Page 77 - Avian Virology: Current Research and Future Trends

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of inducing humoral, cellular and mucosal immunity (Reynolds the F glycoprotein were also protective (Loke et al., 2005). Of
and Maraga, 2000). Mucosal immunity plays an important role these only FPV and HVT vectored vaccines are licensed to use
in preventing virus infection at the site of entry (Al-Garib et al., in the poultry. However, the FPV vectored ND vaccines are not
2003). Both mesogenic and lentogenic strains are used as live vac- widely used because they cannot be applied by mass vaccination
cines. Mesogenic strains are used as booster vaccines in countries methods. The advantages of HVT vectored ND vaccines are that
where ND is enzootic. However, they are prohibited in many they can be used as bivalent vaccines to protect chickens from
countries due to concerns about spreading of virulent virus and ND and Marek’s disease, they can be administered in ovo at the
causing disease in the field. Current live vaccines B1 and LaSota hatchery, and they can produce long term immunity. However,
were derived from field strains isolated in the 1940s that were HVT-vectored ND vaccines require minimum of 4 weeks for pro-
shown to have low pathogenicity for poultry but produced an tective immune response to develop (Palya et al., 2012), which is
adequate protective immune response. B1 has very mild vaccinal not possible in countries where ND is enzootic. Another disad-
reaction and is used widely for initial vaccination of intensive vantage of HVT-vectored ND vaccine is that it is required to be
poultry. In general, LaSota vaccines give better protection than stored in liquid nitrogen and need to be administered within one
B1 vaccines, but produces moderate vaccinal reactions. LaSota hour of thawing.
and B1 are respirotropic vaccine strains, they produce more local It should be noted that NDV is a highly infectious and a fast
immunity in the respiratory tract; whereas VG-GA is an entero- replicating virus. Therefore, a robust immune response is required
tropic vaccine strain, that produces more local immunity in the to completely block virus replication at the site of infection. This
enteric tract. All conventional live vaccines have the disadvantage can probably only be achieved by a live virus vaccine and partici-
of needing to be kept at low temperature because of their thermo- pation of innate, cell-mediated and humoral immune responses.
lability. This is particularly a problem in village flocks in tropical Current live virus vaccines (LaSota and B1) are widely used in
countries. Several thermostable vaccines have been developed, the world, but ND outbreaks still occur in vaccinated flocks, indi-
but the most used thermostable vaccine strain I-2 is a derivative cating the inadequacy of currently used vaccines. Although the
of the avirulent Australian V4 vaccine strain (Spradbrow and current live virus vaccines have been highly effective in protecting
Sabine, 1995). The most common method of mass application of chickens from clinical disease and mortality (Cho et al., 2008),
live vaccines is via drinking water. However, it is difficult to ensure they do not completely prevent virulent virus infection or shed-
delivery of an appropriate dose to each bird by this method. Mass ding, thereby allowing the virulent virus to circulate unnoticed
application of live vaccines by sprays and aerosols is also very in vaccinated chickens and promoting evolution of more virulent
popular. The major limitations of this technique are loss of vac- viruses. Another deficiency of the current vaccines is that they
cine virus and difficulties to maintain uniformity of particle size. do not possess genetic markers to allow differentiation between
Vaccination of individual bird with live vaccines is carried out by infected and vaccinated birds, which is critical for eradication
intranasal instillation, eye drop, and beak-dipping method. purpose. The current vaccines have also reduced efficiency when
Inactivated vaccines are produced by treating infective used in chickens with high level of maternally derived antibodies.
allantoic fluid with an inactivating agent, such as formalin or The current vaccine strains LaSota and B1 were isolated in
beta-propiolactone. An adjuvant, such as mineral oil is added to 1940s in the USA and belong to genotype II, whereas majority of
increase the immunogenicity of the inactivated virus. However, virulent NDV strains circulating worldwide belong to genotype
most adjuvants cause tissue destruction and irritation leading to V-VII (Miller et al., 2010). There is 10 to 14% amino acid differ-
condemnation of meat; hence, they are not suitable for broilers. ence in the F and HN proteins between current vaccine strains
Inactivated vaccines are administered either intramuscularly or and circulating virulent NDV strains. Recent studies indicated
subcutaneously. Because of the cost of the vaccine and the cost that inactivated vaccines (Miller et al., 2007; Jeon et al., 2008;
of individual vaccination, inactivated vaccines are used mainly for Z. Hu et al., 2011) or a live attenuated vaccine (Hu et al., 2009;
revaccinating layers, breeders and some turkeys. Inactivated vac- Xiao et al., 2012; Kim et al., 2013) developed from a currently
cines are extensively used in village poultry. Inactivated vaccines circulating genotype strain induced higher levels of neutralizing
produce high humoral antibody response, but poor cell mediated antibodies and significantly decreased the amount of challenge
immune response. Inactivated vaccines licensed for use in pigeons virus shedding from vaccinated chickens. Although these vac-
are widely used, particularly in racing pigeons. cines did not completely stop virus shedding, they demonstrated
Several recombinant vaccines have been developed, which that a genotype matched vaccine generated by reverse genetics
provide various levels of protection against NDV in experimental can provide better protection than traditional vaccines.
conditions. Fowlpox virus (FPV) (Boursnell et al., 1990a,b; King, Live genotype matched ND vaccines have been developed
1999), vaccinia virus (Meulemans et al., 1988), pigeon pox virus using reverse genetics (Hu et al., 2009; Xiao et al., 2012; Kim et
(Letellier et al., 1991), herpesvirus of turkeys (HVT) (Morgan al., 2017). These vaccine strains are identical to the circulating
et al., 1993; Sakaguchi et al., 1998), Marek’s disease virus, and virulent NDV strains except for F protein cleavage site, which
retrovirus (Morrison et al., 1990; Sonoda et al., 2000) have been has been modified from a polybasic cleavage site to decrease
used as vectors to express F and/or HN proteins of NDV. The virulence. However, development of a reverse genetic system for
F and HN proteins expressed individually using a baculovirus a circulating NDV strain can be costly and time consuming. An
expression system were also found protective (Meulemans et alternative strategy for producing genotype matched vaccine is to
al., 1988; Kamiya et al., 1994). Naked DNA plasmids expressing replace the F and HN genes of a recombinant commercial vaccine