Accelerating or ‘threshold’ immuno-suppression by DWV can create bistable DWV dynamics.

Abstract

<p>The stable (solid line) and unstable (dotted line) equilibrium level of DWV (arbitrary scale) are calculated from equations S4, S5, and plotted as a function of increasing levels of immune depletion (<i>y</i>). Below the dotted line, the virus can be efficiently regulated by the immune-system to some intermediate (potentially cryptic) density, represented by the solid line. Above the dotted line (and for high <i>y</i>, any point to right of intersection with solid line), the virus cannot be efficiently regulated and a viral explosion ensues. Any factor such as mite feeding that depletes the immune system (increasing <i>y</i>) will first cause a gradual increase in copy number, <i>V</i> (moving right along the solid line), and then at a defined point (intersection of solid and dotted lines), a viral explosion will ensue. Parameters are <i>x</i> = 0.09 (<i>y>x</i> ensures that the virus can invade from rare) and <i>z</i> = 0.4.</p