The mosquito-bird cycle – the virus is transmitted through mosquito vectors. • The birds are amplifying hosts – they develop sufficient viral levels to transmit the infection to other biting mosquitoes. • The mosquitoes bite an infected bird and become infected themselves. The virus enters their bloodstream and circulates for a few days, and then settles in the salivary glands. • The infected mosquitoes go on to infect other birds. In the USA the American robin and the American crow are common carriers. • The infected mosquitoes bite animals and humans; virus enters the victim’s bloodstream. Mosquito species that infect animals and humans in the USA: • Eastern USA – Culex pipiens • Southeast USA – Culex quinquefasciatus • Midwest and West USA – Culex tarsalis Mammals – the virus does not multiply as readily in mammals as it does in birds. Experts believe that mosquitoes can infect mammals, but cannot be infected by mammals. Mammals are known as dead-end infections. Why do mosquitoes bi