Can bacteria or parasites take advantage of RNAi against host genomes?

This is an idea I've had floating around in the back of my mind for a few years now.  I keep some ideas secret in the short-run because I hope to be able to investigate them some day. However, I also have to face reality, with my teaching load and limited research funding there are a wide range of projects that I simply won't be able to follow up on.  I figured it would be better to just put the idea out there so someone can check into this if they want.

RNA interference is a widely used and very useful genetics tool where gene expression can be reduced by double stranded RNA that matches the mRNA of the target gene. There is a great deal of literature on this and I am not going to attempt a summary or review of this technology and its natural role here. Instead I want to focus on a curious fact. For a wide range of species you can simply feed bacteria to the organism that have been modified to express double-stranded RNA and this can create phenotypes by silencing certain genes.

Okay, if this is so easy and straightforward (and quite short sequences of RNA can accomplish this) have any bacteria evolved to take advantage of inactivating genes of a host Eukaryote? Perhaps as a pathogen to block immunity and promote infection, or in a more symbiotic role to tweak host phenotypes? For that matter have any Eukaryotic parasites evolved this ability to excrete dsRNA or antisense RNA to alter host phenotypes?  Part of this is a comparative genomics question and if candidate sites are found the follow-up study of activating or inactivating the gene sequence can be done to see if there is a response across species.

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