Introduction
Waterborne transmission is considered a significant route for avian influenza viruses (AIV) dissemination (Lebarbenchon et al., 2011). This indirect route of transmission is known as one of the main determinants of disease dynamics, since water can be simultaneously contaminated by different viral strains and infect multiple animals, including wild birds (Numberger et al., 2019; Roche et al., 2009).
Waterfowl are recognized as the natural reservoir of IAV and play a key role in the viral spread and diversity (Montalvo-Corral & Hernández, 2010). Infected wild ducks shed feces with high AIV concentrations into aquatic environments (Dovas et al., 2010). Viruses shedding into the water have the potential to infect any other waterfowl within bird populations by drinking or filtering the virus-contaminated water (Keeler et al., 2014). Therefore, viral transmission among wild aquatic birds mainly occurs via the fecal-oral route through contaminated water (Brown et al., 2007).
Environmental water where resident and migratory waterfowl congregate has a significant role in the AIV ecology and epidemiology (Numberger et al., 2019). Viruses shed before migration can persist in the water over winter and become a source of infection to birds returning during spring (Brown et al., 2007). This environmental persistence may enable short- and long-term maintenance of AIV among spatially or temporally separated wild bird populations (Lickfett et al., 2018).
Wild bird habitats close to livestock production systems also represent high-risk zones for AIV interspecies transmission (Achenbach & Bowen, 2011; Harris et al., 2017). Barnyard fowls from backyard farms can use open waters and become indirectly infected by wild waterfowls congregated in the same habitats (Cappelle et al., 2011). Likewise, other farm animals, such as pigs, can be AIV infected by contaminated surface water collected from natural or artificial ponds and used for cleaning barns and watering animals (Karasin et al., 2000; Ma et al., 2007).
In addition to the wild duck-livestock animal interface, live bird markets constitute high-risk sites for harboring and spreading AIV (Khan et al., 2018). Wet markets promote close contact between different animal species which facilitates the genetic reassortment of influenza viruses (Vergne et al., 2019). Wild and domestic birds (including infected birds) use to share cages, feed, and water trays. Therefore, birds from live bird markets can be co-infected with different AIV subtypes (Khan et al., 2018).
Globally, multiple subtypes of low- and high-pathogenic avian influenza (LPAI and HPAI) have been identified based on epidemiological surveillance (Machalaba et al., 2015). Environmental AIV surveillance is considered a convenient, noninvasive, and sensitive method for conducting AIV detection (Leung et al., 2007). Avian influenza surveillance in poultry-related environments, including live poultry markets, wild bird habitats, slaughterhouses, and poultry farms is conducted annually in some countries (Zhang et al., 2019). However, the global diversity and distribution of AIVs in environmental water widely used by waterfowl and poultry have been scarcely studied. In this study, we aim to address this knowledge gap by characterizing the AIV diversity and its spatial distribution in water samples from areas considered to be at high risk for avian influenza outbreaks (i.e., wild bird habitats, live bird markets, poultry farms) on a global scale.