Sometimes substrates(Ex. Timberline Top Soil) harbor nematodes that consume decaying organic matter, such as leaves, boluses, and dead animals. It may be possibly that your roaches are not are not infected by a parasitic nematode, rather soil nematodes that happen to facilitate the decomposition of dead roaches. I haven't heard about any problems regarding the presence of these soil nematodes, and personally witness them in some of my containers, however in small populations. I would suggest drying out your substrate since these organisms rely heavily on moisture.