Infectious Agents

The aetiological agents responsible for meningitis are viruses, with bacteria, fungi, and protozoa being the next most common causes. The infection may also occur from a number of non-infectious agents, for instance, malignant or neoplastic meningitis, caused by the spread of malignant (cancer) cells from the tumour site to the meninges (4). Furthermore, certain drugs, such as non-steroidal anti-inflammatory drugs (NSAIDs), antibiotics, and intravenous immune globulins (IVIG), as well as a number of disorders (systemic lupus erythematosus (SLE), sarcoidosis, vasculitis, Behçet's disease etc.) may be responsible for the onset or spread of meningitis (8). Regardless, non-infectious forms of meningitis, alongside fungi- and protozoa-induced infections remain uncommon amongst the human population and are aetiologically dominated by viruses and bacteria (1).

Viral meningitis is the most common form of meningitis globally. Non-polio enteroviruses are currently the leading cause of viral meningitis, accounting for 85 to 95% of all cases in which a pathogen has been identified (3). The virus primarily affects infants and young children, due to their lack of previous exposure and immunity, although only a small percentage of those infected develop meningitis. Other, less common meningitis-causing viruses include type 2 herpes simplex virus, human alpha herpesvirus 3 (HHV-3), usually referred to as the varicella-zoster virus (VZV) (responsible for chickenpox and shingles), measles virus, HIV, arboviruses, such as West Nile virus, Lymphocytic choriomeningitis virus (LCMV), and the Influenza virus (2).

Despite the higher frequency, viral meningitis is a far less serious form, and usually clears up on its own within 7 to 10 days. Contrastingly, bacterial meningitis results in significant morbidity and is potentially fatal, even when diagnosed early and treated accordingly (5). In addition, 10 to 20% of survivors will suffer permanent sequalae including brain damage, hearing loss, and learning disabilities. The frequency of specific pathogens varies in patients with bacterial meningitis depending upon age, race, sex, and geographical locale. Among new-borns, the majority of infections are due to group B Streptococcus agalactiae, Listeria monocytogenes, and, Escherichia coli while most cases in children and adults are caused by Streptococcus pneumoniae and Neisseria meningitidis, responsible for the only form of bacterial meningitis known to cause large-scale epidemics (6 out of the 12 identified serogroups of N. meningitidis cause epidemics) (6). N. meningitidis is a Gram-negative bacterium responsible for meningitis in children and young adults, causing developmental impairment with an overall mortality rate of 3 to 13% of total cases. Approximately 10% of adults are carriers of the bacterium in their nasopharynx, which can be spread through saliva and respiratory secretions of droplets, such as coughing sneezing, kissing etc (9). The bacterium has also been reported to be transmitted through oral sex, leading to urethritis in males. Together with Streptococcus pneumoniae, the two bacteria cause approximately 80% of bacterial meningitis cases in adults. More severe reactions may occur in patients with an impaired immune system, most commonly as a result of underlying health conditions, either cognitive or acquired, these include AIDS, multiple myeloma, hypogammaglobulinemia, chronic liver or kidney disorders and diabetes (7).