All of the symptoms she has point to Leptospirosis. Low platelets are one of the lab abnormalities we can see with this disease. However, low platelets are very common in tick diseases, especially Ehrlichia and Rocky Mountain Spotted Fever (RMSF). Tick borne diseases generally do not attack the liver and kidneys like we are seeing. Leptospirosis does attack the liver and kidneys. I think your vet is on the right track as far as the possible diagnoses.

So, I would certainly at this point be treating for both. If I had a choice, I would have your dog on ampicillin (a penicillin type drug that can be given i.v.) and enrofloxacin right now, until the titers come back. The ampicillin for leptospirosis (which seems more likely to me), and enrofloxacin for RMSF. Penicillin and doxycycline are not recommended to be used together, but, I would DEFINATELY have your dog on i.v. ampicillin at this point until we know for sure what is causing the problem. If it turns out this is a tick borne disease, only then would I stop the penicillin type drugs and go with the doxycycline.

Leptospirosis is not always curable, unfortunately. In most cases if antibiotics are started right away, they can do fine, but if enough liver and kidney damage is done, some dogs do die. I hope this will not be the case for your dog.

Here is some info on Leptospirosis: http://www.veterinarypartner.com/Content.plx?P=A&A=573

The only thing I would change now is the decision to take her off the penicillin. Otherwise, I think your vet is on the right track.

Please reply if you have further questions.