according to the link provided in this thread, here is the anwer to this:
The snake mite (Ophionyssus natricis) is suspected to be a vector, passing the virus from one snake to another. Often, with an outbreak of IBD, the snakes also have concurrent mite problems. Other possible ways that the virus can be transmitted include direct contact between infected and noninfected snakes, contaminated bedding, venereal transmission and intrauterine transmission to developing embryos in viviparous species and to eggs in oviparous species, although the direct route of transmission has not yet been identified.