Discussion
This study describes the molecular identification and phylogenetic characterization of IAV at a wildlife - livestock interface in the Lerma marshes, Mexico. One of the findings in this study was the detection of 22 wild dabbling ducks (3.68%) positive to IAV in the mid-winter. Seasonality, geographical location, and between-year fluctuations can modify the IAV prevalence and subtype distribution. For instance, higher IAV prevalence (>30%) occurs predominantly before and during the fall migration, while lower prevalence (<1%) is related to the spring migration in North America (Lickfett, Clark, Gehring, & Alm, 2018; Wille, Bröjer, Lundkvist, & Järhult, 2018). However, the prevalence of AIVs in their natural hosts also depends on the species (Munster et al., 2007).
Mallards (Anas platyrhynchos ) and other dabbling duck species (Anas spp.) have higher IAV prevalence that diving ducks (Vandegrift, Sokolow, Daszak, & Kilpatrick, 2010). Specie-specific differences in IAV prevalence are related to several factors including intrinsic physiological characteristics, feeding behavior, shedding patterns, population size, and adaptation to the virus (Munster et al., 2007; Papp et al., 2017; J. G. van Dijk, Verhagen, Wille, & Waldenström, 2018; Wilcox et al., 2011). During 2016 - 2017, we detected IAV in four wild dabbling duck species from the Lerma marshes: Mexican Duck (11.10%), Northern Shoveler (6.67%), Green-winged Teal (4.97%), and Blue-winged Teal (4.28%). These results are consistent with multiyear surveillance conducted by Ferro et al. (2010), which found a higher IAV prevalence in Blue-winged Teal, Northern Shoveler, and Green-winged Teal in the United States. Likewise, in our study, none of the diving duck species sampled resulted positive to IAV. This result was in line with the research conducted by Munster et al. (2007), which suggested less-efficient virus transmission by diving ducks.
Previous studies have identified IAV in waterfowl from the Lerma marshes (Cuevas-Domínguez et al., 2009; Ramírez-Martínez et al., 2018). IAV subtypes H7N3, H6N2, and H4N2 have been isolated from anatids, sentinel ducks, and wild bird habitats of the State of Mexico (Barrón-Rodríguez, Chávez-Maya, Loza-Rubio, & García-Espinosa, 2018; Cuevas-Domínguez et al., 2009; Ornelas-Eusebio et al., 2015). Findings in our study revealed the co-circulation of three subtypes: H5N2, H3N2, and H1N1.
The phylogenetic analysis showed that the HA gene sequence of an H5N2 subtype isolated from a Mexican duck (A. diazi ) was closely related to migratory wild duck viruses from North America. This result is consistent with the studies conducted by Barrón-Rodríguez et al. (2018) and Ornelas-Eusebio et al. (2015), who reported influenza viruses highly similar to North America waterfowls IAV strains.
The phylogenetic characterization also revealed that the HA gene sequences of the H5N2, H3N2, and H1N1 subtypes isolated from Northern Shoveler, Green-winged Teal, and Blue-winged Teal shown to be more phylogenetically related to swine and chicken sequences from Mexico. The characterization of a swine-like or avian-like IAV isolated from wild ducks is not very well documented. Nevertheless, Cuevas-Domínguez et al. (2009) previously reported the isolation of an H7N3 subtype from a wild bird that showed 100% identity with an IAV isolated from a quail of the United States. Similarly, Olsen, Karasin, and Erickson (2003) isolated the H1N2 influenza virus (A/Duck/NorthCarolina/91347/01) in a wild duck that was phylogenetically related to swine-like reassortant H1N2 and H3N2 viruses of the United States of America.
Areas with close interactions between humans, livestock, and wild animals have been considered as “hotspots” for IAV interspecies transmission (Wei et al., 2015). Interspecies transmission of IAV is dependent on several factors, including virus shedding, host immune system, host receptor specificity, virus stability in the environment, and the degree of close contact with different host species (Joseph et al., 2017; Yassine, Lee, Gourapura, & Saif, 2010). The presence of IAV in wild migratory birds, along with the proximity of domestic flocks to surface water has been significantly associated with increased risk of local outbreaks (Karasin, Brown, Carman, & Olsen, 2000; Walsh, Amstislavski, Greene, & Haseeb, 2017). However, in our study, all swine and poultry samples were negative to IAV by qRT- PCR.
Negative IAV results could indicate a possible absence of recent viral exposure at the time of sample collection or lack of viral adaptation to alternate hosts such as poultry and swine (Awosanya, Babalobi, Omilabu, Nguku, & Ogundipe, 2013; Bourret et al., 2017). Likewise, some research has reported zero prevalence in backyard systems during winter or discordant detection of AIV subtypes in time and space between poultry and wild birds (Jimenez-Bluhm et al., 2018; Verhagen et al., 2017).
In conclusion, this study identified the co-circulation of three different subtypes of IAV in wild ducks close to backyard farms in the Lerma marshes during the winter season 2016 – 2017. Direct interspecies transmission among wild and livestock animals was no determined. Nevertheless, phylogenetic analysis suggests possible genetic relationships among wild birds, swine, and poultry, as well as the transboundary movement of influenza within the American continent most likely associated with the waterfowl migration. Therefore, increased long-term molecular IAV surveillance throughout the year in the Lerma marshes would further contribute to understanding the contribution of backyard animals, migratory, and resident wild ducks in the disease epidemiology.