This sounds like a dangerous environment to live in. You may be able to move out without being penalized if you follow the terms of the law and the terms of your lease.
First carefully read your lease. Then read the entire Texas statute governing residential tenancies: http://www.statutes.legis.state.tx.us/Docs/PR/htm/PR.92.htm#92.001).. Then re-read your lease. If you comply with the applicable provisions for giving proper notice to your landlord about a dangerous condition rendering the leased premises uninhabitable, and the landlord then fails to remedy the situation, you may be entitled to move out before the lease term ends.
If you can afford a lawyer, I suggest that you hire a lawyer to help you. Find a way out of that place!
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You've been there for over a year. It is going to be difficult to prove liability for the rat problem. You can take care of cleaning, and ask for reimbursement for your expenses. If landlord fails to reimburse, you can sue in small claims court, but keep in mind that you may not win because you waited so long. The filing fee is small, so it wouldn't hurt to try! Take lots of pictures - you'll need evidence and witnesses.