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after 8days of a major surgery is most commonly due to local wound infections, since your drainage
tube showed pus suspicion of local wound infection should be kept in mind. If the pus is still coming from the wound you should get pus culture and sensitivity done.
Post operative pneumonia
could be another cause in that case you should get your x-ray chest and blood investigation done immidiately and if that shows signs of pneumonia or lung infection
you should get admitted in the hospital and appropriate antibiotics should be started.
wishing you good health.