I'll take your word for it. To me, a sudden allergy leading to an actress leaving the show, sounds a bit iffy and, well, convenient. Especially since Kes had spent the last leg of the season with her hair being long and her ears fully covered.
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However, the fact that her ears were fully covered lends credit to the idea that she had become allergic, otherwise, you'd have to admit that it's kind of absurd to create an alien race and hide the only trait that identifies it! For example, the only times that Spock's ears are covered is when the plot demands it.
Just think if Armin Shimmerman had become allergic to his makeup and had a really bad reaction to it. He wouldn't need makeup anymore to look like a Ferengi.
