Based on equation (38), we represent the condition for existence of infected <i>Culex</i> mosquitoes at the endemic equilibrium (EE) state.

<p>The existence of infected <i>Culex</i> is impossible in region I. In region II both <i>Aedes</i> and <i>Culex</i> coexist. The border black line represents the threshold of coexistence, which is exactly .