Chicken anaemia virus (CAV) is a small virus of a unique type with a p
article diameter of 23 to 25 nm and a genome consisting of a circular
single-stranded (minus-strand) DNA. This DNA multiplies in infected ce
lls via a circular double-stranded replicative intermediate, which was
recently cloned. DNA analysis of CAV strains isolated in different co
ntinents revealed only minor differences among the various isolates. A
pparently, all CAV isolates belong to a single serotype. CAV is not re
lated to other known animal single-stranded circular-DNA viruses, such
as porcine circovirus and psittacine beak-and-feather-disease virus.
The major transcript from the CAV genome is an unspliced polycistronic
mRNA of about 2100 nucleotides encoding three proteins of 51.6 kDa (V
T1), 24.0 kDa (VP2) and 13.6 kDa (VP3 or apoptin). All three predicted
CAV proteins are synthesized in CAV-infected cells. Immunization with
(recombinant) VP1 and VP2 synchronously synthesized in the same cells
elicits a protective response and can be used as subunit vaccine agai
nst chicken infectious anaemia. CAV causes clinical and subclinical di
sease in chickens, and is recognized as an important avian pathogen wo
rldwide. In young chickens, CAV causes a transient severe anaemia due
to destruction of erythroblastoid cells in the bone marrow and immunod
eficiency due to depletion of cortical thymocytes. The depletion of th
e cortical thymocytes is considered to cause a (transient) immunodefic
iency resulting in enhanced concurrent infections and to vaccination f
ailures. The depletion of thymocytes and most likely also of erythrobl
astoid cells occurs via CAV-induced apoptosis. The CAV-encoded protein
apoptin is the main inducer of this phenomenon.