Sci-fi Horror Story About Spiders, Psychology, and Aliens

I’m trying to find the name of a short story from a collection of sci-fi horror stories. (NOTE—I’m not talking about a collection of sci-fi AND horror stories, but rather a collection of horror stories that were ALSO sci-fi).

From what I recall, the story’s protagonist is a psychiatrist (and based on the older time period, he might have even been called an "alienist" in a delightfully ironic twist) who’s evaluating a guy who cannot perceive spiders.

Now, he can read the word "spider" and has no trouble identifying a photograph/illustration of a cobweb, but if there’s a spider in that cobweb, he can’t see it. Show him a picture of nothing but a spider, and he’ll tell you with a straight face that it’s blank.

The only other part I remember is the ending, when the psychiatrist is looking out the window of a house in a rural area, and to his horror, he spies an enormous spider-like alien calmly chowing down on a cow right outside the house. His brain then deletes the very concept of "spiders" from his psyche in an attempt to protect him from this sight, and so our shrink ends up just as "crazy" as his patient from earlier.