How Rabies Is Diagnosed

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If you've been bitten by an animal suspected to have rabies, a test may be done on the animal to diagnose rabies. You should also call the local animal control authorities to safely capture a wild or potentially rabid animal.

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© Verywell, 2018

Testing the Animal

According to the Centers for Disease Control and Prevention (CDC), bats, skunks, raccoons, and foxes that bite humans should be euthanized and tested as soon as possible.

In order to do the test, the animal must be euthanized and tissue samples are taken from the brain. In the United States, rabies test results are usually ready within 24 to 72 hours from the time after the animal is euthanized. (The test itself takes two hours, but the sample must be sent to a diagnostic laboratory.)

Not all animals that bite or scratch a human are euthanized and tested. Animals that are considered to be less likely to have rabies (such as a healthy, vaccinated domestic cat or dog) may be observed for 10 days.

Since rabies is a medical urgency and a potentially fatal infection, your doctor and local or state health department will often quickly decide if you need a rabies vaccination based on the type of animal and exposure, as well as information on animal infections in your area.

Labs and Tests

For someone who is exhibiting symptoms but has not been diagnosed, no single test is considered sufficient in diagnosing rabies in a living person, but the following tests may be done in some situations.

Lumbar Puncture

In some cases, providers check the person’s spinal fluid. This involves the use of a lumbar puncture, also known as a spinal tap. With the help of a special needle, doctors can extract a small amount of cerebrospinal fluid (CSF) from the spinal canal then send that sample to a laboratory for analysis.

Although they’re often done in hospitals, lumbar punctures are sometimes performed right at the doctor’s office. The total procedure takes about 15 minutes.

After using a local anesthetic to numb your skin, your doctor will insert a thin needle into the lower part of your lumbar spine. In order to make enough room for the needle, you may be asked to bend forward, usually while sitting down or lying sideways.

Once your spinal tap is completed, you should lie down for at least an hour and spend the next 24 hours resting and drinking plenty of fluids. In many cases, patients will need to stay at the hospital or doctor’s office for up to four hours.

While people rarely experience serious complications after undergoing a spinal tap, you may feel some pain when the needle is inserted. In the hours (or sometimes days) following the procedure, some patients also experience headaches, nausea, rapid heart rate, and/or low blood pressure.

If you experience bleeding or signs of inflammation after a spinal tap, consult your doctor right away.

Skin Biopsies

Skin biopsies are another type of lab test sometimes used to diagnose rabies. After numbing the area with a local anesthetic, your doctor will take a small sample of the skin at the nape of your neck. In the lab, analysts will check the sample for rabies virus proteins.

Other Tests

Healthcare providers may look for antibodies to the rabies virus in samples of your saliva and serum (i.e., the liquid portion of blood that remains after coagulation). The presence of antibodies indicates an infection.

Imaging

Certain imaging tests can aid in the diagnosis of rabies encephalitis (i.e., acute inflammation of the brain resulting from rabies infection). These imaging tests include head MRIs and head CT scans.

Head MRIs

“MRI” stands for “magnetic resonance imaging,” a procedure that involves using magnets and radio waves to create detailed pictures of your brain and the nerve tissues surrounding it.

Before undergoing a head MRI, you may be given a special dye (called “contrast material”), which helps improve image clarity for the radiologist. This dye is generally administered intravenously through the hand or forearm. Although the dye is very safe, some people may experience allergic reactions. What’s more, the most common type of dye (gadolinium) may be harmful to people with kidney problems.

Usually performed at a hospital or radiology center, head MRIs typically last 30 to 60 minutes. The procedure causes no pain and there’s no recovery time.

During an MRI, you’ll lie on a narrow table, which then slides into a tunnel-shaped scanner. If you’re claustrophobic or uncomfortable in close spaces, tell your doctor before undergoing a head MRI. Medication may help to alleviate your anxiety during the procedure.

You should also tell your doctor if you have:

  • brain aneurysm clips
  • artificial heart valves
  • a heart defibrillator or pacemaker
  • inner ear (cochlear) implants
  • kidney disease or dialysis
  • a recently placed artificial joint
  • a blood vessel stent
  • an allergy to iodine, which is used in the contrast material

In addition, make sure to let your doctor know if you’ve worked with sheet metal in the past.

To prepare for your head MRI, you’ll most likely be asked not to eat or drink anything for 4 to 6 hours beforehand. However, you can go back to your normal diet, activity, and medication use immediately after the test.

Head CT Scans

In a head computed tomography (CT) scan, x-rays are used to create pictures of your head. Like head MRIs, head CT scans are performed at hospitals and at radiology centers.

When undergoing a head CT scan, you’ll lie on a narrow table that slides into the center of a CT scanner. While you’re inside the scanner, the machine’s x-ray beam will rotate around you. The complete scan typically takes somewhere between 30 seconds and a few minutes.

As with some head MRIs, certain CT exams require the use of a special dye delivered intravenously through the hand or forearm. Before receiving the dye, tell your doctor if you have kidney problems or take the diabetes medicine metformin.

Although head CT scans are painless, the contrast material may trigger several side effects, including: 

  • a slight burning feeling.
  • a metallic taste in the mouth.
  • warm flushing of the body.

In rare cases, the dye may cause anaphylaxis (a life-threatening allergic response). If you experience any trouble breathing during the test, alert the scanner operator immediately. Tell a healthcare provider if you have any metal in or on your body, and do not enter an MRI room with anything metal.

Unlike head MRIs, head CT scans carry the risk of being exposed to radiation. While there’s some concern that having many x-rays or CT scans over time may increase your cancer risk, the risk from a single scan is small.

Differential Diagnosis

If you’re being evaluated for a possible case of rabies, the following conditions may also be considered during your diagnosis:

Your doctor will use diagnositc criteria for each of these to either confirm or rule out a diagnosis.

How Rabies Is Treated
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