Sources of infection and modes of infection transmission
Introduction.
Sources of infection.
Modes of transmission of infection.
Conclusion.
References.
Man was and is still susceptible to different challenges in
his surrounding environment, some of which were successfully
defeated, but others are still there causing lots of problems.
A living example is infection or infectious diseases which
constitute a minor problem in the developed countries, but a
major one in the developing countries. The outcome of such
problem varies form nothing to a life threatening event. An
infection is an invasion of the body tissue by microorganisms and
their proliferation within the tissue. If the microorganism
produces no clinical evidence of disease, the infection is called
asymptomatic or subclinical. Some asymptomatic conditions can
cause severe damage to the host (1). Microorganisms vary in
their ability to produce disease. This ability is called
virulence. Microorganisms also vary in the source from where
they emerge, the severity of the disease they produce and their
degree of communicability. If the infectious agent can be
transmitted to an individual by direct or indirect contact
through a vector or vehicle, or as an airborne infection, the
resulting condition is called a communicable disease. In
general, five groups of microorganisms can cause disease and
these are: bacteria, viruses, fungi, protozoa, and rickettsia (2).
It is of importance to mention that the human body contains
several types of microorganisms as a part of its normal
architecture and these are called the “Normal Flora”. In
the following paragraphs, the sources of infection and the ways
by which different infections are transmitted are going to be
discussed. In addition, examples of infectious agent will be
presented for each source of infection and mode of transmission.
The sources of infection are numerous, and for each type of
infection a specific source becomes more significant than the
others in the delivery of the infectious agent to the host, which
is man in our discussion. The sources of infection can be
divided into two main groups. These are exogenous and
endogenous sources (3). A source of infection is endogenous
when the infectious agent comes from the patient’s own body,
usually from his own normal flora. Endogenous sources of
infections become important when the person’s own immunity
against his normal flora becomes compromised such as in cases of
contamination during surgery, malnutrition, impairment of blood
supply and debilitating diseases such as AIDS, diabetes or any
other accompanying infection. Examples are the genera of
staphylococci and streptococci which are normally found in the
body, but can become pathogenic in certain circumstances (2).
The exogenous sources of infection introduce organisms from any
where outside to inside the body, which is the case most of the
time. In addition to being exogenous, most of the time
infections are transmitted from person to person or from animal
to man (4). To be more specific, exogenous sources of infections
can be either human, animal, or environmental in origin. Humans
can be a source of infection in three cases, either when they are
clinically infected (symptomatic infection), when they are
asymptomatically infected or when they are carriers (5). In the
first possibility, it is not uncommon that the causal organism
can be shed from the host in large numbers leading to wider
spread of the infection, but fortunately, it is easy to control
the spread of such infections by treatment or by isolation. An
example is N. meningtidis which causes meningitis (4). The
problem is that when the infection is hidden and the host’s
circulation in the community continues to take place as in the
case of being asymptomaticlly infected or being a carrier of the
organism without knowing so as in the case of the carriers of HIV
virus. Humans can be a source of organisms which cause diseases
that are sexually transmitted such as Treponema pallidum which
causes syphilis and N. gonorrhoae which causes gonorrheal
infections or through blood when vectors act as vehicles as in
the case of transmission of Borrelia that causes relapsing
fever (6). Animals are another source of infection, and an
infection derived from this source is called zoonotic infection.
Such infections are usually maintained in animals, and are
acquired accidentally. An example of such infections could be
brucellosis caused by brucella mainly from cows and their
products such as milk , rabies caused by rabies virus from wild
animals, and plague which is caused by Pasteurella pestis.
Moreover, animal products such as meat, milk and eggs can be
sources of infection. Examples are E. Salmonella species and E.
Cambylobacter (4). Environmental sources are numerous and few
environmental saprophytes are pathogenic for man unless in cases
of individuals with severely compromised immune system. But
still some parasites may result in complications if introduced
into the body from the environment. Examples are some sporing
bacilli of the genera Bacillus and Clostridium. Food is another
important and very common source of infection due to the everyday
pattern of dealing with such material. Food can be contaminated
and hence a source of infection at several stages. At its origin
(infected animal or plant), or at the time of processing when
handled with hands or contaminated tools (2). It is not only a
vehicle when transmission is considered, but it is also a good
environment where bacteria or any other pathogen can multiply and
produce toxins (3). Water, the commonest and the most important
material in life is a major source of infection only in case of
being in contact with sewage. Examples of organisms derived from
the previous two sources when contaminated with faeces are
Salmonella typhi associated with food poisoning and Shigella
associated with Bacillary dysentery, Brucella causing undulant
fever, Leptospira and Clostridium causing botulism. Examples of
viruses are Poliomyelitis and Hepatitis A viruses. Protozoa
derived from contaminated food and water are Entamoeba
histolytica which causes dysentery and Giardia lamblia which
causes enteritis. Examples of worms that are food or water
borne are hookworms, pinworms and Ascaris. Faecal bacteria such
as E. coli can produce water borne outbreaks in case of water
contamination with sewage (3). Moreover, Soil, air and dust
usually contain non pathogenic organisms of numerous types, but
this is not always the case since pathogenic organisms can be
introduced through them to humans causing diseases. Soil can be
contaminated with human or animal feaces that contain several
pathogenic organism which have sporing capabilities enabling them
to survive in harsh environments such as Clostridium tetani which
causes tetanus, Cl. botulinum which causes food poisoning and B.
anthracis which causes anthrax in animals mainly. Examples of
pathogenic fungi found in soil are Coccidioides immitis and
Blastomyces (1). Air can be contaminated with organisms shed
from skin or the respiratory tract such as S. pneumoniae which
causes pneumonia and S. pyogens which causes Scarlet fever,
Corynebacterium diphtheriae which causes diphtheria, Haemophilus
Influenzae, meningococci, anthrax bacilli and Measles and Mumps
viruses. Infection with such organisms occurs by inhalation or
ingestion of pathogen-containing droplets produced either orally
or by the respiratory tract. Dust doesn’t differ so much from
the previous two sources in that it can be infected with
organisms especially bacteria and viruses that are shed from
humans or any other source, and can be deposited in dust
particles that can be all the time floating in the atmosphere
such as Chickenpox virus. Fomites are another source of
infection which can be defined as any porous substance that can
absorb and pass on contagion and almost all the previously
mentioned organisms can be derived through this source (4).
Nosocomial infections are infections that were not present or
were incubating at the time of admittance to a health care
facility. They include infections patients acquire during their
stay in a healthcare facility or infections that may manifest
after discharge. These infections are considered more difficult
to prevent and treat, more unpredictable, and more resistant to
cure than infections contracted in the community. Patients
who undergo surgical procedures have a higher incidence of
nosocomial infections than others. The source of microorganisms
that cause nosocomial infections can be the patients themselves,
the healthcare facility, or the healthcare personnel (5).
Modes of
transmission of infections:
As previously mentioned, microorganisms that can infect man
are every where in the air; on dust particles, food, and plants,
on and in animals and humans; in soil and water; and on virtually
every other surface. Transmission of the microorganisms can be
grouped into four main routes: contact, vehicle, airborne, and
vectorborne. The most frequent means of transmission is through
contact with an infected host. There are three types of contact
transmission: direct contact as in cases of physical contact with
an infected person especially in cases of local skin contact.
Diseases transmitted by this route are numerous and an example
could be impetigo caused by Staphylococci. Another type of
contact is the indirect one as in cases of contact with a
contaminated object such as soil which may result in the
transmission of organisms such as S. tetani , and droplet contact
such as the contact with contaminated secretions from an infected
person in saliva or milk as in the transmission of the virus
responsible for Herpes cold sores (2). Vehicle transmission
occurs through a transporting agent or medium, such as food,
water, or blood as in the case of the transmission of Hepatitis B
virus , E. Salmonella or E. coli. Microorganisms that remain on
droplets or dust particles are carried through the air through
airborne transmission (1). The infectious agent is expelled
from the carrier or symptomatically infected host in respiratory
droplets during speaking, sneezing and coughing. These are
inhaled by another host. In addition, droplets may adhere to
dust particles that become a new source of infection when get
into contact with tissues of a new host. Examples of pathogens
transmitted by such method are meningococci, staphylococci,
Cornybacterium diphtheriae and measles virus (6). A vector is
an animal that transfers microorganisms from a reservoir to a
host. A vector picks up disease organisms from a source of
infection, such as blood or feces, carries the infection within
or on its body, and later deposits them where they infect a new
host, directly or indirectly. Mosquitoes, fleas, lice, flies,
and ticks are common vectors of disease to humans such as malaria
or leishmaniasis (5). Sexual contact constitute another
important route of transmission when precautions are not taken as
in the classical example of HIV virus transmission, in addition
to N. gonorrhoae and Hepatitis B infection (1). The faeco-oral
route is important as a route of infection when hygienic
standards are lacking. Bowel excretions that contain the
infectious microorganism can be either ingested directly through
infected fingers or toilets or indirectly via food and water. E.
salmonellae, S. typhi, viral hepatitis A and E and V.
cholerae are few examples of the huge number of organisms that
can be transmitted this way (6). Moreover, vertical
transmission of infections from mother to her child either
prenatally through the placenta, perinatally during passage of
baby through the birth canal, or postnatally through breast
feeding, is another important mode of transmission. An example
is the transmission of T. pallidum that causes syphilis (4).
It is worth mentioning that infectious diseases may be
transmitted by one way or another or by more than one route.
In conclusion, infections can be derived from several sources and transmitted by several means, and as usual prevention is better than cure. The process of preventing infections can be accomplished by several means. One is the prevention through basic cleansing and hygiene procedures, such as keeping the hands clean, washing and covering cuts and grazes, having wounds attended to by a physician, and seeking regular dental treatment. The body itself resists infection by producing substances called antibodies, which act against infectious organisms. The process by which resistance to infection is artificially produced is called immunization. Moreover, safe sex, safe sanitary systems and safe animal keeping will probably always put man on the safe side.
1. Grist, Norman R., et. al., Diseases of infection. 2nd ed. Oxford: Oxford University Press, 1993, P: 2- 11.
2. Duerden, B. I., et. al. A new short text book of Microbial and Parasitic infection. London: Edward Arnold, 1987, P: 32 - 35.
3. Fuerst, Robert. Frobisher & Fuerst’s MICROBIOLOGY in Health and Disease. 15th ed. Philadelphia: W. B. Saunders Company, 1983, P: 354- 357.
4. Ellner, Paul D., and Neu, Harold C. Understanding infectious diseases. St. Louis: Mosby Year Book, 1992, P: 5 - 9.
p>5. Mandal, Bibhat, et. al. Lecture notes on infectious diseases. 5th ed. London: Black Well Sciences, 1995, P: 34.6. Burton, Gwendolyn R. W. Microbiology for the Health
Sciences. 4th ed. Philadelphia: J. B. Lippincott Company,
1992, P: 203 -207.
