SUMMARY
Avian influenza virus of the H9N2 subtype (AIV H9N2) has become one of the major respiratory pathogens of chickens in Asia, the Middle East, and Africa. AIV H9N2 viruses of the G1 sublineage alone could cause very high economic losses to poultry production and can be associated with secondary Escherichia coli infections and/or synergistic effects with other avian pathogens, such as infectious bronchitis virus, avian metapneumovirus, Mycoplasma gallisepticum, and Mycoplasma synoviae. In addition, the AIV H9N2 subtype is considered a potential pandemic threat to humans. Vaccine applications as a control measure, using commercial inactivated vaccines, have been insufficient to stop virus spread. Phylogeny studies of AIV H9N2 in many countries of Africa showed frequent mutations and/or reassortments in the HA gene. Because of the high variability in the HA gene, vaccines do not often match field strains. This review outlines the epidemiology of AIV H9N2 in Africa, highlighting different routes of viral spread between countries and pointing out the role of live bird markets for maintaining virus circulation and infections. The review traces the chronology of AIV H9N2 detections in African countries and the genetic variations between H9N2 viruses, which might be related to different routes and sources of virus origins. This review might underestimate the number of affected countries, because in many parts of Africa the poultry industry is still in its first stage of development; therefore, the economic impacts of this virus were not obvious, which may be why less attention has been paid to the AIV H9N2 subtype. While carrying out this review, we found that most avian influenza surveillance studies in Africa were mainly focused on highly pathogenic avian influenza H5 and H7. The cocirculation of AIV H9N2 viruses along with HPAIV H5 and H7 in some parts of Africa might increase the risk of emergence of zoonotic avian influenza viruses.