Mosquito bites may result in increased stress and pain, which reduces livestock fitness, weight gain, and animal welfare. Furthermore, mosquito feeding may also result in pathogen transmission between livestock reservoirs (epizootics) and incidentally humans (zoonotic diseases). Not all mosquito species are disease vectors and not all individuals within a species will become infected post exposure and these important differences between mosquito genera and species are discussed. The epidemiology (hosts, environment, pathogen, and mosquito vectors) of most significant and frequent pathogens are explained, with particular emphasis on the viruses Rift Valley fever, Japanese encephalitis, West Nile, and the equine encephalitis (Western equine, Eastern equine, and Venezuelan equine). Increased globalization and anthropogenic landscape modification has resulted in widespread emergence and re-emergence of pathogens in old and new habitats. Furthermore, viral adaptation and global climate change will place more animal and human populations at risk of these pathogens.