Lack of interaction between avian leukosis virus subgroup J and fowl adenovirus (FAV) in FAV-antibody-positive chickens

Natural infection and transmission of a retrovirus closely related to myeloblastosis-associated virus type 1 in egg-type chickens

Myeloblastosis-associated virus type 1 (MAV-1) is an exogenous avian retrovirus with oncogenic potential. MAV-1 was detected in young chicks hatching from eggs produced by an experimental genetic line of egg-type chickens. Transmissibility of MAV-1 had not been documented previously. This investigation was intended to partially characterize the virus involved and to study its transmissibility and oncogenicity in naturally and contact-infected chickens. Commercially produced white and brown layer pullets free of exogenous avian leukosis viruses were commingled at hatch with naturally MAV-1-infected chickens. The original MAV-1-infected chickens were discarded after approximately 8 wk, and the contact-exposed chickens were maintained in isolation for 36 wk. Young specific-pathogen-free (SPF) single comb white leghorn chickens were added to the group to study possible horizontal transmission of MAV-1 in young chickens. Upon weekly virus isolation attempts, MAV-1 was readily isolated from the contact-exposed white layers but not from the brown layers between 36 and 53 wk of age (18 wk in total). Three-week-old SPF chickens were readily infected with MAV-1 by contact as early as 1 wk postexposure. Throughout 22 hatches derived from the white and brown MAV-1-contact-exposed layers (between 36 and 53 wk of age), MAV-1 was frequently detected in the white layer progeny, whereas the virus was seldom isolated from the progeny produced by the brown layers during the same 18-wk period. MAV-1 induced a persistent infection in some of the SPF chickens that were exposed by contact at 3 wk of age. Gross tumors were not detected in any of the originally infected experimental chickens at 8 wk of age, in the contact-exposed brown or white layers at the termination of the study at 53 wks of age, or in the contact-exposed SPF chickens at the end of the study at 12 wk of age. Exogenous avian leukosis-related viruses may still be detected in egg-type chickens, emphasizing the importance of thorough screening before incorporation of experimental genetic material into commercial genetic lines of egg-type chickens.

Experimental Infection with Avian Leukosis Virus Isolated from Marek's Disease Vaccines

Recently, avian leukosis virus (ALV) was isolated from four lots of Marek's disease vaccine produced by two laboratories. The ALVs isolated were characterized by examination of their interactions with cells of two phenotypes (C/E and C/A,E), subgroup-specific polymerase chain reaction (PCR), virus neutralization, envelope gene sequencing, and phylogenetic analysis. All four ALVs are exogenous, belong to subgroup A, and appear to be virtually identical to each other based on PCR and envelope gene nucleotide sequences. We describe herein the characterization of the contaminant viruses in vivo by means of experimental infection in chickens. The contaminant viruses established transient viremia in specified pathogen-free (SPF) Leghorn chickens and elicited a robust and lasting antibody response detectable by enzyme-linked immunosorbent assay. None of the contaminant ALVs induced tumors up to 31 wk of age, and mortality was insignificant. Despite a strong antibody response against the contaminant ALVs, vertical (congenital) transmission to the progeny of experimentally infected SPF chickens took place, albeit at a very low rate (< or = 1.6%). Experimental infection in meat-type chicken embryos resulted in viremia at hatch, suggesting that some meat-type chickens are susceptible to infection and support virus replication.