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Thank you for your rescue work. I am a veterinarian and will do my best to help.
If the kittens are just anemic from blood loss to fleas, then warming them, getting warm liquids into them, and treating with Capstar and Frontline may be all you have to do. Giving Pet Tinic to help build their blood may also help. It is available in pet stores or online, no prescription needed.
The other concern is viral/infectious disease. I would test them for feline leukemia; it is a leading cause of anemia in kittens and young cats.
Thanks for your good work and let me know what else I can help with or answer. Rebecca