Sewer rats, entry into house.
The sewer rat, or Norway rat, is a fantastic swimmer and does make the sewers of cities and suburbs its home. The bad thing about sewer rats is that they can swim through pipes and into your home. This can be prevented by buying special grates and valves that are installed in your plumbing. Special toilets also prohibit the entry of sewer rats. If you live in an older home, rats coming through the pipes can be a real issue. The best way to keep rats out of your home is to make sure your plumbing is updated and any drains to the city sewer are covered in a grate. Rats have even been known to come up through sump pump drain. Just like roof rat control, keeping the holes and openings in your home monitored and repaired is the most important part of preventing a rat problem. If you can keep rats outside where they belong, you will never need to learn the intricacies of trapping and removal. Sewer rats are thought to outnumber people in many metropolitan areas. They are smart and evasive, and will take any opportunity to invade the home. If you are concerned about rodent control, consider calling a professional to make sure your preventive methods are up to par.
Plumbing system rats
Yes, it is true that rats can come up through the plumbing system. This is rare in modern facilities; the plumbing structure does not allow it. In older homes and apartment buildings, especially in cities with a lengthy history, old pipes and plumbing do not have the special valves that keep out rats. Sewer rats are a particularly devious and hardy brand of rat, and these animals will swim into unprotected homes. Some buildings cannot have new plumbing installed due to the difficulties and the expense such a project would mean. The best you can do in a situation like this is to install in-home grates over any open drains. A plumber should be able to modify your toilet so that rats popping out from under the lid won't be something you have to worry about. As with any rodent situation, the best methods of prevention have to do with keeping your home fortified and in good repair. Sometimes rats inside of a home will be problematic because of chewing on PVC pipe. Pipes are not the ideal surface to chew on because they are smooth and hard to get a good hold of, but a determined rat can do serious damage if chewing has progressed to the plumbing system.
Read more about rats:
Do rats live in more urban areas, or wild areas?
Rats in New York subway
How do you remove a rat stuck in a dumpster?
Why are there so many rats in NYC?