How do I break my lease in Texas due to unsafe living conditions?

When I first moved in February 2010 we found out we had rats. When I told the owner/landlord they said there was never a problem with rats before & just put out some traps ( I have dogs so I'm reluctant to use poison.) It took a month or so till we stopped seeing them. Then I started hearing chh chh chh in the walls. Turns out we had a family of possums living in the walls. Again, they acted like they had never had the problem before but dropped off a used live trap & showed me how to set it. One mom & four babies later we finally stopped hearing the noises. Then this winter we had a whole new set of rodents. Squirrels...playing chase in our attic & storing nuts in our walls. I informed the landlord & she said there is no way that squirrels were in the walls and anyways, you can't get rid of squirrels. Imagine my surprise when the caulking around the bathtub started dissapearing. One morning I was in the bathroom and I saw pieces of an acorn come shooting out the hole left by (whatever) removed it. My daughter has a friend who's house burned down due to rodents chewing up the wiring.Now summer has rolled around and we are having a major rat problem again(I trapped four last night). So now I have two types of rodents to deal with. I've sent the landlord a notice stating that these issues need to be fixed, but I do not feel safe. I'd like to break the lease & leave as soon as possible. What steps should I take? Thanks, Penny