Nervous System Diseases

Viral Infections of the Nervous System

Viral Nervous System Diseases

Viral infections of the nervous system include the single-causative-agent diseases rabies, polio, and leukoencephalopathy and the disorders encephalitis and meningitis, which are caused by a wide range of viruses.

Symptomatically, viral meningitis is similar to bacterial meningitis, causing inflammation of the membranes (meninges) covering the central nervous system. Infection with many viral pathogens, including poliovirus, several herpesviruses, West Nile virus, measles, mumps, and St. Louis encephalitis virus, may cause meningitis. Viral meningitis is generally not fatal and clears on its own without treatment, but it can cause complications for individuals with compromised immunity. While viral meningitis cannot be treated with antibiotics, its symptoms can be treated. Treatments for symptoms of viral meningitis include rest, hydration, fever-reducing medications (antipyretics), pain medications (analgesics), and anti-inflammatory medications.

Encephalitis is inflammation of the brain. A neuroinvasive pathogen is one that infects cells of the nervous system, such as one that causes encephalitis. Cases range from so mild that the person is unaware they have the disease, to so severe as to be life-threatening. Symptoms of encephalitis include headache, neck and back stiffness, fatigue, fever, confusion, seizures, muscle tingling or weakness, coma, and loss of consciousness. In infants, bulging of the fontanelles, soft portions of the head caused by a lack of skull-bone fusion, may also be present.

Encephalitis is usually caused by viral infection. Many viruses that cause encephalitis are known as arboviruses (arthropod-borne) because they are spread via arthropods, namely insects such as mosquitoes and ticks. Arboviruses causing encephalitis include eastern and western equine encephalitis viruses, West Nile virus, and Japanese encephalitis virus. Herpes simplex virus (HSV) and other human herpesviruses (varicella zoster, Epstein-Barr, and cytomegalovirus) can also cause encephalitis in rare cases. In most cases, encephalitic viruses cause acute encephalitis, meaning that the inflammation presents quickly and also resolves quickly. No viral encephalitis infection can be cured, but the symptoms of encephalitis may be treated. Most patients recover completely within a few weeks. Some symptoms may persist in the longer term and in some cases may even lead to permanent brain damage. Many other viral infections occasionally result in encephalitis. These include but are not limited to infections from St. Louis encephalitis virus, Zika virus, and poliovirus.

Progressive multifocal leukoencephalopathy (PML) is a disease characterized by increasing damage to the white matter of the brain in multiple locations. It is caused by the John Cunningham (JC) virus, an opportunistic pathogen that is normally controlled by the immune system. However, in patients with compromised immune systems, such as those with AIDS or those undergoing chemotherapy treatments, the JC virus can proliferate. Treatment for PML involves reversing immune deficiency so that the body can fight the infection, sometimes supplemented with antiretroviral therapies in AIDS patients.

Rabies

Rabies is a viral infection spread through animal bites that is fatal without treatment.

Rabies is an infection caused by rabies virus. The disease is a zoonosis (plural, zoonoses) meaning it is an infectious disease spread via animal reservoirs, which can be both wild and domestic. Rabies is secreted in saliva and is most commonly spread through bite wounds, but can also be spread when infected saliva comes into contact with mucous membranes, such as in the mouth or eyes. Rabies can infect any mammal. Infected mammals including foxes, raccoons, bats, skunks, and coyotes are most likely to transmit rabies in the United States. However, in developing countries in Africa and Southeast Asia, stray dogs are most likely to transmit rabies to humans. Rabies is a significant public health concern because it is invariably fatal if untreated and treatment must begin before symptoms appear in order to be effective. For this reason all household pets and domestic livestock should receive rabies vaccinations regularly. Humans who have suffered an animal bite also receive a rabies vaccination. Governments of many nations also seek new methods of controlling rabies infection, sometimes vaccinating wild animals when rabies cases increase.

The symptoms of rabies include headache, fever, nausea, and paralysis near the bite wound. The virus directly affects the nerves, spinal cord, and brain. As the disease progresses, paralysis spreads, coupled with uncontrolled muscle spasms. Fear of water, or hydrophobia, is a common symptom resulting from viral replication in the salivary glands. Patients experiencing hydrophobia have increased saliva production and difficulty swallowing and will panic or experience pain at the thought of drinking water. Additional symptoms include hallucinations, confusion, and hyperactivity. Death typically occurs within 14 days of the onset of symptoms.

Treatment of rabies infection includes vaccination immediately after exposure and introduction of antibodies that bind to rabies viral particles and present them to the immune system. Vaccine doses and antibodies are injected at the wound site. Regular vaccination is the best measure to control infection by preventing the disease. Anyone who works near animals, including veterinarians, hunters, and ranchers, is encouraged to get vaccinated regularly. Additionally, all persons are discouraged from approaching wild animals, especially those acting in an unusual or erratic manner.
The direct fluorescent antibody (DFA) test can detect the presence of the rabies virus. If diagnosed before symptoms appear, the virus can be treated.
Credit: CDC (left), Debbie Marshall (right)License: CC BY 4.0 (right)

Polio

Poliomyelitis, or polio, is a viral disease that affects the central nervous system and may or may not be fatal but can have permanent effects.
Poliomyelitis, also called polio, is a disease that occurs when the poliovirus infects the central nervous system. Polio is classified as spinal if it affects only the spinal cord, bulbar if it affects the brain, and bulbospinal if it affects both the brain and the spinal cord. Poliovirus infects cells of the digestive system first, and most polio infections have mild symptoms, including high fever, back pain, and muscle spasms, or are altogether asymptomatic. In the rare case when poliovirus spreads to the central nervous system, more severe symptoms, including muscle paralysis, result. Although some patients do recover from paralysis, occasionally the paralysis becomes permanent. However, not all infections with poliovirus (the pathogen) cause paralysis. Most poliovirus infections do not affect the central nervous system. In fact, most poliovirus infections show no symptoms (are asymptomatic). Of the small number that do cause symptoms, the symptoms listed are temporary, with no paralysis or other damage to the nervous system.

Life Cycle of Polio

The poliovirus cell-infection cycle begins with viral binding, then RNA genome injection, translation, and subsequent processing to yield new viral proteins. Multistep genome replication yields new viral RNA that is packaged in new viral proteins.
Poliovirus is spread via food or water contaminated with fecal matter or, more rarely, via saliva. Even very slight fecal contamination can spread the virus. There are few treatments for infected persons. Famously, the iron lung, a negative pressure ventilator, has been used to treat patients who suffer paralysis of the diaphragm in order to help them breathe. For patients whose paralysis became permanent, the iron lung was necessary for the remainder of their lives. Today, patients who cannot breathe may undergo a tracheotomy, a surgical incision in the trachea that allows for mechanical breathing assistance.

The disease is ancient, appearing in Egyptian paintings dating to 1500 BCE. In the early 20th century a polio pandemic affected Europe, North America, and Australia. In 1955 the polio vaccine developed and tested by American virologist Jonas Salk was officially declared to be safe and effective. Initially, demand outstripped supply, and the vaccine had to be rationed. Today, thanks to the vaccine, polio has been eradicated in the United States, and only a few hundred cases are reported globally each year.