WNF is a vector-borne disease spread by various species of mosquito and there is no direct spread between horses. Indirect transmission has been reported in humans through blood donations, organ transplants or vertical transmission from mother to child. Wild birds are the natural reservoir of the virus and act as an amplifying host by increasing the amount of virus available for transmission.
Mosquitoes are infected when blood feeding on birds and then act as vectors to infect other birds and other animals. When other animals are infected (such as, but not exclusively, humans and horses), they are considered as ‘dead-end’ or 'incidental’ hosts as they cannot transmit the virus due to a low blood viral load compared to birds.