What Are the Most Common Viral Infections of the Brain?
Viral infections of the brain can cause brain inflammation, known as encephalitis, associated with the following viruses [1]:
- Herpes Simplex Virus
- Varicella Zoster Virus
- Enteroviruses
- Epstein-Barr Virus
- Cytomegalovirus
- Adenovirus
- Rubella
- Measles
- Mumps
- Japanese Encephalitis Virus.
Encephalitis mainly manifests with the following symptoms that develop over several days:
- Headaches
- Fever
- Seizures
- Weakness
- Drowsiness
- Speech difficulty
- Memory loss
- Loss of smell (anosmia)
- Vision problems
- Changes in personality and behaviors
- Aching joints and muscles
- Hallucinations
1. What Is a virus?
A virus is a very small infectious agent that hijacks the cellular machinery of organisms to achieve its multiplication and survival through the production of virions.
Viruses can infect any type of living organisms including bacteria, plants, and animals; however, viruses can also target specific tissues, a phenomenon known as viral tropism (e.g., neurotropic virus).
The hijacking of the cellular machinery can involve the integration of a viral DNA into the genome of the host cell, preferential use of cellular transcription (gene expression), metabolism (e.g., energy and nutrients), and translation (production of viral protein).
Like for normal cells, viruses require a messenger RNA (mRNA) to produce proteins that are used for the assembly and generation of new viruses (multiplication).
1.1. What Are the Types of Viruses?
Depending on the viral genome, viruses are classified into DNA viruses and RNA viruses.
- DNA Viruses
DNA viruses are viruses that have a DNA genome and are divided into double-stranded DNA viruses or dsDNA viruses (like the cell genome) and single-stranded DNA viruses or ssDNA viruses.
The viral DNA is used for direct transcription using the host cell transcription machinery (RNA polymerase) and for replication.
Some of the members of DNA viruses include Herpes Simplex Virus (HSV), Varicella-Zoster Virus, Epstein-Bar Virus (EBV), Smallpox Virus, Cytomegalovirus (CMV), and Human Papillomavirus (HPV).
- RNA viruses
RNA viruses are viruses that have an RNA genome and are divided into double-stranded RNA viruses or dsRNA viruses (like the cell genome) and single-stranded RNA viruses or ssRNA viruses.
The RNA viruses RNA is translated into different components of new viruses using the host cell translation machinery and replicating their genome using their own enzyme known as RNA replicase.
Some of the RNA viruses (retroviruses) retrotranscribe their RNA into double-stranded DNA using an enzyme known as reverse transcriptase, which is then integrated into the host genome by an enzyme known as integrase.
Some of the members of RNA viruses include retroviruses (e.g., HIV), coronaviruses (e.g., Covid 19), and Rhabdoviruses (e.g., Rabies virus).
2. What Is Herpes Simplex Encephalitis?
Herpes simplex encephalitis is an inflammation of the brain that is mainly caused by Herpes Simplex Virus-1 (HSV-1); however, it can also be caused by Herpes Simplex Virus-2 (HSV-2) in newborns [2].
HSV-1 generally causes cold sores and HSV-2 is involved in genital herpes with about 90% of adults becoming infected with HSV-1 during their lifetime.
In the United States, 2000 cases of Herpes simplex encephalitis are recorded annually and account for 10% of all encephalitis.
HSV-1 reaches the brain ganglia (e.g., trigeminal ganglia) through sensory nerves of the throat and becomes latent and may occasionally get reactivated and causes cold sores.
The mechanism by which HSV-1 causes encephalitis is unknown but may involve the virus reactivation resulting in complications such as encephalitis.
The treatment of herpes simplex encephalitis involves the use of antiviral drugs such as aciclovir (acyclovir).
3. What Is Varicella Zoster Virus Encephalitis?
Varicella-Zoster Virus (VZV) is responsible for varicella (chickenpox) and shingles [3].
It reaches the brain ganglia through the respiratory system and remains in a latency state until reactivation causes shingles.
Varicella-Zoster Virus Encephalitis affects 1 in 50,000 unvaccinated children which may lead to seizures and coma.
Although vaccines exist, Varicella Zoster Virus Encephalitis is treated with antiviral drugs such as acyclovir, famciclovir, and valaciclovir.
4. What Is Enterovirus Encephalitis?
This type of encephalitis is caused by an enterovirus that is transmitted by the fecal-oral route. In the gastrointestinal system, enteroviruses multiply and spread through the bloodstream to reach the brain after altering the blood-brain barrier (BBB) [4].
Although most enteroviruses do not cause any symptoms, they can also cause mild symptoms such as fever, sore throat, and vomiting, and severe diseases including encephalitis, meningitis, limb weakness, and myocarditis.
Other severe disorders include brainstem encephalitis or meningoencephalitis that manifest with symptoms such as ataxia, hallucinations, lethargy, somnolence, seizures, and coma.
The treatment of enterovirus encephalitis is mostly supportive and there are no specific drugs for this infection.
5. What Is Epstein-Barr Virus Encephalitis?
The Epstein-Barr virus is responsible for mononucleosis and is reactivated after a latency state causing central nervous system complications such as acute encephalitis, demyelinating disease, meningitis, and acute cerebellar ataxia [5].
Unfortunately, they are no effective drugs against Epstein-Barr Virus [6].
6. What Is Cytomegalovirus Encephalitis?
This encephalitis is caused by the cytomegalovirus responsible for mononucleosis and pneumonia.
Cytomegalovirus exists in a latency state where it is kept in check by the immune system; however, in immunocompromised individuals, cytomegalovirus is reactivated which may cause encephalitis [7].
Drugs approved for CMV treatment include ganciclovir, valganciclovir, foscarnet, and cidofovir [8].
7. What Is Adenovirus Encephalitis?
Adenoviruses are responsible for upper respiratory conditions, conjunctivitis, and gastroenteritis [9].
Although rare, adenoviruses can cause encephalitis, hepatitis, hemorrhagic cystitis, nephritis, and pancreatitis.
There are no proven drugs against adenoviruses and the treatment is symptomatic.
8. What Is Rubella Encephalitis?
This type of encephalitis is caused by the rubella virus that causes rash, fever, swollen glands, conjunctivitis, headaches, and joint pain in affected individuals.
However, some cases have reported rubella infections with symptoms of acute encephalitis [10].
Although vaccines exist (MMR vaccine), no treatment is available yet for rubella infections.
9. What Is Measles Encephalitis?
This type of encephalitis is caused by the measles virus responsible for fever, coughing, coryza, conjunctivitis, and maculopapular rash [11].
Measles encephalitis mostly occurs during the rash phase as a complication of the disease.
Although vaccines exist (MMR vaccine), no treatment is available yet for measles infections.
10. What Is Mumps Encephalitis?
This encephalitis is due to an infection with the mumps virus responsible for painful swelling of the side of the face under the ears where parotid glands are located.
Encephalitis is a complication of mumps infection that manifests with difficulties in learning and memory, and loss of hearing and visual acuity [12].
Vaccines (MMR vaccine) exist for the prevention of mumps, but no treatment is available yet for mumps infections.
11. What Is the Japanese Encephalitis Virus?
Japanese Encephalitis is caused by a flavivirus related to West Nile, dengue, and yellow fever viruses. It is transmitted by mosquitos and found in countries in South-East Asia and Western Pacific regions [13].
The symptoms of Japanese encephalitis are characterized by fever, vomiting, headache, seizures, neck stiffness, disorientation, coma, spastic paralysis, and ultimately death.
Unfortunately, no cure exists for this infection and treatment is supportive.
Conclusion
Many cases of viral encephalitis are due to the reactivation of viruses that are found in a latency state within the brain and body. Their reaction may be due to temporary or chronic weakness in the immune system. Early vaccinations of children may also significantly prevent complications associated with these viruses including encephalitis.
