H9N2 viruses have circulated in domestic poultry in mainland China since 1994, and also been detected in different species of wild birds [
24]. A study by Li et al. [
25] showed that six H9N2 viruses found in live poultry markets in southern China could be transmitted in ferrets by respiratory droplet. Thus the widespread dissemination of H9N2 viruses poses a threat, because such droplets can function as “vehicles” for delivering different influenza virus subtypes from avian species to mammals, and H9N2 virus become capable of infecting various mammals [
26,
27]. As the economic market for furs has risen in recent years, the number of mink farms in China has dramatically increased. Minks are obligate carnivores and have a high demand for high-quality protein. In China, owing to their lower costs, raw poultry and poultry byproducts, including heads, bones, viscera and blood are widely used to feed fur animals after they have been mixed with other ingredients [
7,
28]. Therefore, under this condition, it is fully possible that H9N2 viruses could infect minks, and farmed minks might become the carriers of viruses. According to our survey for farmers, diseases with symptoms similar to those of a common cold infect animals in mink farms in the late winter, early spring, and during rainy summers; however, such infections are rarely fatal and thus do not receive much attention by farmers. Lowly pathogenic avian influenza (H9N2) has no obvious clinical symptoms, reduce the possibility of finding timely these cases, providing the virus with adaptive mutations occur in minks. Therefore, under this condition, it is fully possible that AIVs could infect minks and transmit in the minks. To explain how the cross-species contamination arises, Gagnon et al. [
13] have reported maybe because minks are fed with uncooked pork by-products and non-processed, raw remains of swine coming from slaughterhouse facilities (i.e., discarded tissues, such as lung, which is the target of swine influenza virus). Yoon et al. [
14] also believe it is reasonable to speculate that the source of FLUAV might have been uncooked turkey meat, due to the common occurrence of cross-species transmission of SIV to turkeys. Another reason probably of minks infected with influenza virus, farmed mink are kept outdoors in rows of cages covered only by a roof providing shade and shelter. The feed, consisting of uncooked chicken or fish by-products mixed with cereals, is administered on top of each cage and is thus freely accessible for birds perching on the netting. As described by L. Englund [
29], it is well known that mink farms are frequently invaded by sparrows and other birds which forage for food and contaminate mink farms with faecal droppings. This situation is a common occurrence on mink farms in China.
In this study, we report for the first time the seroprevalence of avian influenza viruses in mink populations. Our results showed that antibodies against two H9N2 viruses were found in numerous mink plants, and the rates of serum positivity against more pandemic strains, [e.g., A/chicken/Shanghai/10/01 (H9N2) (47.5 %)] were higher than those against H9N2 [(Ck/HB/4/08) (45.4 %)], suggesting that the H9N2 virus has already been prevailed among the farmed mink population of China. In animal studies, we observed the potential pathogenicity of H9N2 virus and virus shedding in infected minks. After infection, the minks displayed the slight clinical signs, lungs had lesions, virus replicated in the lung tissues and could be eliminated outwards by nasal fluid. The titers of H9-specific antibody began to rise in sera collected at 7 dpi, and continued to increase up to 15 dpi. These results, such as their disease course and body reactions, and nonfatal infection are similar to the ferrets after infected by H9N2 virus [
8,
9].
We provide serological and experimental evidence that strongly suggests farmed minks are susceptible to H9N2 virus infection, strongly suggesting that feeding raw poultry meat is a risk factor for cross-species transmission of influenza A virus from avian to farmed raised small mammals like minks, which could lead to eventual adaption of avian origin new influenza A virus to mammals, and reminding us to pay more attention to detecting its onset in minks. It is very important to do epidemiological surveillance of influenza virus not only within common susceptible animals, like avian populations, but also for all other species where intensive production and high geographic densities of animals may favor the appearance of new influenza virus isolates. Furthermore, through monitoring AIVs in other animal can not only prevent and control new influenza viruses pandemic prevalence, and provide more information for people’s public health.