Hi Arielle,
Parvovirus in dogs is almost invariably a Yang deficiency affecting the Spleen, and in the worst cases, the Kidneys. Since WLT is warming and drying to the Spleen, it makes sense to me that it would have helped.
For prostrate animals, I find homeopathic Veratrum album 30C can be rapidly effective, but must be continued for several days (a few times a day) to pull them back from the brink. Technically, parvovirus appears to be a Shao Yin pathogen, so Shao Yin formulas are appropriate. An example might be Li Zhong Wan, but there are many others in books that categorize formulas that way.
I hope you continue to have fun in herbal land!
Steve