Chickens Succumbing To Virus Formerly Avoided By Vaccination

Live vaccines have been used for more than 50 years to protect commercial poultry against fowlpox – a slow-spreading infection in birds for which there is no treatment. A new viral strain, carrying genes from an unrelated avian virus, has arisen to cause disease in previously vaccinated chickens in Arkansas, California, Indiana, Minnesota, Nebraska, New York and Pennsylvania.

Deoki N. Tripathy, a professor of veterinary pathobiology in the UI College of Veterinary Medicine, summarized the situation in a presentation during the 50th Western Poultry Disease Conference in March at the University of California at Davis.

“In most of these occurrences, the diphtheritic form of the disease, which attacks the eyes, throats and trachea, results in high mortality,” Tripathy said. “Reduced egg production with significant economic losses in affected layer flocks also has been observed.” A molecular analysis of the new form of the virus in the United States and in Australia shows the integration of a retrovirus, avian reticuloendotheliosis virus (REV), into its genome. While fowlpox virus vaccines carry a portion of REV, the new strains contain intact REV.

In the February issue of the Journal of Virology, Tripathy reported that the fowlpox virus also has two other genes. One encodes for an enzyme, photolyase, which allows for the repair of DNA damaged by sunlight, and the other for a protein that shields the virus from environmental damage.

“The virus has acquired several genes that are not vital for its multiplication but provide a good strategy for prolonged survival,” he said. Since fowlpox is among the largest of viruses – more than 200 genes encoding for essential and non-essential proteins – it is not surprising to find “spontaneous, natural genetic engineering” occurring between distinct viruses attacking the same birds, he said. In large poultry operations, birds face multiple pathogens.

Fowlpox occurs as a dry pox, which results in wart-like nodules on the skin that turn to scabs, and/or a usually fatal wet pox that affects the oral cavity and upper respiratory system. Most birds recover from dry pox within three to five weeks but not without losses in egg production or growth.

The scabs from dry pox that fall from infected birds can contain millions of virus particles and survive for months in the poultry environment. Infection can spread into injured or lacerated skin, and by birds inhaling dust containing the particles. Mosquitoes also can transmit the disease.

Tripathy and his colleagues are researching a new generation of poultry vaccines.