Encephalitis is an inflammation of the brain, caused by various agents that include viruses, bacteria, and the bite of an infected insect. Methods of transmission include animals to man (equine encephalitis) and from man to man (herpes simplex encephalitis). The most common strains in North America attributed to mosquitoes are SLE (St.Louis Encephalitis), EEE (Eastern Equine Encephalitis), and CE (California Encephalitis). Another common one found in Asia is JE (Japanese Encephalitis), which can be deadly and a potential risk to travellers. Other forms include: Venezuelan equine encephalitis and Powassan encephalitis, spread by ticks and not mosquitoes. There is no cure or treatment, except time.
Encephalitis is rare in adults, but, in newborns, it can be a fatal infection. This type is usually caused by a bacterium or virus, as from the herpes simplex species, but over thirty flaviviruses cause infections of the brain that lead to encephalitis. About one in 800 infected with measles contract encephalitis. Some recover while others are left with serious impairment of the central nervous system, and some die. It is not known whether the disease is caused by a mutant strain of the measles virus or not. Bacterial encephalitis is rare, but the organisms most associated with it are Legionella pneumophila, Borrelia burgdorferi, and Treponema pallidum. Other rare causes include the yeast Cryptococcus neoformans and the malarial parasite Plasmodium falciparum, as well as trypanosomes, the cause of sleeping sickness.
The mortality rate of Japanese encephalitis (JE) has dropped by 50%, perhaps because of a less virulent strain of the virus, which is now more common. Pigs in endemic areas are testing possitive 100% of the time for anitibodies against JE, but continue to remain healthy as do dogs and other infected animals. Only horses, donkeys, and people become ill with JE.
Symptoms can be mild with a headache, general malaise, and muscle aches that are common to the “flu.” The more acute and serious symptoms include fever, delirium, convulsions, and coma, with a significant number dying as a result. Treatment is essentially symptomatic, but a chemotherapeutic agent is often used. Herpes viral encephalitis is treated with a chemical agent effective only against the herpes virus.
Bunyaviruses are of the family Bunyaviridae, made up of over 200 interrelated viruses most of which are arboviruses. They are single-stranded RNA viruses with helical symmetry and an envelope. They cause such diseases as Korean Hemorrhagic fever, California encephalitis, Sandfly fever, and Rift Valley fever.
California encephalitis (CE) virus is a member of a group called Bunyavirus. It was first discovered in the California Central Valley region in 1943, and is still the most common kind of encephalitis in children younger than fifteen. For some reason, more boys than girls are affected, and it occurs more frequently in rural areas than in urban. Another member of this group is called La Crosse, causing a common encephalitis in young children. The CE virus is a zoonosis, which is a disease passed to humans from animals. Various mosquitoes carry the CE virus, but the La Crosse type is carried only by the Aedes triseriatas mosquito. Mosquitoes are also the vectors for St. Louis encephalitis and the eastern and western equine viruses, favoring wooded areas and discarded tires, where stagnant water is often found. Squirrels and chipmunks are the main animal hosts for the virus. Once the virus is passed on through the bite of a mosquito, it enters the bloodstream and goes directly to the brain and spinal cord. Multiplying in the CNS, it causes inflammation that damages the nerve cells, interferring with signals from the brain to the rest of the body. Most people are not aware that they have contracted the disease even when mild symptoms appear in the form of fever, irritability, drowsiness, headache, and stomach upsets. Later, these victims may suffer convulsions or seizures. There is no special treatment for the disease, except time.
Other forms of encephalitis include:
- Economo’s e. is a form of epidemic encephalitis first described by von Economo. It was observed in various parts of the world between 1915 and 1926. Also known as lethargic encephalitis, it produced symptoms of increasing listlessness, apathy, and drowsiness, passing into lethargy.
- Equine e. is also called encephalomyelitis because it often affects the spinal cord as well as the brain.
- Encephalomyelitis is an inflammation of the brain and the spinal cord which can often result during episodes of encephalitis.
- E. periaxia’lis diffu’sa is a subacute or chronic leukoencephalopathy of children and adolescents, also known as Schilder’s Disease and progressive subcortical encephalopathy. It causes massive destruction of the white substance of the cerebral hemispheres with clinical symptoms of blindness, deafness, bilateral spasticity, and mental deterioration.
- Granulomatous amoebic e. is a chronic and usually fatal opportunistic infection caused by Acanthamoeba species, and usually attacks those who are debilitated or immunocompromised, diabetics, and alcoholics.
- Herpes e. is caused by a herpesvirus and resembles equine encephalomyelitis.
- Lead e. with cerebral edema is caused by lead poisoning.
- Postinfection e. is an acute disease of the CNS seen in patients recovering from other infections, usually viral.
- Postvaccinal e. is an acute disease sometimes occurring after a vaccination.
- Russian spring-summer e. is a form of epidemic encephalitis acquired from infected ticks, but can be acquired in such other ways as the ingestion of meat or milk from infected animals. Severity ranges from mild to fatal, with degenerative changes in organs other than those of the nervous system.
- St. Louis e. is a viral infection acquired through the bite of infected mosquitoes. It was first observed in 1932 in Illinois, and occurs during the late summer and early fall. It is clinically similar to western equine encephalomyelitis.