severe, poisonous, dangerous, itch and other skin diseases
瘌 centers on harmful, severe conditions affecting the body: dangerous, poisonous states and painful skin diseases like severe itching.
瘌 combines 疒 (sickness) with 剌, which likely contributes the sound and may relate to piercing or harmful action, but the exact historical development is uncertain.
The sickness enclosure 疒 wraps around 剌, which looks like a bundle of sharp, piercing things. Imagine a dangerous, poisonous rash that feels like needles under the skin.
For ラツ, picture a rat with a severe, itchy skin disease scratching itself raw: rat -> ラツ, and the sickness enclosure shows it's a dangerous condition.