What are the types of meningitis?
Meningitis may be acute (sudden and short illness) or chronic (slowly starting and long-lasting). Infectious types of meningitis include bacterial meningitis, viral meningitis, fungal meningitis, and parasitic meningitis.
Most infectious meningitis is community-acquired. Very rarely, fungal or bacterial meningitis may be acquired from a hospital or medical procedure. For example, in 2015, over 200 cases of fungal meningitis were reported to the U.S. Centers for Disease Control and Prevention (CDC). The unusual outbreak was linked to tainted steroid medication that was improperly prepared by the New England Compounding Pharmacy.
Viral infection is the most common infectious cause of community-acquired viral meningitis. Most acute viral meningitis (a type of aseptic meningitis) is caused by summer viruses like enterovirus. It is usually not severe. It generally runs its course and goes away without specific treatment. Viral meningitis usually does not cause long-lasting complications or problems. Other community-acquired causes of viral meningitis include influenza, measles, and mumps. Viruses spread by insect bites include the West Nile virus; these viruses often cause meningitis and encephalomyelitis.
Less commonly, herpes simplex viruses, which cause cold sores or genital herpes, can cause viral meningoencephalitis. Hallucinations may be an especially prominent symptom. Varicella zoster virus (the cause of chickenpox, as well as herpes zoster or shingles) may also cause meningoencephalitis. These infections may arise from newly acquired virus or reactivation of virus that infected the body years ago.
Acute bacterial meningitis causes over 4,000 cases and 500 deaths per year in the U.S. A common cause is Streptococcus pneumoniae (pneumococcus), which causes over 50% of cases in the U.S. (about 2,000 per year). It is the leading cause of bacterial meningitis in young children under 5 in the U.S., although immunization with pneumococcal conjugate vaccine has reduced the rate of infection. Pneumococcal meningitis may be complicated by pneumonia (lung infection), bloodstream infection, sepsis, and long-term problems like permanent hearing loss or brain damage. Up to 8% of children and 22% of adults who develop the condition die from it.
Neisseria meningitidis (meningococcus) is known for causing meningococcal disease (bloodstream infection, sepsis) and bacterial meningitis. Meningococcal meningitis is life-threatening and rapidly progressive. Although pneumococcus can do the same, this is the type of meningitis most likely to cause gangrene and amputations of limbs. Up to 15% of people with this infection will die. Up to 19% have long-term complications like permanent deafness or brain damage. It is most likely to occur in children under the age of 1 and youths ages 16 to 23 years, but people of any age may be affected. The most common serogroups (types of the virus that can be identified by antibody testing in the blood) to cause infection in the U.S. are A, C, W, and Y. Serogroup B is more common elsewhere. There is a higher risk of contact with a carrier of serogroup B in areas where groups congregate from many areas, like a college campus or traveling with a group of tourists.
Fortunately, advances in medical care mean that rates of meningococcal disease have been declining for the past few decades. Vaccination of teens added an 80% drop in infections with serogroup C, W, and Y strains. On average in the U.S., there are now 18 cases per thousand people per year. Adolescents are now routinely vaccinated in their early teens, and freshman college students must show proof of meningococcal vaccination before entry.
Less common causes of acute bacterial meningitis include Hemophilus influenzae, which is now prevented very effectively by the Hib vaccine. Group B Streptococcus (GBS) causes life-threatening newborn meningitis and bloodstream infections. It may be spread to the baby during birth. Fortunately, women are screened for this bacterium at 35 to 37 weeks of pregnancy as part of routine maternal care. Those who have it will get antibiotics during delivery to prevent GBS disease and bacterial meningitis in the newborn.
A rarer cause but serious cause of bacterial meningitis, Listeria monocytogenes, may cause meningitis in pregnant women, people over 50, and those with immune problems. It may cause mild illness in a pregnant woman only to cause a severe infection of the fetus, with premature delivery and fetal death. It is acquired from unpasteurized, unaged soft cheeses, and deli meats.
Fungal causes of meningitis are usually limited to people with very weak immunity from medications, immune disorders, or advanced HIV disease. An exception is Coccidioides immitis, which commonly causes valley fever in the American Southwest and can cause meningitis even if immunity is normal. Cryptococcus is a more common cause of fungal meningitis in people with weak immunity.
Rarer still are the parasitic causes of meningitis. Parasites often cause elevation of eosinophils, a specific type of inflammatory cell, in the blood or spinal fluid. Eosinophilic meningitis is caused by parasites that usually infect animals. Humans are accidental hosts for parasites such as
- Angiostrongylus cantonensis (by eating raw or undercooked snails or slugs, often by accident, on leafy greens);
- Bayliscaris procyonis (by accidentally eating soil contaminated with raccoon feces);
- Gnathostoma spinigerum (by eating raw or undercooked freshwater fish, eels, frogs, chicken, or snakes);
- Toxoplasma gondii (by eating raw or undercooked pork, lamb, deer, or shellfish, or accidental ingestion of eggs from cat feces); and
- free-living amoebae like Naegleria fowleri, Acanthamoeba, and Balamuthia mandrillaris (through contact with warm water or soil).
Angiostrongylus meningitis has been reported in Asia and the Pacific, including Hawaii, but infected snails have been reported in Florida, as well. Baylisascaris is most often diagnosed in the U.S. in children who play in areas frequented by raccoons and put their hands in their mouths. Gnathostoma meningitis is most commonly reported in Thailand.
Primary amoebic meningitis (Naegleria fowleri) and granulomatous meningoencephalitis (Acanthamoeba species, Balamuthia mandrillaris) are caused by amoeba found in water and soil. The organisms are forced up the nose in water (primary amoebic meningitis) or inhaled or enter via a break in the skin (granulomatous meningoencephalitis). All are highly fatal even with aggressive and prolonged treatment. Primary amoebic meningitis mimics acute bacterial meningitis in its symptoms and is often fatal within days; swimming in warm freshwater is the clue for appropriate diagnosis and treatment. Granulomatous meningoencephalitis is often chronic.
Chronic meningitis lasts four weeks or longer and is fairly uncommon in the U.S. and developed countries. Most bacterial infection causes acute meningitis, but worldwide the most common cause of chronic meningitis is tuberculosis (Mycobacterium tuberculosis). Other causes include spirochete bacteria like syphilis and Lyme disease, fungi like Cryptococcus, insect-borne viruses like West Nile virus, herpes simplex virus, and varicella zoster virus.