Abstract
Assessing influenza A virus strains circulating in animals and their potential to cross the species barrier and cause human infections is important to improve human influenza surveillance and preparedness. We reviewed studies describing serological evidence of human exposure to animal influenza viruses. Comparing serological data is difficult due to a lack of standardisation in study designs and in laboratory methods used in published reports. Therefore, we designed a scoring system to assess and weigh specificity of obtained serology results in the selected articles. Many studies report reliable evidence of antibodies to swine influenza viruses among persons occupationally exposed to pigs. Most avian influenza studies target H5, H7 and H9 subtypes and most serological evidence of human exposure to avian influenza viruses is reported for these subtypes. Avian influenza studies receiving a low grade in this review often reported higher seroprevalences in humans compared with studies with a high grade. Official surveillance systems mainly focus on avian H5 and H7 viruses. Swine influenza viruses and avian subtypes other than H5 and H7 (emphasising H9) should be additionally included in official surveillance systems. Surveillance efforts should also be directed towards understudied geographical areas, such as Africa and South America.
Highlights
The family Orthomyxoviridae contains three distinct genera of influenza: A, B and C
The rationale for our scoring of the laboratory methods used in the studies that we reviewed is based on the official World Health Organization (WHO) case definitions for human infections with influenza A(H5N1) virus [15]
The weights of variables of the scoring system were divided in an arbitrary manner, the scoring system comprises important factors that should be incorporated in future studies investigating human exposure by animal influenza viruses to improve reliability of human serological evidence
Summary
The family Orthomyxoviridae contains three distinct genera of influenza: A, B and C. Influenza A and B viruses are known to cause high human morbidity and mortality during the yearly seasonal epidemics. In contrast to influenza B viruses, influenza A viruses circulate in many animal species and are able to cross the species barrier, in particular from animal to human. This can occur either directly, or after a unique type of reassortment that results in the generation of viruses that are able to replicate in humans and have haemagglutinin (HA) genes (and less frequently neuraminidase (NA) genes) that are antigenetically distinct from those of seasonal influenza viruses (antigenic shift) [1]. Well-known examples of animal influenza viruses that have recently infected humans include A(H5N1), A(H6N1), A(H7N9), A(H9N2) and A(H10N8) [6]
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