Page 279 - Avian Virology: Current Research and Future Trends

272 | Schat

also decreased in the spleen, but the decrease is often less dra- of age caused severe respiratory effects especially when the vac-
matic than in the thymus (Adair et al., 1993; Hu et al., 1993b; cine was given at one day of age. However, humoral immune
Bounous et al., 1995). Infection after maternal antibodies waned responses were not impaired at 6 weeks of age (De Boer et al.,
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showed similar decreases in CD4 and CD8 T-cells in the spleen 1994). CAV can aggravate bacterial infections probably leading
and blood (Haridy et al., 2012; Wani et al., 2015). Functional to enhanced condemnation rates as suggested by Sommer and
assays have shown that co-infection of REV and CAV causes a Cardona (2003) (Dermatitis leading to ‘blue-wing disease’ is also
decrease in REV-specific CD8 cytotoxic T lymphocytes (CTL) enhanced by CAV infection (Engström et al., 1988).
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(Markowski-Grimsrud and Schat, 2003). The effect on CD4 Th
cells was shown by van Ginkel et al. (2008). Co-infection of CAV
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with infectious bronchitis virus (IBV) reduced the ratio of CD4 / Epizootiology
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CD8 in the Harderian gland and caecal tonsils causing a reduc- CAV infections have been reported in commercial and backyard
tion in IBV-specific IgA secreting cells and a delayed IgA response flocks from all continents (reviewed by von Bülow and Schat,
to IBV. Clearance of IBV was delayed in dually infected birds 1997; Schat, 2009). Publications on isolation of CAV or detec-
compared with IBV infection alone. Clearly, CAV effects antigen- tion of CAV antibodies continue to be added to the literature
specific Th and CTL activity, which provides the explanation for without adding basic new information. Chickens are the major
the CAV-induced decrease in vaccine responses and increase in host as discussed in Host range.
certain infections. Once maternal antibodies wane in commercial broiler flocks,
acquired antibodies can be detected as early as 35 days of age
Interactions with other pathogens and vaccines (Sommer and Cardona, 2003). A cross-sectional study in Ontario
Perhaps due to the possibility of injecting CAV-contaminated showed that 178/231 (77%) of broiler flocks had antibodies to
MDV vaccines in 1-day-old chicks the interaction between MDV CAV between 31 and 53 days of age (Eregae et al., 2014). Unfortu-
and CAV was noted almost as soon as CAV was isolated by Yuasa nately, there was no information provided on the age brackets for
et al. (1979) from birds inoculated with a contaminated MDV the antibody negative flocks. In Georgia (USA), 67 of 68 broiler
vaccine. flocks were positive for CAV antibodies in 2005. The age of these
Von Bülow et al. (1983) noted that dual infections of CAV flocks ranged from 34 to 53 days (L. Dufour-Zavala, unpublished
and MDV enhanced the anaemia as well early MD lesions. data quoted by Smith, 2006). These data clearly indicate that the
HVT-vaccinated chickens dually infected with MDV and CAV majority of broiler flocks in North America become infected with
experienced increased mortality with severe thymus and bursal CAV.
atrophy and increased MD lesions. Experimental infection with Most of the breeder and layer flocks also become antibody
CAV and MDV isolated from the diseased birds confirmed the positive by natural exposure between 5 and 10 weeks of age. If
interaction between the two viruses (Otaki et al., 1987). In a breeder flocks are not seropositive by 8 to 10 weeks of age, vac-
subsequent study, it was shown that inoculation of HVT plus cination is recommended to provide maternal immunity to their
CAV aggravated the depression of mitogen stimulation compared offspring.
with CAV alone (Otaaki et al., 1988) suggesting decreased cell-
mediated immunity, which is of crucial importance for vaccine
induced protection to MDV (Schat and Xing, 2000). Otaki et Diagnosis
al. (1988) suggested that CAV infection can cause MD vaccine The diagnosis of clinical disease caused by CAV infection can be
breaks. Fehler and Winter (2001) analysed MDV isolates from 26 problematic because there are no specific lesions associated with
flocks experiencing MD vaccine breaks and found CAV in 70% of CAV infection with the exception of eosinophilic refractile intra-
the isolates. Miles et al. (2001) studied the impact of CAV infec- nuclear inclusion bodies in thymocytes, which are rarely seen in
tion on the early pathogenesis of RB-1B, a very virulent (vv) MDV diagnostic cases (Smyth and Schat, 2013). Severe thymus atrophy
isolate, and 584A, a vv+ MDV isolate. Co-infection increased the and pale bone marrow provide some indication that CAV may
replication of RB-1B to as early as 4 days pi, but the degree of be involved but these lesions are not specific for CAV-induced
increase was less severe with 584A. The latter replicated also to a disease. For example, infection with vv or vv + MDV can also
higher titre than RB-1B in the absence of CAV. It is likely that the cause severe thymus atrophy (Calnek et al., 1998). Thus, in addi-
impact of CAV on the early pathogenesis of MDV depends on the tion to pathology, the flock history and especially the antibody
virus dose as suggested by Jeurissen and de Boer (1993) or the status of the parent flock will provide further evidence that CAV
degree of MDV replication. may be the cause. Detection of viral antigens in affected tissues
The interactions between IBDV and CAV have been described by immunohistochemistry or immunofluorescence, preferably
in the section Pathogenesis in young chickens. CAV infection can using monoclonal antibodies to VP3, will confirm the involve-
increase the incidence of inclusion body hepatitis and hydrop- ment of CAV in clinical disease. Molecular tests, especially qPCR
ericardium syndrome (Toro et al., 2000). Combined vaccination and qRT-PCR assays, may be used to further demonstrate the
with an inactivated fowl adenovirus vaccine and live attenuated involvement of CAV. Positive results with the qRT-PCR assay is
CAV vaccine protected against these diseases (Toro et al., 2002). the most important one because it will indicate that CAV is rep-
Spray vaccination against NDV using La Sota clone 30 vaccine licating. Positive qPCR tests only show the presence of CAV and
at 1 or 10 days of age combined with CAV infection at one day low copy numbers may reflect the presence of latent viral DNA.