Hi Neil:

Well, we’re in realm of guesswork I think – Nigerian outbreak is new.

But I suspect poultry; Nigeria clamped down on poultry imports to guard against H5N1 and, ironically, perhaps so boosted demand for smuggled birds.
(This outbreak on battery farm; if they couldn’t supply all demand, maybe turned to outside source.)

More info needed.
But for spread to nw China, thence across Russia to e Europe, I’m more confident re poultry trade/smuggling (even Moscow Zoo’s chief vet said he thought smuggling).