A dangling modifier causes ambiguity in a sentence, which means it makes room for misinterpretation to happen. If we change the modifier's position, the ambiguity is eliminated and only one interpretation will be possible. In this case, it is not clear if it is the cat that has worms or if it is the veterinarian. To make is clear that it is the cat, we could rewrite the sentence in the following way:
The cat that has worms should be treated by a veterinarian.
