General Practice and Population Health practice

A 34-year-old man presents with five days of fever, drenching sweats, severe headache behind the eyes and aching muscles. He started work at a rural Queensland abattoir eight weeks ago and has not travelled overseas. On examination his temperature is 39.2°C; there is no rash, no jaundice, no red eyes and no neck stiffness. Investigations show ALT 142 U/L (<40), AST 118 U/L (<45), bilirubin 14 micromol/L (<20), creatinine 78 micromol/L (60–110) and platelets 118 x 10^9/L (150–400). Which one of the following is the most likely diagnosis?