Encephalitis and Meningitis Health Article

Definition

Encephalitis is an acute inflammatory process that affects brain tissue and is almost always accompanied by inflammation of the adjacent meninges (tissues lining the brain). There are many types of encephalitis, most of which are caused by viral infections.

Meningitis is an inflammation of the membranes (meninges) that surround the brain and spinal cord. Meningitis may be caused by many different viruses and bacteria, or by diseases that can cause inflammation of tissues of the body without infection (such as systemic lupus erythematosus). Viral meningitis is sometimes called aseptic meningitis to indicate it is not the result of a bacterial infection.

Description

Encephalitis can be divided into two forms, primary and secondary encephalitis, according to the two methods by which the viruses infect the brain. Primary encephalitis occurs when a virus directly invades the brain and spinal cord. Primary encephalitis can happen to people at any time of the year (sporadic encephalitis), or can be part of an outbreak (epidemic encephalitis). Secondary, or post-infectious encephalitis occurs when a virus first infects another organ and secondarily enters the brain.

Meningitis is an inflammation of the membranes that surround the brain and spinal cord, and may be caused by many different viruses and bacteria, or by non-infectious inflammatory diseases. Encephalitis is a distinct disease from meningitis, although, clinically, the two often share signs and symptoms of inflammation of the meninges.

Demographics

Determining the true incidence of encephalitis in the United States is difficult because reporting policies are neither standardized nor rigorously enforced. Several thousand cases of viral encephalitis are reported yearly. HSE (herpes simplex encephalitis), the most common cause of sporadic encephalitis in other western countries, is still relatively rare in the United States, with an overall incidence of two cases per one million persons per year.

Arboviruses (viruses transmitted to humans by blood-sucking insects such as mosquitoes and ticks) are the most common causes of episodic encephalitis. These statistics may be misleading because most people bitten by arbovirus-infected insects do not develop clinical disease, and only 10% of those develop overt encephalitis. Among less common causes of viral encephalitis, varicella-zoster encephalitis (a complication of the condition commonly known as shingles) has an incidence of one in 2000 infected people.

Internationally, Japanese virus encephalitis (JE), occurring principally in Japan, Southeast Asia, China, and India, is the most common viral encephalitis outside the United States.

In 1995, there were 5755 cases of bacterial meningitis reported in United States. This is a dramatic decrease from the 12,920 cases reported in 1986, probably due to the decrease in Haemophilus influenzae meningitis since the introduction of the Hib vaccine. The occurrences by infectious agents in 1995 are as follows:

• Streptococcus pneumoniae: 1.1 per 100,000 persons
• Neisseria meningitides: 0.6 per 100,000 persons
• Streptococcus: 0.3 per 100,000 persons
• Listeria monocytogenes: 0.2 per 100,000 persons
• Haemophilus influenzae: 0.2 per 100,000 persons

The incidence of meningitis in newborns has shown no significant change in the last 25 years. Viral meningitis is the most common form of aseptic meningitis and, since the introduction of the mumps vaccine, is caused by enteroviruses in up to 85% of cases. The incidence of encephalitis is more difficult to estimate because of difficulty in establishing the diagnosis. One report estimates an incidence of one in 500–1,000 infants and in the first six months of life.

Causes

The causes of encephalitis are usually infectious, but may also be due to some noninfectious causes. Three broad categories of viruses—herpes viruses, viruses responsible for childhood infections, and arboviruses (viruses harbored by mosquitoes and ticks, and transferred through their bite)—typically trigger encephalitis.

ENCEPHALITIS AND HERPES VIRUSES Some herpes viruses that cause common infections may also cause encephalitis. These include:

• Herpes simplex virus. There are two types of herpes simplex virus (HSV) infections. HSV type 1 (HSV-1) causes cold sores or fever blisters around the mouth. HSV type 2 (HSV-2) causes genital herpes. HSV is the most common cause of sporadic encephalitis, with HSV-1 being the more common culprit. When untreated, the mortality rate from herpes simplex encephalitis is between 60–80%. That number drops to 15–20% with treatment.
• Varicella-zoster virus. This virus is responsible for chicken pox and shingles. It can cause encephalitis in adults and children, but the cases tend to be mild.
• Epstein-Barr virus. This herpes virus causes infectious mononucleosis. If encephalitis develops, it's usually mild, but more severe forms can result in death in up to 8% of cases.

ARBOVIRUSES The mosquito season varies according to geographic location. Arbovirus transmission, therefore, also varies according to season, the cycle of viral transmission, and local climatic conditions. Six encephalitis disease groups caused by arboviruses are monitored by the United States Centers for Disease Control (CDC) and include:

• St. Louis encephalitis
• West Nile encephalitis
• Powassan encephalitis
• Eastern equine encephalitis
• Western equine encephalitis
• California serogroup viral encephalitis, which includes infections with the following viruses: La Crosse, Jamestown Canyon, snowshoe hare, trivittatus, Keystone, and California encephalitis viruses.

OTHER CAUSES OF ENCEPHALITIS Bacterial pathogens (disease-causing organisms), such as rickettsial disease, mycoplasma, and cat scratch disease, are rare, but often involve inflammation of the meninges. Encephalitis can be due to parasites and fungi. Insects, such as mosquitoes in the eastern and southeastern United States can also spread encephalitis.

CAUSES OF MENINGITIS Viral meningitis is the most common infection of the Central Nervous System (CNS). It most frequently occurs in children younger than one year of age. Enteroviruses (viruses that causes infections of the gastrointestinal tract) are the most common causative agent and are a frequent cause of febrile illnesses in children. Other viral pathogens include paramyxoviruses, herpes, influenza, rubella, and adenovirus. Meningitis may occur in up to half of children younger than three months with enteroviral infections. Enteroviral infections can occur any time during the year, but are normally associated with outbreaks in the summer and fall. Viral infections cause an inflammatory response, but to a lesser degree than bacterial infections. Damage from viral meningitis may be due to an associated encephalitis and increased intracranial pressure.

Bacterial meningitis is fairly uncommon, but can be extremely serious. There are two main types of bacterial meningitis, which cause most of the reported bacterial cases: meningococcal and pneumococcal. Haempohilus influenzae type b (Hib), which was recently a major cause of bacterial meningitis, has now been almost eliminated by the vaccination of infants. The most common causative organisms in the first month of life are Escherichia coli and group B streptococci. Listeria monocytogenes infection also occurs in patients in this age range and accounts for 5–10% of cases. In people older than two months, S. pneumoniae and N. meningitides currently cause the majority of the cases of bacterial meningitis. H. influenzae may still occur, especially in children who have not received the Hib vaccine.