Vacant property next door has created a roach issue - help!

[Nicole Heasley Beitenman]

Hello BP! One of my rentals is in a older, C+/B- neighborhood. The house next door was owner-occupied when we purchased. Our tenant is in year 2 of her 2-year lease, and she's been a dream. 

House next door then became a rental. Tenants vacated and left a MESS. Our house, and several others nearby, are getting roaches. Our tenant is finding them in the dirt when she pulls weeds. We keep sending an exterminator over, but until the source is dealt with, they aren't going to go away. 

Looking at the county auditor's site, it looks like Fannie Mae has the house now. There was info for a realtor posted on the door, but he's since removed that info and stopped answering calls as the whole neighborhood has been after him. 

We're trying to get the city and health department to do something, but that's an ongoing battle. We plan to consult an attorney about next steps to recover our costs.

Has anyone been in a similar situation? Does anyone have any advice? We're desperate. We really don't want to lose this tenant. TIA!