26.1 Anatomy of the Nervous System
- The nervous system consists of two subsystems: the central nervous system and peripheral nervous system.
- The skull and three meninges (the dura mater, arachnoid mater, and pia mater) protect the brain.
- Tissues of the PNS and CNS are formed of cells called glial cells and neurons.
- Since the blood-brain barrier excludes most microbes, there is no normal microbiota in the CNS.
- Some pathogens have specific virulence factors that allow them to breach the blood-brain barrier. Inflammation of the brain or meninges caused by infection is called encephalitis or meningitis, respectively. These conditions can lead to blindness, deafness, coma, and death.
26.2 Bacterial Diseases of the Nervous System
- Bacterial meningitis can be caused by several species of encapsulated bacteria, including Haemophilus influenzae, Neisseria meningitidis, Streptococcus pneumoniae, and Streptococcus agalactiae (group B streptococci). H. influenzae affects primarily young children and neonates, N. meningitidis is the only communicable pathogen and mostly affects children and young adults, S. pneumoniae affects mostly young children, and S. agalactiae affects newborns during or shortly after birth.
- Symptoms of bacterial meningitis include fever, neck stiffness, headache, confusion, convulsions, coma, and death.
- Diagnosis of bacterial meningitis is made through observations and culture of organisms in CSF. Bacterial meningitis is treated with antibiotics. H. influenzae and N. meningitidis have vaccines available.
- Clostridium species cause neurological diseases, including botulism and tetanus, by producing potent neurotoxins that interfere with neurotransmitter release. The PNS is typically affected. Treatment of Clostridium infection is effective only through early diagnosis with administration of antibiotics to control the infection and antitoxins to neutralize the endotoxin before they enter cells.
- Listeria monocytogenes is a foodborne pathogen that can infect the CNS, causing meningitis. The infection can be spread through the placenta to a fetus. Diagnosis is through culture of blood or CSF. Treatment is with antibiotics and there is no vaccine.
- Hansen’s disease (leprosy) is caused by the intracellular parasite Mycobacterium leprae. Infections cause demylenation of neurons, resulting in decreased sensation in peripheral appendages and body sites. Treatment is with multi-drug antibiotic therapy, and there is no universally recognized vaccine.
26.3 Acellular Diseases of the Nervous System
- Viral meningitis is more common and generally less severe than bacterial menigitis. It can result from secondary sequelae of many viruses or be caused by infections of arboviruses.
- Various types of arboviral encephalitis are concentrated in particular geographic locations throughout the world. These mosquito-borne viral infections of the nervous system are typically mild, but they can be life-threatening in some cases.
- Zika virus is an emerging arboviral infection with generally mild symptoms in most individuals, but infections of pregnant women can cause the birth defect microcephaly.
- Polio is typically a mild intestinal infection but can be damaging or fatal if it progresses to a neurological disease.
- Rabies is nearly always fatal when untreated and remains a significant problem worldwide.
- Transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease and kuru are caused by prions. These diseases are untreatable and ultimately fatal. Similar prion diseases are found in animals.
26.4 Fungal and Parasitic Diseases of the Nervous System
- Neuromycoses are uncommon in immunocompetent people, but immunocompromised individuals with fungal infections have high mortality rates. Treatment of neuromycoses require prolonged therapy with antifungal drugs at low doses to avoid side effects and overcome the effect of the blood-brain barrier.
- Some protist infections of the nervous systems are fatal if not treated, including primary amoebic meningitis, granulomatous amoebic encephalitis, human African trypanosomiasis, and neurotoxoplasmosis.
- The various forms of ameobic encephalitis caused by the different amoebic infections are typically fatal even with treatment, but they are rare.
- African trypanosomiasis is a serious but treatable disease endemic to two distinct regions in sub-Saharan Africa caused by the insect-borne hemoflagellate Trypanosoma brucei.
- Neurocysticercosis is treated using antihelminthic drugs or surgery to remove the large cysts from the CNS.