Why aren't interfering RNA's used by pathogens?

The world is full of things that want to kill me, and then without any conscience, eat my conveniently packaged and substantial store of proteins and fats; even when they don't want to kill me completely, they want to use my body as a well stocked larder and home, sometimes for a long time, and then move on to the body of others. I unconciously spend substantial time and energy fending off these bacteria, fungi, viruses, helminths, protozoa, insects, and so forth, and huge portions of my highly elaborate biological sub-systems (the innate and acquired immune system, stomach acids, membranes, up to my learnt social hygiene habits, etc.) are assigned to this never ending battle. When I die, within a matter of hours the hungry hoards will win - this biological battle and arms race has been going on for hundreds/thousands of millions of years.

Oh yes, there are, in turn, many things that try and protect themselves from my personal need for their fats and proteins, as the incovenient, distasteful and poisonous nature of many fungi, plants, and the guile and cunning of wild animals show.

Many pathogens have evolved molecules to suppress my defenses, or to allow better access to my 'resources' (or interfere more generally with normal life, like the wonderous self-healing circulatory blood system), and allow them to enter unobserved, or to maintain long term 'silent running'. These molecules fall into two general classes... (I have been broad in including various classes of bioactives in this list, and not all of these are directly involved in human pathogenic processes, but hopefully you get the point).

  • Proteins and Proteins - things like hirudin that prevent my blood clotting when a hungry leach wants me - functionally analogous (but structurally distinct and polyphyletic) proteins are found in vampire bat saliva, the blood of ticks, and so on; another example would be a large number of secreted proteins in helminths that suppress the normal process of antigen recognition and clearance, an example here would be Neutrophil Inhibitory Factor from hookworms, or ES-62 again from a nematode.
  • Small molecules - things like statins, made by fungi to kill other fungi, aflatoxins from various fungi, Tacrolimus/FK-506 used as an immunosuppresant drug. These types of molecules are generally known as natural products.
  • DNA/RNA - ?

    Both of these classes of molecules have provided many life-saving and enhancing drug classes, their curative mechanisms often exploiting, these evolved toxic, or immuno-suppressive activities.

    So, given the huge power over biological processes initiated by various form of interfering RNA, and the triviality of making and encoding them for a pathogen (especially when compared to the very complex set of genes and metabolites required to make natural product toxins) why aren't there huge numbers of diverse RNAi-based mechanisms exploited by pathogens? There certainly is a lot of interest and money going into siRNA drugs. Pathogens, with all their fancy mechanisms for integration and often hijacking of cells, would also seem to be an ideal delivery vector for RNAi weapons. It seems so simple for these organisms to just evolve them, allowing immortalisation, suppression of their growth, etc., etc..

    There are two simple explanations for this - 1) maybe they do exist, we just haven't looked for or found them yet, or 2) maybe they can't because of some systems-level defense that we haven't fully characterised yet. Maybe the Toll-like receptors do this defense role for the more highly evolved eukaryotes. However, the lack of a TLRs in lower animals, plants and so forth, and especially fungi, the venerable masters of long-term, hand-to-hand chemical and biological warfare, may suggest a deeper-embedded system for defense from manipulation from small xeno-RNAs.

    Both 1) and 2) would have profound impact on the likely future success of RNA-based therapies.

    Maybe the answer is obvious to those that know; but it isn't to me, and I would be interested in knowing.