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Chicken Infectious Anaemia Virus | 269

positive semen can be a source of infection resulting in vertical pi. Maximum virus titres were obtained between 7 and 21 days
transmission. Parent and grandparent flocks can also transmit pi when VN antibodies were present. Rectal contents were posi-
viral DNA to the embryos even in the presence of high titres tive from 1 to 49 days pi, thus well after the development of VN
of VN antibodies (Brentano et al., 2005; Hailemariam et al., antibodies. At 42 and 49 days pi CAV could still be detected in a
2008) like the transfer of viral DNA in SPF flocks. The practi- few organs. Using histology and immunohistochemistry, Smyth
cal importance of the transfer of viral DNA to the embryos in et al. (1993) detected CAV antigen at 3 days pi in the bone
commercial settings is unknown at the current time. marrow and at 4 days pi in thymus, spleen and lung. At 11 days
pi most of the other organs had positive aggregates of lymphoid
Transmission through contaminated vaccines cells. Antigen remained present in the thymus until 20 days pi,
Since the first description of CAV by Yuasa et al. (1979) reports but other organs were negative at 16 days pi. It is unknown which
have been published that SPF flocks seroconverted to CAV (e.g. cells become infected first in the host after intramuscular, oral or
McNulty et al., 1989). Recently, Li et al. (2016b) reported a 20% ocular inoculation.
seroconversion in a SPF flock and isolated CAV from one of the Anaemia typically develops between 8 and 10 days pi and
antibody positive chickens. The reasons that SPF flocks experi- recovers between 20 and 28 days pi (Taniguchi et al., 1983; Smyth
ence breaks has been discussed in a previous section. et al., 1993). In parallel with the decrease in haematocrit values
Nicholas et al. (1989) and Kulcsar et al. (2010) examined the numbers of white blood cells and thrombocytes declined
24 and 27 batches of live vaccines, respectively, without finding (Taniguchi et al., 1983).
evidence for CAV contamination. In contrast, recent testing of The first histological changes are detected in the bone marrow
vaccines showed 2/14 (Li et al., 2017) and 7/31 (Varela et al., and thymus between 4 and 5 days (Taniguchi et al., 1983; Goryo
2014) batches to be contaminated with CAV. The latter group et al., 1989a; Jeurissen et al., 1989; Hu et al., 1993b; Smyth et al.,
also showed that 8/31 batches were contaminated with AGV2. 1993). In the bone marrow some enlarged hemocytoblasts are
The difference in detection of CAV in vaccines may reflect the present, which may contain eosinophilic intranuclear inclusion
different methods used to examine vaccine batches for CAV bodies. Over the next few days hypoplasia develops followed
contamination. An accepted method for the detection and by aplasia with replacement of the hemocytoblasts by fatty cells
identification of extraneous CAV by PCR can be found on the (Fig. 9.11). Between 18 and 28 days pi the bone marrow becomes
USDA-APHIS website under number VIRPRO0118.05. hypercellular with extensive hemopoietic activity leading to res-
CAV contaminated MD vaccines are an excellent vehicle toration of haematocrit values and normal levels of granulocytes
to introduce CAV into poultry flocks as demonstrated by the and thrombocytes. Changes in the thymus start around 4 to 5
original isolation of CAV by Yuasa et al. (1979). MD vaccines days pi with the appearance of some enlarged lymphoblasts in the
are produced in primary CEF prepared from SPF embryos. If cortex followed by lymphoblast depletion starting between 5 to 6
CAV is present in the embryos it is likely to be present in the days pi (Fig. 9.9). Between 14 and 21 days pi, the percentage of
+
+
vaccine, which is injected in ovo at ED18 or at one day of age. If CD4 and CD8 cells is sharply reduced compared with controls.
CAV-contaminated vaccine is inoculated into embryos or 1-day- Regeneration of the thymus cortex starts around 21 days pi and at
old chicks lacking maternal antibodies, clinical disease is likely to 28 days there is no difference in cell populations between infected
occur. and control birds.
The return to normal histology for the bone marrow and
Pathogenesis in young chickens (1 to 7 days thymus coincides with the development of VN antibodies, which
of age) starts around 14 days pi. However, virus can be shed for 2 to 5
The pathogenesis of CAV has been studied in 1-day-old SPF weeks after the development of VN antibodies (e.g. Yuasa et al.,
chickens infected by parenteral injection or by the oral route. The 1983b; Imai et al., 1999; Drén et al., 2000) and can remain present
route of exposure and the dose of virus are important determi- as latent virus for much longer (Cardona et al., 2000b). Interfer-
nants for the outcome of infection. In general, oral inoculation ence with antibody development by infectious bursal disease virus
reduced or delayed viral replication and severity of the lesions can prolong the persistence of CAV and increase the morbidity
such as the decrease in weight gain and haematocrit values and mortality even when birds are infected with CAV at an older
compared with exposure by injection (Rosenberger and Cloud, age (Yuasa et al., 1980b; von Bülow et al., 1986b; Rosenberger
1989a; van Santen et al., 2004a; Tan and Tannock, 2005). and Cloud, 1989a; Imai et al., 1999). Simultaneous infections
The first pathogenesis studies were conducted using virus with reovirus (McNeilly et al., 1995), MDV or REV (von Bülow
isolation and/or staining of tissues for viral antigens (Yuasa et al., et al., 1986b) also aggravates the severity of CAV infections.
1983b; Smyth et al., 1993). After the development of qPCR more
specific information on virus titres was obtained (van Santen et Pathogenesis in older chickens
al., 2004a; Tan and Tannock, 2005; Kaffashi et al., 2006; Wani Exposure to CAV after 2 weeks of age results in most instances in a
et al., 2016) but the use of qPCR did not fundamentally change subclinical infection except when the humoral immune response
the data. Yuasa et al. (1983b) infected 1-day-old SPF chickens by is compromised. Smyth et al. (2006) infected 3- and 6-week-old
intramuscular injection and used virus isolation between 1 and SPF chickens by the oral route. In 3-week-old birds, CAV antigen
49 days pi. Spleen, liver and bone marrow had the highest titre was detected in the thymus starting at 8 days pi and was abun-
at 1 day pi. Thymus and kidney samples were positive on day 2 dant in the outer cortex of all birds between 13 and 17 days pi.