Epidemiology, modes of transmission control of comm. disease

1. UNIT – I
Epidemiology
Mr. S. N. Mendhe
Department of Microbiology,
Shri Shivaji Science and Arts College,
Chikhli

2. EPIDEMIOLOGY
a) Definition, classification & scope of
epidemiology
b) Infection- Types of infections and modes of
transmission
c) Normal flora of human body
d) Infection process, pathogenicity and
virulence, Microbial virulence factors: toxins
and enzymes
d) Control of communicable diseases

3.  Epidemiology is defined as the study of disease
distribution in population & the factors that
influence this distribution
 classification : Two main areas are involved in
the above definition.
 Study of the distribution of disease in human
population. This is known as descriptive
epidemiology.
 Study of determinants or underlying causes of
disease prevalence. This is known as Analytical
epidemiology.
 Epidemiologists are detectives who try to identify
etiological agents & discover the events that
contribute to the disease.

4. Infection
 Parasites are microbes that can establish
themselves & multiply in hosts. Parasite microbes
may be either pathogens or commensals.
 Infection: The lodgment & multiplication of a
parasite in or on the host tissues constitutes the
process of infection.
OR
 An infection is the entry & development or
multiplication of an infectious agent in the body of
man or animal.
 Disease:- departure from normal status of health
or recognizable interaction between host &
parasite is known as disease.

5. Scope/Uses of epidemiology
 To study the history of the health of
population & rise & fall of disease & change
in their characters.
 To diagnose the health of the community.
 For planning & evaluation of health services.
 To estimate the individual risk & chances.
 To identify syndromes.
 To search for causes.
.

6.  Infestation: invasion of the tissues by animal
parasites (e.g. Round worm infestation).
 Infectious disease:- disease of man or animal
resulting from an infection.
 Contagious disease:-transmitted through direct
contact e.g. (STDS) (syphilis,etc).
 Communicable disease:-illness due to specific
infections agent or its toxic products.
 Non-communicable disease:-illness not due to a
specific agent. e.g. cancer, heart attack

7. Types of infections
 Primary infection:- Initial infection with a
parasite in a host is termed primary infection.
 Re-infection:-Subsequent infection by the same
parasite in host is termed as re-infection.
 Secondary Infection:-When in a host resistance
is lowered by a pre- existing infection, parasite
produces infection then this is termed as
secondary infection.
 Cross- infection:-When in a patient already
suffering from disease, a new infection is set up
from another external source or host, it is termed
as cross infection.

8.  Nosocomial infection:- Cross-infections occurring in
hospital are called nosocomial infections.
 Focal infection:-In such case, infection is present at
localized sites,but symptoms so produced are
generalized, e.g. Appendicitis, tonsilitis.
 Inapparent/ Subclinical infection:- Infections in
which clinical symptoms or effects are not apparent
or clear.
 Atypical infection:-It is one in which the typical or
characteristic clinical manifestations of a particular
disease are not present.e.g. (Atypical pneumonia).
 Endogenous or Exogenous infection:-Depending on
whether the source of infection is from the host’s
own body or from external source, infections are
classified as endogenous or exogenous respectively.

9.  Iatrogenic/ Lab infection:- It refers to
physician induced infections resulting from
investigative, therapeutic or other procedures.
 Latent infection:- Some parasites following
infection may remain in the tissues in a latent
or hidden form proliferating and producing
infection or disease when the host resistance
is lowered.

10. Types of infectious disease
 Bacteraemia:-Circulation of bacteria in the
blood is known as bacteraemia. e.g., transient
bacteramia is a frequent event & may occur
during chewing, brushing of teeth.
 Septicaemia:-It is the condition where
bacteria circulate & multiply in the blood,
and form toxic products.
 Pyaemia:- Is the condition where pyogenic
bacteria produce septicaemia with multiple
abscesses in internal organs such as spleen,
liver & kidneys.

11. Periodicity of disease Occurrence
 Depending on the spread of infectious diseases
that may be classified into different types.
 Epidemic disease:-It is the one that spreads rapidly
involving many persons in an area at same time.
 The occurrence in the community of group of
illness of similar nature, clearly in excess of normal
expectation.
 Endemic disease:-When disease is constantly
present in a particular area, then it is said to be
endemic.
 Pandemic disease:-Is an epidemic that spreads
through out the world involving very large number
of persons within a short period.

12.  Sporadic disease:-It refers to the
occasional or infrequent or scattered
cases.
 Exotic disease:- Disease which is
imported into country in which it does
not other wise occur is called exotic
disease, e.g. Yellow fever, HIV, etc in
India.
 Zoonotic disease (Zoonoses):- Disease
which is transmitted from animal to
man., e.g. rabies.

13. Modes or routes or channels of transmission
 Communicable diseases are transmitted from the
source or reservoir of infection to susceptible persons
by the various routes. Some infections like respiratory
or intestinal tract diseases can spread from host to
host by variety of mechanisms, while others e.g.
veneral & arthropod borne infections are normally
transmitted by a single mechanism for which the
parasite is specifically adopted. There are six methods
of transmission. These are as follows:
1)Contact transmission 2)vehicle transmission
3)Vector transmission 4)Air borne transmission
5)Congenital transmission
6)Laboratory & Iatrogenic transmission

14. 1)Contact transmission:
i) Direct contact:- The disease agent is transmitted
directly from an infected individual to susceptible host
during physical contact. e.g. sexually transmitted diseases
or STDS(syphilis, AIDS gonorrhea), certain skin infections,
etc.
ii) Indirect contact:- Often the disease agent is carried by
a third person or contaminated objects. The term fomite
is used for such objects. fomites may be defined as
objects or substances capable of absorbing, retaining or
transferring infections. e.g. beddings, cloths, utensils,
cups, pens, pencils, books & toys. Diseases transmitted by
this method include T.B., diphtheria, small pox, etc.
Pencils shared by school children may act as fomites in
transmission of diphtheria, and face towels in trachoma.

15. 2)Vehicle transmission:- A large number of infections are
transmitted in the absence of contact. However, the organism
must be capable of surviving outside the body for certain
periods & there must be some
vehicle which will transmit the organisms from one place to
another. The vehicles are nonliving objects which become
contaminated & transfer infection from one person to another.
These are generally water, food &milk.
i)Water borne infection may occur when excreta contaminate
a supply of water like river or well which is used without
purification for drinking purposes. Water is the common vehicle
of infection in intestinal diseases like typhoid & cholera. It is
not always possible to isolate the disease agent in water.
ii) Milk may be vehicle for some infections like bovine
tuberculosis, typhoid & rickettsial fever (Q fever).

16. In milk borne infections those who consume the
contaminated milk are affected. In a family, all may be
attacked at one & same time. Cases are confined to those
families that consume the particular milk supply. The
common source of milk supply is traceable.
iii) Food borne infections may occur as a result of carrier
handling the food, by the preparation of food in utensils
contaminated by handling or washing in contaminated water
or by flies landing on food after feeding infected feces. Food
borne disease are of two types – food poising & food
infection. Food poisoning or food intoxication results due to
preformed toxin in the food. For the bacterial food poisoning,
it is necessary that favorable growth conditions should be
there for bacterial multiplication & production of enterotoxin.
Though it is not the case with food borne infection which
may result from the ingestion of only a few pathogenic
organisms, e.g, Salmonellosis, bacillary dysentery.

18. 3)Vector transmission- In this infections microbes are
abundantly present in the blood& are spread to other
individuals by blood sucking arthropods by directly
inoculation in to the skin, i.e. by biting. Also these
invertebrate hosts spread the disease agents by depositing
them on skin or food or other objects. Blood sucking insects
such as Lice, flea, mites, ticks & mosquitoes that transmit the
diseases are known as vectors and the diseases so
transmitted by vectors are known as vector borne or
arthropod borne diseases. The vectors transmit the disease in
to two ways:
I) Mechanical transmission:-The disease agent is carried
mechanically on the leg or body of insects. e.g. diarrhea,
dysentery & typhoid fever by housefly.
2) Biological transmission:-Inside vector body pathogen
multiplies, often undergoing part of developmental cycle in it.

21. 4)Air borne transmission:-
Three types of air borne transmission have been
distinguished.
A)Droplet infection:-Some microbes are spread in to the
environment by infected person in secretions from the nose
or throat during sneezing, coughing or talking(speaking)
loudly.
Large droplets over 0.1mm in diameter fly forwards &
downwards from the mouth to a distance of few feet. They
reach the floor within a few seconds. If a susceptible person
is within the range of the spray of infected droplets, he/she is
likely to inhale some of these droplets, before they fall. This
type of infection is known as droplet infection.
The diseases spread by droplet infections are
tuberculosis, diphtheria, small pox, whooping cough,
influenza & others. The conditions which favors droplets
infections are over crowding & inadequate ventilation.

22. B)Droplet nuclei:-During coughing, sneezing & other
forceful expiratory activities, a large number of droplets
are a sprayed from mouth of infected persons or patients.
Large droplets play role in transmission of droplet
infections. Small droplets, under 0.1mm(<0.1mm)
evaporate immediately to become minute solid residues or
droplet nuclei(1-10u in dia). These are also known as
aerosols i.e. biological contaminants suspended in air.
Being very light, they may remain air borne for
considerable periods, may be swept by air currents
from their source & inhaled into the nose, throat &
lungs. Diseases spread by droplet nuclei are small pox
,chickenpox, measles, tuberculosis & several others.

23. C)Infected dust:-Large droplets from the infected persons
mouth & nose, carrying pathogenic microorganisms settle
on the floor within a few seconds.
Some of the droplets also settle on the cloths,
furniture, bedding. These become what is known as
infected dust. A wide variety of bacteria like
Staphylococcus , Peumococci & Tubercle bacilli have
been found in the dust of hospital wards & living rooms.
These organisms survive in the dust for variable
periods depending upon conditions of temp.& moisture.
During the act of dusting, sweeping & bed making
the infected dust is released into the atmosphere, &
bacterial population of air is thus greatly increased.
Infected dust is primarily inhaled, but may also
settle on uncovered food & milk.

26. 5) Transplacental or Congenital
transmission
 Some pathogens are able to cross the placental
barrier & infect the fetus in utero & may result in
various consequences like abortion, miscarriage &
still birth.
 Also live infants may be borne with manifestation
of disease as in congenital syphilis.
 Intrauterine infection with rubella virus, may
interfere with organogenesis & lead to congenital
malformation.
 Other organisms which can pass placenta are small
pox & measles.

28. 6)Laboratory &Iatrogenic transmission:
 Laboratory workers occasionally become infected
from artificial cultures or from infected material
collected from patients during laboratory procedure
especially during pipetting infected liquids by
mouth, leading to accidental ingestion or accidental.
 Self inoculation with a syringe when it becomes
loose & conjunctiva is spread.
 Many lab. Procedures atomize liquids & may
contaminate the air with infected droplet nuclei.
Laboratory personal are at risk & special care
should be taken to prevent laboratory infections.

29. Control of communicable disease:-
 Diseases results from interaction of agent, host &
environment. The effective control requires
recognition of its multifactorial causation.
 Broadly, general preventive measures may be
discussed as under:-
1) Controlling the source of infection.
2) Blocking the channels of transmission.
3) Protecting the susceptible population.
1) Controlling the source of infection:-First link in the
chain of causation-existence of infected persons,
cases or carriers, who constitute the primary source
of infection.

30. The general measure of control of reservoirs of
infections are:-
a)Early diagnosis & notification
b)Isolation & treatment
c)quarantine & surveillance and
d)Disinfection
a)Early diagnosis & notification:-
 Once a communicable disease is detected, it must
be notified immediately to the local health
authority. Notifiable disease is one for which an
official report is required.

31. The notification of diseases is usually the duty of
attending physician, where this is not possible, it
becomes the duty of the parents or intelligent or
social minded people, school teachers, village
headman, paramedical staff, etc.
b)Isolation & Treatment:-
 Purpose of isolation-to limit the spread of
disease.
 Isolation -best done in a hospitals because the
patient receives better care & secondly, hospital
isolation is more effective than home isolation.
 Home isolation is indicated where facilities for
hospitalization are not available or possible.

32.  Treatment is a part of prevention.
 It reduces the communicability of the disease,
cuts the periods of illness & prevents the
development of secondary cases.
 Not only the cases even the carriers of
infection should be treated.
 In certain diseases, e. g, filarial, mass
administration of drugs is undertaken to
control the disease.
 All the people in the community are
administered the drug whether they have the
disease or not.

33. c) Quarantine & surveillance:-
 Quarantine means isolation of well persons who
have come in contact with an infectious disease
for a period of time equal to the longest
incubation period of disease, to prevent effective
contact with those not exposed to disease.
 Surveillance means prompt field investigation of
disease to identify the causative agent and causes
of disease to prevent further spread of disease.

34. d) Disinfection:-
 An important procedure for controlling the
spread of infection is disinfection of all the
excretions of the patient & the articles
contaminated by the patient.
 Disinfection means immediate destruction
of infectious materials (e.g. feces, urine,
sputum, fomites) through out the course of
illness.

35. 2)Blocking the Channels of Transmission:-
 Communicable diseases spread by 6
important routes. If the mode of spread of
a particular disease is known, it is easy to
exercise control over the spread of
disease.
a) Vehicle transmission
b) Vector transmission
c) Control of air borne transmission
d) Contact transmission
e) Congenital Transmission
f) Iatrogenic transmission

36. a)Vehicle transmission:-
 The four important agents or vehicles of
transmission which carry disease agents are
water, milk, food & human excreta.
i) Control of water route is easy by disinfecting
water or providing safe in place of unsafe
water.
ii)Control of milk route is easy by subjecting
the milk to boiling or pasteurization.
iii) Food borne infections may be controlled by
stepping up standards of food hygiene,
exclusion of sick persons from food handling.

37.  strict attention to personal hygiene,
promotion of hand washing.
 protection of food against flies & rodents &
providing facilities for refrigeration.
iv) Safe disposal of excreta will block the
transmission of diseases by the faecal oral
route.
In short, control of vehicle transmission
implies control of man’s physical
environment.

38. b) Vector transmission:-
 The most important vectors which spread
disease are mosquitoes, flies, fleas, lice, mites
& ticks.
 By controlling their growth & development
through environmental sanitation measures
such as malaria, plague, etc. have been
brought under control or eradication.
 Control of vector transmission implies control
of man’s biological environment.

39. c) Air borne Transmission:-
 Control of air borne transmission is far from
easy. Air borne infection occurs in three
ways—by droplet infection, droplet nuclei &
infected dust.
 The methods available for control of air borne
transmission are:-
 i) use of masks:-Direct droplet infection can
be reduced by covering the nose & mouth with
a handkerchief during the act of coughing &
sneezing & by wearing masks by those who
are exposed to risk of air borne infection.

40.  ii)Bed spacing:-In isolation hospitals, a
distance of 12 feet's between bed centers is
generally aimed to reduce hazards from
droplet infections.
 iii)Screening:-Screens, curtains or cubicles
offer additional physical isolation but these
should not be allowed to interfere with
adequate ventilation.
 iv) Dust control:-Dust should be prevented
from being lifted up into the air during
sweeping, dusting and bed making by oiling of
floors, vacuum sweeping and damp dusting.

41.  v) Over crowding:- Adequate space for
each person will reduce hazards from
droplets infection.
 vi) Ventilation:- Adequate natural
ventilation is enough in itself to maintain a
supply of pure clear air. It will decrease the
chances from droplet infection in most
instances.
 vii) Spitting:-Spitting should be prohibited.
 viii) Sunlight:-It is one of most potent
natural disinfectants of air.

42.  ix) Disinfection of air:- Chemo sterilizers
in the form of spray or vapors have been
used in the disinfection of air, viz. sodium
hypochlorite, resorcinol, hexyl resorcinol,
propylene, and triethylene glycols.
 Ultraviolet rays have also been used in the
disinfection of air.
 d) Contact transmission:-Control of
contact transmission, particularly social
contact is difficult to achieve. It requires co-
operation of all people in the community.

43. 3) Protecting the susceptible
population
i) Active immunization
ii) Passive immunization
iii) Chemoprophylaxis
iv) Health education

44. Active immunization(vaccination):-
 Immunization or vaccination is the most
effective method in control of
communicable diseases whose control is
solely based on immunization, e.g. small
pox, poliomyelitis, diptheria, whooping
cough, TB, tetanus and measles.
 Therefore, vaccination against these
diseases is given as a routine to all children.

45. Passive immunization:-
 Administration of antisera and gamma
globulin is called seroprophylaxis (passive
immunization).
 Passive immunization has a limited value in
the mass control of disease because of short
duration of immunity.
 It is recommended for non-immune persons
under special situations.

46. Chemoprophylaxis:-
 It is the administration of a specific
drug before the occurrence of disease
for primary prevention.
 In this way, susceptible persons may be
protected against specific diseases, e.g.
use of antimalarials in malaria,
sulphonamides in plague, etc.

47. Health education:-
 Health education is an important element in
a communicable disease control
programme.
 Successful control of any disease involves
enlistment of public co-operation.
 This necessities heath education of the
public appropriate to the situation.

48.  Variation in virulence:-
 The virulence of a strain is not constant & may undergo
spontaneous or induced variation.
 Enhancement or increase in the property of virulence is
known as exaltation.
 This can be demonstrated experimentally by passing
organisms of relatively low virulence from one animal or
person to another.
 Their virulence is greatly increased.
 This was first demonstrated by Pasteur by repeated
inoculation of rabies virus in rabbit & virus was exalted.
 This increase in virulence probably result from faster
growth of some individual mutant cells which are most
capable of multiplying under condition of cultivation.
 Reduction of virulence is known as attenuation, which is
brought about by growing them in certain unfavorable
conditions.

49. Different methods of attenuation :-
1)Passing the organism through a less susceptible host:-
Virulent strain of B. anthracis is passed through guinea pig,&
attenuation of the organism occurs & subsequent inoculation
of B.anthracis may not kill the animal..
2)Sub culturing the organism in artificial media outside the
animal body:- It causes attenuation of the organism as in case
of streptococci, D. pneumoniae & Sal. typhosa. In these cases,
attenuation probably occurs by loss of surface antigen with
each sub culture or production of less capsular material.
3)Growing the organism under high temp:-When B- anthracis
is grown at 40C, its virulence is reduced & it can be utilized as
a vaccine, Pasteur vaccine thus prepared is used for
immunization of sheep or cattle.
4)Growing the organism in presence of an inhibitory
substance:- Bovine strains of M. tuberculosis are attenuated by
sub culturing in presence of bile. Vaccine is prepared from
such an attenuated strain.

50. Measurement of virulence:-
 Successful infections require that an adequate
number of microorganisms should gain entry.
 A single bacterium, even though highly virulent
may not be able to establish infection in healthy
body.
 If number of such organisms so that they might
overcome the strong resistance of the healthy
body.
 Virulence of organisms can be measured by
conducting experiments with susceptible
animals like mouse & guinea pig.
 These animals can be well standardized in terms
of age, sex, weight & some genetic factors.

51.  Virulence is measured in terms of minimum
infecting dose (MID) or minimum lethal dose
(MCD) which are respectively defined as a
number of microorganisms or micrograms of
toxin necessary, to infect or kill a given host.
 However assessment of virulence is usually
expressed as ID50 or LD50 ( Average infecting
dose & Average lethal dose).
 Because individual animals show marked
differences in their susceptibility. ID50 &LD50
are expressed as number of pathogens required
to cause disease or kill appro. 50% infected
animals.