Lethargy, hindlimb weakness, peripheral oedema and green urine in ferret?

Hi. My veterinary nurse has a 2yr old male castrated ferret who has suddenly become very lethargic, is eating only small amounts, has developed green urine and peripheral oedema. I have x-rayed him and no sign fb. he has an enlarged spleen. Seemed tender on palpation of his bladder and is pyrexic-temp 39.5-40.5. He is currently on synulox and gentamicin. Help...where do we go from here- my knowledge is very limited???