I'd seen this film well before I got around to reading the book it was based on, the debut novel from British horror author Graham Masterton.
It was also the final film for schlock director William Girdler, (responsible for such "classics" like Day of the Animals, the Jaws-on-land ripoff Grizzly, Project:Kill and 3 on a Meathook) it was his first and ultimately only solo production before his untimely death.