Randy
You are correct in the case of RNA viruses, gene silencing is not what is going on. RNAi silences genes by destroying the transcription products, and RNA viruses don't really have their own "genes" (DNA). But its use would not be limited to viruses in any case:

> RNAi applications are being developed as control strategies for the Asian citrus psyllid and Varroa mites. The gene encoding glutathione S-transferase, involved in detoxification of pesticides used to control mites in the hive, was targeted successfully in Varroa destructor with dsRNA. Two different dsRNA delivery methods were evaluated, microinjection and soaking mites in dsRNA solution. -- RNAi in Agriculturally-Important Arthropods, Katherine Aronstein, et al

> We used controlled laboratory experiments to show that double-stranded RNA homologous to specific N. ceranae ADP/ATP transporter genes can specifically and differentially silence transcripts encoding these proteins. For gene silencing to be effective as a preventive or curative strategy, amplification and systemic spread of the silencing signal is important.-- Effective gene silencing of a microsporidian parasite associated with honey bee (Apis mellifera) colony declines. Paldi, et al

Also, Paldi states:

> Orally ingested, unprotected, undelivered (by vectors) and unmodified dsRNA cannot cause specific silencing of endogenous genes in vertebrates

New work Hervé Vaucheret & Yves Chupeau suggests otherwise:

In certain organisms, a simple and convenient manner to induce RNA silencing consists in feeding the recipient host with bacteria expressing dsRNA homologous to the cognate target.  

Concentration of plant miRNAs was higher in the serum of rice-fed mice compared with chow diet-fed mice. Moreover, adding plant miRNAs to chow diet resulted in an increase of plant miRNA concentration in mouse serum.

Given that six nucleotides of perfect complementary between the “seed” region of a small RNA and its target is sufficient to promote RNA silencing in mammals, how many plant miRNAs have the potential to actually regulate gene expression in animals?  

* This study also implies precise awareness from biotechnologists who intend to make use of dsRNA, especially in the field of plant protection against pests.

SOURCE: Ingested plant miRNAs regulate gene expression in animals, Hervé Vaucheret, Yves Chupeau

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