Page 393 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART IV Clinical Virology
382
delivery and that the remainder is split roughly equally
transmission of HIV from health care personnel to patients
between transplacental transmission and transmission via
is exceedingly rare.
breast feeding. There is no evidence for airborne, water-
borne, or insect transmission of HIV.
Infection occurs by the transfer of either HIV-infected
Pathogenesis & Immunity
cells or free HIV (i.e., HIV that is not cell-associated).
Although small amounts of virus have been found in other
them, resulting in suppression of cell-mediated immu-
fluids (e.g., saliva and tears), there is no evidence that they
nity. This predisposes the host to various opportunistic
play a role in infection. In general, transmission of HIV HIV infects helper T cells (CD4-positive cells) and kills
infections and certain cancers such as Kaposi’s sarcoma and
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follows the pattern of hepatitis B virus (HBV), except that
lymphoma. HIV does not directly cause these tumors
HIV infection is much less efficiently transferred (i.e., the
because HIV genes are not found in these cancer cells. The
dose of HIV required to cause infection is much higher
than that of HBV). People with sexually transmitted dis-
that line the mucosa (Langerhans’ cells), after which the
eases, especially those with ulcerative lesions such as syphi-
local CD4-positive helper T cells become infected. HIV is
lis, chancroid, and herpes genitalis, have a significantly
first found in the blood 4 to 11 days after infection.
higher risk of acquiring HIV. Uncircumcised males have a
HIV infection also targets a subset of CD4-positive cells
higher risk of acquiring HIV than do circumcised males.
called Th17 cells. These cells are an important mediator of
Transmission of HIV via blood transfusion has been
mucosal immunity, especially in the gastrointestinal tract.
greatly reduced by screening donated blood for the pres-
ence of antibody to HIV. However, there is a “window”
Th17 cells produce interleukin-17 (IL-17), which attracts
period early in infection when the blood of an infected
neutrophils to the site of bacterial infection. The loss of
person can contain HIV but antibodies are not detectable. Many mucosal Th17 cells are killed early in HIV infection.
Th17 cells predisposes HIV-infected individuals to blood-
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Blood banks now test for the presence of p24 antigen in an
stream infections by bacteria in the normal flora of the
effort to detect blood that contains HIV.
colon, such as Escherichia coli.
The Centers for Disease Control and Prevention (CDC)
HIV also infects brain monocytes and macrophages,
estimates that at the end of 2011, there were approximately
producing multinucleated giant cells and significant central
1.1 million people infected with HIV living in the United
nervous system symptoms. The fusion of HIV-infected
States. The transmission rate has declined markedly, pri-
cells in the brain and elsewhere mediated by gp41 is one of
marily due to increased prevention efforts and improved
the main pathologic findings. The cells recruited into the
treatments for HIV; the latter reduces the number of people
syncytia ultimately die. The death of HIV-infected cells is
with high titers of HIV. CDC estimates that approximately
also the result of immunologic attack by cytotoxic CD8
50,000 people new infections occur each year. CDC also
estimates that 15% of those who are infected with HIV do
limited by the ability of the viral Tat and Nef proteins to
not know it because they have not been tested.
Approximately 630,000 people have died of AIDS in the lymphocytes. Effectiveness of the cytotoxic T cells may be
Another mechanism hypothesized to explain the death
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United States since 1981, when AIDS was first recognized.
of helper T cells is that HIV acts as a “superantigen,” which
As of 2011, it is estimated that approximately 34 million
indiscriminately activates many helper T cells and leads to
people worldwide are infected, two-thirds of whom live in
their demise. The finding that one member of the retrovi-
sub-Saharan Africa. Three regions, Africa, Asia, and Latin
America, have the highest rates of new infections. AIDS is
superantigen lends support to this theory. Superantigens
the fourth leading cause of death worldwide. (Ischemic
are described in Chapter 58.
heart disease, cerebrovascular disease, and acute lower
Persistent noncytopathic infection of T lymphocytes
respiratory disease are ranked first, second, and third,
also occurs. Persistently infected cells continue to produce
respectively.)
HIV, which may help sustain the infection in vivo. Lym-
In the United States and Europe during the 1980s, HIV
HIV infection.
infection and AIDS occurred primarily in men who have
In addition, a true latent infection can occur in which
sex with men (especially those with multiple partners),
intravenous drug users, and hemophiliacs. Heterosexual phoid tissue (e.g., lymph nodes) is the main site of ongoing
no HIV is produced. This occurs in resting CD4-positive
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transmission was rare in these regions in the 1980s but is
memory T cells within which an integrated HIV genome is
found. The latent period can last for months to years but if
now rising significantly. Heterosexual transmission is the
the resting cell is activated, HIV can be produced. HIV
predominant mode of infection in African countries.
Very few health care personnel have been infected
in activated, but not resting, CD4-positive cells.
despite continuing exposure and needle-stick injuries, sup-
A person infected with HIV is considered to be
porting the view that the infectious dose of HIV is high.
infected for life. This seems likely to be the result of
The risk of being infected after percutaneous exposure to
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