Encephalitis
Encephalitis, any infectious disease of the human central nervous system characterized by inflammation of the brain. The typical symptoms are headache, fever, and extreme lethargy, which lead eventually to coma; double vision, delirium, deafness, and facial palsy often occur in the acute stage of the disease. Aftereffects of encephalitis may include deafness, epilepsy, and dementia.
Several types of encephalitis are caused by viral infection of the central nervous system. These types fall into two main groups, primary neurotropic-virus infections (original virus infections in nerve tissues), and secondary infections occurring as complications of a primary virus infection elsewhere in the body.
NEUROTROPIC-VIRUS ENCEPHALITIS
This group comprises several epidemic diseases that primarily affect wild and domestic animals and birds. The disease is transmitted to humans from these animals by insect vectors. St. Louis encephalitis, first recognized in 1933 during an epidemic in St. Louis, Missouri, is transmitted to humans by mosquito bites. Other mosquito-borne types of encephalitis are Japanese B encephalitis, California encephalitis, and equine encephalomyelitis. Ticks are the insect vectors of Russian spring-summer encephalitis.
SECONDARY VIRUS INFECTIONS
This group includes two types, postinfective encephalitis and postvaccinal encephalitis. The first type occurs as an occasional complication of certain viral diseases, including mumps, measles, influenza, and yellow fever. Occasionally infections with the herpes virus involve the brain and cause brain damage or death. The second type may occur, although very infrequently, following the first vaccination with such attenuated-virus vaccines as those against smallpox and yellow fever (see Immune System).
OTHER TYPES
Encephalitis may occur as a result of an infection from a sporozoa called Toxoplasma, which is parasitic in animals, birds, and human beings. It can also be due to infection with the protozoan Trypanosoma cruzi, transmitted by the bite of the tsetse fly (see Trypanosomiasis). In addition, forms of apparently noninfectious encephalitis occur occasionally as a complication in cases of poisoning from contact with heavy metals, particularly lead (see Meningitis).
Several types of encephalitis are caused by viral infection of the central nervous system. These types fall into two main groups, primary neurotropic-virus infections (original virus infections in nerve tissues), and secondary infections occurring as complications of a primary virus infection elsewhere in the body.
NEUROTROPIC-VIRUS ENCEPHALITIS
This group comprises several epidemic diseases that primarily affect wild and domestic animals and birds. The disease is transmitted to humans from these animals by insect vectors. St. Louis encephalitis, first recognized in 1933 during an epidemic in St. Louis, Missouri, is transmitted to humans by mosquito bites. Other mosquito-borne types of encephalitis are Japanese B encephalitis, California encephalitis, and equine encephalomyelitis. Ticks are the insect vectors of Russian spring-summer encephalitis.
SECONDARY VIRUS INFECTIONS
This group includes two types, postinfective encephalitis and postvaccinal encephalitis. The first type occurs as an occasional complication of certain viral diseases, including mumps, measles, influenza, and yellow fever. Occasionally infections with the herpes virus involve the brain and cause brain damage or death. The second type may occur, although very infrequently, following the first vaccination with such attenuated-virus vaccines as those against smallpox and yellow fever (see Immune System).
OTHER TYPES
Encephalitis may occur as a result of an infection from a sporozoa called Toxoplasma, which is parasitic in animals, birds, and human beings. It can also be due to infection with the protozoan Trypanosoma cruzi, transmitted by the bite of the tsetse fly (see Trypanosomiasis). In addition, forms of apparently noninfectious encephalitis occur occasionally as a complication in cases of poisoning from contact with heavy metals, particularly lead (see Meningitis).
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