st_2013 epidem infectious

7/29/2019 St_2013 Epidem Infectious

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EPIDEMIOLOGY

INFECTIOUSDISEASES

OF


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OUTLINE

Chain of infection

Factors Affecting Communicability

Herd Immunity Iceberg Concept of Infection


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The Chain Model of

Infectious Diseases

Fig. 1. Chain of infection.Disease or infection occurswhen the 6 key components

in the chain are linked.

Disease transmissioninvolves a sequence ofevents that involvesconveyance of the diseaseagent from its reservoir orsource through a portal of

exit, its spread by one ofseveral modes oftransmission, to asusceptible host throughan appropriate portal of

entry (Fig. 1).

We shall illustrate thisusing AI as an example(Fig.2.

Mode oftransmission

Causativeagent

Epidemiologic Methods for the Study of Infectious Diseases, Oxford University Press2001

CHAIN OF INFECTION


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Reservoir iswild migratorywaterfowl.

AIV exits the wild waterfowl through the feces.

AIV is transmittedthrough directcontact withinfected materialsor through dropletnuclei.

Enters the susceptible host(chickens)

The pathogenis AvianInfluenza

virus.

1

23

4

5

AIV leaves itsreservoir orsource throughthe portal of exit

Fig. 1. Chain of Infection for AIV


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Components of the Chain

INFECTIOUS AGENT -

bacteria, virus, parasite,fungus, prion or any

organism that can cause

infection/disease.

RESERVOIR - The medium

or habitat in which

infectious agents thrive,

propagate and multiply.SOURCE - Often the place

where the agent originates -

where it lives, grows, and

multiplies.

PORTAL OF EXIT- pathway by

which a pathogen leaves itshost.

MODE OF TRANSMISSION -

method by which the pathogen

gets from the reservoir to thenew host

PORTAL OF ENTRY- pathway

through which the pathogen

enters its new host

SUSCEPTIBLE HOST- animal

that is at risk for developing an

infection from the disease.


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RESERVOIRS AND

SOURCES OF INFECTION


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Reservoirs

Important in control

Animate or inanimate >1 species Unidentified

Varies with the locale


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Bird-mosquito cycleRESERVOIRS

Ardeola grayii (pond heron)

Bubulcus ibis (cattle egret),

Dead-end hosts

Rice fields Farms

Endemic / Natural Cycle Amplification cycle

Rural infections Rural & peri-urban infections

JAPANESEENCEPHALITIS


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Amplifying hosts

Bird mosquito cycle

is important in

maintaining

and amplifying JEV

in the environment.

Reintroductionof infectedmosquitoes orvertebrates

Pig/ducklingmosquitopig/duckling cycle exist in nature.

Vertical transmission

Infected Vertebrate Reservoirs

Japanese encephalitis Life Cycle


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Taeniasis

DefinitiveHost

Intermediate Host DefinitiveHost

Cysticercosis

Taeniasis

Fig. Taeniasis saginata and cysticercosis transmission cycle

Leakage of raw

sewage ontopasture followingfloods

Muncipal sewagesludge applicationon the farm

Defecation of infeed/water

Ingestion ofinfected meat

Infection of cattleprior to arrival


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Incubatory

Carriers

Convalescent

Silent infection Not essential forperpetuation Caution- some:

vaccinated Important incontrol


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Fomites

- some examples

Inanimate objects


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ROUTES AND MODES OF

TRANSMISSION


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Respiratory tract

Gastrointestinal

Mammary

Urogenitaltract

Circulatory system

Anus

Placenta

Semen

Skin, Hair

Conjunctiva

Mouth, nose

Fig. Portals (routes) of Entry and Exit in VertebrateAnimals

Some examples of ports of entry and exit are described in

the next slides.


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Oral Route

B. abortusRotavirus

Feeds, water


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Respiratory route

< 5 nm

Poor ventilation

High population density

Dusts

Droplets

Droplet nuclei

>5 nm


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Cornea

localized M. bovis

generalized - NDV

Mucous membrane

Labile agents:

Trypanosoma equiperdum

urogenital tract


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Skin route Intact

Cuts or abrasions Bites by animals and vectors

Direct contact w/ organisms e.g. ringworm

Penetration e.g., hookworms

Localized - e.g. B. anthracis

Generalized - leptospires

Veterinary Procedures


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Veterinary Procedures

Catheterization/endoscopywith contaminated equipment

Transfusion withcontaminated fluids

Mass vaccination without

changing needles

Rectal palpation withouchanging sleeves


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There can be more than one route of entry/exitWhat is the primary route?

Do not overlook other routes

Veterinary procedures

CAUTION


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Source: AVET Manual. FAO.

Portal of exit Portal of entry

Susceptiblehosts

Agent(s) insource

Fig. Ways in which infectious agent are transmitted fromaffected to susceptible animals

HORIZONTAL

DirectContactDroplet spread

Vehicle

Vector-borne MechanicalBiological

Indirect

Airborne Droplet nucleiDust

In utero

TransovarialColostralVERTICAL

MODE OF TRANSMISSION

PropagativeDevelopmentalCyclopropagative


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VERTICAL TRANSMISSION

Transmission of disease agents from one

generation to another through sperm,

placenta, milk, egg or vaginal fluids.

HORIZONTAL TRANSMISSION

Transmission of disease agents from an

infected individual to a susceptiblecontemporary.


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Two general modesof horizontal transmission

1. Direct transmission

The direct and immediate transfer of an agent from a

host/reservoir to a susceptible host.

Can occur through direct physical contact such as

exposure to an animal or its waste products.

EXAMPLES

Anthrax from direct contact with an infectedanimal, across placenta (toxoplasmosis), fecal-oral

and ingestion of infected food (trichinosis)


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Horizontal Transmission

2. Indirect transmission

Transfer or carriage of an agent by some

intermediate item, organism, means or process to a

susceptible host, resulting in disease.

MEANS OF INDIRECT DISEASE TRANSMISSION

Fomites, vectors, air currents, dust particles, water, food,

oral-fecal contact and other mechanisms that effectively

transfer disease-causing organisms

MODE OF TRANSMISSION Disease examples


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MODE OF TRANSMISSION Disease examples

Direct Transmission

Direct Contact: bites, licks, touch, sexualintercourse, droplet spread

Rabies, cutaneous anthrax

Droplets sneezing, coughing, sniffing orbellowing

Mycoplasmosis

Indirect Transmission

Airborne:droplet nuclei

dust: particles from soil, clothes, bedding,contaminated floors

Histoplasmosis

Vehicle inanimate objects Salmonellosis, Paratuberculosis,

FMD

Vector invertebrate animalsMechanical: flies, roaches, Schistosomiasis, Filariasis, Dengue

Biological : lice, mites, mosquitoes, ticks Tick infestation

VERTICAL TRANSMISSION

Transovarial via the egg

In utero

within uterusColostral via colostrum or milk

Salmonellosis in chickens


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Airborne transmission

A form of indirect transmission

Conveyance of droplets or dust particles containingthe pathogen to the host to cause infection.

MEANS OF AIRBORNE TRANSMISSION

Sneezing, coughing, bellowing, barking, spraying

microscopic pathogen carrying droplets into the air that can

be breathed in a nearby susceptible hosts.Through conveyance of droplets through a buildings heating

or airconditioning ducts spread by fans


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Vector-borne transmission

Conveyance of the infectious agent through a vector

such as lice, mice, ticks, mosquitoes.

a) Mechanical vector-borne transmission

The pathogen, in order to spread, uses a host (e.g., fly,

flea, louse) as a mechanism for a ride, fornourishment, or as part of a physical transfer process.

b) Biological transmission

The pathogen undergoes changes as part of its life

cycle while within the host/vector and before being

transmitted to the new host


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Contaminated hair ?

Contaminated soil isconsidered the traditional

means of transmission.

BOX 1. Direct contact as a mode of transmission

forToxocara canisinfection


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May have >1 vector

Important in control, prediction

Flying Non-flying Swimming

Types of Vectors

M it V t


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Mosquitoes as Vectors

Flight range

Biting patterns

- indoor/outdoor- nocturnal/diurnal

Biting frequency

Host preference Habitat

V hi l T i i


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DEVELOPMENTAL

Vehicle Transmission

PROPAGATIVE

Multiplication

staphylococci

in milk

CYCLOPROPAGATIVE

PROPAGATIVE

Development ofnematodeeggs in soil

Multiplication & developmentofStrongyloidesin soil


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Controlling or

eliminating agent atsource of transmission

Reservoir /Source

identification & control

Protectingportals of entry

Increasing hosts defenses

Recognition of Susceptible Host

Control portals of exit

Control of agent

Pathogen identification

Prevent a route

of transmission

Interrupt

transmission


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The choice of methods dependson.

Availability of proper tools and techniques

Relative cost effectiveness, efficiency &

acceptability Stakeholders participation or involvement

eg., producers, community, industry

Political support Intersectoral coordination


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Knowledge of the chain of infection provides the

basis for determining the appropriate controlmeasures.

Control measures can be delayed due to: Incomplete knowledge of the disease

agents;

Reservoirs or sources of infection

Routes of transmission

The susceptible hosts

Every disease has some weak points in the

chain that are susceptible to attack.


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Implications to disease control

Identifying the weak points break the

weakest links in disease transmission.

Weakest Links?

Controlling the reservoirs

Interruption of transmission

Protect the susceptible host


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FACTORS AFFECTING

COMMUNICABILITY

The ease with which a disease agent is spread within a

population


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BASIC REPRODUCTIVE NUMBER

The average number ofsecondary infectionsgenerated by one primarycase in a susceptible

pop1ulation

Average no. of secondary

infections generated by oneprimary case in a susceptiblepopulation

R = Ro

x (1 P)R = effective reproductive no.

Ro = Basic reproductive no.P = proportion that is immune

R=1 Conditions supporting endemic disease exist.

R>1 No. infected animals can increase (epidemic?)

R


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Depends on:Duration of infectious

period

Probability that a contact

between an infective andsusceptible individual willlead to an infection

No. of new susceptibleanimals contacted per unit

of time.

BASIC REPRODUCTIVE NUMBER


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Ave. no. susceptible animals

infected by each infected

animal.

Ease of transmission

FMD Ro=70

IBR Ro=7

Oth F t


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AGENT

ENVIRONMENT

Other Factors

Life cycle

Minimal infectious dose

Heterogeneity

Immunity

HOST FACTORS


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Prepatent period- Agent in secretions,excretions, blood or tissues

Communicable periodtime during

which a disease agent may betransferred directly or indirectly fromone infected animal to another,including invertebrate vectors.

Extrinsic Incubation PeriodTime between infection of a biological vectorand acquisition of the biological vector of the

ability to transmit the agent to anothersusceptible vertebrate host.

AG

E

N

T


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Minimal Infective Dose

The lower the minimal infective

dose, the more readily the agent

is transmitted.

A

GE

N

T


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HeterogeneityIndividual variation in disease

susceptibility: some are particularlysusceptible.

H

O

ST

FA

C

T

O

R

S

ImmunityVertebrates develop stronger immune

response than do metazoans.


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Dust Droplet Droplet nuclei

Sources Solid matter,fabrics, etc

Fluids fromnose, throat

Solid residuesof evaporateddroplets

Particlediameter

10-100m >100 m 2-10 m

Flightrange

Hovers inclouds

Immediatein space

< 1 m

Dispersed thruout confinedatmospheres

Removal

from air

Filtration and

electrostaticprecipitation

Best by face

mask

Electrostatic

precipitation

Control Air cleanliness,moistening,oiling, etc.

Spacing out Ventilation (airchange&equivalent airdisinfection)

D t D l t D l t l i


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Dust Droplet Droplet nuclei

Vulnerabi-lity

Resistant Indeterminate Vulnerable tochemical and

physicalagents

Mode ofinfection

Endemicinfection of

nose andthroat

Contactinfection

Epidemiccontagion


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HERD IMMUNITY


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Herd Immunity

When proportionof immuneanimals in thepopulation isabove thethreshold level,the infection willdie out in thatpopulation.


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ICEBERG CONCEPT OF INFECTION


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PhP

PhPPhP

Susceptible in the future or

resistant as a consequence

of past exposure


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CELL RESPONSE HOST RESPONSELysis of cell Fatal

Cell transformationor cell dysfunction

Clinical & severe disease

Exposurewithout cell entry

Exposurewithout infection

Incomplete viralmaturation

Infection withoutclinical illness

Discernibleeffect Moderate severity

Mild illness

Belowvisualchange

Clinicaldisease

Sublclinicaldisease

Fig. Iceberg concept of infection (Friies & Sellers, 2009)

Iceberg concept


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Iceberg concept

Clinical cases are seen- subclinical infections are not

If only clinical cases are given an intervention

- mistake: Recovery as due to intervention

st_2013 epidem infectious

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