Nervous System infections

ADVANCE ORGANIZER

The nervous system does not have any normal flora associated with it. Infectious agents must either cross the blood-brain barrier or enter by way of peripheral nerves or directly from wounds in the middle ear or sinuses. Treatment is difficult because many drugs do not cross the blood -brain barrier. These infections are uncommon compared with infections in other parts of the body.

ANATOMY AND PHYSIOLOGY
The brain and the spinal cord make up the central nervous system. There are two kinds of nerves: motor neurons and sensory neurons. The brain makes cerebrospinal fluid with flows out over the brain, spinal cord and nerves. Fluid samples can be tested for infection.
Meninges are three membranes that cover the surface of the brain and spinal cord. Cerebrospinal fluid flows between the two innermost membranes.

BACTERIAL INFECTIONS

Neisseria meningitdis is a gram negative diplococcus. It causes meningococcal menigitis. Release of endotoxin can cause shock and rapid death. Disease can occur at any age but is most common in infants, it can occur in epidemics.

Haemophilus influenzae is a leading cause of bacteria meningitis, is a tiny Gram-negative coccobacillus that requires X and V virulence factors for growth. On plates it often grows with satellites. Some strains have acquired a plasmid that codes for penicillinase.

Streptococcus pneumonia is the most common cause of meningitis in adults. This is a gram-positive, encapsulated diplococcus. Pneumonia and ear and/or sinus infection often accompanies the meningitis.

E. coli , a gram negative rod, is the most common cause of meningitis in newborns. It is normal enteric flora.

Streptococcus agalactiae is another leading cause of meningitis of newborns. Characteristic in terms of diagnosis is its Lancefield group B cell wall polysaccharide.

Listeria moncytogenes is a motile aerobic, gram positive rod which causes meningitis in newborns and in some people with other diseases.

]Leprosy is characterized by invasion of peripheral nerves by acid -fast bacillus, Mycobacterium leprae. This organism has never (yet) been cultivated in vitro but it does grow in armadillos. There are two major forms of this disease, tuberculoid and lepromatous. The form depends upon the immune status of the individual: tuberculoid type occurs in those with strong cell mediated immunity; lepromatous in those whose cell mediated immunity is overcome.

VIRAL DISEASES OF THE NERVOUS SYSTEM
Sporadic encephalitis is most frequently caused by herpes simplex virus. That is, about 1 in 2000 reactivation events results in the infection traveling up the nerve instead of down. Other cases occur at lower frequency, some are attributable to mumps, measles and other human viruses.

Arboviruses usually cause epidemic encephalitis. La Crosse encephalitis virus is harbored in Aedes mosquitoes and rodents. St Louis encephalitis is maintained in Culex mosquitoes and birds.

Poliomyelitis is caused by three picornaviruses (1, 2 and 3). Infection involves motor nerve cells of both the brain and the spinal cord. A WHO (World Health Organization) program is actively trying to eradicate this disease by 2000. Polio can lead to paralysis, muscle wasting and failure of normal bone development. There is also a post polio syndrome which involves death of nerves that had probably compensated for ones killed by the initial infection.

Rabies is mainly transmitted by the bite of an infected animal. The incubation can be months or years. The virus multiplies in muscle and then travels through nerve cells to the brain. Then it spreads outward to infect other body tissues and causes rapid death. Fortunately because there is a long incubation period, the disease can often be prevented by vaccination after exposure to the virus.
FUNGAL NERVOUS SYSTEM DISEASES
Cryptococcal meningoencephalitis in caused by Filobasidiella (Cryptococcus) neoformans. It causes an infection of the meninges and brain. The organism is often associated with pigeon dropping. The infection starts in the lungs after inhaling dust containing the fungi. The disease is a common and serious complication of AIDS. Pathogenesis depends on the presence of a large capsule.
PROTOZOAN CENTRAL NERVOUS SYSTEM DISEASES
Trypanosoma brucei , a slender flagellated protozoa causes African sleeping sickness. Transmission is via the bite of tsetse flies. The organism shows bursts of growth, each occurring with different surface proteins. Each of these different surface proteins is a variant that requires that the host respond with a new set of antibodies since the previous immune response is no longer effective.

slow virus or PRION caused central nervous system diseases
Spongiform is a disease whose transmission among humans has occurred in the past through cannibalism. Cases have been reported following corneal and dura grafts and through growth hormone prepared from human cadavers.
The normal function of prion proteins is just now being accessed.

Toxoplasma gondii causes severe infections in immunocompromised people including fetuses and newborns. This protozoan is the reason why HIV infected individuals are asked to give up their cats. Cats are the major reservoir of this disease. Infection in the first trimester can cause epilepsy. Acute disease includes chill, fever, headaches, myalgia, and fatigue.