that's a common misconception. most people think you can't wash dreads, or that they're formed from filth or something, but no. just no.
dreads are formed from a variety of ways. there's back-combing, palm rolling, twist & rip... and if you dont comb or brush your hair long enough it will knot up into dreads. that's the lazy method. there's no reason to think dreadlocks are unsanitary. dreads can be washed just like normal hair, just preferably with residue-free shampoo. they're as clean as your hair.
let me give an example. my sister got lice from jail, and brought it into the house. my little brother caught it. they both have 'normal' hair.
i have dreads, yet i never caught them. i've never had lice in my life, and this is my second time with dreads. my hair is uninhabitable :D