Page 84 - Textbook of Pathology, 6th Edition

68 together comprise about 50% of all HIV positive cases (mostly These genes and viral components act as markers for the
contracted sexually), while North-East state of Manipur laboratory diagnosis of HIV infection. Besides, there is tat
accounts for 8% of all cases (mostly among intravenous drug (transcription activator) gene for viral functions such as
abusers). amplification of viral genes, viral budding and replication.
ETIOLOGIC AGENT. AIDS is caused by an RNA retrovirus ROUTES OF TRANSMISSION. Transmission of HIV
called human immunodeficiency virus (HIV) which is a type infection occurs by one of following three routes:
of human T cell leukaemia-lymphoma virus (HTLV). 1. Sexual transmission. Sexual contact in the main mode
HIV resembles other HTLVs in shape and size and both of spread and constitutes 75% of all cases of HIV
have tropism for CD4 molecules present on subpopulation of transmission. Most cases of AIDS in the industrialised world
SECTION I
T cells which are the particular targets of attack by HIV. like in the US occur in homosexual or bisexual males while
However, HIV differs from HTLV in being cytolytic for T cells heterosexual promiscuity seems to be the dominant mode of
causing immunodeficiency (cytopathic virus) while HTLV may HIV infection in Africa and Asia. Other sexually transmitted
transform the target cells into T cell leukaemia (transforming virus) diseases (STDs) may act as cofactors for spread of HIV, in
(Chapter 8). Two forms of HIV have been described, HIV1 particular gonorrhoeal and chlamydial infection.
being the etiologic agent for AIDS in the US and Central Transmission from male-to-male and male-to-female is more
Africa, while HIV2 causes a similar disease in West Africa potent route than that from female-to-male.
and parts of India. Both HIV1 and HIV2 are zoonotic
infections and their origin can be traced to a species of 2. Transmission via blood and blood products. This mode
chimpanzees who are natural reservoir of HIV and most of transmission is the next largest group (25%) and occurs in
likely source of original infection. 3 groups of high-risk populations:
HIV-I virion or virus particle is spherical in shape and 100- i) Intravenous drug abusers by sharing needles, syringes etc
140 nm in size (Fig. 4.4): comprise a large group in the US.
It contains a core having core proteins, chiefly p24 and p18, ii) Haemophiliacs who have received large amounts of
two strands of genomic RNA and the enzyme, reverse clotting factor concentrates from pooled blood components
transcriptase. from multiple donors.
The core is covered by a double layer of lipid membrane iii) Recipients of HIV-infected blood and blood products who have
derived from the outer membrane of the infected host cell received multiple transfusions of whole blood or components
during budding process of virus. The membrane is studded like platelets and plasma.
with 2 envelope glycoproteins, gp120 and gp41, in the positions
General Pathology and Basic Techniques
shown. 3. Perinatal transmission. HIV infection occurs from infected
Besides various other genes, three important genes code mother to the newborn during pregnancy transplacentally, or
for the respective components of virion: in immediate post-partum period through contamination with
i) gag (group antigen) for core proteins, maternal blood, infected amniotic fluid or breast milk.
ii) pol (polymerase) for reverse transcriptase, and 4. Occupational transmission. There have been a small
iii) env (envelope) for the envelope proteins. number of health care workers (HCW), laboratory workers
and those engaged in disposal of waste of sharps who have
developed HIV infection by occupational exposure to HIV-
infected material. It is imperative that these workers follow
CDC guidelines for universal precautions which include
disinfecting and sterilizing all reusable devices and use of
bleaching solution for disinfecting all blood spillage.
5. Transmission by other body fluids. Although besides
blood, HIV has been isolated and identified from a number
of body fluids such as saliva, tears, sweat and urine, semen,
vaginal secretions, cervical secretions, breast milk, CSF,
synovial, pleural, peritoneal and pericardial fluid, there is
no definite evidence that HIV transmission can occur by any
of these fluids; isolated cases of such infection reported are
in likelihood due to concomitant contamination with HIV-
infected blood.
It may, however, be understood regarding spread of HIV
infection that AIDS cannot be transmitted by casual non-sexual
contact like shaking hands, hugging, sharing household
facilities like beds, toilets, utensils etc.
Figure 4.4 Schematic representation of HIV virion or virus particle. It should also be appreciated that HIV contaminated
The particle has core containing proteins, p24 and p18, two strands of waste products can be sterilised and disinfected by most of the
viral RNA, and enzyme reverse transcriptase. Bilayer lipid membrane is chemical germicides used in laboratories at a much lower
studded with 2 viral glycoproteins, gp120 and gp41, in the positions concentration. These are: sodium hypochlorite (liquid
shown.