Delay in the diagnosis of both listerial meningitis and meningoencephalitis frequently occurs. A monocytic response in the cerebrospinal fluid and a negative cerebrospinal fluid Gram's stain can lead the clinician to confuse listerial meningitis and meningoencephalitis with herpes and other forms of viral encephalitis, viral meningitis, tuberculous meningitis, Lyme disease, syphilis, cryptococcal meningitis, Wegener's granulomatosis, or CNS sarcoidosis. Particularly in the immunocompromised host, the possibility of Listeria infection must always be considered as a possible cause of CNS infection. Bacteremia Bacteremia without meningitis or focal infections occurs in five to 30 percent of adult cases. There are no distinctive clinical features except for peripheral monocytosis, which is present in a small percentage of patients.
Diagnosis is based on blood culture findings.
Isolated brain abscesses have also been reported. A diphtheroid-like organism discovered in blood or CSF cultures is frequently misinterpreted as a contaminant. In an immunocompromised host, treatment of listeriosis should be initiated pending the laboratory staff's final diagnosis. Diphtheroids can be rapidly differentiated from Listeria organisms by using a microscopic bacterial motility test. Tumbling motility is indicative of Listeria organisms.