Break lease due to cat allergies

We have recently rented our rural vacant house to a couple with three children. They required a three-year lease because the husband is in the military and they will be stationed in Abilene, TX for 3 years. There is no early buy-out clause in the lease and no early termination clause with the exception of the Military PCS if he is transferred. I found the standard Texas lease document online and it was signed by both parties June 17, 2009. They moved into the house July 29, 2009. They were told BEFORE they moved in that a cat had lived in the house. They also said they had viewed the house and surrounding property in person a month earlier and loved it. We have had the house professionally cleaned, the carpets deep cleaned, the exterior landscaped, and the exterior and interior exterminated. They have lived in the house for one week and are now asking to be released from the lease because one of the children has had an allergic reaction which they think is because of the cat that previously lived in the house. They are also concerned that the creek bed next to the house is a hazard because of snakes. Can they legally break the lease due to their son's cat allergies? We have told them we will have the carpets cleaned again and have instructed the property manager to change the A/C filters (which was done 3 weeks prior to them moving into the house) and we have asked the landscaper to give us a bid on cleaning out the creek bed. There is a paragraph in the lease that says they accept the house as acceptable by signing the lease. I guess my ultimate question... Can they legally break the lease?

[sunlover]

No they cannot. However you might offer them an exchange of lost of deposit and couple months rent to release them from lease. This will have no happy ending if left as is. You are in the right as you have a signed lease. It just might be easier and less time consuming to come to an agreement, if you can. If you do come to an agreement on early move out get details in writing signed by al parties

Other good ideas to suggest to them: Since the carpet was already deep cleaned, the carpet is not the issue. But dander is still in the house. They need to wipe down baseboards, vacuum out register vents, wash windows, sills, and window coverings, wash corners, & wipe off the inside of doors (bottom couple of feet). Cats climb doors when they want out, sit in windows, drop dander into vents and along floorboards, and rub on the corners of walls. Also, change filters to a hepa particulate filter (which removes dander) and instruct them to run the fan as often as possible to catch that dander in the filter.

They had knowledge of the cat prior to moving in and saw the creek bed when they viewed the property. I wouldn't touch it. Snakes will come out to sun themselves on rocks, even when there is no brush around. These are wild creatures that you can't eliminate (they live in the ground!) You are not responsible for the wildlife. They need to live inside town if they are worried about snakes. They may not break the lease for this.