Viral antigens can be demonstrated in the corneal smear, skin biopsy collected from the face or neck, and saliva (antemortem) or in the brain tissue (postmortem) for diagnosis of rabies. Direct fluorescent antibody (DFA) test using specific mono-clonal antibodies is a specific method to demonstrate rabies antigen in clinical specimens.
Viruses can be isolated from brain tissue, CSF, saliva, and urine by culture in cell lines or in animals.
Culture: Isolation of virus in cell lines (WI-38, BHK-21, andCER) is a sensitive method. The cytopathic effect produced by the virus is minimal; hence, the presence of virus in the inoculated cell cultures is made by demonstration of viral anti-gen within 2–4 days. The DFA test using monoclonal rabies antibodies tagged with fluorescein isothiocyanate is a fre-quently used specific method to demonstrate the antigen.
Animal inoculation: Mouse is the animal of choice. CSF,saliva, and urine are inoculated intracerebrally. The inoculated mice are observed for the signs of clinical illness. After death of the mice or after 28 days of inoculation, the brain tissues are examined for the presence of Negri bodies by microscopy or for viral antigen by the DFA test.
Rabies antibodies are found in the serum as well as in the CSF of human cases. A high titer of antibodies is found in the CSF. Antibodies in the serum are demonstrated by a rapid fluores-cent focus inhibition test titer in which results are positive in 50% of cases. In the CSF, positive antibody titers (2–25% of serum titer) are found after the first week of illness.
WBC count ranges from normal to elevated, with 6–8% atypical monocytes. Albuminuria and sterile pyuria may be observed. Respiratory alkalosis due to hyperventilation develops in the prodromal and early acute neurologic phases, which is followed by respiratory acidosis as respiratory depression progresses. In the CSF, after the first week of illness, 80% monocytosis is observed. Protein and glucose test results are normal.
Until recently, most of the cases of rabies were fatal and patients used to die of the rabies. But with intensive support care and better management of patients, few cases have survived. Hence in such situations, laboratory methods for antemortem diag-nosis of rabies in humans are proving to be more useful.