Why have we been able to largely control viscerotropic Newcastle disease with vaccination but not respiratory Newcastle disease?
Why have we been able to largely control viscerotropic Newcastle disease with vaccination but not respiratory Newcastle disease?
The reason why control of viscerotropic Newcastle disease (VVNDV) has been more successful with vaccination than respiratory Newcastle disease (e.g. pneumotropic or tracheotropic) is due to several main reasons.
1. Location of virus focus and type of immunity required
Vescerotropics mainly attack internal tissues (intestines, viscera, immune system). The humoral immunity (systemic, serum antibodies) that vaccines induce is very effective in controlling it.
In comparison, respiratory forms mainly replicate in the mucosa of the upper respiratory tract where they are out of reach of the systemic immune system and require local mucosal immunity (IgA, T cells in respiratory tissues, hardin glands, CALT and BALT), which conventional vaccines do not fully induce this level of immunity.
2. Herd conditions and management factors
Respiratory infections are usually exacerbated by high ammonia, dust, high crowding, and poor ventilation. In such conditions, even appropriate vaccination cannot completely suppress respiratory symptoms.
In contrast, viscerotropic infections are more intracellular and environmental factors have less influence on the severity of the disease.
3. Vaccine quality and administration
To control respiratory Newcastle disease, a potent mucosal vaccine (spray or eye drops with appropriate strains such as Clone30, B1, VG/GA) should be used at the right time.
Errors in the administration method (weak spray, drinking in inappropriate conditions, low dose) cause insufficient immunity in the respiratory tract.
4. Diversity of strains and genomic changes of NDV when the shedding virus is mainly respiratory.
Viscerotropic strains are less antigenically diverse and injectable and classic vaccines (La Sota, B1, clone 30) provide adequate coverage. However, in respiratory forms, more diverse strains with genotypic differences (especially in the F and HN proteins) are often seen, which current vaccines do not always fully match.
5. There are concurrent infections with other viruses, mainly with influenza and bronchitis in the respiratory form, which practically renders the vaccine ineffective due to immunosuppression. This situation does not exist in visceral Newcastle.
