3rd year/Analytical Invest.
Dr.Hawraa A.Ali Ad-Dahhan
Transmitted Diseases
Lecture 2-3
WHEN DEFENSES ARE IMPAIRED
Impaired host defenses can invite infection.Conditions that may weaken a person’s
defenses include poor hygiene, malnutrition, extremes of age, climate, inadequate physical
barriers, inherited and acquired immune
deficiencies,
emotional
and
physical
stressors,
chronic disease, medical and surgical
treatments, and inadequate immunization.
HYGIENE
Poor hygiene increases the risk of
infection because untended skin is more
likely to crack and break, allowing
microorganisms to enter. Also, dirty skin
harbors transient microorganisms, and
microbial colonization of the skin increases.
Good hygiene promotes normal host
defenses. Washing (and using topical
moisturizers, if necessary) lubricates the skin
and protects the epidermis against breaks.
Removing microbial buildup on clothing
through frequent laundering is another
important step in controlling infection.
NUTRITION
The body produces antibodies from
proteins, which are obtained through nutritional intake.With inadequate protein, a
malnourished person lacks the energy to mount an adequate attack against invading
organisms. In addition, many nutrients (such as zinc, selenium, and vitamins A and E) are
necessary for an optimal immune system response.
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AGE
The very young and the very old
are at higher risk for infection. The
immune system
doesn’t fully develop until about age
6 months. During the infant’s first
exposure to an infectious agent, the
infection usually wins out—
especially if it’s an upper respiratory
infection, the most common type
among infants and toddlers. Also,
toddlers tend to put toys and other
objects in their mouths, play in the
dirt, or soil their clothes with urine
and feces, bringing further exposure
to microorganisms. Exposure to
communicable diseases continues
throughout childhood. Preschoolers
are exposed in day-care facilities;
school-age children are exposed in school. Skin diseases, such as impetigo and lice (scabies),
commonly travel from one child to the next. Childhood accidents, such as abrasions,
lacerations, and fractures, may also allow microorganisms to enter the body. Lack of
immunization contributes to childhood infection.Measles, virtually eradicated in the United
States by the measles vaccine several decades ago, resurfaced in community outbreaks in
2008. Its revival has been linked to unvaccinated individuals, many of whom were children
whose parents chose not to have them vaccinated.Most cases of measles occurring in the
United States are imported from endemic areas. At the opposite end of the age spectrum,
advancing age is associated with weakened immune system function as well as with chronic
diseases that reduce host defenses. What’s more, the increased use of long-term care
facilities, such as nursing homes and personal care units, has added to the risk of disease
transmission among the elderly.
Infection in health care facilities
In health care facilities, patients of all ages stand a higher chance of developing an
infection. Invasive procedures and devices, drugs that suppress the immune system,
increased use of blood products, and inhalation therapy add to the potential threat. Heroic
techniques such as massive radical surgery with prolonged anesthesia and organ transplants
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also place tremendous stress on the patient’s immune system. Likewise, poor aseptic
technique by health care providers also increases the risk of infection.
HEALTH CARE–ASSOCIATED INFECTIONS
Every year, 5% to 10% of hospitalized patients in the United States develop health care–
associated infections.Health care– associated infections are estimated to more than double
the mortality and morbidity risks of any admitted patient and probably result in as many as
90,000 deaths per year in the United States. Costs for diagnosing and treating these
infections reach billions of dollars annually. The longer a patient remains in the hospital, the
greater his or her chance will be for developing a health care–associated infection.
The microorganisms that flourish in health care settings, along with patients’ weakened
defense mechanisms, help set the stage for health care–associated infections. The
microorganisms responsible for health care–associated infections may be either endogenous
(from normal flora) or exogenous (from external sources). (See A dangerous combination.)
Invasion sites
Health care–associated infections most commonly invade the body through the urinary tract.
Other common portals of entry include surgical wounds, the respiratory tract,and the
bloodstream.
URINARY TRACT
Urinary tract infections (UTIs) are the most common health care–associated infections in
the United States, accounting for 35% to 45% of reported cases and affecting an estimated
600,000 patients each year. Of these infections, 80% follow introduction of instrumentation
into the urinary tract, primarily indwelling urinary Catheters .Health care–associated UTIs
may lengthen a patient’s hospital stay and are costly to treat. It is estimated that the annual
cost of health care–associated UTIs in the United States ranges from $424 to $451 million.
Normal bowel flora is also implicated in UTIs. These microorganisms can gain access to the
urinary tract through the use of contaminated equipment or irrigate solutions, through
inadequate cleaning at the time of catheter insertion, or from the unwashed hands of health
care providers.
RESPIRATORY SYSTEM
Most respiratory health care–associated infections are linked o respiratory devices used
to aid breathing or administer medications. Pneumonia is the second most common health
care–associated infection in the United States. It is also the most dangerous, causing more
deaths than any other type of health care–associated infection. For those who become
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infected while in the intensive care unit, pneumonia is the primary cause of death.
Respiratory system–related infections typically lengthen a patient’s hospital stay by 1 to 2
weeks and add billions to U.S. health care costs every year. Health care–associated
pneumonia most commonly stems from gram-negative bacteria, although it can also result
from other bacteria, fungi, and viruses. Typically, the pathogen invades the lower respiratory
tract by one of three routes:
◗ Aspiration of oropharyngeal organisms
◗ Colonization of the aerodigestive tract
◗ Use of contaminated equipment or Medications.
SURGICAL WOUNDS
The third most common type of health care–associated infection, accounting for about
25% of cases, are surgical wound infections. Occurring in about 2% to 5% of surgical
patients, these infections lengthen hospital stays by about 6 days. Surgical wound infections
can occur in the incision as well as in the deep tissue of a wound.Most are thought to
originate from bacteria that enter the wound during surgery.
BLOODSTREAM
Called bacteremia, health care–associated infections of the bloodstream account for
about 6% of health care–associated infections, although thisnumber is decreasing due to
recent initiatives.Bacteria and fungi are common culprits.Although local infections outside
thebloodstream are sometimes the source of the infection, more than 75% of bacteremias
seen in hospitals are related to intravascular devices such as I.V. catheters.Microorganisms
can move from the patient’s skin to the catheter tip and along the outer surface of the
catheter and then enter the bloodstream. The sicker a patient is, the more central and
peripheral lines he or she is likely to have, bringing more opportunities for microorganisms
to invade the bloodstream.
Infection control and treatment
BREAKING THE WEAKEST LINK
The best way to control infections is to break the weakest link in the chain of infection
(usually the mode of transmission).Many strategies exist to prevent or control the
transmission of infectious agents, and they fall into four general categories:
◗ Control or elimination of infectious agents by appropriate sanitation, disinfection, and
sterilization
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◗ Control of transmission through proper hand hygiene, effective ventilation, and aseptic
technique
◗ Reservoir control. In health care settings, a number of interventions are directed at
controlling or destroying infectious reservoirs.
◗ Isolating infected patients, according to Centers for Disease Control and Prevention
(CDC) recommendations, to limit the chance that they will transmit the infection
TREATMENT
Treatment for infections can vary widely. Vaccines may be administered to induce a primary
immune response under conditions that won’t cause disease. If infection occurs, treatment is
tailored to the specific microorganism causing the infection. Drug therapy should only be
used when appropriate. Supportive therapy can play an important role in fighting infections.
◗ Antibiotics work in a variety of ways, depending on the class of drug used. Antibiotic
action is either bactericidal or bacteriostatic. Antibiotics may inhibit cell wall synthesis,
protein synthesis, bacterial metabolism, or nucleic acid synthesis or activity, or they may
increase cell membrane permeability.
◗ Antifungal drugs destroy the invading microorganism by increasing cell membrane
permeability. The antifungal binds sterols in the cell membrane, resulting in leakage of
intracellular contents, such as potassium, sodium, and nutrients.
◗ Antiviral drugs stop viral replication by interfering with DNA synthesis.
Bacterial resistance
Certain gram-positive microorganisms have become resistant to many of the antimicrobial
drugs previously used to treat them. Resistant microbial strains that pose a serious challenge
to health care facilities (especially acute-care hospitals and long-term care facilities) include
methicillin-resistant Staphylococcus aureus (MRSA), resistant Streptococcus pneumoniae,
and vancomycin-resistant Enterococcus (VRE). These strains, which are rapidly becoming
part of the flora in many health care facilities,must be controlled to prevent health care–
associated infections. (See Mechanisms of resistance.)
RESISTANT S. AUREUS INFECTIONS
S. aureus commonly occurs on the skin without producing any disease, but it can produce
a variety of signs and symptoms ranging from a skin pustule to bloodstream infections to
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death. It’s also a frequent cause of pneumonia, septicemia, and surgical siteinfections in
hospitalized patients. Disease can also be caused by community-associated infections.
Methicillin is an antibiotic commonly used to treat staphylococcal (staph) infections.
Although methicillin is very effective in treating most staph infections, some staph bacteria
have developed resistance to methicillin and can no longer be killed by this antibiotic.
According to the CDC, MRSA infections accounted for 2% of all staph infections in 1974,
22% in 1995, and 63% in 2004. The antibiotic used to treat MRSA infections is
vancomycin.However, in 1996, an infection caused by a strain of S. aureus with reduced
susceptibility to vancomycin was diagnosed in a patient in Japan. This strain of S. aureus
was referred to as vancomycinintermediate- resistant S. aureus (VISA).
Since then, several more infections with VISA have been identified. A great concern
among infectious disease experts is the emergence of a strain of S. aureus with full resistance
to vancomycin. This could leave doctors with no antibiotics for the treatment of S. aureus
infections. Some persons are asymptomatic carriers of resistant S. aureus, with colonies in
the nose and on the skin. The infection spreads from patient to patient, mainly on the hands
of health care providers as they become contaminated during patient care. Appropriate
control measures to prevent the spread of MRSA and VISA include strict hand hygiene,
barrier protection (gloves, gowns, and masks), and contact precautions.
RESISTANT S. PNEUMONIAE INFECTIONS
S. pneumoniae is the most common cause of bacterial pneumonia in the United States and
is a major cause of ear infections, sinusitis, bloodstream infections, and meningitis. Until
2000, S. pneumoniae infections caused 100,000 to 135,000 hospitalizations for pneumonia, 6
million cases of otitis media, and 60,000 cases of invasive disease, including 3,300 cases of
meningitis. Disease figures are now decreasing due to the introduction of a conjugate
vaccine. The emergence of drug-resistant S. pneumonia has complicated the treatment of
these infections. Infections involving these microorganisms may eventually necessitate
treatment with costly broad-spectrum antibiotics.
VRE INFECTIONS
Enterococci exist as part of the normal flora of the GI tract and female genital tract.Most
enterococcal infections can be traced to endogenous sources.However, in the health
care setting, patient-to-patient transmission of Enterococcus can occur through direct
contact with the hands of health care workers or indirectly through contaminated surfaces.
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This highlights the need for strict adherence to infection control measures such as hand
hygiene and the use of gowns,gloves, and disinfectants. Responsible for a rising number of
health care–associated infections over the past 5 years, VRE is of great concern among
infectious disease specialists today. Doctors lack effective antibiotics to treat these
infections; most VRE microorganisms also resist other drugs used against enterococcal
infections. Also, there’s a possibility that vancomycin-resistant genes present in the
microorganisms may be transferred to other gram-positive microorganisms such as S.
aureus. The risk of VRE colonization and infection is associated with the following
conditions:
◗ Previous vancomycin therapy,multiple antimicrobial therapy, or both
◗ Severe underlying disease
◗ Immunosuppression ◗ Intra-abdominal or cardiac surgery
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