April 23, 2010

Poultry disease vaccine may bring long-term problems

Attenuated live vaccines that protect poultry against Newcastle Disease may be altering the genetic makeup of the wild virus strains, which could make future outbreaks unpredictable, according to US biologists.

Newcastle disease is an economically devastating poultry disease that costs the industry millions of dollars.

"Many vaccines in the animal industry are developed by modifying a virulent live virus," said Mary Poss, professor of biology and veterinary and biomedical sciences, Penn State. "These vaccines elicit a strong protection against disease."

However, vaccinated birds can shed the vaccine virus to infect other birds, and live virus vaccines do not always protect birds from infection from other viral strains of Newcastle disease.

Poss and her Penn State colleagues Yee Ling Chong, graduate student in biology; Abinash Padhi, post-doctoral fellow and Peter J. Hudson, Willaman professor of biology, found that one vaccine strain recombined with at least three wild strains, creating new viruses. These viruses are found in both domestic and wild birds.

"Our findings indicate that birds can be simultaneously infected with the live virus vaccine and several other strains of this avian virus," said Poss. "This raises concerns that modified live virus vaccines, though effective, may combine with circulating viruses to create unpredictable new strains."

A modified live virus vaccine is essentially a weakened virus that does not cause disease but mimics a natural infection that in turn evokes a strong immune response from the infected host. But Poss argues that vaccination may be unwittingly increasing the diversity of Newcastle disease viruses that are circulating in wild birds.

For instance, many poultry farmers typically vaccinate the flock by mixing the vaccine in the birds' drinking water or by aerosol, which means wild birds and pigeons can also become infected with the vaccine virus.