Suspected Human Rabies

Symptoms of rabies

After a rabies exposure, the rabies virus must travel to the brain before it can cause symptoms. This time between exposure and the appearance of symptoms is the incubation period. It may last for weeks to months. The incubation period may vary based on:

• the location of the exposure site (how far away it is from the brain),
• the severity of exposure (how much virus entered the body),
• the age of the patient (younger children may have shortened incubation periods), and
• whether the person had ever received a rabies vaccine.

The first symptoms of rabies, called prodrome, may be like the flu, including weakness, discomfort, fever, or headache. There also may be discomfort, prickling, or an itching sensation at the site of the bite. These symptoms may last for several days.

Usually, severe disease appears within two weeks of the first symptoms, when the rabies virus causes brain dysfunction. Rabies symptoms in humans may include one or more of the following:

• Anxiety
• Insomnia
• Confusion
• Agitation
• Delirium
• Hallucinations
• Hydrophobia (fear of water)
• Hypersalivation
• Seizures

Once clinical signs of rabies appear, the disease is nearly always fatal, and treatment is typically supportive. Less than 20 cases of human survival from clinical rabies have been documented. Only a few survivors had no history of rabies vaccination before disease onset.

When to consider rabies in an ill patient

Rabies causes encephalitis in infected patients. Rabies should be considered in patients with encephalitis of unknown origin, particularly if there is a history of animal exposure.

Patient history, duration of illness, and laboratory tests for other common etiologies of encephalitis will help determine if rabies diagnostic testing should be pursued.

• Patient history is important to identify possible exposure to rabies; however, rabies should never be ruled out based solely on the absence of definite exposure history.
• Rabies presents as a severe, acute illness with hospitalization required within days to just several weeks of initial symptom onset.
• Rabies is rare in the U.S., and other more common causes of encephalitis should be ruled out before pursuing rabies testing.

Positive Indicators for rabies

• Nonspecific prodrome phase prior to onset of neurologic signs
• Neurologic signs consistent with encephalitis or myelitis:
  • Includes Dysphagia, Hydrophobia, Paresis, and Autonomic instability.
• Progression of neurologic signs
• Negative test results for other etiologies of encephalitis:
  • Includes Herpesvirus, Arboviruses, Enteroviruses, Autoimmune disorders, Certain cancers.

Negative Indicators for rabies

• Improvement or no change in neurologic status after several weeks of illness
• Illness persisting greater than 4 weeks
• A diagnosis of an alternative cause for encephalitis

Antemortem testing

The state health department should always be consulted before collecting and submitting samples to the National Rabies Reference Laboratory at the Centers for Disease Control and Prevention (CDC). After consulting with the state health department, any remaining questions can be directed to the Rabies Duty Officer at CDC by calling 404-639-1050.

For more information about how to submit specimens to CDC, see Submitting Specimens for Rabies Testing. The following instructions will guide you in collecting samples for rabies testing.

Antemortem samples

To rule out rabies before death, all four of the listed samples must be collected. Without all samples, a definitive rule-out cannot be provided. Testing should only be conducted at a public health laboratory qualified to conduct this testing. Commercial laboratories should not be used for testing patients suspected of having rabies.

Saliva

Saliva is intermittently shed in patients with rabies. Serial collection of 4 or more samples over 24 hours may be necessary. Using a sterile eyedropper pipette, collect saliva and place it in a small sterile container that can be sealed securely. No preservatives or additional material should be added.

The laboratory should conduct tests to detect rabies RNA by reverse transcription polymerase chain reaction (RT-PCR) of extracted nucleic acids. Tracheal aspirates and sputum are not suitable for rabies tests, and contamination with blood can reduce test accuracy.

Skin Biopsy

A section of skin 5 to 6 mm in diameter should be taken from the posterior region of the neck at the hairline. The biopsy specimen must contain a minimum of 10 hair follicles and be of sufficient depth to include the cutaneous nerves at the base of the follicle. Place the specimen on a piece of sterile gauze moistened with sterile water and place in a sealed container. Do not add preservatives or additional fluids.

Laboratory tests to be performed include RT-PCR and immunofluorescent staining for viral antigen in frozen sections of the biopsy.

Serum

At least 0.5 ml of serum should be collected; no preservatives should be added. Do not send whole blood. If no vaccine or rabies immune serum has been given, the presence of antibodies to rabies virus in the serum can confirm a diagnosis of rabies.

Laboratory tests for antibodies include an indirect fluorescent antibody test and a virus neutralization test.

Cerebral Spinal Fluid (CSF)

At least 0.5 ml of CSF should be collected; no preservatives should be added. Antibody to rabies virus in the CSF, regardless of the immunization history, suggests a rabies virus infection.

Laboratory tests for antibodies include an indirect fluorescent antibody test and a virus neutralization test.

Rabies virus, antigen, and RNA are rarely found in CSF; therefore antigen and molecular test methods are not routinely performed.

Brain Biopsy

The rarity of rabies and the lack of an effective treatment make the collection of a brain biopsy for antemortem testing unwarranted; however, biopsy samples negative for herpes encephalitis should be tested for evidence of rabies infection. The biopsy is placed in a sterile sealed container; do not add preservatives or additional fluids.

Laboratory tests to be performed include RT-PCR and immunofluorescent staining for viral antigen in touch impressions.

Postmortem samples

In certain cases, human samples may need to be tested for rabies after the patient has died. Before sending any samples to the Rabies Laboratory at the CDC, it's important to consult with the state health department. Fresh tissue samples from the central nervous system (brain) should be submitted.

Postmortem diagnosis of rabies is made by immunofluorescent staining of viral antigen in touch impressions of the medulla (brain stem), the cerebellum, and the hippocampus. Fresh, non-formalin-fixed tissues are preferred. Preservation of tissues by fixation in formalin may reduce the accuracy of rabies diagnosis.