Rabies Workup

Updated: Sep 11, 2017
  • Author: Sandra G Gompf, MD, FACP, FIDSA; Chief Editor: Michael Stuart Bronze, MD  more...
  • Print
Workup

Approach Considerations

If the patient presents after an acute or recent bite

When the patient presents with a bite, wound cleansing, debridement, and careful exploration for foreign body (eg, broken tooth) are essential; this should take at least 10 minutes. Generally, leave wounds to heal by secondary intention to permit drainage of wound fluids and prevent infection. [15, 16]

If the animal to which the patient was exposed has been captured, it should be delivered to a veterinarian for further evaluation or euthanasia; state health departments can then test the unfixed brain tissue. [17]

Consult immediately with public health authorities regarding need for prophylaxis.

If the patient presents with encephalitis and suspected rabies

Skin biopsy from the nape of the neck

Rabies antigen can be detected in cutaneous nerves by direct fluorescent antibody. Consult with public health authorities, as these require specialized laboratories and shipping.

Corneal touch impression

Less preferably, scraping of corneal epithelia, or corneal touch impression, for direct fluorescence antibody can be used. This requires topical ocular anesthetic and is best performed by an ophthalmologist, under public health authority guidance on specimen preparation and transport. Corneal impression is obtained by pressing the surface of sterile glass slide gently but firmly onto the cornea. Corneal scrapings should be performed by an ophthalmologist unless none is available; epithelial cells are gently collected using a sterile loop or spatula and smeared carefully on a glass slide.

Viral cultures and polymerase chain reaction (PCR) assay

Consult with public health authorities, as these require specialized laboratories and shipping. The following may be used:

  • Saliva - Results of saliva culture for rabies virus are positive in low yield within 2 weeks of illness onset
  • Cerebrospinal fluid - After the first week of illness, 80% monocytosis is observed; protein and glucose test results are normal
  • Brain tissue - Often postmortem; staining with immunohistochemical or florescent antibody staining is definitive. Negri bodies are pathognomonic (cytoplasmic inclusion bodies reflective of accumulated virions within rabies-infected neurons). They are found in the horn of Ammon of the hippocampus and cerebral cortex

Blood gas analysis

Respiratory alkalosis resulting from hyperventilation develops in the prodromal and early acute neurologic phases of rabies; this is followed by respiratory acidosis as respiratory depression progresses

Hematology studies

Results of the white blood cell (WBC) count range from normal to elevated, with 6-8% atypical monocytes

Urinalysis

Albuminuria and sterile pyuria may be observed

Imaging studies

As the neurologic phase of rabies progresses, chest radiographs may reveal infiltrates due to aspiration, nosocomial pneumonia, acute respiratory distress syndrome, or congestive heart failure.

Findings from magnetic resonance imaging (MRI) and computed tomography (CT) scanning of the brain often indicate that no abnormalities are present.

Electroencephalography

Electroencephalography (EEG) findings include encephalopathic changes. Due to generalized vasospasm of cerebral arteries during the first week of illness, EEG amplitude may drop precipitously and mimic brain death. This may be further suggested by papillary reflex abnormalities such as anisocoria or fixed pupils due to dysautonomia. These findings may reverse with return of blood flow. A more reliable means of determining brain death in the case of rabies may therefore be cerebral arterial flow scanning that demonstrates absent flow. Brain biopsy is another option. [18, 19, 20, 21, 22]

Cardiac monitoring

Supraventricular tachycardia may be observed during cardiac monitoring. Eventually, bradycardia and cardiac arrest occur.

Future tests

The nucleic acid sequence ̶ based amplification (NASBA) technique on urine samples may be used in the future. [23, 24] The NASBA technique on saliva and CSF can be used for rapid diagnosis as early as 2 days after symptom onset.

Next:

Serology

Serum rapid fluorescent focus inhibition test (RFFIT) titer results are positive in 50% of rabies cases. Results of the CSF RFFIT are antibody-positive (2-25% of serum titer) after the first week of illness.

Detection of viral RNA from saliva using PCR assay and viral antigen from brain biopsy specimens yields 100% specificity. Viral antigen assessment involving nuchal skin and corneal touch impressions have sensitivities of 67% and 25%, respectively.

In true rabies cases, however, the rise in specific neutralizing antibodies is often not documented through an RFFIT, because the victims succumb to the disease prior to mounting a response. Serologic testing is more useful to ascertain serostatus in immunized animals and humans. [15]

Previous
Next:

Skin Biopsy

Nuchal skin biopsy is the most reliable test of rabies infection during the first week. Results from nuchal skin punch biopsy for immunofluorescent antibody staining are 50% positive within the first week.

Obtain a full-thickness punch biopsy from the nape of the neck and include hair follicles. Place the specimen in a sterile container with saline-soaked sterile gauze, store it at -70°C, and obtain shipping instructions for a laboratory that performs the examination.

Previous
Next:

Histologic Findings

General findings on pathology include cerebral congestion and inflammation typical of encephalitis. Neuronal cell death is uncommon histopathologically.

Immunohistochemical or fluorescent antibody staining of nervous tissue, usually of unfixed brain or skin biopsy specimens with sensory nerve endings, reveals deposition of virion in the cytoplasm.

Negri bodies, seen in the image below, are observed in neurons on light microscopy and represent round cytoplasmic inclusions of assembling nucleocapsid. Only 70% of brain biopsy tissue exhibits this finding in human rabies encephalitis. Electron microscopy is more sensitive than light microscopy and reveals the characteristic bullet-shaped virion.

Hematoxylin and eosin stain of Negri body in a rab Hematoxylin and eosin stain of Negri body in a rabies-infected neuron. Courtesy of the US Centers for Disease Control and Prevention.
Previous