Transcript Freeman 1e: How we got there

CHAPTER 26
Person-to-Person Microbial Diseases
Airborne Transmission of
Diseases
Airborne Pathogens
• Many respiratory pathogens are grampositive Bacteria. Because gram-positive
Bacteria are resistant to drying, they are easily
transmitted in air.
• Most respiratory pathogens are transferred
from person to person via respiratory aerosols
generated by coughing, sneezing, talking, or
breathing.
• Figure 26.2 illustrates the human respiratory tract.
Streptococcal Diseases
• Diseases caused by streptococci include
streptococcal sore throat and pneumococcal
pneumonia. Occasionally, Streptococcus
pyogenes infections develop from pharyngitis
into serious conditions such as scarlet fever
and rheumatic fever.
• Pneumonia caused by Streptococcus
pneumoniae is a serious disease with high
mortality.
• Definitive diagnosis for both pathogens is by
culture. Infections with both pathogens are
treatable with antimicrobial drugs, but drugresistant strains are known, especially for
Streptococcus pneumoniae.
Corynebacterium and
Diphtheria
• Diphtheria is an acute respiratory disease
caused by the gram-positive bacterium
Corynebacterium diphtheriae. Early
childhood immunization (DTP) is very
effective for preventing this serious
respiratory disease.
Bordetella and Whooping
Cough
• In the United States, there has been an
increase in the number of annual cases of
whooping cough. From an average of less
than 2000 pertussis cases per year in the
1970s, the number of cases has now risen to
over 8000 per year. Inadequately immunized
children are at high risk for acquiring
pertussis.
Mycobacterium, Tuberculosis,
and Leprosy
• Tuberculosis is one of the most prevalent
and dangerous single diseases in the world. Its
incidence is on the increase in developed
countries, in part because of the emergence of
drug-resistant strains.
• The pathology of tuberculosis and leprosy is
influenced by the cellular immune response.
• In most cases of tuberculosis, acute infection
does not occur. The infection remains
localized, is usually not apparent, and appears
to end. But this initial infection
hypersensitizes the individual to the bacteria
or their products and consequently alters the
response of the individual to subsequent M.
tuberculosis exposures.
• A diagnostic test, called the tuberculin test,
can be used to measure this hypersensitivity.
• Chemotherapy of tuberculosis has been a
major factor in control of the disease. Initial
success in chemotherapy occurred with the
introduction of streptomycin, but the real
revolution in tuberculosis treatment came with
the discovery of iisoniazid (Figure 26.9).
• This drug, virtually specific for
mycobacteria, is effective, inexpensive, and is
readily absorbed when given orally.
Neisseria meningitidis,
Meningitis, and
Meningococcemia
• Neisseria meningitidis is a common cause of
meningococcemia and meningitis in young
adults and occasionally occurs in epidemics in
closed populations.
• Bacterial meningitis and meningococcemia
are serious diseases with very high mortality
rates. Treatment and prevention strategies are
in place to deal with epidemic outbreaks, but
an effective universal vaccine is not yet
available.
Viruses and Respiratory
Infections
• Viral respiratory diseases are highly
infectious and may cause serious health
problems. However, the common childhood
viral diseases measles, mumps, rubella, and
chickenpox are all controllable with
appropriate immunization procedures.
Although once a common childhood illness, measles generally occurs
nowadays in rather isolated outbreaks because of widespread
immunization programs begun in the mid-1960s (Figure 26.13).
Colds and Influenza
• Colds and influenza, or flu, are the most
common infectious diseases (Figure 26.16).
• Although they are not usually lifethreatening by themselves, they can lower
resistance and allow serious secondary
bacterial infections.
• Most antiviral drugs are ineffective against colds, but
a pyrazidine derivative (Figure 26.18a) has shown
promise for preventing colds after virus exposure.
• Antigenic shift occurs when major changes
in antigens occur due to gene reassortment in
influenza virus.
• Antigenic drift occurs when minor changes
in antigens occur due to gene mutation in
influenza virus.
• Influenza outbreaks occur annually, and
more serious epidemics and pandemics occur
periodically (Figure 26.20).
Direct Contact Transmission of
Diseases
Staphylococcus
• Although staphylococci are usually harmless
inhabitants of the upper respiratory tract and
skin, several serious diseases can result from
pyogenic infection, including some caused by
staphylococcal superantigens.
• Staphylococci can cause acne, boils (Figure
26.21), pimples, impetigo, pneumonia, osteomyelitis,
carditis, meningitis, and arthritis.
• Certain strains of S. aureus have been
implicated as the agents responsible for toxic
shock syndrome (TSS), a serious outcome of
staphylococcal infection characterized by high
fever, rash, vomiting, diarrhea, and
occasionally death.
Helicobacter pylori and Gastric
Ulcers
• Helicobacter pylori infection appears to be
the most common cause of gastric ulcers.
Treatment of gastric ulcers now involves
antibiotics, which seem to promote a
permanent cure.
Hepatitis Viruses
• Hepatitis caused by viruses can cause
cirrhosis, an acute liver disease.
• Symptoms include fever and jaundice
(production and release of excess bilirubin by
the liver due to destruction of liver cells,
resulting in yellowing of the skin), among
others.
• Table 26.2 lists the hepatitis viruses. HBV
and HCV can cause chronic infections leading
to liver cancer.
• Vaccines are available for HAV and HBV.
The overall prevalence of hepatitis has
decreased significantly in the last 20 years in
the United States (Figure 26.23), but viral
hepatitis is still a major public health problem
because of the high infectivity of the viruses.
Sexually Transmitted
Infections
Gonorrhea and Syphilis
• Table 26.3 lists sexually transmitted
diseases and treatment guidelines.
• Gonorrhea and syphilis, caused by Neisseria
gonorrhoeae and Treponema pallidum,
respectively, are sexually transmitted
infections (STIs) with potential serious
consequences if not treated.
• During pregnancy, syphilis can be
transmitted from an infected woman to the
fetus; the disease acquired by the infant is
called congenital syphilis.
• Although the incidence of these diseases has generally
declined in recent years, there are still over 350,000
cases of gonorrhea and 6000 cases of syphilis annually
in the United States (Figure 26.25).
Chlamydia, Herpes, and
Trichomoniasis
• Chlamydia, the most prevalent STI, is
caused by infection with the bacterium
Chlamydia trachomatis. Untreated
chlamydial nongonococcal urethritis causes
serious complications in males and females.
• Herpes lesions can also be transmitted
sexually and are caused by herpes simplex virus
type 1 and herpes simplex virus type 2. HSV-2
is generally associated with sexual transmission
and infection of the anogenital regions.
• Genital herpes infections are presently
incurable, although a limited number of drugs
have been successful in controlling the
infectious blister stages.
• The guanine analog acyclovir (Figure
26.31), given orally and also applied topically,
is particularly effective in limiting the shed of
active virus from blisters and promoting the
healing of blistering lesions.
• Trichomonas vaginalis is a protozoan
responsible for trichomoniasis, another STI.
In general, these STIs are widespread and are
more difficult to diagnose and treat than
gonorrhea or syphilis.
Acquired Immunodeficiency
Syndrome: AIDS and HIV
• AIDS is now one of the most prevalent
infectious diseases in the human population.
HIV destroys the immune system, and
opportunistic pathogens then kill the host.
• HIV can infect cells displaying the CD4
cell-surface protein. The two cell types most
commonly infected are macrophages and Thelper cells.
• Infected macrophages and T cells produce
and release large numbers of HIV particles,
which in turn infect other cells that display
CD4 (Figure 26.35).
• The HIV infection results in a progressive
decline in CD4 cell number.
• In a normal human, CD4 cells constitute about 70%
of the total T-cell pool; in AIDS patients, the number
of CD4 cells steadily decreases, and by the time
opportunistic infections become established, CD4
cells may be almost absent (Figure 26.36).
• The RT-PCR test estimates the number of
viruses present in the blood, or the viral load.
• After initial discovery of infection, the RTPCR test is used to monitor progression of
AIDS and the effectiveness of chemotherapy
(Figure 26.37).
• There is still no effective vaccine for HIV. However, several
antiviral drugs slow the progress of AIDS (Table 26.4).
• AZT and the other nucleoside analogs are nucleoside
reverse transcriptase inhibitors (NRTIs), and they stop
HIV replication (Figure 26.38a).
• The second category of anti-HIV drugs is the
nonnucleoside reverse transcriptase inhibitors (NNRTIs)
(Figure 26.38b).
• These compounds directly inhibit the action
of reverse transcriptase by interacting with the
protein and altering the conformation of the
catalytic site.
• Another category of anti-HIV drugs is the protease
inhibitors (Figure 26.38c).
• The protease inhibitors are computer-designed
peptide analogs that inhibit processing of viral
polypeptides by binding to the active site of the
processing enzyme, HIV protease.
• A final category of anti-HIV drugs is
represented by a single drug, enfuvirtide, a fusion
inhibitor composed of a 36–amino acid synthetic
peptide that acts by binding to the gp41
membrane protein of HIV.
• The only prevention for the spread of HIV
infection is avoidance of behavior such as
intravenous drug use (needle sharing) and unsafe
sexual practices.