Google announced Thursday that it has made a major change to its algorithm to push down the rankings of low-quality websites and “reward” high-quality content.

While not explicit, this change is Google’s long-anticipated response to the rising prevalence of content farms such as eHow and Demand Media.

Google tweaks its algorithm around 500 times a year, but most changes are so subtle that users do not notice them. This latest change, according to a Google blog post, is “a pretty big algorithmic improvement…a change that noticeably impacts 11.8% of [Google search] queries.”

For the past year, Google has been struggling to prevent low-quality, irrelevant articles from appearing prominently in search results.

Matt Cutts, Google’s Principal Engineer, has said in a recent interview that “in general, there are some content farms that I think it would be fair to call spam, in the sense that the quality is so low-quality that people complain.”

Content farms generate ad revenue by producing articles that respond to the most frequently searched keywords.  While not all content from sites such as eHow and Demand Media is low-quality, their writers are unpaid, or given next to nothing, and have to churn out articles at a mad pace.

Yesterday, Larry Fitzgibbon, Demand Media's EVP of Media and Operations, responded to Google in a blog post, where he “applaud[ed] changes search engines make to improve the consumer experience – it’s both the right thing to do and our focus as well.”

Fitzgibbon also said that Demand Media has not yet seen a net impact on its business.

Google said it was continuing to work on more updates to improve search quality results.

Whether or not these changes will affect the average users’ experience may become more apparent in the coming months.