A mass spectrometer 10 comprises an ion source 12 which generates nebulized ions which enter an ion cooler 20 via an ion source block 16. The address information transmitted from the data processing apparatus in response to the request is registered in a communication control apparatus. The invention also provides a method for the prevention or amelioration of West Nile disease, such as encephalitis, in equidae by administering the vaccine composition herein set forth.