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Classification of infectious diseases
•According to onset and duration
 •According to location
 •According to item present
 •According to sequence of appearance
 •According to epidemiological factors
Classifications according to onset and duration
Acute disease -- Rapid onset and short
duration. E.g. common cold, measles
 Chronic disease -- Slower onset and longer
duration. eg TB, leprosy
 Subchronic disease -- ermediate to acute and
chronic both in onset and duration. eg gingivitis
 Latent disease -- One characterized by periods
of activity erspersed with periods of inactivity.
E.g.: malaria, herpes simplex.
Classification according to location
Local infection -- Confined to
a specific area of the body.
E.g.: cystitis, vaginitis,
Focal infection -- Infection
that started in one place and
later on spread to other
areas. E.g. tuberculosis,
sinus infection, infected
Systemic (generalized)
infection - Occurring
throughout the body
Classifications according to item present
Septicemia -- A microbe is present in the blood, is
continuously being delivered from tissues to blood and
is actively multiplying in the blood. Typical of systemic
diseases. Often fatal.
Bacteremia -- Presence of bacteria in the blood
Viremia -- Presence of virus in the blood of the body
Pyemia -- Poisoning of the blood by pus-producing
bacteria released from an abscess
Toxaemia -- Presence of bacterial toxins in the blood.
Can result in fever, diarrhea and vomiting.
Classification according to sequence of
Primary infection - Initial infection in a healthy person
Secondary infection - Occurs after primary infection
Superinfection - A secondary infection due to the
destruction of the protective normal flora of the body
by the use of a broad spectrum antibiotic
Also defined as a secondary infection facilitated by a
primary infection e.g. HIV and AIDS
Epidemiological factors - mode of appearance,
number of cases, trends of diseases in
Classifications according to epidemiological factors
Endemic disease (endemia) - Present regularly in particular area of the
world, and total number and severity are low
Ecdemic disease (ecdemia) - A foreign disease brought to a new area by
travelers or immigrants from a foreign country
Pandemic disease (pandemia) - A disease that affects many people and
which occurs across neighboring cities, countries or continents. Affects
many people.
Epidemic disease (epidemia) - Appears suddenly, affects many people and
is confined to a particular, often the same, area. Morbidity rate and
mortality rate above what is normal.
Sporadic disease (epidemia) - Appears suddenly, in a random and unpredictable manner, affects only a few people, and limited to a few, usually
unrelated, places.
Outbreak - Few cases, often related in time and location, and sharing same
Threat of emerging infectious diseases
Due to constant changes in our lifestyles and environments,
there are constantly new diseases that people
are susceptible to, making protection from the threat of
infectious disease urgent. Many new contagious diseases
have been identified in the past 30 years, such as
AIDS, Ebola, and hantavirus. Increased travel between
continents makes the worldwide spread of disease a bigger
concern than it once was. Additionally, many common
infectious diseases have become resistant to known
Problems of antibiotic resistance
Because of the overuse of antibiotics, many bacteria
have developed a resistance to common antibiotics. This
means that newer antibiotics must continually be developed
in order to treat an infection. However, further
resistance seems to come about almost simultaneously.
This indicates to many scientists that it might become
more and more difficult to treat infectious diseases. The
use of antibiotics outside of medicine also contributes to
increased antibiotic resistance. One example of this is the
use of antibiotics in animal husbandry. These negative
trends can only be reversed by establishing a more rational
use of antibiotics through treatment guidelines.
Beta-Lactam Antibiotics
A penicillin culture
Cephalosporins and cephamycins are similar to
penicillins chemically, in mechanism of action,
and in toxicity. Cephalosporins are more stable
than penicillins to many bacterial -lactamases
and therefore usually have a broader spectrum
of activity. Cephalosporins are not active against
enterococci and Listeria monocytogenes.
These are drugs with a monocyclic -lactam ring . They are
relatively resistant to lactamases and active against gramnegative rods (including pseudomonas and serratia). They have
no activity against gram-positive bacteria or anaerobes.
Aztreonam is the only monobactam available in the USA. It
resembles aminoglycosides in its spectrum of activity.
Aztreonam is given intravenously every 8 hours in a dose of 1–
2 g, providing peak serum levels of 100 g/mL. The half-life is
1–2 hours and is greatly prolonged in renal failure.
Penicillin-allergic patients tolerate aztreonam without reaction.
Occasional skin rashes and elevations of serum
aminotransferases occur during administration of aztreonam,
but major toxicity has not yet been reported. The clinical
usefulness of aztreonam has not been fully defined.
These substances resemble beta-lactam molecules but
themselves have very weak antibacterial action. They are
potent inhibitors of many but not all bacterial lactamases and
can protect hydrolyzable penicillins from inactivation by these
enzymes. Beta -Lactamase inhibitors are most active against
Ambler class A lactamases (plasmid-encoded transposable
element [TEM] - lactamases in particular) such as those
produced by staphylococci, H influenzae, N gonorrhoeae,
salmonella, shigella, E coli, and K pneumoniae. They are not
good inhibitors of class C -lactamases, which typically are
chromosomally encoded and inducible, produced by
enterobacter, citrobacter, serratia, and pseudomonas, but they
do inhibit chromosomal lactamases of legionella, bacteroides,
and branhamella.
The indications for penicillin- -lactamase inhibitor combinations
are empirical therapy for infections caused by a wide range of
potential pathogens in both immunocompromised and
immunocompetent patients and treatment of mixed aerobic
and anaerobic infections, such as intraabdominal infections.
Doses are the same as those used for the single agents except
that the recommended dosage of piperacillin in the piperacillintazobactam combination is 3 g every 6 hours. This is less than
the recommended 3–4 g every 4–6 hours for piperacillin alone,
raising concerns about the use of the combination for
treatment of suspected pseudomonal infection.
Adjustments for renal insufficiency are made based on the
penicillin component.
The carbapenems are structurally related to beta-lactam antibiotics.
Ertapenem, imipenem, and meropenem are licensed for use in the USA.
Imipenem has a wide spectrum with good activity against many gramnegative rods, including Pseudomonas aeruginosa, gram-positive
organisms, and anaerobes. It is resistant to most lactamases but not
metallo- lactamases. Enterococcus faecium, methicillin-resistant
strains of staphylococci, Clostridium difficile, Burkholderia cepacia, and
Stenotrophomonas maltophilia are resistant. Imipenem is inactivated
by dehydropeptidases in renal tubules, resulting in low urinary
concentrations. Consequently, it is administered together with an
inhibitor of renal dehydropeptidase, cilastatin, for clinical use.
Meropenem is similar to imipenem but has slightly greater activity
against gram-negative aerobes and slightly less activity against grampositives. It is not significantly degraded by renal dehydropeptidase and
does not require an inhibitor.
Ertapenem is less active than meropenem or imipenem against
Pseudomonas aeruginosa and acinetobacter species. It is not
degraded by renal dehydropeptidase.
The usual dose of imipenem is 0.25–0.5 g given intravenously every 6–
8 hours (half-life 1 hour). The usual adult dose of meropenem is 1 g
intravenously every 8 hours. Ertapenem has the longest half-life (4
hours) and is administered as a once-daily dose of 1 g intravenously
or intramuscularly. Intramuscular ertapenem is irritating, and for that
reason the drug is formulated with 1% lidocaine for administration by
this route. A carbapenem is indicated for infections caused by
susceptible organisms that are resistant to other available drugs and
for treatment of mixed aerobic and anaerobic infections.
Carbapenems are active against many highly penicillin-resistant
strains of pneumococci. A carbapenem is the beta- lactam antibiotic
of choice for treatment of enterobacter infections, since it is resistant
to destruction by the lactamase produced by these organisms.
Strains of Pseudomonas aeruginosa may rapidly develop resistance
to imipenem or meropenem, so simultaneous use of an
aminoglycoside is recommended for infections caused by those
organisms. Ertapenem is insufficiently active against P aeruginosa
and should not be used to treat infections caused by that organism.
Imipenem or meropenem with or without an aminoglycoside may be
effective treatment for febrile neutropenic patients.
Chloramphenicol is a potent inhibitor of microbial protein synthesis. It
binds reversibly to the 50S subunit of the bacterial ribosome.
Because of potential toxicity, bacterial resistance, and the
availability of other effective drugs (eg, cephalosporins),
chloramphenicol is all but obsolete as a systemic drug. It may be
considered for treatment of serious rickettsial infections, such as
typhus or Rocky Mountain spotted fever, in children for whom
tetracyclines are contraindicated, ie, those under 8 years of age. It is
an alternative to a -lactam antibiotic for treatment of meningococcal
meningitis occurring in patients who have major hypersensitivity
reactions to penicillin or bacterial meningitis caused by
penicillinresistant strains of neumococci. The dosage is 50–100
mg/kg/d in four divided doses.
Chloramphenicol is occasionally used topically in the treatment of eye
infections because of its wide antibacterial spectrum and its
penetration of ocular tissues and the aqueous humor. It is ineffective
for chlamydial infections.
Newborn infants lack an effective glucuronic acid
conjugation mechanism for the degradation and
detoxification of chloramphenicol. Consequently,
when infants are given dosages above 50
mg/kg/d, the drug may accumulate, resulting in
the gray baby syndrome, with vomiting, flaccidity,
hypothermia, gray color, shock, and collapse. To
avoid this toxic effect, chloramphenicol should be
used with caution in infants and the dosage limited
to 50 mg/kg/d or less (during the first week of
life) in full-term infants and 25 mg/kg/d in remature
Tetracyclines are broad-spectrum
bacteriostatic antibiotics that inhibit protein
synthesis. They are active against many grampositive and gram-negative bacteria, including
anaerobes, rickettsiae, chlamydiae, mycoplasmas,
and L forms; and against some protozoa, eg,
amebas. The antibacterial activities of most
tetracyclines are similar except that tetracyclineresistant strains may remain susceptible to
doxycycline or minocycline, drugs that are less
rapidly transported by the pump that is
responsible for resistance
A tetracycline is the drug of choice in infections with Mycoplasma
pneumoniae, chlamydiae, rickettsiae, and some spirochetes. They are
used in combination regimens to treat gastric and duodenal ulcer
disease caused by Helicobacter pylori. In cholera, tetracyclines rapidly
stop the shedding of vibrios, but tetracycline resistance has appeared
during epidemics. Tetracyclines remain effective in most chlamydial
infections, including sexually transmitted diseases. Tetracyclines are no
longer recommended for treatment of gonococcal disease because of
resistance. A tetracycline—usually in combination with an
aminoglycoside—is indicated for plague, tularemia, and brucellosis.
Tetracyclines are sometimes employed in the treatment of protozoal
infections, eg, those due to Entamoeba histolytica or Plasmodium
falciparum. Other uses include treatment of acne, exacerbations of
bronchitis, community-acquired pneumonia, Lyme disease, relapsing
fever, leptospirosis, and some nontuberculous mycobacterial infections
(eg, Mycobacterium marinum). Tetracyclines formerly were used for a
variety of common infections, including bacterial gastroenteritis,
pneumonia (other than mycoplasmal or chlamydial pneumonia), and
urinary tract infections. However, many strains of bacteria causing these
infections now are resistant, and other agents have largely supplanted
Clarithromycin (is derived from erythromycin)
Azithromycin (differs from erythromycin and clarithromycin
mainly in pharmacokinetic properties). The drug is
slowly released from tissues (tissue half-life of 2–4
days) to produce an elimination half-life approaching 3
days. These unique properties permit once-daily dosing
and shortening of the duration of treatment in many
cases. For example, a single 1 g dose of azithromycin is
as effective as a 7-day course of doxycycline for
chlamydial cervicitis and urethritis. Community-acquired
pneumonia can be treated with zithromycin given as a
500 mg loading dose, followed by a 250 mg single daily
dose for the next 4 days.
Ketolides (Telithromycin)
Streptomycin, neomycin, kanamycin,
amikacin, gentamicin, tobramycin, sisomicin,
The pharmacodynamic properties of aminoglycosides are:
 Concentration-dependent killing
 Significant post-antibiotic effect
Aminoglycosides eliminate bacteria quickest when their
concentration is appreciably above the MIC for an organism,
this is referred to as concentration dependent activity. The
aminoglycosides also exhibit a significant post-antibiotic effect
(PAE). PAE is the persistent suppression of bacterial growth
following antibiotic exposure. Practically speaking this means
that trough levels can drop below the MIC of targeted bacteria
for a sustained period without decreasing efficacy.
Sulfonamides are infrequently used as single
agents. Formerly drugs of choice for infections
such as Pneumocystis jiroveci (formerly P carinii)
pneumonia, toxoplasmosis, nocardiosis, and
occasionally other bacterial infections, they have
been largely supplanted by the fixed drug
combination of trimethoprim-sulfamethoxazole.
Many strains of formerly susceptible species,
including meningococci, pneumococci,
streptococci, staphylococci, and gonococci, are
now resistant. Nevertheless, sulfonamides can be
useful for treatment of urinary tr
Sulfisoxazole and sulfamethoxazole are short- to
medium-acting agents that are used almost
exclusively to treat urinary tract infections. The usual
adult dosage is 1 g of sulfisoxazole four times daily or 1
g of sulfamethoxazole two or three times daily.
Sulfadiazine achieves therapeutic concentrations in
cerebrospinal fluid and in combination with
pyrimethamine is first-line therapy for treatment of acute
toxoplasmosis. Pyrimethamine, an antiprotozoal agent,
is a potent inhibitor of dihydrofolate reductase.
Sulfadoxine is available only as Fansidar, a combination
formulation with pyrimethamine, which is used as a
second-line agent in treatment for malaria
Sulfasalazine (salicylazosulfapyridine) is widely
used in ulcerative colitis, enteritis, and other
inflammatory bowel disease
Topical Agents
Sodium sulfacetamide ophthalmic solution or
ointment is effective treatment for bacterial
conjunctivitis and as adjunctive therapy for
trachoma. Mafenide acetate is used topically to
prevent bacterial colonization and infection of
burn wounds.
The important quinolones are synthetic
fluorinated analogs of nalidixic acid. They are active
against a variety of gram-positive and gram-negative
bacteria. Quinolones block bacterial DNA synthesis by
inhibiting bacterial topoisomerase II (DNA gyrase)
and topoisomerase IV.
Earlier quinolones (nalidixic acid, oxolinic acid,
cinoxacin) did not achieve systemic antibacterial
levels. These agents were useful only for treatment of
lower urinary tract infections.
Fluorinated derivatives (ciprofloxacin, levofloxacin,
and others) have greatly improved antibacterial
activity compared with nalidixic acid and achieve
bactericidal levels in blood and tissues.
Ciprofloxacin, enoxacin, lomefloxacin,
evofloxacin, ofloxacin, and pefloxacin comprise a
second group of similar agents possessing
excellent gram-negative activity and moderate to
good activity against grampositive bacteria.
Gatifloxacin, moxifloxacin, sparfloxacin, and
rovafloxacin comprise a third group of
fluoroquinolones with improved activity against
gram-positive organisms, particularly S
pneumoniae and to some extent staphylococci.
Antifungal Agents
The antifungal drugs presently available
fall into several categories: systemic
drugs (oral or parenteral) for systemic
infections, oral drugs for mucocutaneous
infections, and topical drugs for
mucocutaneous infections.
Terbinafine Hydrochloride