HIV is a rather insidious virus. It does not attack just any cells in the body; it specifically attacks those that are meant to defend against infection and does so with amazing efficiency. In the final stages of the infection, the third stage, the virus obliterates the immune system to such an extent that many latent pathogens are able to reactivate and gain a foothold. One kind of pathogen that is frequently reactivated is the opportunistic central nervous system infection. This particular pathogen may lie latent in a healthy person's system for many years however HIV makes it possible for the pathogen to take hold where a previously strong immune system would have overcome it. Let us take a look at a few of the opportunistic CNS infections that may take hold of the HIV positive individual once the immune system reaches a CD4-cell count of 200 cells per millilitre.
Tuberculosis meningitis and brain abscesses
The World Health Organisation has estimated that approximately one in three people are infected with this bacterium. In most people the bacteria is latent, however contracting HIV increases the risk that an active form will develop. An individual becomes infected with the bacteria through the inhalation of the airborne bacterium which then moves from the alveoli of the lungs into the bloodstream. The bacterium causes the formation of granulomas in the brain which can then expand to form tuberculomas or abscesses or they may rupture and cause meningitis. It is vital that the treatment course be completed so as to try to prevent the development of drug resistant strains of this bacterium.
Toxoplasmic encephalitis
This is the most commonly reported opportunistic central nervous system infection. Fortunately a decline in incidence has been seen since 1997. This condition is most frequently caused by the reactivation of encysted bradyzoites which is made possible by the suppressed immune system in HIV patients rather than by primary infection. These latent bradyzoites transform into active tachzyoites when the immune system is no longer able to keep the latent infection under control. These tachzyoites produce necrotising cerebritis with a wall of histiocytes and inflamed vessels. A more diffuse form of encephalitis may occur if the immune system is sufficiently suppressed.
Cryptococcal meningitis
This condition is a major concern in sub-Saharan Africa as well as in south and south-east Asia. The pathogen is found in encapsulated yeast which is found in soil as well as in the excrement of pigeons. The pathogen enters the system through the inhalation of spores and this can then lead to a disseminated infection, a latent infection or a pulmonary infection. The current theory is that HIV patients with suppressed immune systems develop the active infection as a result of the reactivation of the latent infection which may have been in their system for many years. The pathogen causes the formation of cysts in the Virchow-Robin spaces of the brain as well as in the deep blood vessels of the brain and cranial nerves.
There are many other opportunistic central nervous system infections which wreak havoc with the HIV positive individual once the immune system is sufficiently suppressed. Early detection and treatment is a vital part of recovery as all these conditions have a high mortality rate.
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