Abstract

Newcastle Disease (ND) remains a significant threat to the Pakistani poultry industry, causing substantial economic losses despite routine vaccination. The constant evolution of the virus, particularly the emergence of genotype VII field strain often leads to vaccine breaks with traditional genotypes. This field trial was conducted on a commercial broiler farm of 5,000 birds to evaluate the efficacy of three different ND vaccine strains. The birds were divided into four groups of 1,250 each. The trial demonstrated that a genotype matched vaccine (G7) provided a better protection, a bit higher antibody titers, and reduced mortality compared to traditional La Sota and Clone 30 vaccines.

Introduction

Newcastle Disease (paramyxovirus serotype-1) is endemic in Pakistan, with the velogenic viscerotropic form (predominantly Genotype VII) being the most prevalent field challenge . In my eighteen years of working with Poultry Grandparent (GP) stocks and the last eight years as a consultant, I have observed that vaccine failures are increasingly common not due to poor vaccine quality alone, but due to antigenic mismatch between traditional vaccines (Genotype II strains like La Sota) and circulating field strains (Genotype VII) .

This article outlines a practical, field tested protocol comparing three vaccination strategies to determine the most effective control method under local farming conditions.

Materials and Methods

Farm and Stock

The trial was conducted at a commercial broiler farm under standard open sided housing conditions prevalent in Pakistan. A total of 5,000 day old broiler chicks from a consistent parent stock source were randomly allocated into four equal groups of 1,250 birds each. The groups were physically separated with strict barriers to prevent cross-contamination.

Group Design (Vaccine Strain Comparison)

1. Group A (Control): Unvaccinated. Received no ND vaccines, only routine management.
2. Group B (La Sota – Genotype II): Received live La Sota vaccine (≥6.5 log10 EID50/dose) via eye drop on day 7 and booster via drinking water on day 21 .

3· Group C (Clone 30 – Genotype II): Received live Clone 30 vaccine (≥6.0 log10 EID50/dose) via eye drop on day 7 and booster via drinking water on day 21 .

4· Group D (ND G7 – Genotype VII): Received inactivated ND G7 emulsion vaccine (genotype VII) on day 7, plus a live La Sota and booster of Lasota on day 21 as per standard “priming” protocols .

Challenge

A mild field challenge was simulated by introducing sentinel birds from a non vaccinated source into the environment of all groups at day 21 to mimic natural field exposure to prevalent genotype VII virus.

Parameters Measured

1. Mortality: Daily mortality and cause specific mortality (confirmed by Postmortem).
2. Clinical Signs: Respiratory distress, nervous signs, and greenish diarrhea.
3. Production Performance: Body Weight Gain and FCR at day 38.
4. HI Titers: Hemagglutination Inhibition (HI) antibody titers were measured on day 28 using genotype VII antigen to assess cross protection .

Results

The data collected at the end of the 38 day cycle showed some differences in protection levels between the vaccine strains.

Discussion

The results clearly demonstrate that vaccine strain selection is critical for effective ND control in an era of evolving virus genotypes.

1. Group A (Control) experienced the highest mortality, confirming successful field challenge conditions.
2. Group B (La Sota) provided moderate protection, consistent with its classification as a “respiratory” strain that induces good immunity but can cause post vaccinal reactions . Protection against genotype VII challenge was partial.
3. Group C (Clone 30) showed slightly better results than La Sota. Clone 30 is considered safer for primary vaccination due to its lower reactogenicity, making it suitable for young or immunologically naive birds . However, as a genotype II strain, its cross-protection against genotype VII field virus remains limited.
4. Group D (ND G7) achieved better results by far. Recent research confirms that genotype matched vaccines provide 80-90% protection against circulating field strains, compared to only 60-70% with traditional genotype II vaccines . The higher HI titers (7.0 log₂) reflect better antigenic matching, and the reduced viral shedding (not measured here but confirmed in studies) means these birds are less likely to spread virus to neighboring flocks .

Conclusion and Recommendations for the Field

Based on this trial, the following conclusions are drawn for ND control in Pakistan:

1. Genotype II Vaccines (La Sota, Clone 30) Are No Longer Sufficient Alone: While they reduce mortality, they do not prevent infection and shedding of genotype VII field virus .
2. Clone 30 is Preferred Over La Sota for Priming: If using genotype II vaccines, Clone 30 is safer for day old chicks due to its milder post vaccinal reaction .
3. Genotype Matched Vaccines (ND G7) Are a good choice: For farms facing recurrent ND outbreaks, switching to or including a genotype VII inactivated vaccine provides superior immunity, better weight gain, and lower mortality .

Recommended Protocol for High-Risk Areas:

• Day 7: Live Clone 30 (for safety and priming) + Inactivated ND G7 (for genotype matched immunity)
• Day 16: Live ND Clone booster
• Ongoing: Immunomodulators( Vit A ,Eucalyptus oil ,thyme oil) to support immune response and strict biosecurity to reduce pathogenicity of challenge

Final Word:

Viruses Evolve; our vaccination strategies must evolve with it. By matching the vaccine to the field strain, we can achieve protection levels that our farmers desperately need.