Recurrence of highly pathogenic avian influenza (HPAI) virus subtype H7 in humans and poultry continues to be a serious concern to public health. Before 2002, only occasional case reports of human H7 influenza virus infections occurred as a result of direct animal-to-human transmission or laboratory accidents and most of these infections resulted in conjunctivitis and/or mild influenza-like illness . In 2003, an HPAI H7N7 outbreak in the Netherlands infected 89 people who were in close contact with affected poultry, including one fatal case, and led to the culling of over 30 million birds . The most recent outbreak of H7N9 strains in China resulted in more than 130 human cases, including 36 deaths, making H7 subtype HPAI viruses the focus of public attention . WHO has listed HPAI H7N9 as one of the most lethal viral pathogens . Most of the infected patients had a history of poultry contact, indicating the transmission from poultry to human. The scale of poultry outbreaks and its association with cases of human infection with H7 viruses highlights the need for efficient diagnosis and continued surveillance of this virus subtype .
Conventional laboratory methods for influenza virus detection include virus isolation in embryonated eggs or Madin-Darby canine kidney (MDCK) cells, followed by subsequent HA subtype identification using serological methods. Molecular detection methods such as real-time PCR assays have been widely applied for the laboratory diagnosis of influenza infections [6, 7] and HA subtype identification . However, both conventional and laboratory methods are technically demanding and are not suitable for on-site use in field investigations. The development of rapid H7 subtype influenza virus detection tests in dot ELISA (enzyme-linked immunosorbent assay) , AC-ELISA (antigen-capture ELISA), and chromatographic strip formats  using H7 monoclonal antibodies (MAbs) is hence preferred.
Serological investigations to detect specific antibodies from H7 infection in poultry and humans are critical to the success of disease prevention and control programs. However, due to the lack of a specific and sensitive monoclonal antibody, there are no serologic tests available against H7 AIV. Microneutralization is currently used as the “gold standard” for subtyping. However, the test is labor-intensive and its sensitivity is limited, rendering it impractical for rapid and high-throughput diagnostics. The HI test and indirect ELISA are considered to be simple serology tests. However, low sensitivity and subtype cross-reactivity significantly limit the value of these assays . Competitive ELISAs (cELISA), also called epitope blocking ELISAs, are widely used for serological detection of antibodies to influenza viruses , mainly due to their sensitivity and simplicity. The cELISA makes it possible to provide general assays for testing sera from different avian species, humans, and other species without changing any of the test reagents .
It is a challenge to combine AC-ELISA and cELISA on the same plate with the same amount of antibodies. The selected Mabs are required to target conserved antigenic epitopes and compete to host antibodies in infected sera for the epitope binding. In this study, two H7 Mabs were identified to meet these requirements and assembled in a dual-function-ELISA for universal H7 diagnosis via either antigen or antibody detection. The sensitivity and specificity for both functions were evaluated. The results indicated that for the first time, antigen and antibody detection could be performed with the same device and Mabs for specific and sensitive H7 AIV detection.