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Newcastle Disease Virus | 69

infection has been shown to activate several aspects of host innate cells susceptible to fusion, as well as in nonfusing cells. It has been
immunity including type I IFN (Elankumaran et al., 2010), apop- shown that the F protein is the major contributor to induction
tosis (Ravindra et al., 2008; Harrison et al., 2011), and autophagy of neutralizing antibodies and protective immunity, followed by
(Sun et al., 2014). HN protein, which provides partial protection (Kim et al., 2013).
The IFN system is the most important host defence mecha- Passive immunity is conferred by transfer of maternal antibodies
nism during viral infection because it controls viral infection to chicks via egg yolk (Heller et al., 1977). These maternal anti-
and modulates innate immune responses. In chickens, a weak bodies provide protection in the first two weeks of life, but during
induction of IFN has been correlated with higher virus titres and vaccination can neutralize the vaccine virus rendering the vaccine
a longer shedding period, whereas, strong IFN expression results less effective (Awang et al., 1992). Levels of maternal antibody
in lower virus titres and a shorter shedding period (Cauthen et al., in day-old chicks directly correlate to the antibody titre of the
2007; Liang et al., 2011). It was shown that chicken cells induce parent.
higher levels of IFNs and other pro-inflammatory cytokines than The mucosal immune responses are characterized by the pro-
duck cells (Kang et al., 2016). The high levels of IFNs in chickens duction of secretory IgA that plays an important role in providing
rapidly clears viral infection and shortens the shedding period (3 protection to NDV both by restricting primary replication at the
days), whereas ducks exhibit a longer shedding period (14 days). site of entry and also preventing shedding of the virus (Holmes,
It was suggested that the host innate immune responses may 1979). Vaccination with live virus induces an IgA response at
be responsible for the differences in the pathogenicity of NDV mucosal surfaces of the respiratory and gastrointestinal tracts, and
between chickens and ducks (Kang et al., 2016). in the Harderian gland of eye, whereas parenteral administration
Both cell-mediated immunity (CMI) and humoral immunity of vaccine rarely induces a local IgA response (Parry and Aitken,
play a role in the protection against NDV. CMI is the initial 1977; Powell et al., 1979). Secretory IgA is particularly important
immune response that is detected as early as 2–3 days after infec- in protecting the upper respiratory tract, which is accessed inef-
tion with live vaccine strains (Ghumman and Bankowski, 1976; ficiently by serum IgG. In summary, although it is not known
Timms and Alexander, 1977). The role of CMI response to NDV which arm of the immune response nor which viral envelope gly-
in non-vaccinated chickens is not clear. Depletion of B cells by coprotein is more important in providing protection against ND,
cyclophosphamide resulted in undetectable levels of IgG, IgM, it is known that a robust immune response is required to com-
and IgA antibodies, but NDV was still cleared (Lam and Hao, pletely cease virus replication and provide long-term protection,
1987). Furthermore, bursectomized chickens vaccinated with which can only be induced by both the envelope glycoproteins
a live virus vaccine had little humoral response but were fully and participation of local and systematic humoral and cellular
protected from a virulent NDV challenge (Marino and Hanson, immune responses.
1987). These results suggest that CMI plays an important role in
virus clearance, but alone it may not be sufficient for protection
against virulent NDV infection (Reynolds and Maraqa, 2000). Control of ND
The humoral immune response plays a major role in virus Current strategies to control ND include (1) culling of infected
clearance and in protection against re-infection. Passive transfer and contact birds, (2) good biosecurity practices, and (3) pre-
of immunoglobulin from vaccinated chickens to unvaccinated ventative vaccines. In most NDV-free countries, outbreaks of ND
chickens induces protection against virulent NDV challenge are controlled by culling or stamping-out programs, whereby all
indicating that antibodies can provide protection. Serum IgM infected and contact birds are euthanized. Carcasses of dead birds
antibodies are detected as early as 4 days post infection; whereas, and bird products are sanitized, and affected premises are disin-
the IgG antibodies are detected between 6–10 days with peak fected and left without birds for a period. This is the best method
responses at about 3–4 weeks. The antibodies may last up to for removing the virus and virus reservoir in a disease-free area or
one year in the blood of infected birds, but their titre may not be country. In enzootic area or country, ND is controlled by good
sufficient to provide protection against virulent NDV infection. biosecurity practices and vaccination. Biosecurity is achieved by
The protective immune response to NDV is usually assessed by addressing three goals: isolation, sanitation and movement con-
HI test because the HI antibody titre generally parallels well with trol. The poultry farms and flocks should be well separated. Bird
the virus neutralization antibody titre. However, some studies houses, feed stores and water reservoirs should be bird-proofed.
have shown that HI titre is not always directly correlated with All equipment, particularly vehicles, should be properly disin-
the chickens’ level of resistance (Gough and Alexander, 1973; fected before entry into farm site. To prevent spread of disease,
Holmes, 1979). Therefore, virus neutralization antibody titer the movement of poultry, poultry products, equipment and
of the serum should be determined by virus neutralization test people must be controlled. In addition to biosecurity, vaccination
not by HI test. The protective antibody responses to NDV are is used to prevent ND.
directed against both the surface glycoproteins F and HN as
these proteins are readily accessible to antibodies on the virions
and on infected cells. Antibodies to HN block virus adsorption; Vaccination
therefore, is effective only when there is no cell fusion and dis- Vaccination against ND is performed using either live, inactivated
semination occurs through released progeny virions. Whereas, or vectored vaccines. Among these vaccines, live vaccines are
antibodies to F are capable of preventing spread of infection in most widely used to control ND. Live vaccines have the advantage