Fleas in Duplex, Tenants are upset!

[Mitchell Morwood]

I a have a duplex where the front and back exterior doors are shared.  Both have their own interior door leading to each side of the duplex.  The back yard is shared.  Tenant A brings in a cat without asking me.  Tenant A admits to Tenant B the cat has fleas.  The fleas spread to both tenants dogs.  

Tenant B is very upset because they have to treat their side of the duplex and dog.  They want me to pay for it.  I feel like Tenant A should pay for it.  Tenant A now claims to me their dog doesn't have fleas.  The cat is gone.  

What should I do???