Thank you for your patience. These symptoms are definitely a big concern, but it definitely sounds like your veterinarian and their team have done a lot of work trying to diagnose and treat your girl already. It sounds as though she has a good regenerative response to losing these red blood cells, but it sounds as though the question as to why her RBC count (and platelet count) is so low, hasn't yet been answered. It sounds like a number of diagnostic tests have been run which is great, and this has ruled out a large number of potential issues here including parasites, however I would definitely recommend further diagnostics here if a solid diagnosis hasn't yet been found.
Sorella is on a range of treatments now which is good and should cover off potential issues like rat bait ingestion (the Vitamin K), bacterial infection (amoxicillin) and an immune issue (the steroid prednisone to help here). Hopefully your vet has run a full clotting profile to look to see if there is a clotting issue here. A full CBC (complete blood count) and cytology of a blood smear to check for abnormal cells like spherocytes and a Coombs test will help to rule out an immune mediated issue like immune mediated hemolytic anemia and/or immune mediated thrombocytopenia, and a full biochemistry may also give clues to rule this in our out (such as a raised bilirubin level). An internal bleed should also be ruled out via ultrasound and an abdominal 'tap'.
I would definitely recommend a blood transfusion in your girl's case if the RBC is continuing to drop. This really will be the key to giving her a chance of surviving this. Whole blood will likely be ideal as she needs RBC's and plasma it seems. This may give your vet enough time to make a definitive diagnosis and start the appropriate treatment course for your girl. A blood transfusion is a relatively common procedure in veterinary medicine and definitely isn't something to be afraid or wary of.
Best of luck with your girl. It definitely sounds like your vet is doing all he or she can for your girl, and there have certainly been a lot of diagnostics done already. Do follow up on the above just in case one of the diagnostics mentioned hasn't yet been done. I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
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