I don't think there are any in Houston that are strictly on a 24-hour emergency basis. Most of them open at 7 AM.
Try calling the Best Care Animal Hospital. The address and phone number follow:
10850 Louetta Rd., Suite 1200
Houston, TX 77070
Klinik Veterinar & Surgeri Goh
No. 1, Slr Wah Keong, Taman Wah Keong, 31400 Ipoh, Perak
Tel: 05-545 4880
Klinik Veterinar Seri Ampang
No. 29, Jln Seri Ampang 1, Taman Seri Ampang, 31350 Ipoh, Perak
Tel: 05-313 0191
Quick...go to vet now..hope your pet's gonna be fine soon..
but i don't know if they open 24hrs or not..