Hello this is Dr. Scheer and I am an equine veterinarian. I reviewed your question about your horse and hopefully can offer some suggestions.
The blood counts and fever suggest an infection and since they are low, not high, than it is likely an infection that has been present for sometime or is very severe. Infection can be due to bacteria or a virus. An antibiotic is not going to help a viral infection but would be important for preventing secondary bacterial infections while her immune system is down. Different bacteria are sensitive to different antibiotics.
Does she have any other symptoms? Lameness or swollen limbs
? Is she eating and drinking normally? Any diarrhea? Any coughing? Does she get out of breath easily when exercised? Are her gums pink? How long has she been sick? Has she been vaccinated for anything?
Having that information will help me get some idea of what organ systems are effected by her infection.
The most ideal thing to do is to send blood samples to a lab and test it for different diseases, particularly any that are prevalent in you area. Some suggestions would be testing her for Equine Infectious Anemia (Coggin's test), Trypanosomiasis, Equine Herpes virus, Glander's Disease, West Nile Virus. All these diseases have different treatments so it's invaluable to know which (if any) she has. Some of them are also contagious to people.
Tribersin is an uncommon antibiotic and 2 days is a very short course for a horse that has blood cell changes as severe as your horse. I would recommend putting her on a more broad spectrum antibiotic for a longer course of treatment if you are not able to do further diagnostics. The cause of her blood cell counts may be an infection less severe than the ones I previously listed and putting her on an antibiotic and monitoring for improvement is not a wrong thing to do.
I hope that is helpful and please respond with answers to the questions I asked you.