  We present a linear stability analysis of the perturbation modes in
anisotropic MHD flows with velocity shear and strong magnetic field.
Collisionless or weakly collisional plasma is described within the 16-momentum
MHD fluid closure model, that takes into account not only the effect of
pressure anisotropy, but also the effect of anisotropic heat fluxes. In this
model the low frequency acoustic wave is revealed into a standard acoustic mode
and higher frequency fast thermo-acoustic and lower frequency slow
thermo-acoustic waves. It is shown that thermo-acoustic waves become unstable
and grow exponentially when the heat flux parameter exceeds some critical
value. It seems that velocity shear makes thermo-acoustic waves overstable even
at subcritical heat flux parameters. Thus, when the effect of heat fluxes is
not profound acoustic waves will grow due to the velocity shear, while at
supercritical heat fluxes the flow reveals compressible thermal instability.
Anisotropic thermal instability should be also important in astrophysical
environments, where it will limit the maximal value of magnetic field that a
low density ionized anisotropic flow can sustain.
