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.