Encephalitis generally falls into two categories... viral and autoimmune.
- Arboviruses – The most common types of encephalitis is spread by mosquitoes and ticks and typically have a sudden onset of symptoms;
- Enteroviruses – Viruses that enter the body through the gastrointestinal tract;
- Herpes viruses – Viruses that cause an infection and then lie dormant. In some cases, the virus reactivates, causing encephalitis;
- Autoimmune encephalopathy (also sometimes called “autoimmune encephalitis”)
- Other rare causes – Childhood viral diseases, drug reactions and bacterial infections.
Arboviruses are transmitted by mosquitoes and ticks. A bird usually develops a viral infection, which is passed on by the insects feeding on its infected blood. The insects spread the virus when they bite a human or animal. Infected humans do not pass the virus to another human except through blood transfusions.
Different arboviruses cause different forms of encephalitis. Although the overall disease is the same, there are subtle differences in symptoms and the type of brain damage they produce. These are some of the more common forms of mosquito-transmitted viral encephalitis:
- Japanese encephalitis is the most common arbovirus in the world and is found mostly throughout Asia. JE kills one in three infected humans. This strain of mosquito-borne virus is preventable through the JE vaccination. Symptoms present five to fifteen days post infection and typically include fever, headache, lethargy and rash.
- LaCrosse encephalitis affects mostly children in the upper mid-western states. Symptoms, which typically present after five to 10 days, include fever, headache, fever, lethargy and vomiting.
- St. Louis encephalitis affects more adults than children, especially elderly adults. Symptoms, such as fever and headache, typically present seven to ten days after infection.
- Western Equine encephalitis is more common in farming areas in the western and central states and typically affects children more than adults. Symptoms begin five to ten days after infection. Death occurs in about 3 percent of cases.
- West Nile encephalitis, the newest to reach the United States, is spreading rapidly in the warmer climate. Like other arboviruses, symptoms are flu-like and include fever, headache, and joint pain. Some patients may develop a skin rash and swollen lymph glands, while others may not show any symptoms.
- Tick-borne encephalitis covers a large geographical region, including Asia, Europe, the former Soviet Union, but rarely in the United States. Powassan encephalitis is the main tick-borne encephalitis found in the United States and Canada.
- Venezuelan equine encephalitis has reached epidemic stages in South and Central America. Children are most at risk for this virus, which often leaves patients with severe neurological damage.
Enteroviruses are common, but rarely serious viruses that spread through contaminated food or water, sneezing or coughing. They enter the body through the gastrointestinal tract and account for between 10-20% of viral encephalitis cases.
The herpes virus group includes a number of common infections that typically affect patients with compromised immune systems. The incidence is low of these viruses causing encephalitis.
- Cytomegalovirus encephalitis is common and usually mild. In immunocompromised patients, such those with AIDS, it can be dangerous, with severe complications including encephalitis.
- Epstein-Barr virus, which typically affects people under 20 years old, is the cause of infectious mononucleosis. Symptoms are fatigue, fever, headache and sore throat. The incidence of encephalitis is rare and most have a full recovery.
- Herpes simplex virus (HSV) is the most common cause of encephalitis in developed countries and is the most treatable of all types of encephalitis through intravenous acyclovir. Treatment must be administered within the first few days of symptoms. With treatment, the mortality rate is 30%; without treatment, the mortality rate is 70%.
- Varicella-Zoster virus is responsible for both chickenpox (varicella) and shingles (herpes zoster). The chickenpox virus typically remains dormant for a lifetime. In the cases where it erupts, this is referred to as shingles. Encephalitis caused by varicella tends to be more severe than herpes zoster cases, but both are rare.
Autoimmune encephalopathy (also sometimes called “autoimmune encephalitis”) occurs when a person’s own immune system mounts a defense (autoimmune) against parts of that person’s own brain, leading to pathological damage to the brain (encephalopathy). Autoimmune encephalopathies comprise a large number of syndromes, many of which have only recently been identified. Unfortunately, for many years, such syndromes were not readily classified, and many persons who mounted these immune responses were not treatable. Indeed, many were considered delusional and were institutionalized. Fortunately, today, there are many treatments for this diverse set of syndromes, even when the underlying cause is not known. (please click here to read more on this topic)
OTHER RARE CAUSES
- Adenoviruses can cause respiratory or gastrointestinal infections that are usually mild. In rare cases, they cause severe cases of encephalitis that are fatal in 30% of patients.
- Bacteria rarely cause encephalitis. The two bacteria that have caused encephalitis are mycobacterium tuberculosis (TB) andBartonella henselae (cat scratch disease).
- Brain injury
- Brain tumor
- Childhood diseases, such rubella, measles, and mumps are rare in developed countries, typically only presenting in immunocompromised children. Rarely, influenza has caused acute encephalitis, usually in children and therefore flu vaccinations are recommended for prevention.
- Drug reactions
- Immunocompromised patients, such as those with HIV-AIDS, cancer therapies, or organ transplantation, are more susceptible than other individuals to any form of encephalitis. Varicella and cytomegalovirus encephalitis are more common and deadly in these patients.
- Rabies, transmitted from the saliva of an infected wild animal, is extremely rare, but almost always fatal. Only one or two cases are typically reported each year.
- Parasites – Encephalitis may be caused by parasitic infections, such as pork tapeworm eggs or roundworms. These tend to cause milder forms of encephalitis.
- Raccoon roundworm is a large parasitic worm that lives in the intestines of raccoons. Humans are exposed by ingesting or coming into contact with soil, wood chips, or tree bark contaminated with raccoon feces.
- Toxoplasmosis, transmitted by contact with a cat's feces or eating contaminated food, causes very mild symptoms in most people and can be treated with antibiotics. Toxoplasmosis is dangerous for pregnant women as it can cause severe problems in the fetus’ central nervous system and eyes.