It's really not, though most of the research seems to be on refugee populations: http://voices.yahoo.com/researchers-examine-ptsd-depression-...

It's a global disease, it just receives much, much more attention in the west (for example, all the antidepressant ads during prime-time television).

Lab mice can get depressed as well - there is specific way by which it gets achieved, so it's not even a human-only problem.