Fleas/mites a few days ago, now what is this skin growth?!

[knownothinghomesteader]

We found many fleas or mites (cant tell, they are too small but scampered when i would part feathers so thought fleas) on this one 3 days ago and treated with permethrin spray. Today we bathed her (didn't see any more parasites) and found this gnarly growth. It is hard and I couldn't remove it gently. What is this?!?

Perhaps important context is that we have had terrible luck and had several recent deaths in the last couple months from what we now think was Mareks based on loss of leg and neck function.

Thanks!
Second photo is of the lice/mites. Best I could get, sorry.

 

[LaFleche]

Where is the "gnarly growth" situated?
In case it is at the base of the tail it should be the oil gland and not to be messed with.

 

[knownothinghomesteader]

Where is the "gnarly growth" situated?
In case it is at the base of the tail it should be the oil gland and not to be messed with.

A ways below the vent and a bit off center.

 

[LaFleche]

A ways below the vent and a bit off center.

Might be the umbilicus.
Maybe try to get a better picture for us

 

[knownothinghomesteader]

A ways below the vent and a bit off center.

I posted a better photo and started a new thread with the subject being flystrike because the neighbor suggested that is something to consider. Would so appreciate your expertise

 

[Wyorp Rock]

https://www.backyardchickens.com/threads/flystrike.1636812/