On discreteness of spectrum of a second order functional differential operator

Necessary and sufficient conditions for discreteness of spectrum for the singular second order functional differential operator of the form
$$\frac{1}{\rho(x)}\left(-(p(x)u')'+q(x)u-\int_a^b u(s)r(x,ds)\right), \; x\in (a,b),\; -\infty\le a<b\le\infty$$
are obtained.