Page 281 - Avian Virology: Current Research and Future Trends

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immunosuppression. The presence of maternal antibodies will Two research teams have developed DNA vaccines. Moeini et
interfere with the replication as discussed before. Thus, vaccina- al. (2011a) used a bicistronic vector expressing VP1 and VP2 to
tion needs to be done when maternal antibodies have waned to vaccinate SPF chickens at 2 weeks of age followed by two booster
the level permitting virus replication, which may take up to 4 vaccinations at 2-week-intervals. Ten-days after the final vacci-
weeks of age (Markowski-Grimsrud and Schat, 2003). The devel- nation VN titres associated with protection were present in the
opment of protective immunity minimally requires 6 to 7 days vaccinated chickens, which also had elevated levels of IFNγ and
during which time exposure to wild-type CAV is likely to occur IL-2. By linking VP1 to VP22 of MDV-serotype 1, VN antibody
as well (Sommer and Cardona, 2003; Eregae et al., 2014). Schat titres were significantly increased compared with vaccination
et al. (2011) showed that SPF chickens receiving an experimental with the original vector expressing VP1 and VP2 (Moeini et
immune complex vaccine at 1 day of age protected against chal- al., 2011b). Sawant et al. (2015) co-administered DNA vectors
lenge at 2 or 3 weeks of age. Immune complex vaccines for IBD expressing VP1 and VP2 together with a vector expressing the
are successfully used in commercial birds (Haddad et al., 1997; chicken high mobility group box 1 protein (HMGB1ΔC) as an
Jeurissen et al., 1998). adjuvant. SPF chickens received three vaccinations and serum
A second problem in the development of a broiler vaccine samples were collected up to 14 weeks post primary vaccination.
that can be used in ovo or in 1-day-old chicks is demonstration The use of the adjuvant boosted the antibody titres measured by
of efficacy. Current commercial vaccines have shown to be effica- ELISA. Challenge experiments have not been reported for any of
cious by challenging 1-day-old chickens which are protected by the DNA vaccines. Unless DNA vaccination induces solid immu-
maternal antibodies. To show efficacy after vaccination at 1 day nity after a single injection, this method will not be of interest to
of age, challenge at 14 days or even 7 days of age will most likely the commercial poultry industry.
not show clinical signs in the unvaccinated controls due to the
rapid development of age resistance to disease. Protection against
viral replication can be used as an alternate method. In the case Perspectives
of the experimental immune complex vaccine a strain-specific CAV is an economically important pathogen for the commercial
qRT-PCR assay (Markowski-Grimsrud et al., 2002) was used to poultry industry, which is often not recognized due to the subclin-
discriminate between the virus used in the immune complex and ical nature of the immunosuppressive infections. The subclinical
the challenge virus. infections not only affect responses to vaccinations and increase
Experimental inactivated vaccines have been shown to pro- susceptibility to other pathogens but also may negatively impact
duce protection (Pagès-Manté et al., 1997; Zhang et al., 2015) feed conversion and growth potential of broilers. For these reasons
and may be used in some countries. However, production of the development of a recombinant vaccine that can be adminis-
high-titered inactivated vaccines is problematic and probably not tered in ovo or at hatch is highly desirable. CAV, and by extension
economical. AGV2, also has an important impact on the SPF industry. Once
Subunit vaccines have been investigated by several research birds seroconvert, the flock is no longer considered SPF and the
groups. As early as 1995, Koch et al. (1995) produced a recom- flock has a diminished economic value. Besides strict isolation,
binant vaccine in baculovirus expressing VP1 and VP2, but this SPF producers have limited tools to prevent infection especially
has not been commercialized. Moeini et al. (2011c) constructed in view of the findings by Cardona et al. (2000b) and Miller et
clones of VP1 and VP2 in frame with the acmA gene of Lactoba- al. (2003, 2004, 2005) that viral DNA, which can be transmitted
cillus lactis. N-acetylmuramidase (AcmA) binds to the cell wall vertically, can be present in chickens that are antibody negative.
of L. acidophilus. The recombinant AcmA-VP1 and -VP2 were Perhaps transgenic chickens can be developed for the SPF indus-
purified and bound to L. acidophilus, which was then fed for 5 try expressing multiple short-hairpin (sh)RNAs interfering with
days to 21-day-old SPF chickens and again for 5 days at 35 days of replication of CAV. Hinton and Doran (2008) have shown that
age. VN antibodies were present 7 days after the second feeding that specific shRNAs can silence CAV replication in MSB1 cells.
period. Challenge experiments were not reported and the need
for prolonged feeding of the bacteria makes this approach less References
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proteins were purified and used to immunize SPF chickens with Adair, B.M. (2000). Immunopathogenesis of chicken anemia virus infection.
rVP1 or rVP1+rpIFNγ at 7 and 17 days of age. In addition, a Dev. Comp. Immunol. 24, 247–255.
group of chickens was vaccinated with an inactivated CAV vac- Adair, B.M., McNeilly, F., McConnell, C.D., Todd, D., Nelson, R.T., and
McNulty, M.S. (1991). Effects of chicken anemia agent on lymphokine
cine (iCAV). At 21 days of age chickens were challenged. The production and lymphocyte transformation in experimentally infected
group receiving rVP1+rpIFNγ had significantly higher ELISA chickens. Avian Dis. 35, 783–792.
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nated groups. This group also had normal haematocrit values at 7 Characterization of surface markers present on cells infected by chicken
anemia virus in experimentally infected chickens. Avian Dis. 37, 943–
days post challenge. It is not clear if the protection was caused by 950.
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an important control group receiving only rpIFNγ was missing. Knowles, N.J., Kropinski, A.M., Krupovic, M., Kuhn, J.H., Mushegian,
A.R., et al. (2016). Ratification vote on taxonomic proposals to the