C h a p t e r 1 6
Disorders of the Immune Response
367
Opportunistic Infections
Opportunistic infections involve common organisms
that do not typically produce infection unless there is
impaired immune function. As the number of CD4
+
T
cells declines, the risk of these infections increases. In
addition, the baseline HIV RNA level contributes and
serves as an independent risk factor.
2,3
Opportunistic infections are most often catego-
rized by the type of organism (e.g., fungal, proto-
zoal, bacterial and mycobacterial, viral). Bacterial
and mycobacterial opportunistic infections include
bacterial pneumonia, salmonellosis, bartonellosis,
Mycobacterium tuberculosis
(TB), and
Mycobacterium
avium
complex (MAC). Fungal opportunistic infec-
tions include candidiasis, coccidioidomycosis,
cryptococcosis, histoplasmosis, penicilliosis, and pneu-
mocystosis. Protozoal opportunistic infections include
cryptosporidiosis, microsporidiosis, isosporiasis, and
toxoplasmosis. Viral infections include those caused
by cytomegalovirus (CMV), herpes simplex and zoster
viruses, human papillomavirus, and JC virus, a virus
that is the causative agent of progressive multifocal
leukoencephalopathy (PML).
RespiratoryTract Infections.
The most common causes
of respiratory disease in persons with HIV infection are
bacterial pneumonia,
Pneumocystis jiroveci
pneumo-
nia, and pulmonary tuberculosis. Other organisms that
cause opportunistic pulmonary infections in persons
with AIDS include CMV, MAC,
Toxoplasma gondii,
and
Cryptococcus neoformans.
2
Pneumonia also may
be caused by more common bacterial pulmonary patho-
gens, including
S. pneumoniae, Pseudomonas aeru-
ginosa,
and
H. influenzae.
Some persons may become
infected with multiple organisms, causing a polymicro-
bial infection. Kaposi sarcoma (to be discussed) also can
occur in the lungs.
P. jiroveci
(formerly known as
P. carinii
) pneu-
monia (PCP) was the most common presenting
manifestation of AIDS during the first decade of the
epidemic.
P. jiroveci
is an organism common in soil,
houses, and many other places in the environment,
and in healthy persons does not cause infection or
disease. In persons with HIV infection,
P. jiroveci
can
multiply quickly in the lungs and cause pneumonia.
As the disease progresses, the alveoli become filled
with a foamy exudate that forms cup-shaped cyst
walls within the exudate (Fig. 16-10). Since highly
active antiretroviral therapy (HAART) and prophy-
laxis for PCP were instituted, the incidence of PCP
has decreased.
61
P. jiroveci
(formerly known as
P.
carinii
) pneumonia still is common in people unaware
of their HIV-infected status, in those who choose not
to treat their HIV infection or take prophylaxis, and
in those with poor access to health care. The best pre-
dictor of PCP is a CD4
+
cell count below 200 cells/
μ
L,
61
and it is at this point that antimicrobial pro-
phylaxis with trimethoprim-sulfamethoxazole or an
alternative agent (in the case of adverse reactions to
sulfa drugs) is strongly recommended. The symptoms
of
P. jiroveci
pneumonia may be acute or gradually
progressive. Patients may present with complaints of
a mild cough, fever, shortness of breath, and weight
loss. Diagnosis is made by identifying the organism in
pulmonary secretions.
Tuberculosis (TB) is the leading cause of death for
people with HIV infection worldwide, and is often the
first manifestation of HIV infection. In 2011, 23% of
those with TB tested positive for HIV.
44
In the United
States, the number of TB cases decreased from the
1950s to 1985; then, in 1986, the number began to
increase (see Chapter 22).
62
Several factors contributed
to this increase, but the most profound factor was HIV
infection. The lungs are the most common site of
M.
tuberculosis
infection, but extrapulmonary infection of
the kidney, bone marrow, and other organs also occurs
in people with HIV infection. Whether a person has
pulmonary or extrapulmonary TB, most persons pres-
ent with fever, night sweats, cough, and weight loss.
Persons infected with both HIV and TB are more likely
to have a rapidly progressive form of TB, and usually
have an increase in viral load, which decreases the suc-
cess of TB therapy. They also have an increased num-
ber of other opportunistic infections and an increased
mortality rate.
Since the late 1960s, most persons with TB have
responded well to therapy. However, in 1991, there
were outbreaks of multidrug-resistant (MDR) TB. Since
the original outbreak of MDR TB in the early 1990s,
new cases of MDR TB have declined, largely because of
improved infection control practices and the expansion
of directly observed therapy programs.
Gastrointestinal Manifestations.
Diseases of the
gastrointestinal tract are some of the most frequent
complications of HIV infection and AIDS. Esophageal
candidiasis, CMV infection, and herpes simplex virus
infection commonly cause esophagitis in people with
FIGURE 16-10.
Pneumocystis jiroveci pneumonia.
Histopathology of lung shows characteristic cysts with cup
forms and dotlike wall thickening (methenamine silver stain).
(From the Centers for Disease Control and Prevention Public
Health Image Library. No. 960. Courtesy of Edwin P. Ewing, Jr.)
