A neurotropic virus is a virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 which is capable of infecting nerve cells, or which does so preferentially. Such viruses thereby largely evade the usual immune response—which acts only within the blood system.

Terminology

A disease agent is said to be neuroinvasive if it is capable of entering or infecting the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

, and neurovirulent if it is capable of causing disease within the nervous system. The most common neuroinvasive viruses are rabies

Rabies

Rabies is a viral disease that causes acute encephalitis in warm-blooded animals. It is zoonotic , most commonly by a bite from an infected animal. For a human, rabies is almost invariably fatal if post-exposure prophylaxis is not administered prior to the onset of severe symptoms...

, which has both high neuroinvasiveness and high neurovirulence, and herpes simplex virus

Herpes simplex virus

Herpes simplex virus 1 and 2 , also known as Human herpes virus 1 and 2 , are two members of the herpes virus family, Herpesviridae, that infect humans. Both HSV-1 and HSV-2 are ubiquitous and contagious...

, which has low neuroinvasiveness and high neurovirulence. Others, like poliovirus

Poliovirus

Poliovirus, the causative agent of poliomyelitis, is a human enterovirus and member of the family of Picornaviridae.Poliovirus is composed of an RNA genome and a protein capsid. The genome is a single-stranded positive-sense RNA genome that is about 7500 nucleotides long. The viral particle is...

, can spread neurally, but primarily spread by via the blood system in what is called hematogenous dissemination

Viremia

Viremia is a medical condition where viruses enter the bloodstream and hence have access to the rest of the body. It is similar to bacteremia, a condition where bacteria enter the bloodstream.- Primary versus Secondary :...

.

Examples

Neurotropic viruses that cause acute infection include Japanese Encephalitis

Japanese Encephalitis

Japanese encephalitis —previously known as Japanese B encephalitis to distinguish it from von Economo's A encephalitis—is a disease caused by the mosquito-borne Japanese encephalitis virus. The Japanese encephalitis virus is a virus from the family Flaviviridae. Domestic pigs and wild birds are...

, Venezuelan Equine Encephalitis

Venezuelan equine encephalitis virus

Venezuelan equine encephalitis virus is a mosquito-borne viral pathogen that causes Venezuelan equine encephalitis or encephalomyelitis . VEE can affect all equine species, such as horses, donkeys, and zebras. After infection, equines may suddenly die or show progressive central nervous system...

, and California encephalitis

California encephalitis virus

California encephalitis virus was discovered in Kern County, California and causes encephalitis in humans. Encephalitis is an acute inflammation of the brain that can cause minor symptoms, such as headaches, to more severe symptoms such as seizures...

 viruses; polio, coxsackie

Coxsackie A virus

Coxsackie A virus is a cytolytic coxsackie virus of the Picornaviridae family, an enterovirus .-Diseases:...

, echo

Echovirus

An ECHO virus, is a type of RNA virus that belongs to the genus Enterovirus of the Picornaviridae family...

, mumps

Mumps

Mumps is a viral disease of the human species, caused by the mumps virus. Before the development of vaccination and the introduction of a vaccine, it was a common childhood disease worldwide...

, measles

Measles

Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses...

, influenza

Influenza

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

, and rabies as well as diseases caused by members of the family Herpesviridae

Herpesviridae

The Herpesviridae are a large family of DNA viruses that cause diseases in animals, including humans. The members of this family are also known as herpesviruses. The family name is derived from the Greek word herpein , referring to the latent, recurring infections typical of this group of viruses...

 such as herpes simplex

Herpes simplex virus

Herpes simplex virus 1 and 2 , also known as Human herpes virus 1 and 2 , are two members of the herpes virus family, Herpesviridae, that infect humans. Both HSV-1 and HSV-2 are ubiquitous and contagious...

, varicella-zoster, cytomegalo and Epstein-Barr viruses.

Those causing latent infection include herpes simplex and varicella-zoster viruses. Those causing slow virus infection include measles, rubella

Rubella

Rubella, commonly known as German measles, is a disease caused by the rubella virus. The name "rubella" is derived from the Latin, meaning little red. Rubella is also known as German measles because the disease was first described by German physicians in the mid-eighteenth century. This disease is...

 and JC

JC virus

The JC virus or John Cunningham virus is a type of human polyomavirus and is genetically similar to BK virus and SV40. It was discovered in 1971 and named using the two initials of a patient with progressive multifocal leukoencephalopathy...

 viruses, and retrovirus

Retrovirus

A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...

es such as human T-lymphotropic virus

Human T-lymphotropic virus

Human T-cell Lymphotropic Virus Type 1 , also called the Adult T-cell lymphoma virus type 1, a virus that has been seriously implicated in several kinds of diseases including HTLV-I-associated myelopathy, Strongyloides stercoralis hyper-infection, and a virus cancer link for leukemia...

 1 and human immunodeficiency virus.

Research use

Neurotropic viruses are increasingly been exploited as research tool, and for their potential use in treatment. In particular they are being used to improve the understanding of the nervous systems circuits

Other neurotropic infections

Several diseases, including transmissible spongiform encephalopathy

Transmissible spongiform encephalopathy

Transmissible spongiform encephalopathies , also known as prion diseases, are a group of progressive conditions that affect the brain and nervous system of many animals, including humans. According to the most widespread hypothesis they are transmitted by prions, though some other data suggest an...

, Kuru

Kuru (disease)

Kuru is an incurable degenerative neurological disorder that is a type of transmissible spongiform encephalopathy, caused by a prion found in humans...

 and Creutzfeldt-Jakob disease

Creutzfeldt-Jakob disease

Creutzfeldt–Jakob disease or CJD is a degenerative neurological disorder that is incurable and invariably fatal. CJD is at times called a human form of mad cow disease, given that bovine spongiform encephalopathy is believed to be the cause of variant Creutzfeldt–Jakob disease in humans.CJD...

 resemble a slow neurotropic virus infection—but are in fact caused by the infectious proteins known as prions.