Q. Can horses contract West Nile virus from humans, or humans from horses?

—Via email

A. While horses and humans can get West Nile virus (WNV) from the bite of mosquitos (that have previously bitten a WNV-infected bird), both horses and humans are what we refer to as dead-end hosts. That means that once the human or the horse becomes infected with the virus, as a general rule the virus doesn’t replicate at high enough levels in the bloodstream, which is called “viremia.” So horses and humans don’t get high enough viremia to subsequently infect a mosquito so that that mosquito could transfer the disease to another horse or human. The only way for a horse or human to get West Nile virus is through a mosquito bite.