Most emerging diseases are of zoonotic origin, with wild and domestic animals acting as natural reservoirs [
1]. Globalization and intensive animal farming have led to an increased spread of zoonotic infections [
2]. Influenza type A viruses include several distinct subtypes based on the antigenic properties of the two major surface glycoproteins, the hemagglutinin (HA) and the neuraminidase (NA). To date, 18 subtypes of HA (H1-H18) and 11 subtypes of NA (N1-N11) have been described [
3]. A number of influenza A subtypes have successfully crossed the species barrier and have established in the mammals and human population, causing yearly seasonal epidemics or they have sporadically been directly transmitted from poultry to humans causing zoonotic infections [
4,
5]. The influenza A viruses of the H9N2 subtype are classified as low pathogenic avian influenza (LPAI) viruses. They cause infections both in wild birds and in the poultry population worldwide, including several countries in Asia, Europe, North Africa and North America [
6,
7]. A significant proportion of recent H9N2 avian influenza (AI) isolates contains the L226Q (H3 numbering) amino acid substitution in their hemagglutinins (HAs) showing preferential binding to analogs of receptors with sialic acid linked to galactose by α2,6 linkage (SAα2,6Gal), a phenotypic portrait which is characteristic of human influenza viruses. Thus, these AI viruses might possess one of the key elements for infection in humans [
8‐
10]. Indeed, H9N2 viruses were isolated for the first time from humans in Hong Kong in 1999 and further human infections were reported in 2003 [
11,
12]. These studies have shown that avian H9N2 viruses isolated from chickens are closely related to the H9N2 viruses responsible for human infection [
13]. One human case of H9N2 AI was reported in Bangladesh [
14] and the World Health Organization (WHO) in 2015 has reported new cases in Egypt and Bangladesh [
15,
16]. In 1998, domestic pigs from Hong Kong were confirmed as being infected with H9N2 influenza, and infections have been reported also in recent years in swine along with other mammals [
17,
18]. Furthermore, H9N2 viruses can contribute with gene segments during reassortment events leading to the generation of novel avian influenza virus that can infect humans (e.g. recent Chinese H7N9 and H10N8 viruses) [
19,
20]. Recent transmission studies have demonstrated that some natural isolates of H9N2 viruses can acquire the ability to transmit efficiently between ferrets via respiratory droplets. In addition, it has been reported that serial passages of an H9N2 virus through guinea pigs can result in the introduction of amino acid substitutions, which increases contact transmission efficiency in this mammalian model [
21,
22].
The wide circulation of H9N2 viruses throughout Eurasia, along with their ability to cause direct infections in mammals and humans, raises public health concerns on their potential role as candidates for the next influenza pandemic [
23]. H9N2 human infection is generally asymptomatic or responsible for mild clinical signs. This may explain the scarcity of evidence accounting for the circulation and transmission of this virus subtype [
24]. Nonetheless, human sera positive for H9 subtype were identified in China, India, Iran, Thailand, Cambodia, Romania, Egypt and Pakistan [
25‐
34].
In Iran, the H9N2 subtype was identified for the first time in 1998 and is still circulating in the poultry population. In the affected farms the mortality rate ranges between 20 and 60 %, although this may also be attributable to co-infections with other pathogens, such as IBV or Mycoplasma gallisepticum [
35]. In spite of the implemented national control measures, which include the mass vaccination of poultry, the virus has rapidly spread and can be considered endemic in the Iranian poultry [
36].
Exposure to H9N2 AI viruses in Iranian poultry workers was previously revealed by means of HI test, using serum titre
≥20 as positive cut-off [
28,
29]. In previous reported studies, the H9N2 AI seroprevalence, assessed by means of HI test, in Iranian poultry workers ranged from 1.6 to 15.7 % (Median 9.5). Amongst the Middle-Eastern and Southern Asia countries, the highest seroprevalence was observed in Pakistan (47.8 %) by means of HI test [
34] and 7.5 % by means of MN test in Egypt [
33]. In the present study, two different serological tests were used and compared to screen 200 individuals from the Fars province, Iran to better understand the risk of infection with the H9N2 virus in the poultry sector. The two different diagnostic tests were applied to assess whether (i) the human exposure to this virus subtype can be confirmed and whether (ii) the poultry operators in the Iranian endemic areas (the Fars province, in this study) are at risk of exposure to the H9N2 infection.