I'm sorry that your question wasn't answered in a timely manner and I'm sorry to hear of this with Monty. When the cause of a fever can't be confirmed with basic diagnostics and imaging, we call the condition a fever of unknown origin (FUO). Advanced testing might consist of the following:
1. Laboratory tests such as blood cultures to detect bacteremia associated with discospondylitis, endocarditis, or other foci of bacterial infection; cytology and cultures of cerebrospinal fluid or synovial (joint) fluid); cytology of lymph nodes or affected organs; serologic tests: antibody titers or antigen tests for specific infectious agents; PCR (DNA-based) testing for specific infectious agents; immune tests such as ANA, Coomb's, and serum protein electrophoresis
2. Imaging such as abdominal ultrasound to examine for evidence of pyelonephritis, prostatitis and to identify +/- aspirate enlarged abdominal organs or masses; echocardiogram; CT/MRI
3. Diagnostic procedures may include arthrocentesis looking for polyarthritis, CSF tap, muscle biopsy, abdominocentesis, transtracheal wash or bronchoalveolar lavage, laparoscopy, thorascopy, or exploratory surgery for biopsies.
Monty's age and clincial presentation will dictate which of the above should be considered. If there's evidence of altered mentation (mental status) such as disorientation, confusion, or ataxia ("drunken sailor") CSF tap would be reasonable. Please respond with further questions or concerns if you wish.