Encephalitis means inflammation of the brain. This inflammation usually is triggered by a viral infection, although sometimes it can be caused by a bacterial infection of the brain, such as Lyme disease. In some cases, symptoms are caused by direct infection of the brain. In other cases, the brain inflammation is caused by the immune system's response to the brain infection. Even if the immune system attack succeeds in eliminating the infection, it may injure the brain in the process. This is called post-infectious encephalitis.
Often, viruses that cause encephalitis also cause inflammation of the delicate tissues that cover the brain and spinal cord, which are called the meninges. This condition is meningitis. When encephalitis and meningitis occur together, it is called meningoencephalitis.
Of the many different viruses that can cause meningoencephalitis, enteroviruses (particularly coxsackievirus and echovirus) are the most common cause in the United States, particularly if the illness occurs in the summer or fall. Encephalitis also can be caused by the herpes simplex virus, which also causes cold sores and genital herpes. This type of encephalitis is less common but tends to be more severe. The mumps and measles viruses also can cause encephalitis, with mumps occurring most often in the winter or spring.
Other viruses that can cause encephalitis include several related viruses: varicella-zoster virus (the cause of chickenpox and shingles), cytomegalovirus, Epstein-Barr virus (the most common cause of infectious mononucleosis) and human herpesvirus-6 (a cause of transient encephalitis in very young children). HIV also can cause encephalitis, particularly in the early stages of infection.
Other viruses that cause encephalitis are transmitted directly or indirectly from animals to humans. Arboviruses are indirectly transmitted from animals and birds to humans by insects, especially mosquitoes and ticks.
The West Nile virus, one of the arboviruses, is widespread in Africa, Central Europe, the Middle East, and Asia. Since 1999, it has become increasingly common in the United States. The virus commonly infects birds. Mosquitoes that bite an infected bird and then bite a human can transmit the virus. West Nile virus does not cause encephalitis in most humans who are infected. This virus does not spread directly from human to human.
Arboviruses that can infect horses are called equine viruses, such as Eastern equine encephalitis (EEE or triple E). A mosquito that bites an infected horse can carry the virus to a human. Fortunately, human infection is rare because unlike West Nile virus infection in humans, triple E infection is often much more serious. Like West Nile, triple E does not spread through direct contact with an infected human.
Lymphocytic choriomeningitis (LCM) virus rarely infects humans. When it does, it can occur through contact with small animals.