Download An Introduction to the Infectious Diseases

1
An Introduction to the
Infectious Diseases
2
F
or Health: A state of complete physical, mental, and social
well-being and not merely the absence of disease or infirmity.
[The World Health Organization (WHO) defined health in its broader
sense in 1946]
An infectious disease is an infection that is capable of
spreading from person to person. Not all infections are
communicable diseases. The term infectious means communicable.
Infectious diseases may be transmitted by many routes: direct
person-to-person transfer; respiratory transmission; parenteral
inoculation; by way of formites; sexual or mucosal contact; and by
insect vectors. Infectious diseases occur as the result of
interactions between pathogenic (disease- producing)
microorganisms and the host (Fig. 1-1). All infectious disease begin
at some surface of the host, whether it be the external surfaces
such as the skin and conjunctiva or internal surfaces such as the
mucous membranes of the respiratory tract, intestine, or urogenital
tract.
Chapter 1
Figure 1-1
An Introduction to the Infectious Diseases
Individual infection progression (WHO, 2007)
Infectious disease:
Of the thousands of species of viruses, bacteria, fungi, and parasites,
only a tiny portion is involved in disease of any kind. These are called
pathogens. There are plant pathogens, animal pathogens, fish pathogens, as
well as the subject of this book, human pathogens. Among pathogens, there
are degrees of potency called virulence, which sometimes makes the
dividing line between benign and virulent microorganisms difficult to draw.
Many bacteria and some fungi are part of a normal flora that colonizes the
skin and mucosal surfaces of the body, where most of the time they appear
to do no harm. In extreme circumstances, a few of these organisms are
associated with mild disease, making them low-virulence pathogens at best.
Other pathogens are virtually always associated with disease of varying
severity. Yersinia pestis, the cause of plague, causes fulminant disease and
3
4
death in 50 to 75% of individuals who come in contact with it. It is highly
virulent. Understanding the basis of these differences in infectious agent,
occurrence, reservoir, mode of transmission, incubation period, period of
communicability, susceptibility, and methods of control et al., is the
fundamental goal of this book.
Infectious diseases kill more than 14 million people each year, mainly in
the developing world. In these countries, approximately 46% of all deaths
are due to infectious diseases, and 90% of these deaths are attributed to
acute diarrhea and respiratory infections of children, AIDS, tuberculosis,
malaria, measles, and recent SARS. Other diseases, that rarely kill, maim
millions. Large populations living in remote areas of the developing world
are at risk of disabling diseases, such as poliomyelitis, leprosy, lymphatic
filariasis, and onchocerciasis. For these diseases, the toll of suffering and
permanent disability is compounded by a double economic burden (Fig. 1-2).
For all these reasons, concern about the impact of infectious diseases has
increased, with some encouraging results. Lack of access to effective
vaccines and drugs has been a long-standing problem in the developing
world. Major new initiatives, including the Global Fund to Fight AIDS,
Tuberculosis and Malaria, the Global Alliance for Vaccines and
Immunization, and the Roll Back Malaria and Stop TB partnerships, have
formed to attack the main infectious diseases that kill and are delivering
badly needed drugs and vaccines. The concern of international community is
also evident in time-limited drives to eradicate or eliminate polio, leprosy,
lymphatic filariasis, onchocerciasis and other diseases that maim. While
microbial agents will always deliver surprises, the shock of SARS has
encouraged a number of countries to give infrastructures for protecting
public health much higher priority. All health care will benefit.
Chapter 1
Figure 1-2
An Introduction to the Infectious Diseases
The trend of global infectious disease (JICA, 2004)
(www.jica.jp/english/publications/report/network/archive_20
04/vol_24_3.html)
Causes of infectious disease:
In order to cause infectious disease a pathogen must accomplish the
following: It must enter the host, it must metabolize and multiply on or in
the host tissue, it must resist host defenses, and it must damage the host.
The simple illustration as follows:
Infectious dose of microbe → (Contact) → Portal of entry → (Invasion)
→ Host defense barrier → Body compartment → Target tissue →
Portal of exit → Microbe spreads to other host.
5
6
An infection is a disease caused by a pathogen. The human body is
colonized on the skin and mucosal surfaces with numerous microorganisms
that form the normal flora of the body.
The medical terms of infection:
The medical terms used in discussion of infectious diseases are
constantly changing to keep pace with changes in our knowledge and
understanding. We will start by defining some of the common terms used in
this book.
1. Communicable disease: an infection that is capable of spreading from
person to person.
2. Epidemiology: the study of the distribution and determinants of diseases
in population.
3. Incubation period: is the time interval between initial contact with the
infectious organism and the first appearance of symptoms associated with
the infection.
4. Infection: a disease caused by a pathogen.
5. Infectious agent: identifies the specific agent or agents causing the
disease; classifies the agent; and may indicate its important
characteristics.
6. Infectiousness: the ease with which a pathogen can spread in a
population.
7. Methods of control: contain preventive measures and control of patient,
contacts and the immediate environment.
8. Mode of transmission: describes the mechanisms by which the
infectious agent is spread to humans.
Chapter 1
An Introduction to the Infectious Diseases
9. Occurrence: provides information on where the disease is known to
occur. Information on past and current outbreaks may also be included.
10. Outbreak: an occurrence of a disease clearly in excess of normal
expectancy.
11. Pathogen: an organism that can invade the body and cause disease.
12. Pathogenicity: the ability to cause disease.
13. Period of communicability: is the time during which an infectious
agent may be transferred directly or indirectly from an infected person
to another person; from an infected animal to humans; or from an
infected person to animals, including arthropods.
14. Reservoir: indicates any person, animal, arthropod, plant, substance in
which an infection agent normally lives and multiplies, on which it
depends primarily for survival, and where it reproduces itself in such a
manner that it can be transmitted to a susceptible host.
15. Susceptibility: provides information on human or animal populations at
risk of infection, or that are resistant to either infection or disease.
Information on subsequent immunity consecutive to infection is also
given.
16. Vector: a living creature that can transmit infection from one host to
another.
17. Virulence: a pathogen’s power to cause severe disease.
18. Zoonosis: an animal disease that can spread to humans.
Clinical aspects of infectious disease:
Fever, pain, and swelling are the universal signs of infection. Beyond
this, the particular organs involved and the speed of the process dominate
7
8
the signs and symptoms of disease. Cough, diarrhea, and mental confusion
represent disruption of three different body systems. On the basis of clinical
experience, physicians have become familiar with the range of behavior of
the major pathogens. However, signs and symptoms overlap considerably.
Skilled physicians use this knowledge to begin a deductive process leading
to a list of suspected pathogens and strategy to make a specific diagnosis
and provide patient care. Through the probability assessment, an
understanding of how the diseases work is a distinct advantage in making
the correct decisions.
Reporting of communicable diseases:
Reporting of some communicable diseases is required within countries
and in some instances internationally to world health organization (WHO).
Reporting can take the form of either a case report or an outbreak report.
Class I: Case report required internationally to WHO by the international
health regulations or as a disease under surveillance by WHO.
Class II: Case report regularly required wherever the disease occurs.
Class III: Selectively reportable in recognized endemic areas.
Class IV: Obligatory report of outbreaks only – no case report required.
Class V: Official report not ordinarily justifiable.
Case reports: Case reporting provides diagnosis, age, sex and date of
onset for each person with the disease. Sometimes it includes identifying
information such as the name and address of the person with the disease.
Additional information such as treatment provided and its duration are
required for certain case reports. If there is a requirement for
international case reporting, national governments report to WHO.
Chapter 1
An Introduction to the Infectious Diseases
Outbreak reports: Outbreak reporting provides information about an
increase above the expected number of persons with a communicable
disease that may not be included in the list of diseases officially
reportable, or it may be of unknown etiology if it is newly recognized or
emerging. If there is a requirement for outbreak reporting, international
national governments report to WHO (Tab. 1-1).
Infectious diseases remain as important and fascinating as ever. Where
else do we find the emergence of new diseases, together with improved
understanding of the old ones? At a time when the revolution in molecular
biology and genetics has brought us to the threshold of new and novel
means of infection control, the perpetrators of bioterrorism threaten us with
diseases we have already conquered. Meeting this challenge requires a
secure knowledge of the pathogenic organisms and how they produce
disease, as well as an understanding of the clinical aspects of those diseases.
In the collective judgment of the authors, this book presents the principles
and facts required for students of Public Health to understand the most
important infectious diseases.
Table 1-1
Current WHO phases of pandemic alert (WHO, 2007)
Phase
Definition
Interpandemic phase
Low risk of human cases
New virus in animals, no Higher risk of human cases
human cases
Pandemic alert
No or very limited human-to- human
New virus causes human transmission
cases
Evidence of increased human-to- human
transmission
Pandemic
Level
1
2
3
4
Evidence of significant human-to- human
transmission
5
Efficient and sustained human-to- human
transmission
6
9
10
1. Communicable disease:
2. Pathogenicity:
3. Virulence:
4. Zoonosis:
5. Fever:
6. Pain:
7. Swelling:
8. Cough:
9. Diarrhea:
10. Mental confusion:
I.
Multiple-choice questions (five selected one):
1. (
) Infectious diseases may be transmitted by the routes of (a)
parenteral inoculation, (b) by way of formites, (c) sexual or
mucosal contact, (d) by insect vectors, (e) all of the above.
2. (
) Many countries in the developing world, approximately (a) 15%,
(b) 25%, (c) 36%, (d) 46%, (e) 90% of all deaths are due to
infectious diseases.
3. (
) The study of the distribution and determinants of diseases in
population means: (a) communicable disease, (b) epidemiology,
(c) infection, (d) mode of transmission, (e) outbreak.
Chapter 1
4. (
An Introduction to the Infectious Diseases
) A pathogen’s power to cause severe disease; means: (a)
communicable disease, (b) epidemiology, (c) virulence, (d) mode
of transmission, (e) susceptibility.
5. (
) Case report required internationally to WHO by the international
health regulations or as a disease under surveillance by WHO,
which belongs to the (a) class I, (b) class II, (c) class III, (d)
class IV, (e) class V.
II. Simple answer:
1. In order to caused infectious disease, a pathogen must accomplished
which 4 steps?
2. Would you please tell me the clinical aspects of infectious disease?
11
12
2
The Infectious Disease
Process
2-1
Types of Infection
2-2
Chain of Infection
2-3
Mode of Transmission
14
1. Acute infection:
2. Chronic infection:
3. Fulminating infection:
4. Localized infection:
5. Generalized infection:
6. Polymicrobial infection:
7. Inapparent infection:
8. Horizontal transmission:
9. Vertical transmission:
10. Blood-borne transmission:
T
he infectious disease process provides a scientific explanation
for the factors that determine the relationships among humans,
the many microorganisms that are in the environment, and normal
flora. There are many variables that determine the relationship
between microorganisms and human host. This interaction has
illustrated in various ways, including the epidemiologic chain; Host
←→ Agent ←→ Environment. The chain portrays the three key
elements in infection – agent, host, and environment – and their
dynamic interaction. This model demonstrates this interaction for
other types of disease as well. In this chapter, we discuss specific
characteristics or changes in any one of the three elements that
Chapter 2
The Infectious Disease Process
can lead to the development of an infectious disease. In order to
cause infectious disease a pathogen must accomplish the following:
1) It must enter the host, 2) It must metabolize and multiply on or in
the host tissue, 3) It must resist host defenses, 4) It must damage
the host.
15
16
2-1
Types of Infection
The term infection refers to the presence and multiplication of
microorganisms in the tissue of a host. The host’s response to this invasion
and replication varies. When there are signs and symptoms caused by this
invasion, such as fever, swelling, pain, and inflammation, we recognize
these as characteristics of infection. Some infectious diseases are evident
and produce acute signs and symptoms. Infectious diseases occur as the
result of interactions between pathogenic (disease-producing)
microorganisms and the host. All infectious disease begin at some surface of
the host, whether it be the external surfaces such as the skin and conjunctiva
or internal surfaces such as the mucous membranes of the respiratory tract,
intestine, or urogenital tract. The types of infection content as follows:
1. Infection: It can result in little or no illness.
2. Acute infection: Has a short and relatively severe course. For example,
Streptococcal pharyngitis (Sore throat caused by Streptococcus
pyogenes).
3. Chronic infection: Has a long duration. For example, Tuberculosis is
caused by Mycobacterium tuberculosis.
4. Fulminating infection: Occurs suddenly and with severe intensity. For
example, Cerebrospinal meningitis is caused by Neisseria meningitidis.
5. Localized infection: Restricted to a limited area of the body. For
example, Urinary tract infection is caused by Escherichia coli.
6. Generalized infection: Affects many or all parts of the body. For
example, Blood infections, such as typhoid fever is caused by Salmonella
typhi.
Chapter 2
The Infectious Disease Process
7. Mixed, or polymicrobial infection: More than one kind of
microorganism contributes to the infection. For example, Gaseous
gangrene, in which a combination of Clostridium species may occur.
8. Primary infection: An initial localized infection that decreases
resistance and thus paves the way for further invasion by the same
microorganisms. For example, Viral influenza.
9. Secondary infection: Infection that is established after a primary
infection has caused a decreased resistance. For example, Pneumococcal
pneumonia is following viral influenza.
10. Inapparent infection: It is termed subclinical, and the individual is
sometimes referred to as a carrier.
11. Carrier: It can be asymptomatics, but infectious to others.
12. Horizontal transmission: It is direct or indirect infection from personto-person.
13. Vertical transmission: It is direct infection, which occur from
transplacentally, during birth, or through breast milk.
14. Zoonotic transmission: It is direct or indirect infection from animals to
humans.
2-2
Chain of Infection
The interaction among the agent, the host, and the environment leading
to infection has been described in a model called the chain of infection (Fig.
2-1).
17
18
Figure 2-1
The chain of infection
Specific conditions and characteristics affecting each element in the
chain and element’s interaction with the other elements determine whether
an infection will result.
1. Susceptible host: means human or animal that easily influenced or
affected with microorganisms causing infectious disease.
2. Causative agent: The causative agent of infection may be one of several
classes of microorganisms causing human infectious disease. These
include bacteria, viruses, fungi, protozoa, rickettsia, Chlamydia,
mycoplasma, prions, and helminthes. Each has characteristics that
influence its ability to cause an infection.
3. Reservoir: The source of a causative agent is referred to as the reservoir.
In a reservoir, the organism can survive but may or may not multiply.
Organisms causing infectious disease may be found in other humans,
animals, or the physical environment. Human reservoirs may have a
symptomatic or asymptomatic infection, or they may be colonized with a
potentially infective agent. In health care settings, the human reservoir
includes other patients as well as health care providers.
4. Portal of entry: The portal of entry is the means by which the infectious
agent enters the susceptible host. The portal of entry can be the
respiratory tract, GI tract, genitourinary tract, or skin or mucous
membranes. Breaks in the skin or mucous membranes can create a portal
of entry, such as can occur with injuries from needle sticks or sharps.
Chapter 2
The Infectious Disease Process
5. Portal of exit: If human is serving as the reservoir of an infectious agent,
the means by which the agent leaves the original host is the portal of exit.
Orifices that are part of each body system can portal of exit.
2-3
Mode of Transmission
The mechanism by which an organism moves from the portal of exit or
environmental reservoir through a susceptible host’s portal of entry is
referred to as the mode of transmission.
Table 2-1
The common routes of transmission
Route of exit
Route of transmission
Example
Respiratory
Aerosol droplet inhalation
Influenza virus
Salivary
Direct salivary transfer; animal
bite
Oral-labial herpes
Rabies
Gastrointestinal
Stool-hand-mouth
Stool-object-mouth
Enterovirus infection;
hepatitis A
Skin
Skin discharge-air-respiratory
tract; skin to skin
Human papilloma virus;
syphilis
Blood
Transfusion or needle prick;
Insect bite
Hepatitis B; AIDS; malaria;
relapsing fever
Genital
secretions
Urethral or cervical secretions;
semen
Gonorrhea; herpes
simplex; CMV
Urine
Urine-hand-catheter
Hospital-acquired urinary
tract infection
Eye
Conjunctival
Adenovirus
Zoonotic
Animal bite; contact with
carcasses; arthropod
Rabies; tularemia; plague;
Lyme disease
19
20
1. Respiratory spread: Many infections are transmitted by the respiratory
route, often by aerosolization of respiratory secretions with subsequent
inhalation by others. Droplet particle in diameter: 100 um: requires only
seconds to fall the height of a room. 10 um: remains airborne for about
20 minutes. 0.6~5.0 um: attach to mucous sites at various levels along
the URT or LRT.
2. Salivary spread: Transmission of infectious secretions, are by direct
contact with the nasal mucosa or conjunctiva. Above often accounts for
the rapid dissemination of agents such as respiratory syncytial virus
(RSV) and adenovirus.
3. Fecal-Oral spread: Fecal-oral spread involves direct or finger-to-mouth
spread, the use of human feces as a fertilizer, or fecal contamination of
food or water. Reduced gastric hydrochloric acid can facilitate enteric
infections.
4. Skin-to-Skin transfer: Skin is the portal of entry, such as: Syphilis,
caused by Treponema pallidum, Impetigo, caused by strains of gr. A
Streptococci, Ringworm and athlete’s foot that caused by dermatophyte
fungi.
5. Blood-borne transmission: Blood-borne transmission may caused by
Insect vector, human blood transfusions, illicit drugs by intravenous or
subcutaneous route, and using shared non-sterile equipment. Hepatitis B
and C viruses as well as HIV were frequently transmitted in this way
prior to the institution of blood screening tests.
6. Genital transmission: Spread can occur between sexual partners or from
or from the mother to the infant at birth. A major factor in these
infections has been the persistence, high rates of asymptomatic carriage,
such as Chlamydia trachomatis, cytomegalovirus (CMV), herpes simplex
virus (HSV), and Neisseria gonorrhoeae.