# New Insights into the Biology of the Emerging Tembusu Virus
## 3 Epidemiology
### Transmission  
TMUV transmission occurs during mosquito biting on a vertebrate host (Figure 4). However, although the bite of an infected mosquito is probably the main mode of transmission, the existence of other modes may exist. Vertical transmission within mosquitoes was examined for _Cx. tritaneionlynchus_ and _Cx. Quinquefasciatus_, but the progeny was not infected [35]. Nevertheless, further studies are needed to clearly assess the vertical maintenance of infection in mosquitoes.  
Birds are the main hosts of the virus, and vectors of the _Culex_ genus associated with birds play a major role in transmission. Observation of TMUV-affected animals shows that the virus has been detected in the ovaries of birds and in eggs. Intrauterine transmission from mother to chick is therefore possible and leads to abortion of the gestating eggs. This clinical presentation is reminiscent of the ability of ZIKV to be transmitted in humans from mother to child during pregnancy, resulting in abortion or congenital malformations [74]. Although sexual transmission has not yet been demonstrated, non-vector transmissions were reported in birds by airborne contamination or by close contact between animals [72].  
\begin{table}
\begin{tabular}{c c c c} \hline
**Country/Location of Origin** & **Species** & **Year** & **References** \\ \hline \multirow{2}{*}{China} & Mallard & \multirow{2}{*}{2014} & [68] \\  & _(Ana s platylynchos)_ & & \\ \hline China & broiler duck & 2015 & [69] \\ \hline China & Layer duck & 2015 & [70] \\ \hline Guangdong Province, China & Muscovy duck & 2015 & [71] \\ \hline Thailand & duck & 2015 & [72] \\ \hline Shandong Province, China & Meat duck & 2016 & [40] \\ \hline Inner Mongolia Autonomous & \multirow{2}{*}{Meat duck} & \multirow{2}{*}{2017} & [40] \\ Region, China & & & \\ \hline Thailand & Free-grazing duck & 2018 & [42] \\ \hline \end{tabular}
\end{table}
Table 2: _Cont._Maintenance of TMUV was observed in duck farms in China during the winter season when mosquitoes are inactive. To confirm non-vector transmission, naive ducks were exposed to direct contact with infected animals or exposed to infected groups by airborne  
Figure 4: Summary of the transmission cycle for TMUV.  
Figure 3: Geographic distribution of TMUV in Asia. Map representing countries where TMUV has been reported. Created with mapchart.net.  
transmission. Exposed ducks developed specific acute signs of infection with increased viremia in serum and seroconversion [75]. In 2020, [PERSON] et al. evaluated the effect of duck age on TMUV infection and contact transmission. Overall, TMUV was transmitted from all age groups of infected animals to contact-naive ducks. However, naive groups in contact with older infected ducks had higher levels of the virus in the oropharyngeal and cloacal swabs than those exposed to younger infected ducks [76].  
Therefore, the regular presence of wild birds on farms, especially outdoors, could allow transmission between domestic birds and maintenance of the virus on farms without the intervention of an arthropod vector. This original mode of transmission for an arbovirus needs to be evaluated in depth because it could facilitate the spread of the virus in a region without a vector. Finally, the transmission of TMUV to non-human primates and humans has been demonstrated by serological studies, as well as by the presence of viral RNA [45,46]. The role of vertebrate hosts other than birds in viral transmission is not known at present but should be evaluated.