Here's why:
* Rabies is a serious and deadly disease. Actors are not intentionally exposed to rabies on set, and the risk of contracting it from an animal on a film set is extremely low.
* Animal handling on film sets is strictly regulated. Animal trainers and handlers are professionals who ensure the safety of both the animals and the actors.
* Rabies is not a common plot point in Westerns. While Roy Rogers was known for his Westerns, rabies wouldn't be a typical element in those films.
It's possible you are thinking of a different story, or that the information you have is incorrect.
If you can provide more details about what you are remembering, I might be able to offer further assistance.