dog with low platelet count
Does anyone know of causes for thrombocytopenia - low platelet counts - in dogs?
The dog in question is a seven and a half year old spayed female flat coated retriever. She was diagnosed with hypothyroidism about four years ago (as was her sister). She has had several unresolved "issues" her whole life including bouts of low energy levels. She is allergic to corn and has been on a corn-free food for many years. We just recently (a couple of months ago) switched to a grain-free food (Wellness CORE reduced fat) and she has seemed to be thriving on that until a few days ago.
In January I took her in to have her teeth cleaned and the pre-anesthetic blood work showed very low platelet levels. The vet said there are two reasons she knows of for low platelets - rat poison or autoimmune disorders. She took another blood sample to test clotting times (for rat poison) and autoimmune markers. Both came back negative - that is, her clotting time was normal and none of the autoimmune markers showed up. The vet sent her home with plans to check her again at an undefined later date.
Yesterday evening I commented to DH that she seemed awfully quiet. He agreed. During the night, at 4:30 in the morning, she wanted to go out - unusual but not unheard of. I took her out (and the other two dogs as well) and she squatted multiple times to defecate. She had moved away from me and it was snowing and windy so I stayed on the patio - the point being that I could not tell if anything passed or not (I know, I feel like a bad mom for not knowing this - in retrospect). Eventually she scratched at the grass to cover up, so I assumed it was fine.
This morning she was very lethargic. She did not get up when DH went downstairs which is very unusual. She did not get up to get her morning thyroid pill - he fed it to her on her dog bed. She was slow to get up to eat breakfast although she did eventually do so, or at least most of it.
This all concerned me enough that I took her in to the vet. They just called and said that her platelets are again (or still) quite low. I do not know how low, I didn't ask at the moment. Previously they were significantly below normal, but not in the "critical" range.
She is not anemic, her RBC and WBC counts are normal.
The vet is still checking her out. I believe they did an x-ray to check for blockage, even though I don't really think that's the cause.
I have read that some of the tick-borne diseases can also cause thrombocytopenia, but usually have other abnormal labs as well. Even so, they are checking heartworm, Lyme disease, etc. today also. The vet does not think it is any of those because of the normal RBC.
Does anyone know of other causes of low platelet counts or do these vague symptoms ring any bells? I would be happy to hear of other things we can check out.
Thanks so much in advance,