I have a "Särna" chair. "Sarna" in Spanish is "Scabies".
Fun fact: Although "Sarna" comes from Latin, seems like it's only used in Romance languages present in the Iberian Peninsula (Galician, Catalan, Portuguese, Spanish...) and not for example in Italian, Romanian or French, as it's an Iberian loan word. Most other languages derive from another different Latin word, "scabiēs".
The umlaut does mean that to a Swede it is not the same word sound at all. I can only guess that is why it was allowed?