Addressing evolving velogenic Newcastle disease virus strains
While vaccines have reduced the incidence and virulence of Newcastle disease, outbreaks are prevalent, leading to high rates of mortality and culling of infected birds, along with associated negative impacts on food security and economic stability.
Costs associated with Newcastle disease range from approximately US$70 million to US$2.5 billion worldwide. “This emphasises the need for constant attention and vaccination,” says CK Mah, Newcastle disease expert at Zoetis.
Velogenic Newcastle disease virus (NDV) is endemic in areas of Central and South America and widely spread across Asia, the Middle East and Africa with sporadic outbreaks in Europe and the US. Lentogenic NDV strains are worldwide in their distribution.
Mah: “With improved biosecurity standards on most farms, some modernised commercial poultry operations have remained free from Newcastle disease, although outbreaks are still occurring in operations where biosecurity protocols are not as robust.”
Newcastle disease can cause death in susceptible chickens within 3-4 days, resulting in mortality rates of 55-100%. The infectious nature of the Newcastle disease virus together with high mortality rates can pose constraints on worldwide poultry production.
Virus strains
Lentogenic subtypes (e.g., LaSota and B1) are low pathogenic forms of the Newcastle disease virus. They are widespread but cause few disease outbreaks. Lentogenic strains are used as vaccines to prevent Newcastle disease in poultry.
Mesogenic subtypes are moderately pathogenic forms of the Newcastle disease virus. They cause respiratory symptoms and occasional nervous system symptoms but have low mortality.
Velogenic subtypes are highly pathogenic forms of the Newcastle disease virus and must be officially reported to the World Organization for Animal Health. There are 2 forms of velogenic NDV: Viscerotropic velogenic (haemorrhagic intestinal lesions) and Neurotropic velogenic (respiratory and nervous system symptoms).
Genotype VII velogenic Newcastle disease virus strains are the predominant concern in Asian countries. They are also prevalent in the Middle East, as well as in northern and southern Africa. Ensuring protection against a broad range of Newcastle disease virus types is important. In unvaccinated birds, Genotype VII can cause high morbidity and mortality, reaching 100% in commercial broilers, layers and breeders, said Mah.
He continues: “Newcastle disease virus Genotype VII is often further divided into subgenotypes, some of which are extremely velogenic (aggressive and dangerous). The symptoms and lesions induced by Newcastle disease virus vary, but without timely and appropriate interventions, significant losses are inevitable.”
Vector vaccines
“The introduction of recombinant vector vaccines with a herpesvirus of turkeys (HVT) carrier made them valuable tools for veterinarians and production managers to address this evolving threat because they have proven to be effective at inducing a robust immune response,” said Sing Rong, PhD, senior research director, Zoetis.
“Several key factors are involved in the development of a good recombinant vector vaccine, including where the F protein will be inserted into the HVT genome,” Rong said. “Where you insert not only impacts the stability of the viruses, it impacts the efficacy so the insertion site is important.”
Other important factors include selection of the best expression cassette that includes the F protein antigen sequence, the promoter — which goes in front of the sequence to drive expression of the antigen — and a poly A DNA sequence at the end of the cassette. “You want a strong promoter to have good antigen expression, but you don’t want it too strong,” Rong said.
“If it is too strong, that can lead to instability, so different promoters have to be tried to find the best one.” Vector vaccines do not interfere with maternal antibodies, and in the case with Poulvac Procerta HVT-ND, onset of immunity is seen at 19 days of age.
Genotype VII protection
“With the launch of the latest ND vector vaccine in 2020, Zoetis studied the vaccine’s ability to address NDV Genotype VII as a stand-alone vaccination protocol or in combination with other live or killed ND vaccines,” Rong said. Studies demonstrated critical Newcastle disease virus Genotype VII protection, including the Poulvac Procerta HVT-ND vaccine:
Can be used as part of a multiple vaccination strategy to help alleviate production loss and mortality in chickens challenged with Newcastle disease at 21 and 28 days of age. Can be used as a stand-alone program or as an addition to an existing commercial vaccination protocol either administered in ovo or subcutaneously. Can provide protection under most velogenic Newcastle disease virus challenges, while the addition of an inactivated Newcastle disease vaccine may be more beneficial during high velogenic Newcastle disease virus challenge times of the year.
These studies provide more insights that will help veterinarians enhance protection against Genotype VII as they tailor vaccination plans according to regional challenges. Mah concludes: “Besides vaccination, additional prevention and control measures for Newcastle disease include stamping out, disease monitoring, and imposing biosecurity measures, for example, minimising the number of visitors and strict hygiene measures have proven to be valuable tools besides vaccination.”
References available upon request
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