This is dangerous ground to tread on at this point. 

Nuisance would include creating danger to property and other residents.

What can make it dangerous is when something occurs to cause it to grow outside of it's normal levels.There are swabs for surface mold for testing and air tests for airborne.

I find using schools has helped to remove any bias I may have had and gives very distinct criteria of what I'm honing in on.

I think as long as you pick quality tenants and are providing nice units which are not dangerous or in total il repair you should be fine. 

Now your accountant did not say that, but I want to make it clear how dangerous an informal partnership is. 

I'm willing to take a risk, but I don't want to put us in danger any further financially.Thanks in advance for any advice or direction.

As this has now become a potentially proven dangerous liability, the tenant should be more than willing to provide you that level of assurance that the animal is in compliance of local laws and regulations in consideration for the health and safety of all.Seems it would be incumbent on you to alert any prospective future tenants, especially those with children, of the potential danger based on the history of this animal.None of this may have impact based on where you are, but asking the right questions of the right people may get answers that position you for success. 

Any contracts signed in your name can be dangerous if sued.

@Steve Babiak  another junior lender or gap funder has learned first hand how dangerous those types of transactgions are for the lender