Avian leukosis virus – new mutations: a threat for the upcoming century

Abstract

New strains of avian leukosis virus (ALV) and new threats to the poultry industry such as the appearance of subgroup J ALV arise as a result of virus mutation. Avian retroviruses are classified as exogenous (transmitted as infectious virus) and endogenous (transmitted genetically). Exogenous species are the avian leukosis-sarcoma viruses, reticuloendotheliosis virus, and lymphoproliferative disease virus of turkeys. Endogenous virus families include the ev loci, EAV, ART-CH and CR 1. Evolutionary relationships between different species of retrovirus are determined from their nucleotide sequences and construction of phylogenetic trees. Similar analyses are conducted to determine the relationships between endogenous and exogenous retroviruses and the various retroelements from which retroviruses are believed to have evolved. Retroviruses are subject to several types of mutation. Point mutations are the most common and occur mainly because of errors during reverse transcription. The frequency of point mutations is of the order of one mutation per virion per replicative cycle. Point mutations occur most frequently in the env gene of ALVs resulting in changes in antigenicity and host range. Antigenic variation enables mutant viruses to avoid the host immune response. Mutation also occurs from genetic recombination among exogenous and endogenous ALVs, and between exogenous ALVs and non-retroviral host genes such as cellular oncogenes, giving rise to acute avian leukaemia and sarcoma viruses. Thus, continuing viral mutation will result in the appearance of ALVs with new disease producing properties.