Discussion
Since TMUV was first reported in 2010, it had widely spread in China
(Liu et al., 2013; Z. Xie et al., 2014; Yan et al., 2017; Yun et al.,
2012; X. Zhou et al., 2016; Zhu et al., 2012). Many studies have focused
on the mechanism of virus action and the establishment of detection
methods (Bai et al., 2015; L. Chen et al., 2019; S. Chen et al., 2018;
Cui et al., 2018; Fu, Ji, Liu, Dafallah, & Zhu, 2015; Hu et al., 2020;
Jiang et al., 2012; Wu et al., 2016). In this study, we provide new
insights into the origin, genetic diversity, and codon usage of TMUV.
Compared with the previous studies, we provide better evidence for the
origin, genotype diversity, and codon usage bias of TMUV (Dai et al.,
2015; H. Zhou et al., 2015). We found that mosquito TMUV, which was
isolated from 2015 to 2019, shared the most recent common ancestor with
avian TMUV. Furthermore, we found a new potential clade that was
isolated in the Shandong province; this new clade also showed high
homology with the mosquito TMUV. Overall, the mosquito TMUV is the most
likely origin of avian TMUV based on the currently available sequences.
Mosquitoes are considered the main transmission hosts ofFlaviviridae . TMUV host jumps pose a major threat to the duck
industry and public safety. According to previous research, TMUV virus
has also been detected in humans (Cao et al., 2011).
Previously, Tao et al. used ML trees to type TMUV (Dai et al., 2015). In
this study, we explored the phylogenetic history of TMUV isolated in
China from 2010 to 2019. To identify the genotype of TMUV, three
different methods (NJ, ML, and MCC) were used. All three methods showed
that Chinese TMUV can be divided into three different clades (TMUV 1,
TMUV 2, and TMUV 3). Additionally, within the TMUV 1 clade, 3 stable
subclades were identified (TMUV 1a, TMUV 1b, and TMUV 1c), and in the
TMUV 2 clade, 2 stable subclades were identified (TMUV 2a and TMUV 2b).
Compared with the previous study (Dai et al., 2015; Ninvilai, Tunterak,
Oraveerakul, Amonsin, & Thontiravong, 2019), the TMUV in the current
study showed more genetic diversity. Nonetheless, in recent years, the
rate of emergence of new genotypes has slowed, which is also supported
by the Bayesian skyline plot analysis.
The codon usage bias can be used to understand the changes in codon
usage and its influencing factors during the evolution of TMUV. Gene
mutation pressure, secondary protein structure, natural selection
pressure, and external environment were found to be the main factors
causing codon bias (Bera et al., 2017; Hershberg & Petrov, 2008). In
this study, ENC-plot analysis and neutrality plot analysis showed that
mutation pressure played a decisive role in codon usage bias, which is
similar to the results of previous research (H. Zhou et al., 2015).
Notably, the preferred codons consisted of the optimal codons for human
cells, which indicates that TMUV has the potential to infect humans
after cross-species transmission; this is also supported by previous
studies that have detected TMUV in humans (Tang et al., 2013; H. Zhou et
al., 2015).
Cross-species transmission of viruses is a serious threat to public
safety. There are already many such cases that have adversely affected
public health, including SARS, MERS, SARS-CoV-2, and Zika virus
infections (Dye & Gay, 2003; Lazear, Stringer, & de Silva, 2016;
Verdoni et al., 2020; Zumla, Alagaili, Cotten, & Azhar, 2016). TMUV has
the potential to spread across species and cause great damage. With the
rapidly increasing population and increasing constant contact of humans
with nature, there is an increasing possibility of an interaction
between different hosts (avian to mosquito to human). Therefore, it is
likely not a matter of if but when TMUV will cause a major infection
outbreak in the human population.
In summary, we collected the sequences of TMUV that were reported in
China over the past 10 years. We assigned the possible mosquito origin
of TMUV and divided the TMUV into three clades. Furthermore, we raised
the possibility that in future TMUV may infect people and cause serious
consequences. Future research should focus on the following aspects:
recombinant TMUV in animals, cross-species transmission of TMUV within
the animal, and finally, establishing a detection network to prevent the
sudden emergence of TMUV with high virulence that can infect humans.
Overall, our study characterized TMUV spread in China and provided
insights that may aid in future prevention and control of the virus
spread.