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Figure 9.11 (A) Bone marrow with severe hypoplasia of both erythroid and myeloid cells with replacement by adipose tissue in a chicken
infected with chicken anaemia virus. (B) Uninfected control hatch-mate. H&E 12.6 ×. Source: Shivaprasad and Lucio, 1994, with permission
from the American Association of Avian Pathologists.

Bone marrow, spleen, lung and proventriculus occasionally had by intraabdominal injection of 1-day-old SPF chickens with
a few positive cells. Histopathological changes were seen in the CIAV-VAC® [Intervet (now Merck), Millsboro, DE]. Vaccina-
cortex starting at 10 days pi and could be extensive. Infection of tion did not cause clinical or gross lesions with the exception of
+
+
6-week-old birds resulted also in some damage to the thymus. a decrease in blood heterophils. A decrease in small CD4 CD8
Haematocrit values were normal in both age groups. Haridy et al. cells in the thymus was noted at 14 and 28 days post vaccination.
(2012) reported a moderate depletion of CD4+ and CD8+ cells The decrease was correlated with a decrease in CD4 but not in
in the thymus and spleen after im inoculation of 4-week-old SPF CD8 expression. Vaccine virus persisted in some birds up to
chickens with the TK5803 strain at passage 10 in MSB1 cells. the termination of the experiments at 28 days post vaccination.
Interestingly, they also found mild to moderate lymphoid deple- Antibody responses remained low in most vaccinated birds and
tion in the bursa of Fabricius and some other organs. The only did not persist after 18 days post vaccination, probably reflecting a
clinical sign was some reduced weight gain at 4 weeks pi in some lack of Th cells. When chickens vaccinated with CIAV-VAC® were
of the infected chickens. Comparative qPCR studies by Kaffashi challenged with an intermediate strain of IBDV there was a slight
et al. (2006) using 1-day-old and 6-week-old chickens showed increase in the number of birds positive for CAV DNA (Vaziry et
similar levels of virus genome copies in liver, spleen and thymus al., 2013). However, the numbers of birds in this experiment were
tissues with the highest genome concentrations in the thymus. small and the relevance of the results is not clear.
A reduction of weight gain was the only clinical difference in the
6-week-old infected group compared with age-matched controls. Animal models
Chickens are the natural host for CAV and there is no need for an
Pathogenesis of attenuated CAV strains animal model for CAV. When and if AGV2 becomes available as
Some attenuated CAV strains are used as vaccines and therefore it infectious virus preparations, chickens may become a model for
is important to describe the pathogenesis of infection with these HGyV infections in humans.
strains. McKenna et al. (2003) compared the pathogenesis of two
attenuated cloned Cux-1 isolates with the pathogenic Cux-1 by
im inoculation of 1-day-old SPF chickens. The two attenuated iso- Clinical features and pathology
lates did not cause anaemia in contrast with the wild-type Cux-1, Clinical disease is a rare event in commercial flocks and only
the latter also caused severe thymus atrophy and bone marrow occurs when chickens lacking maternal antibodies are infected
cell depletion. Moderate lesions in the thymus were noted for vertically or horizontally before 1 to 2 weeks of age or in older birds
the attenuated isolates, which were also reflected in a significant when the humoral antibody responses are compromised. The
reduction in the number of CD4+ and CD8+ cells in the thymus. major clinical feature of CAV-induced disease is anaemia, which
Hussein et al. (2003) reported that inoculation of 1-day-old SPF manifests itself in pale combs and wattles and low haematocrit
chickens with an unidentified commercial CAV vaccine resulted values. Haematocrit values are infrequently used in chickens and
in decreased haematocrit values and depletion of lymphocytes reference values may change over time due to genetic selection.
in thymus and spleen. Seroconversion rates were variable during Reference values were established in the early 1990s by Good-
the 10-week experimental period. Vaziry et al. (2011) vaccinated win et al. (1991, 1992) showing differences by breeds and age