So, we got back from Italy yesterday. It's now 1.5 weeks since Lada had his vitamin injection.
He's not gotten any worse. He's also not ok. His neck fur is still thinner than the rest of it, in the same circular area. But at at least he's not bald. ;-)
I decided to go with the vet's idea about a vitamin deficiency and change his feed. He already gets fresh greens of various kinds almost every night, and dog biscuits as a protein supplement. Other than that, his staple food has been "beapher Care+" hamster feed, same as Tanita got. I have now bought "Oxbow Essentials Hamster & Gerbil food", which is the only other brand of hamster food pellets in my pet store. His Care+ will be gradually replaced with the Essentials.
I also bought the beapher Hamster Vitamin Solution to add to his drinking water. Syrian hamsters technically do not need vitamin supplements, but this is at the vet's suggestion, so should do no harm.
If he keeps losing neck fur over the next couple of weeks, it'll be back to the vet again. But I hope not. Fingers crossed that this feed change is enough. I really don't want to take him to the vet for the 4th time in 3 months!