What Causes Meningitis? 3 Pathogens Responsible for the Infection

Meningitis
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Meningitis is an inflammation of the meninges, the membrane covering of the brain and spinal cord. It is a serious infection that can cause brain damage, stroke, nerve damage, and even death. Meningitis may develop from pathogenic or non-pathogenic sources, but most incidences of meningitis result from infection, and the pathogens most often responsible are viruses, bacteria, and fungi. Non-microbial causes of meningitis include certain types of cancers, drugs, and head injury. 

How Meningitis Develops

Meningitis
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Becoming infected with a pathogen that causes meningitis does not mean that you will develop meningitis. Meningitis may develop if the infecting pathogen gains access to the bloodstream and travels to the brain or spinal cord, where it can infect the cerebrospinal fluid (CSF). The CSF is produced by the meninges, and its function is to protect and nourish the brain and spinal cord. If the CSF is infected, the meninges can become inflamed. In order to determine if meningitis is the result of a pathogenic infection, a CSF examination must be performed. 

Most bacterial and viral pathogens that cause meningitis are found in body fluids of the infected person are spread in a variety of ways, depending on the causative pathogen. They may be spread through person-to-person contact, coughing, sneezing and sharing utensils. Some pathogens can also be contracted through the consumption of contaminated food, or passed from mother to child during birth.

Fungal meningitis is not spread by direct contact with an infected individual. The fungi that cause meningitis are often contracted by inhalation of soil contaminated with animal droppings (bird or bat) or decaying matter. These fungi spread from the lungs to the brain via blood circulation.

Bacterial Meningitis

Neisseria meningitidis
This is a colored transmission electron micrograph of the bacteria Neisseria meningitidis, the agent which causes meningitis. Dr Kari Lounatmaa/Science Photo Library/Getty Images

One of the most serious forms of meningitis is bacterial meningitis. This form of meningitis develops as a result of a bacterial infection that spreads to the central nervous system after some type of trauma or a respiratory system infection like sinusitis. Some of the bacteria that cause meningitis are part of the normal human microbiome and are able to gain access to the bloodstream through the mucus membranes. The most common causes of bacterial meningitis are Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae.

Meningococcal Meningitis

Neisseria meningitidis bacteria cause meningococcal meningitis. This very serious infection may result in death within several hours of the manifestation of symptoms. Meningococcal bacteria are found is saliva and can be spread through contact like sneezing, coughing, or kissing. Meningococcal meningitis most commonly occurs in teenagers and young adults, especially those living in close contact. Outbreaks typically occur in shared living environments like college dorms, military bases, and prisons. Getting a meningitis shot or vaccine is the most effective way to prevent meningococcal meningitis.

Pneumococcal Meningitis

The causative agent of pneumococcal meningitis is Streptococcus pneumoniae. This bacterial species also causes pneumonia and is part of the normal throat microbiota in many children. S. pneumoniae is the most common cause of bacterial meningitis in adults and one of the major causes in infants. A pneumococcal vaccine is available for prevention of this infection.

Haemophilus influenzae

Haemophilus influenzae Type b (Hib) bacteria are also part of the normal human throat microbiota. Hib infection was once the leading cause of bacterial meningitis in children up to five years of age. Thanks to the Hib vaccine, the numbers of individuals with this type of meningitis have been greatly reduced.

Viral Meningitis

Echovirus
This is an illustration of an Echovirus (EC), a highly infectious enterovirus which primarily targets children. Laguna Design/Oxford Scientific/Getty Images

Viral meningitis is typically not as severe as bacterial meningitis, but occurs much more frequently. There are several viruses that can cause meningitis, which develops after an initial infection. Among these viruses are non-polio enteroviruses, influenza, herpes, measles, mumps, and arboviruses (West Nile virus).

Individuals at greater risk for viral meningitis include young children and individuals with compromised immune systems as a result of disease, transplantation (bone marrow or organ), or certain medications (chemotherapy). Most people who become infected with a virus that causes meningitis do not actually develop meningitis. Those that do develop meningitis typically improve within a week without treatment. In other cases, antiviral drugs may be used to improve symptoms. Vaccinations against mumps and measles can reduce the risk of developing viral meningitis.

The most common causes of viral meningitis are non-polio enteroviruses. Among these viruses are Coxsackie A viruses, Coxsackie B viruses, and echoviruses. These viruses are very contagious and cause millions of infections each year. The viruses are found in the saliva and stool of an infected person and are spread by contact with infected body secretions. To prevent the spread of these viruses, you should wash your hands properly, disinfect surfaces, and avoid contact with infected individuals.

Fungal Meningitis

Cryptococcus neoformans fungus
C. neoformans is a yeast-like fungus that reproduces by budding. It can cause the disease cryptococcosis and meningitis, especially in immune deficient patients. Kateryna Kon/Science Photo Library/Getty Images

Fungal meningitis is much rarer than bacterial and viral meningitis and is not contagious. Fungal meningitis does not typically occur in healthy individuals; rather, it occurs most often in those with compromised immune systems. The most common cause of fungal meningitis is Cryptococcus neoformans, a fungus found in dried bird and bat droppings.

Individuals may become infected with C. neoformans by inhaling spores that become airborne when contaminated soil is disturbed. The fungi can cause meningitis by infecting the lungs and spreading to the central nervous system through the blood. Other types of soil fungi that may cause meningitis include Histoplasma, Blastomyces, and Coccidioides.

Key Takeaways

  • Meningitis is an infection of the membrane covering of the brain and spinal cord known as the meninges.
  • Most instances of meningitis result from infection caused by bacteria, viruses, or fungi.
  • Meningitis typically develops as a secondary infection after pathogens causing an initial infection enter the bloodstream and travel to the central nervous system.
  • Bacterial meningitis is the most serious of the three pathogenic causes of meningitis. It can cause severe brain injury and even death.
  • Haemophilus influenzae Type b bacterial meningitis, meningococcal meningitis, and pneumococcal meningitis are vaccine-preventable.
  • Viral meningitis is the most common type of meningitis. 
  • Fungal meningitis is very rare and is not spread through person-to-person contact.

Sources

  • "Meningitis." Centers for Disease Control and Prevention, 9 Apr. 2018, www.cdc.gov/meningitis/.
  • Parker, Nina, et al. Microbiology. OpenStax, Rice University, 2017.