**2. Diagnostics**

Once the patient has become infected, the incubation period can be as short as 5 days or as long as 2 years. However, it is common for symptoms to occur 20–90 days after initial exposure [6]. Nevertheless, there are several techniques for diagnosing a patient during and after the incubation period.

#### **2.1 Physical symptoms**

As mentioned previously, the two ways the rabies virus may present itself is paralytic and furious symptoms. A patient with paralytic rabies will show progressive paralysis until death. If infection occurred due to a bite, the paralysis typically starts around the wounded area, spreading outwards. It is also common for patients to have a fever, vomiting, weakness in muscles, and myalgia prior to the paralysis [7].

Furious rabies often presents itself with more obvious symptoms. The infected individual commonly displays mood swings, unregulated consciousness, phobias, especially hydrophobia, as well as spasms of the respiratory system [1, 8]. Other symptoms may include a cough, psychosis, delirium, and difficulties swallowing [9].

#### **2.2 Neuroimaging**

For both furious and paralytic rabies, magnetic resonance imaging (MRI) will produce the same key diagnostic images. When symptoms first arise in the prodromal phase, progression in hypersignal T2 changes can be seen around the brachial plexus and the spinal nerve roots associated with the extremity of infection origin. The main MRI feature is increased T2 signal (seen on T2 and FLAIR sequences) in the affected parts of the brain and spinal cord, with a predilection for grey matter structures including basal ganglia, thalami, hypothalami, limbic system, and brainstem. The abnormal hypersignal T2 changes will continue to progress as the patient enters a coma. Once in the comatose phase, the contrast will enhance around the spinal cord, nerve roots located at the sine and cranial region, deep grey matter, brain stem, limbic structures, and thalamus (**Figure 1**) [8].

Another study noted that lesions could be seen throughout different areas of the neuroaxis. They, too, found that paralytic and furious rabies present the same MRI indications, however, they are more noticeable in the paralytic form. The blood–brain barrier often shows no sign of damage until the patient reaches the comatose state.

*The Diagnosis, Clinical Course, Treatment, and Prevention of the Rabies Virus DOI: http://dx.doi.org/10.5772/intechopen.97691*

**Figure 1.**

*Magnetic resonance images with arrows pointing to high focal areas (A. dorsal medulla, B. pons, C. hypothalamus, and D. splenium of corpus callosum) of an infected patient with furious rabies [10].*

Imaging of the blood–brain barrier has greatly improved as new techniques such as diffusion-weighted and diffusion tensor imaging can capture objective and subjective data [11].

### **2.3 Testing techniques**

There are several techniques used to diagnose a patient with a rabies infection; including reverse transcription polymerase chain reaction (rtPCR) analysis, fluorescent antibody test (FAT), tissue culturing, and viral antibody neutralization [12, 13]. The FAT assay has long been a microbiological standard for diagnosing rabies (and other viral infections). Fluorescent antibody virus neutralisation (FAVN) test is also used to diagnose rabies [13]. The CDC recommends a direct form of FAT to detect the rabies virus in animals ante-mortem, however, the animal is usually euthanized after detection and brain tissue samples taken to solidify the diagnosis post-mortem. However, diagnosis in humans requires several different types of methods such as direct rapid immunohistochemistry testing, the use of electron microscopy, and reverse transcriptase-polymerase chain reactions on biological samples (**Figure 2**) [15].

#### **Figure 2.**

*These images show the comparison of a positive direct FAT result (left) and a negative direct FAT result (right) [14].*

Another test utilizing immunohistochemistry, known as the direct rapid immunohistochemistry test (dRIT) is a specific form of histology because it employs antibodies unique to RABV. While this testing method can give a reliable result in less than an hour, retrieving brain samples is very invasive [16].

Samples can also be viewed using electron microscopy. When Negri bodies are located in samples, electron miscopy can give a clear depiction of the bullet-shaped rabies virus being produced. A colloid can be used to compare the virus size to further interpret the image, however shape and size alone are not enough to identify with confidence [17].

Saliva and skin samples can be used for identifying the presence of RABV. rtPCR can be used to confirm or oppose the results of FAT test results. Because RABV is a single-stranded RNA virus, rtPCR can help transform RNA into DNA through amplification for analysis of a complement to a rabies-specific primer. This is often achieved by inoculating suckling mice and retrieving brain or kidney samples after death [15, 18].
