The American Association of Equine Practitioners indicates West Nile is a core vaccine, meaning all horses in the United States should be vaccinated for it. West Nile is transmitted by mosquitoes, causes neurological disease in horses, and has an overall death rate of 33% so it is important to vaccinate your horse for West Nile especially in areas with a heavy mosquito population. Today there are many companies that make West Nile vaccine, either alone as an individual antigen or in combination with other vaccines such as tetanus and encephalitis. The advantage of the combination vaccines is that the horses get less injections.
The only disadvantage to combination vaccines is that if your horse has a reaction and you are giving multiple vaccines in the same injection, you will not know which vaccine caused the problem. However, a recent report in the Journal of Equine Veterinary Science indicates there may be another reason to give West Nile vaccine separately. Zoetis Animal Health performed a study by using over 300 horses that did not have titers to West Nile. Some were vaccinated with combination West Nile vaccines, and some with vaccines containing only West Nile; that group was given other vaccines at the same time but as a different injection. The horses given the West Nile vaccine separately had an antibody titer that remained higher for a longer period than horses receiving the combination vaccines. It is believed antigen interference decreased the response to West Nile in the combination products. It is not known if the decrease is significant but it seems to me giving West Nile vaccine separately from other vaccines is probably a good idea.