chickens characterised by increased mortality, anaemia associated with atrophy of the haematopoietic tissues in the bone marrow, subcutaneous and intramuscular haemorrhages, and atrophy of the lymphoid system. CAA spreads both vertically and horizontally. Vertical transmission occurs following primary infection of in-lay breeding stock, and results in clinical disease in their progeny around 2 weeks of age. Horizontal spread usually results in subclinical disease. Both clinical and subclinical disease cause economic loss. Current control measures often involve controlled exposure of breeding flocks to CAA during the rearing period. A vaccine is indicated.