A noncytopathic EIAV-target cell limited interaction dynamical model system is
extended by including the spatial effects of dispersion and chemotaxis. A linear
stability analysis of its two equilibrium states and both one-dimensional longitudinal
and two-dimensional rhombic planform nonlinear stability analyses of the infectious
state are performed on a quasiequilibrium version of this model system. This system
only depends on two dimensionless ratios: the basic reproductive number and a
chemotaxis coefficient, which is a measure of the attraction of the uninfected target
cells to density gradients in the infected target cells. For sufficiently large
values of the chemotaxis coefficient, a morphological infection sequence of the
uninfected state to sparse homogeneous distributions and isolated spots to
periodic spots to dense homogeneous distributions is predicted as the basic
reproductive number increases from zero. The patterned region is identified with
the occurrence of petechial hemorrhages or minute blood-red spots on the
anemic mucous membranes of horses during the chronic degree phase of EIAV
infectiousness. The concept of higher threshold rhombic patterns based on
the mean density deviation level of the infected target cells is introduced
to make the interpretation of spots in the periodic patterning region. The
distance between these adjacent spots decreases as the basic reproductive
number increases, consistent with this predicted morphological infection
sequence.