As soon as rats show in

daylight not hiding and exploring anymore to be active in the late evenings and nights having grown in population so bad that they have surfacing ratnests in these brown nograss fields, they will attack humans and cause a deadly local epidemic. 

Last time in the Kairo Slums before the central waste dump was closed while rats needed to be hunted with 22lrs in the 80ies. 

I am serious, they just won't get that big here, Cat size. 

In both cases, past and future, Street Hygiene systems collapsed soon by no urban cleaning staff along with a drop it on the spot attitude in Berlin crating a wide spread problem over a one spot problem. 

But, if that gets out of hand the rats will connect and noone can out of protest put one giant rat hill on fire killing most and creating a dark stinky wave over all town, you would have to burn down half the entire city.

Chemical Warfare. Fits in almost every bag and available online