New vet, new theory.
Lada went to the same clinic as before, but got to see a new vet. Third appointment, third vet. And this one agreed that his fur is thinner in the neck area. However, she couldn't see any indication of mites. Last time he got a mix of vitamins and medication, so she wants to see if maybe it's just a vitamin deficiency. So this time he got a shot of vitamins, no medication.
If it's a vitamin deficiency it means I need to change his diet around, maybe try another brand of hamster food or something. I told her what he eats and she said it's fine, but this particular hamster might need something else.
We're off to Italy tonight, so will check on Lada again when we're back next Thursday. If he keeps losing fur, it wasn't the vitamins and he will need to go back to the vet again. If he's better then, I'm going to have to track down some place that sells a different brand of hamster food.