TICK-BORNE ENCEPHALITIS-VIRUS TRANSMISSION BETWEEN TICKS COFEEDING ONSPECIFIC IMMUNE NATURAL RODENT HOSTS

To determine whether the portion of a vertebrate host population havin g specific immunity to tick-borne encephalitis (TEE) virus can partici pate in the TEE virus transmission cycle, natural hosts immunized agai nst TEE virus were challenged with infected and uninfected ticks. Yell ow-necked field mice (Apodemus flavicollis) and bank voles (Clethriono mys glareolus) were either immunized with TEE virus by subcutaneous in oculation of the virus, or they were exposed to virus-infected Ixodes ricinus ticks. One month later, when serum neutralizing antibody was d etectable, the animals were infested with infected (donor) adult femal e ticks and uninfected (recipient) nymphal ticks; recipients were allo wed to feed either in close contact (chamber 1) or physically separate d (chamber 2) from the infected donor ticks. Following challenge with infected (and uninfected) ticks, viremia developed in all the control, nonimmune animals, whereas viremia was undetectable in all those anim als naturally immunized by previous exposure to infected ticks. Despit e the presence of neutralizing antibodies in all the immunized animals , 89% (24/27) immune animals supported virus transmission between infe cted and uninfected cofeeding ticks. Most transmission was localized, occurring within chamber 1; disseminated transmission from chamber 1 t o chamber 2 was reduced. Immunization by tick bite was more Effective than immunization by syringe inoculation in blocking cofeeding virus t ransmission Nevertheless 75% (9/12) animals with ''natural'' immunity still supported transmission. The results demonstrate that natural hos ts having neutralizing antibodies to TEE virus (and no detectable vire mia) can still support virus transmission between infected and uninfec ted ticks feeding closely together on the same animal. These observati ons have important epidemiological implications relating to the surviv al of TEE virus in Nature. (C) 1997 Academic Press.