Actually, if you are using any material from the original article, then you do need by law to cite the original work. If not it might well be seen as plagiarism.
Incidentally, it is up to copyright holder to enforce copyright. And copyright attaches to all work, even when not declared. Though, the failure to declare copyright weakens your claims. Summarizing an article without credit is not illegal and should never get you in trouble. However, "Trivial changes in copied text, in an attempt to avoid copyright infringement, are specifically prohibited by law in the USA," according to the article at the following link: http://www.rbs2.com/plag.htm#anchor333333