Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
Encephalitis is defined as inflammation of the brain. This definition means encephalitis is different from meningitis, which is defined as inflammation of the layers of tissue, or membranes, covering the brain. Unfortunately, in some people, both of the diseases may coexist and lead to a more complex diagnosis and treatment plan; in addition, both conditions share many of the same symptoms so they may be difficult to distinguish.
However, most doctors consider encephalitis to be a viral illness. Viruses such as those responsible for causing
cold sores, mumps, measles, and chickenpox can also cause encephalitis; they will not be further discussed in this article because their major disease manifestations, symptoms, and complications are detailed in other articles. Major causes of viral encephalitis are
herpes viruses and certain families of viruses, the arboviruses. Arboviruses are spread by insects such as mosquitoes and ticks. The equine (meaning horse), West Nile, Japanese, La Crosse, and St. Louis encephalitis viruses are all mosquito-borne arboviruses. Although viruses are the most common source of infection, bacteria, fungi, and parasites can also be responsible for encephalitis. The focus of this article will be on the most common causes (viral) of encephalitis but will also mention other less frequent causes.
Viral encephalitis resembles the flu in terms of its symptoms and usually lasts for
two to three weeks. It can vary from mild to life-threatening and even cause death. Most people with a mild infection can recover fully. Those with a more severe infection can recover although they may have damage to their nervous system. This damage can be permanent. Some other general features of viral-caused encephalitis are as follows:
Age, season, geographic location, regional climate conditions, and the strength of the person's immune system play a role in development of the disease and severity of the illness.
Herpes simplex (the virus causing cold sores) remains the most common virus involved in encephalitis in the United States and throughout the world. These viruses are usually transmitted from person to person.
In the United States, there are five main encephalitis-causing viruses spread by mosquitoes: West Nile, Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), La Crosse, and St. Louis encephalitis. Two types of Powassan viruses, an infrequent cause of encephalitis, are transmitted by at least two types of ticks.
Venezuelan equine encephalitis is found in South America. It can be a rare cause of encephalitis in Southwestern United States, particularly Texas. The infection is very mild, and nervous system damage is rare.
Japanese encephalitis virus is the most common
arbovirus in the world (virus transmitted by blood-sucking mosquitoes or ticks) and is responsible for 50,000 cases and 15,000 deaths per year
worldwide. Most of China, Southeast Asia, and the Indian subcontinent are affected.