The dynamics of disease transmission

1. THE DYNAMICS OF DISEASE TRANSMISSION
Dr Su Su Hlaing
Assistant Lecturer
Department of Epidemiology
University of Public Health
Yangon, Myanmar

2. LEARNING OBJECTIVES
 To introduce concepts related to disease transmission using the epidemiologic
approach to communicable diseases as a model.
 To define important terms related to the occurrence of disease in a population.
 To calculate an attack rate and illustrate how it may be used to measure person-
to-person transmission of a disease.
 To describe the steps in an outbreak investigation and introduce how cross-
tabulation may be used to identify the source.

3. CHAIN OF INFECTIOUS DISEASE
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4. AGENT
 A substance, living or non-living,
OR
 A force, tangible or intangible, the excessive presence or relative lack of which
may initiate or perpetuate a disease process
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5. CLASSIFICATION
1. Biological agents - Infectivity: :invade and multiply
- Pathogenicity :induce apparent illness
-Virulence :Proportion of severe cases (C.F.R)
2. Physical agents - Heat, cold, pressure, radial ion, etc.
3. Chemical agents - Endogenous : Urea, Creatinine, Uric acid, Bilirubin
- Exogenous : Dust, allergens, fumes
4. Mechanical agents -chronic friction, sprain, disclocation,etc.
5. Social agents - poverty, smoking, abuses, unhealthy lifestyle, etc.
6. Nutritional agents - CHO, Proteins, Fats,Vitamins, Minerals6/26/2017 5

6. CLASSIFICATION
7. Excess, insufficiency or absence of a factor necessary to health;
 Chemical factors (Hormones)
 Nutritional factors
 Lack of structure – Congenital (-) ofThymus
 Chromosomal factors-Trisomy 21 : Down's syndrome
 Immunological factors -agammaglobulinaemia
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7. SOURCE OF INFECTION
• the person, animal, object or substance from which an infectious agent
passes or is disseminated to the host
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8. RESERVOIR
• any person, animal, arthropod, plant, soil or substance (or combination of these)
in which an infectious agent lives and on which it depends primarily for survival,
and where it reproduces itself in such manner that it can be transmitted to a
susceptible host
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9. RESERVOIR
• Natural habitat - in which the organism metabolites and multiplicates
• For example, in hookworm infection, the resevoir is man but the source of
infection is the soil contaminated with infective larvae
• In tetanus, the resevoir and the source are the same, that is soil
• In typhoid fever, the resevoir may be case or carrier but the source of
infection may be faeces or urine of patients or contaminated food, milk or
water
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10. TYPES OF RESERVOIR
 Homologous Reservoir
 Heterologous Reservoir
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11. HOMOLOGOUS RESERVOIR
 when another member of the same species is the victim, man is the resevoir of
some enteric pathogens e.g. vibrio cholerae
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12. HETEROLOGOUS RESERVOIR
 when an infection is derived from a reservoir other than man E.g. animals and
birds infected with salmonella
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13. THREETYPES OF RESERVOIR
 Human Reservoir
 Animal Reservoir
 Reservoir in non-living things
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14. HUMAN RESERVOIR
I. Case
II. Carrier
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15. I. CASES
 a person in the population or study group identified as having the particular
disease, health disorder or condition under investigation
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16. CASES
 May be Clinical, Subclinical or Latent
 These variations in the manifestations of disease are referred to as "spectrum of
disease" or "gradient of infection
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17. CLINICALCASES
• may be mild or moderate, typical or atypical, severe or fatal depending upon the
gradient of involvement
• mild cases may be more important sources of infection than severe cases because
they are ambulant and spread the infection wherever they go, whereas severe
cases are usually confined to bed
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18. SUBCLINICAL CASES
 Referred as inapparent, covert, missed or abortive cases
 The disease agent may multiply in the host but does not manifest itself by signs
and symptoms
 The disease agent is eliminated and it contaminates the environment in the same
way as clinical cases
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19. SUBCLINICAL CASES
 Subclinical cases play a dominant role in maintaining the chain of infection
(endemicity) in the community
 can be detected only by laboratory tests, e.g., recovery of the organism, antibody
response, biochemical and skin sensitivity tests
 E.g. Polio, Influenza, Hepatitis A and B
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20.  Whatever may be the "gradient of infection", all infected persons,
whether clinical or subclinical, are potential sources of infection
 because the disease agent is leaving the body through frequent
stools, vomiting, coughing, sneezing or other means and is
potentially available for transfer to a new host
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21. LATENT CASES
 In latent infection, the host does not shed the infectious agent which lies
dorminant within the host without symptoms and often without demonstrable
presence in blood, tissues or bodily secretions of the host
E.g. Herpes simplex
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22. INDEX CASE
 refers to the first case to come to the attention of the investigator
 it is not always the primary case
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23. PRIMARY CASE
 refers to the first case of a communicable disease introduced into
the population unit being studied
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24. SECONDARY CASE
 Secondary cases are those developing from contact with primary
case
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25. II. CARRIER
 A carrier is an individual who harbors the organism but is not infected as
measured by serologic studies (no evidence of an antibody response) or by
evidence of clinical illness.

26. CARRIER
 As a rule, carriers are less infectious than cases
 but epidemiologically, they are more dangerous than cases
 because they escape recognition, and continuing as they do to live a normal life
among the population or community
 they readily infect the susceptible individuals over a wider area and longer period
of time, under favourable conditions
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27. ELEMENTS IN A CARRIER STATE
a) the presence in the body of the disease agent
b) the absence of recognizable symptoms and signs of disease, and
c) the shedding of the disease agent in the discharges or excretions, thus acting
as a source of infection for other persons
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28. CARRIERS MAY BE CLASSIFIED AS:
A.Type
(a) Incubatory
(b) Convalescent
(c) Healthy
B. Duration
(a)Temporary
(b) Chronic
C. Portal of Exit
(a) Urinary
(b) Intestinal
(c) Respiratory
(d) Others
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29. INCUBATORY CARRIERS
 are those who shed the infectious agent during the incubation period of
disease
 That is, they are capable of infecting others before the onset of illness
usually occurs during the last few days of the incubation period E.g.,
measles, mumps, polio, pertussis, influenza, diphtheria and hepatitis B
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30. CONVALESCENT CARRIERS
 those who continue to shed the disease agent during the period of convalescence
 E.g., typhoid fever, dysentery (bacillary and amoebic), cholera, diphtheria and
whooping cough
 In these diseases, clinical recovery does not coincide with bacteriological recovery
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31. CONVALESCENT CARRIERS
 a serious threat to the unprotected household members and those in the
immediate environment
 as in the case of a typhoid fever patient who may excrete the bacilli for 6-8 weeks
 This highlights the importance of bacteriological surveillance of carriers, after
clinical recovery
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32. HEALTHY CARRIERS
 Healthy carriers emerge from subclinical cases
 They are victims of subclinical infection who have developed carrier state without
suffering from disease, but are shedding the disease agent
 E.g., poliomyelitis, cholera, meningococcal meningitis, salmonellosis, and
diphtheria
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33. HEALTHY CARRIERS
 It is well to remember that a person whose infection remains subclinical may or
may not be a carrier
 E.g. in Polio, the infection may remain subclinical and the person may act as a
temporary carrier by virtue of shedding the organism
 On the other hand, inTuberculosis, most persons with positive tuberculin test do
not actively disseminate tubercle bacilli and therefore are not labelled as carrier
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34. TEMPORARY CARRIERS
 are those who shed the infectious agent for short periods of time
 In this category may be included the incubatory, convalescent and healthy carriers
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35. CHRONIC CARRIERS
 one who excretes the infectious agent for indefinite periods
 E.g., typhoid fever, hepatitis B, dysentery, cerebra-spinal meningitis, malaria,
gonorrhoea, etc
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36. CHRONIC CARRIERS
 The longer the carrier state, the seater the risk to the community
 Some carriers excrete the infectious agent only intermittently and some
continuously
 The duration of the carrier state varies with the disease
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37. CHRONIC CARRIERS
 in typhoid fever and hepatitis B, it may last for several years
 in chronic dysentery, it may last for a year or longer
 in diphtheria, the carrier state is associated with infected tonsils
 in typhoid fever with gall bladder disease
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38. CHRONIC CARRIERS
 Chronic carriers are known to reintroduce disease into areas which are otherwise
free of infection (e.g., malaria)
 early detection and treatment are essential to limit the spread of infection
 Carriers of avirulent organisms are called pseudo-carriers
 Pseudo-carriers are not important epidemiologically
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39. ANIMAL RESERVOIR
 The source of infection may be animals and birds
 The diseases and infections which are transmissible to man from vertebrates are
called Zoonoses
 There are over (100) zoonotic diseases which may be conveyed to man from
animals and birds
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40. ANIMAL RESERVOIR
 The best known examples are rabies, yellow fever and influenza
 The role of pigs and ducks in the spread of epidemic and pandemic influenza both
as reservoirs, carriers and "amplifying hosts”
 Pigeons in cities can lead to infection with chlamydia; dust mites from them can
cause allergy in man
 Ornithosis and arboviruses can be transmitted to man from various birds
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41. ANIMAL RESERVOIR
 Wild birds are important hosts in the transmission cycles of most of the mosquito-
borne encephalitis and several mosquito-borne undifferentiated febrile diseases
 Histoplasmosis is carried all over the world by birds
 As birds migrate from one locality to another, they may carry ticks infected with
viruses and rickettsiae that may cause disease in humans
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42. ANIMAL RESERVOIR
 the migrations and movements of animals and birds may carry serious
epizootiological and epidemiological risks
 There is evidence that genetic recombination between animal and human viruses
might produce "new" strains of viruses (e.g., influenza viruses)
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43. RESERVOIR IN NON-LIVINGTHINGS
 Soil and inanimate matter can also act as reservoirs of infection
 For example, soil may harbour agents that cause tetanus, anthrax, etc
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44. BY PORTAL OF EXIT
 urinary carriers, intestinal carriers, respiratory carriers, nasal carriers, skin
eruptions, open wounds and blood, etc
 In typhoid fever, the urinary carrier is more dangerous than an intestinal
carrier
 A typhoid carrier working in a food establishment or water works is more
dangerous than a typhoid carrier working in an office establishment
 Thus the portal of exit and the occupational status of the carrier are important
epidemiological considerations
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45. MODES OFTRANSMISSLON
 Depend upon the infectious agent, portal of entry and the local ecological
conditions
 As a rule, an infectious disease is transmitted by only one route, e.g., typhoid
fever by vehicle transmission and common cold by direct contact
 But there are others which may be transmitted by several routes e.g., AIDS,
salmonellosis, hepatitis B, brucellosis, Q fever, etc
 The multiple transmission routes enhance the survival of the infectious agent
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46. CLASSIFICATION
 A. DIRECT TRANSMISSION
1. Direct contact
2. Droplet infection
3. Contact with soil
4. Inoculation into skin or mucosa
5.Transplacental (vertical)
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47. CLASSIFICATION
B. INDIRECTTRANSMISSION
1.Vehicle-borne
2.Vector-borne
a. Mechanical
b. Biological
3. Air-borne
a. Droplet nuclei
b. Dust
4. Fomite-borne
5. Unclean hands and fingers
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48. DIRECT CONTACT
 Infection may be transmitted by direct contact from skin to skin, mucosa to
mucosa, or mucosa to skin of the same or another person
 This implies direct and immediate transfer of infectious agents from the reservoir
or source to a susceptible individual, without an intermediate agency
 e.g., skin-to-skin contact as by touching, kissing or sexual intercourse or
continued close contact
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49. DIRECT CONTACT
 Direct contact not only reduces the period for which the organism will have to
survive outside the human host but also ensures a larger dose of infection
 Diseases transmitted by direct contact include STD and AIDS, leprosy,
leptospirosis, skin and eye infections
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50. DROPLET INFECTION
 direct projection of a spray of droplets of saliva and naso-pharyngeal
secretions during coughing, sneezing or speaking and spitting,
talking in to the surrounding atmosphere
 The expelled droplets may impinge directly upon the conjunctiva,
oro- respiratory mucosa or skin of a close contact
 Particles of (10) mmm or greater in diameter are filtered off by nose
 Those (5) mmm or less can penetrate deeply and reach the alveoli
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51. DROPLET INFECTION
 The droplet spread is usually, limited to a distance of 30-60cm
between source and host
 In infectious diseases, the droplets may contain millions of bacteria
and viruses can be a source of infection to others
 When a healthy susceptible person comes within the range of these
infected droplets, he is likely to inhale some of them and acquire
infection
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52. DROPLET INFECTION
 Diseases transmitted by droplet spread include many respiratory infections,
eruptive fevers, many infections of the nervous system, common cold,
diphtheria, whooping cough, tuberculosis, meningococcal meningitis, etc
 The potential for droplet spread is increased in conditions of close proximity,
overcrowding and lack of ventilation
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53. DROPLET DISPERSAL FOLLOWING A
VIOLENT SNEEZE
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54. CONTACT WITH SOIL
 The disease agent may be acquired by direct exposure of susceptible tissue to the
disease agent in soil, compost or decaying vegetable matter in which it normally
leads a saprophytic existence
 e.g,, hookworm larvae, tetanus, mycosis, etc
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55. INOCULATION INTO SKIN OR MUCOSA
 The disease agent may be inoculated directly into the skin or mucosa
 e.g., rabies virus by dog bite, hepatitis B virus through contaminated needles and
syringes, etc.
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56. TRANSPLACENTAL ORVERTICALTRANSMISSION
 This is another form of direct transmission
 E.g.TORCH agents (Toxoplasma gondii, rubella virus, cytomegalovirus and herpes
virus), varicella virus, syphilis, hepatitis B, Coxsackie B and AIDS
 Some of the non-living agents (e.g., thalidomide, diethylstilbestrol) can also be
transmitted vertically
 In these cases, the disease agent produces malformations of the embryo by
disturbing its development
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57. B. INDIRECTTRANSMISSION
 5 F's – “flies, fomites, food, fluid and fingers”
 An essential requirement for indirect transmission is that the
infectious agent must be capable of surviving outside the human
host in the external environment and retain its basic properties of
pathogenesis and virulence till it finds a new host
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58. INDIRECTTRANSMISSION
 This depends upon the characteristics of the agent, the inanimate object and the
influence of environmental factors such as temperature and humidity
 If the disease agent acquires drug resistance, it will further facilitate its spread
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59. 1.VEHICLE-BORNETRANSMISSION
 transmission of the infectious agent through the agency of water, food (including
raw vegetables, fruits, milk and milk products), ice, blood, serum, plasma or other
biological products such as tissues and organs
 Of these water and food are the most frequent vehicles of transmission, because
they are used by everyone
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60.  The infectious agent may have multiplied or developed in the vehicle
(e.g., S. aureus in food) before being transmitted
 or only passively transmitted in the vehicle (e.g., hepatitis A virus in
water)
 Organ transplantation may result in the introduction of the disease
agent such as cytomegalovirus in association with kidney transplants
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61.  Diseases transmitted by water and food include chiefly infections of the
alimentary tract, e.g., acute diarrhoeas, typhoid fever, cholera, polio,
hepatitis A, food poisoning and intestinal parasites
 Those transmitted by blood include hepatitis B, malaria, syphilis, brucellosis,
trypanasomas (Chaga's disease), infectious mononucleosis and
cytomegalovirus infection
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62. EPIDEMIOLOGICAL FEATURES OF
VEHICLETRANSMISSION
(a) if the dose of contamination is heavy, the outbreak may be explosive as in
the case of cholera and hepatitis A epidemics
(b) cases are initially confined to those who are exposed to the contaminated
vehicle, in some infections
(c) when secondary cases occur, the primary case may be obscured
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63. (d) the distance travelled by the infectious agent may be great, e.g., outbreaks of
food poisoning
(e) it is not always possible to isolate the infectious agent in the incriminated
vehicle, e.g., typhoid bacilli in contaminated water
(f) when the vehicle is controlled or withdrawn, the epidemic subsides, e.g.,
epidemics of cholera
(g) the common source of infection is often traceable
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64. VECTOR-BORNETRANSMISSION
 In infectious disease epidemiology, vector is defined as an arthropod or any living
carrier (e.g., snail) that transports an infectious agent to a susceptible individual
 Transmission by a vector may be mechanical or biological
 In the latter case, the disease agent passes through a developmental cycle or
multiplication in the vector
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65. EPIDEMIOLOGICAL CLASSIFICATIONOF
VECTOR-BORNE DISEASES
I. By vector :
a) Invertebrate type : Arthropod vectors fall into seven orders largely
(1) Diptera - flies and mosquitoes
(2) Siphonaptera - fleas
(3) Orthoptera - cockroaches
(4) Anoplura - sucking lice
(5) Hemiptera - Bugs, including kissing bugs
(6) Acarina - ticks and mites
(7) Copepoda - cyclops
b)Vertebrate type - Mice, rodents, bats
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66. 6/26/2017 66
II. By transmission chain :
a) Man and a non-vertebrate host
1) Man-arthropod-man (malaria)
2) Man-snail-man (schistosomiasis)
b) Man, another vertebrate host, and a non-vertebrate host
1) Mammal-arthropod-man (plague)
2) Bird-arthropod-man (encephalitis)
c) Man and 2 intermediate hosts
1) Man-cyclops-fish-man (fish tape worm)
2) Man-snail-fish -man (Clonorchis sinensis)
3) Man-snail-crab-man (Paragonimiasis)

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III. By methods in which vectors transmit agent (Portal of entry) :
a) Biting
b) Regurgitation
c) Scratching-in of infective faeces
d) Contamination of host with body fluids of vectors

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IV. By methods in which vectors are involved in the transmission and
propagation of parasites
(a) Mechanical transmission
(b) Biological transmission

69. (A) MECHANICALTRANSMISSION
 The infectious agent is mechanically transported by a crawling or flying arthropod
through soiling of its feet or proboscis or by passage of organisms through its
gastro-intestinal tract and passively excreted
 There is no development or multiplication of the infectious agent on or within the
vector
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70. (B) BIOLOGICALTRANSMISSION
 The infectious agent undergoing replication or development or
both in vector and requires an incubation period before vector can
transmit
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71. BIOLOGICALTRANSMISSION IS OFTHREE
TYPES:
i ) Propagative :The agent merely multiplies in vector, but no change
in form, e.g., plague bacilli in rat fleas
ii) Cyclo-propagative :The agent changes in form and number, e.g.,
malaria parasites in mosquito
iii) Cyclo-developmental :The disease agent undergoes only
development but no multiplication, e.g. microfilaria in mosquito
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72.  When the infectious agent is transmitted vertically from the infected
female to her progeny in the vector, it is known as trans-ovarial
transmission
 Transmission of the disease agent from one stage of the life cycle to
another as nymph to adult is known as trans-stadial transmission
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73.  The factors which influence the ability of vectors to transmit disease are :
(a) host feeding preferences
(b) infectivity, that is ability to transmit the disease agent
(c) susceptibility, that is ability to become infected
(d) survival rate of vectors in the environment
(e) domesticity, that is degree of association with man, and
(f) suitable environmental factors
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74.  Seasonal occurrence of some diseases (e.g., malaria) may be related to intense
breeding and thereby greater density of the insect vector during certain periods of
the year
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75. 3. AIRBORNETRANSMISSION
a) Droplet nuclei
b) Dust
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76. (A) DROPLET NUCLEI
 a type of particles implicated in the spread of airborne infection
 They are tiny particles (1 -10 microns range) that represent the dried
residue of droplets
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77.  They may be formed by:
i. evaporation of droplets coughed or sneezed into the air or
ii. generated purposefully by a variety of atomising devices
(aerosols)
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78.  They may also be formed accidentally in microbiological
laboratories, in abattoirs, rendering plants or autopsy rooms
 The droplet nuclei may remain airborne for long periods of time,
some retaining and others losing infectivity or virulence
 They not only keep floating in the air but may be disseminated by air
currents from the point of their origin
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79.  Particles in the (1-5) micron range are liable to be easily drawn into
the alveoli of the lungs and may be retained there
 Diseases spread by droplet nuclei include tuberculosis, influenza,
chickenpox, measles, Q fever and many respiratory infections
 airborne spread of toxic air pollutants including "smog" resulting in
air pollution epidemics
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80. (B) DUST
 Some of the larger droplets which are expelled during talking,
coughing or sneezing, settle down by their sheer weight on the
floor, carpets, furniture, clothes, bedding, linen and other objects in
the immediate environment and become part of the dust
 A variety of infectious agents (e.g., streptococci, other pathogenic
bacteria, viruses and fungal spores) and skin squamae have been
found in the dust of hospital wards and living rooms
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81. 6/26/2017 81
 Some of them (e.g., tubercle bacilli) may survive in the dust for
considerable periods under optimum conditions of temperature and
moisture
 During the act of sweeping, dusting and bed-making, the dust is
released into the air and becomes once again airborne
 Dust particles may also be blown from the soil by wind, this may
include fungal spores
 Coccidioidomycosis is an example of a disease spread through
airborne transmission of fungal spores

82.  Other diseases carried by infected dust include streptococcal and
staphylococcal infection, pneumonia, tuberculosis, Q fever and
psittacosis
 Airborne dust is primarily inhaled, but may settle on uncovered food
and milk
 This type of transmission is most common in hospital-acquired
(nosocomial) infection
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83. 4. FOMITE-BORNETRANSMISSION
 Fomites are inanimate articles or substances other than water or
food contaminated by the infectious discharges from a patient and
capable of harbouring and transferring the infectious agent to a
healthy person
 Fomites include soiled clothes, towels, linen, handkerchiefs, cups,
spoons, pencils, books, toys, drinking glasses, door , handles, taps,
lavatory chains, syringes, instruments and surgical dressings
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84. 5. UNCLEAN HANDS AND FINGERS
 Hands are the most common medium by which pathogenic agents
are transferred to food from the skin, nose, bowel as well as from
other foods
 The transmission takes place both directly (hand-to-mouth) and
indirectly
 E.g. staphylococcal and streptococcal infections, typhoid fever,
dysentery, hepatitis A and intestinal parasites
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85.  Unclean hands and fingers imply lack of personal hygiene
 Lack of personal hygiene coupled with poor sanitation favour
person-to-person transmission of infection,
 the most recent example is the 1984 Dysentery epidemic in India
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86. HOST
 Human host acts as SOIL
 Disease agent acts as SEED
 A person or other living organism that can be infected by an infectious agent
under natural condition
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87. 6/26/2017 87

88. CLASSIFICATION
1. Demographic characteristics - age, gender, ethnicity
2. Biological characteristics - Genetic factors
- Blood levels (Serum cholesterol level, Blood glucose level)
- Blood groups
- Physiological functions (B.P/FEV)
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89. CLASSIFICATION
3. Social and economic characteristics
 Socioeconomic status
 Education
 Occupation
 Housing
 Stress
4. Lifestyle factors - Personality traits
 Habits (healthy or unhealthy)
 Behavioural patterns
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90. SUSCEPTIBLE HOST
Successful parasitism
 Four stages have been described in successful parasitism :
(a) - First, the infectious agent must find a “PORTAL OF ENTRY” by
which it may enter the host
-There are many portals of entry, e.g., respiratory tract,
alimentary tract, genitourinary tract, skin, etc
- Some organisms may have more than one portal of entry, e.g.,
hepatitis B, Q fever, brucellosis
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91. (b) On gaining entry into the host, the organisms must reach the
appropriate tissue or "site of election" in the body of the host where
it may find optimum conditions for its multiplication and survival
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92. (c)-Thirdly, the disease agent must find a way out of the body (portal
of exit) in order that it may reach a new host and propagate its
species
- If there is no portal of exit, the infection becomes a dead-end
infection as in rabies, bubonic plague, tetanus and trichinosis
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93. (d) - After leaving the human body, the organism must survive in the
external environment for sufficient period till a new host is found
- In addition, a successful disease agent should not cause the death
of the host but produce only a low-grade immunity so that the host
is vulnerable again and again to the same infection
-The best example is common cold virus
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94. THE EPIDEMIOLOGICTRIAD OF A DISEASE
Host
Vector
Agent Environment

95. FACTORSTHAT MAY BE ASSOCIATED WITH
INCREASED RISK OF HUMAN DISEASE
Host Characteristics
• Age
• Sex
• Race
• Religion
• Customs
• Occupation
• Genetic profile
• Marital status
• Family background
• Previous diseases
• Immune status
Types of Agents and
Examples
• Biologic- Bacteria, viruses
• Chemical - Poison, alcohol,
smoke
• Physical -Trauma,
radiation, fire
• Nutritional - Lack, excess
Environmental Factors
• Temperature
• Humidity
• Altitude
• Crowding
• Housing
• Neighborhood
• Water
• Milk
• Food
• Radiation
• Air pollution
• Noise

96. ENDEMIC, EPIDEMIC, AND PANDEMIC
Endemic
- is defined as the habitual presence of a disease within a given geographic
area. It may also refer to the usual occurrence of a given disease within such an area.
Epidemic
- is defined as the occurrence in a community or region of a group of illnesses of
similar nature, clearly in excess of normal expectancy, and derived from a common or
from a propagated source.
Pandemic
- refers to a worldwide epidemic.

97. HERD IMMUNITY
 defined as the resistance of a group of people to an attack by a disease to which a
large proportion of the members of the group are immune.

98. INCUBATION PERIOD
 The incubation period is defined as the interval from receipt of infection to the time
of onset of clinical illness.

99. The three critical variables in investigating an outbreak or epidemic are:
(1)When did the exposure take place?
(2) When did the disease begin?
(3)What was the incubation period for the disease?

100. SECONDARY ATTACK RATE
 It is defined as the attack rate in susceptible people who have been exposed to a
primary case. It is a good measure of person-to-person spread of disease after the
disease has been introduced into a population, and it can be thought of as a ripple
moving out from the primary case.We often calculate the secondary attack rate in
family members of the index case.

101. ATTACK RATE
Attack rate =
Number of people at risk in whom a certain illness develops
Total number of people at risk

102.  For example, food-specific attack rate
Attack rate =
Number of people who ate a certain food and became ill
Total number of people who ate that food

104. THANK YOU!!
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