Researchers with the U.S. Department of Agriculture's Agricultural Research Service (ARS) have developed an improved Newcastle disease virus (NDV) vaccine evaluation procedure that could be used to select better vaccines to treat the disease.

Newcastle disease, one of the most important poultry diseases worldwide, can cause severe illness in chickens and other birds. Severe, or virulent, strains rarely occur in poultry species in the U.S., but they are regularly found in poultry in many foreign countries, ARS explained.

Available commercial NDV vaccines perform well in chickens infected with virulent NDV under experimental conditions. They also perform well under field conditions where virulent virus is not common. However, they often fail in countries where virulent viruses are endemic.

At the ARS Southeast Poultry Research Laboratory (SEPRL) in Athens, Ga., microbiologist Claudio Afonso and veterinary medical officer Patti Miller have updated the traditional vaccine evaluation method, which does not compare vaccines or take into account suboptimal field conditions.

Under perfect conditions, vaccines should work, but conditions are not always perfect in the field, according to Miller. Chickens sometimes get less than the required vaccine dose and don't always have the minimum amount of time required to develop an optimum immune response.

The improved vaccine evaluation procedure compares vaccines made using genes from the same viral strain (genotype) that the birds are exposed to in the field to vaccines made with a strain that differs from the virus birds are exposed to, ARS said.

Using the improved procedure, scientists inoculated chickens with different vaccine doses before exposure to a high dose of virulent NDV. Birds given the genotype-matched vaccine had reduced viral shedding, superior immune responses, reduced clinical signs and increased survival than the birds vaccinated with a different-genotype vaccine, ARS reported. By using genotype-matched vaccines, viral shedding and death were significantly reduced.