Is this possibly gapeworm?

[RoyalChick]

I recommend that you cull her. ILT is contagious and infected birds are carriers for life. The virus can be carried on your person and spread via that method as well.

Yes they can be carriers for life - though not necessarily - and it actually survives poorly outside the body unlike Mareks.
It is clear that someone else in my flock is already a carrier and passed it to her.
New chicks can be vaccinated for it relatively easily (eye drops) though it is a controversial vaccine.