Vaccines against mildly pathogenic
avian influenza (AI) have been used in turkeys within the United States as part of a comprehensive control strategy. Recently, AI
vaccines have been used in control programs against highly pathogenic (HP) AI of chickens in Pakistan and Mexico. A recombinant
fowl pox-AI
hemagglutinin subtype (H) 5 gene insert
vaccine has been shown to protect specific-pathogen-free chickens from HP H5 AI virus (AIV) challenge and has been licensed by the USDA for emergency use. The ability of the recombinant
fowl pox vaccine to protect chickens preimmunized against
fowl pox is unknown. In the current study, broiler breeders (BB) and white leghorn (WL) pullets vaccinated with a control fowl poxvirus
vaccine (FP-C) and/or a recombinant fowl poxvirus
vaccine containing an
H5 hemagglutinin gene insert (FP-HA) were challenged with a HP H5N2 AIV isolated from chickens in Mexico. When used alone, the FP-HA
vaccine protected BB and WL chickens from lethal challenge, but when given as a secondary
vaccine after a primary FP-C immunization, protection against a HP AIV challenge was inconsistent. Both
vaccines protected against virulent
fowl pox challenge. This lack of consistent protection against HPAI may limit use to chickens without previous
fowl pox vaccinations. In addition, prior exposure to field fowl poxvirus could be expected to limit protection induced by this
vaccine.