David Gerrold talked about this at a convention in Richmond, VA, back within the first season or two of the series.
He said it was basically Tracy Torme saying, "Let's write a good old fashioned horror-story!" And he did.
But apparently, at the time, he had not yet made plans to follow it up.
And, beyond any comments from Gerrold that were directly about this story, there were other complications. At the end of the first season there was a writer's strike. The last script of the season, for example, "The Neutral Zone," came from nowhere. (No writer could officially work on it during a strike, so any work on it until after the strike was unofficial.)
Then, in the second season, there were a number of changes, such as moving Geordi to Engineering and replacing Dr. Crusher with Dr. Pulaski. I don't know all their plans, but there were budgeting and other issues that resulted in that season being 4 episodes shorter than usual.
It's very possible that Torme planned on following up on the episode, but with these issues, may not have been able to. He was not with the series after the second season, so any of his plans would have been dropped in favor of the plans of the writers on the staff at the time.
I do have to question the statement, from Memory Alpha, about the intent for the parasites to be agents of the Borg. While Gerrold wasn't on staff at the time, he was familiar with what was going on, and was quite clear that, at the time, the episode wasn't part of an already planned "official" arc.
It's a lot easier for people, when remembering, to look back and consider integrating what was already done and say, "Yeah, this was the basic plan."
As to a canon explanation of what was going on, there is none.