  We investigate combined effects of nontrivial topology, induced by a cosmic
string, and boundaries on the fermionic condensate and the vacuum expectation
value (VEV) of the energy-momentum tensor for a massive fermionic field. As
geometry of boundaries we consider two plates perpendicular to the string axis
on which the field is constrained by the MIT bag boundary condition. By using
the Abel-Plana type summation formula, the VEVs in the region between the
plates are decomposed into the boundary-free and boundary-induced contributions
for general case of the planar angle deficit. The boundary-induced parts in
both the fermionic condensate and the energy-momentum tensor vanish on the
cosmic string. Fermionic condensate is positive near the string and negative al
large distances, whereas the vacuum energy density is negative everywhere. The
radial stress is equal to the energy density. For a massless field, the
boundary-induced contribution in the VEV of the energy-momentum tensor is
different from zero in the region between the plates only and it does not
depend on the coordinate along the string axis. In the region between the
plates and at large distances from the string, the decay of the topological
part is exponential for both massive and massless fields. This behavior is in
contrast to that for the VEV of the energy-momentum tensor in the boundary-free
geometry with the power law decay for a massless field. The vacuum pressure on
the plates is inhomogeneous and vanishes at the location of the string. The
corresponding Casimir forces are attractive.
