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INTRODUCTION OF EPIDEMIOLOGY
Pradhuman YadavB.V.Sc & A.H
ONE WORLD ONE HEALTH
One World: Earth One Health: Humans, Animals,
Environment Emerging Infectious Disease (EID)
Avian Influenza (H5N1), SARS (Severe Acute Respiratory Syndrome) Nipah virus Influenza A (H1N1)
EMERGING INFECTIOUS DISEASES
infectious diseases with an increasing i n patient report over the past 20 years
infectious diseases with an increasing possibility in the near future
AIDS, Avian Influenza, and drug resista nt tuberculosis
Antimicrobial resistant organisms
RE-EMERGING INFECTIOUS DISEASES
infectious diseases that used to create outb reak in the past and subsided for a number o
f years but are occurred again tuberculosis, hemorrhage fever and malaria
Emerging and re-emerging Infectious Diseases
Global of emerging and re-emerging infectious diseases
FACTORS OF EID
Humans (Africa, Asia and Latin America)
Wildlife (Forest encroachment) Climate change Pathogens
Spread of pathogen (air or insect) Virus (mutation)
EPIDEMIOLOGY
Epi = on, upon Demos = people Logos = knowledge
HISTORICAL OF EPIDEMIOLOGY
Hippocrates (400 BC) John Graunt (1662) John Snow (1854)
Out break of cholera occurred in a small area of central London (Golden Square)
WHAT IS EPIDEMIOLOGY?
The study of the distribution and de - terminants of health related states
or events in specified populations a nd the application of this study to t he control of health problems (CDC)
WHAT IS EPIDEMIOLOGY?
focused on the health and disease status of a population
the study of how disease is distributed in populations and the factors that influence or determine this distribution
Epidemiology
is a scientific disciplinediscipline
that involves the studystudy of
the frequencyfrequency and distributiondistribution
of healthhealth and diseasedisease
in populationspopulationsin order to find risk factorsrisk factors
for preventionprevention and controlcontrol
Discipline: the general approach is to creating order and structure from incomplete knowledge
Study: combines learning about epidemiology theory with on the job field application
Frequency: means that we count characteristics in a population of people or animals
Distribution: describes the patterns of disease in a population, in a particular place during a period of time
Health: refers to measures of optimum productivity due to lack of disease (meat, eggs or milk)
Disease: refers generally to an imbalance in the health status of individuals or populations that result in decreased productivity, illness or death
Population: refers to the group of individual animals or people that are considered or affected
Prevent: means not providing the opportunity for a disease to occur
Control: method to reduce the extent of disease in a population or area
Risk factors: risk is the probability that a factor the population is exposed to be associated with the occurrence of disease
OBJECTIVE OF EPIDEMIOLOGY
To identity the etiology (cause) of disease and the relevant risk factors
To determine the extent of disease found in the community
To study the natural history and prognosis of disease
OBJECTIVE OF EPIDEMIOLOGY
To evaluate both existing and newly developed preventive and therapeutic measure and modes of health care delivery
To provide the foundation for developing public policy relating to environmental problems
Use of Epidemiology
• Describe the distribution of disease• Describe the natural history of disease• Identify factors that increase/decrease risk• Predict trends• Consider mechanisms of transmissions• Test efficacy & evaluate interventions• Identify health needs
Epidemiology Clinical medicine
Population People (Case)
Prevention and control Treatment
Epidemiologist Case
Healthy in population Healthy in people
Epidemiology VS Clinical medicine
FIELD EPIDEMIOLOGY
Field Epidemiology is the front linefront line There is health emergency or an
immediate need to understand the health status of a population
Emerging Infectious Disease (EID): no information, very limited
FIELD EPIDEMIOLOGY
Attempts to gather and organize data to bring order and meaning to it
Can be applied to disease outbreaks, situation assessments and policy evaluation.
Relies on a systematic approach to gather and organize data in a way that will support a better understanding of a disease situation
GOAL OFVETERINARY FIELD EPIDEMIOLOGY
Prevention and control disease agents Health of animals, humans and environment
Concepts and methods of epidemiology Practical and information
EPIDEMIOLOGY APPROACH
Try and understand what factors may be increasing or reducing the risk of disease
Promoting and protecting the health of animal and human populations
ENDEMIC
the constant occurrence of a disease that commonly presentscommonly presents in a particular place with stability in the level of infection Sporadic: An irregular occurrence of a disease
that commonly presents in a particular place
Endemic pattern
Sporadic pattern
EPIDEMIC
the occurrence of a disease that the level of infection exceeds that normal expectancy in a specific region, spreads rapidly and usually lasts for a limited period of time Pandemic: widespread epidemic that affects a
large part of population in many countries Epizootic: epidemic that involves animal host
population
Epidemic pattern
EPIDEMIC PATTERNS
D ISEASE OUTBREAK
survey of disease data count of cases describe
person / animal place time
Epidemiology triad: explain why diseases occur in a population
Environment
Host
Agent
Environment
Host
Agent
Environment
Host
Agent
Environment
Host
Agent
Environment
Host
Agent
AGENTS
Biological Viruses Bacteria Parasites or prions
Chemical Toxins Man-made (Dioxins and melamine) Inorganic/organic: zearalenone
Physical Foreign bodies Trauma Radiation
AGENT FACTORS
Dose Environmental
hardiness Virulence (microbial) Infectivity (microbial) Toxicity (poisons)
HOST
Natural host: agent has adapted itself and co-
exists in balance in the host Atypical host:
agent is not normally encountered
HOST Demography
Age, Sex, Species, Breed Production type / level, Density
Biology Genetics, behavior
Management Intensive (housing) / extensive (free
roaming) Nutrition Hygiene Husbandry Vaccination / medication
HOST Marketing
Profitability related to prices (economics) Distance from market
Herd immunity Innate (genetic capability) Acquired through vaccination or deliberate
exposure Proportion of total population that is
resistant to a disease agent Susceptibility
Lack of resistance to the disease agent
HOST FACTORS
Innate resistance (e.g. gastric barrier, mucocilliary transport mechanism)
Previous exposure Passive immune status (neonates) Vaccination status and response Age Gender
HOST FACTORS
Behavior (e.g. mutual grooming, dominance, pica) Production status (e.g., lactating vs. non-
lactating) Reproductive status (e.g., pregnant vs. non-
pregnant, sterile vs. intact) Genetics
ENVIRONMENT
1) N atural environment Geography Climate Season pH Ammonia concentration Water activity Ultraviolet light Organic matter
2) H uman aspects Animal management systems Marketing systems and economics Government policies
ENVIRONMENTAL FACTORS
Animal stocking density Animal movement between groups Housing (e.g. ventilation, sanitation) Environmental conditions (e.g.
temperature, humidity, wind velocity, precipitation)
Nutrition (protein, energy and macromineral and micromineral adequacy)
EXAMPLE
"Bovine mastitis is a disease of man with signs in the cow." "Bad management will overwhelm the best immunology."
• Increased animal density may lead to increased microbial load in the environment
•a roof may prevent exposure of microbe to killing UV
• low ventilation
• increase humidity
• increases environmental survival of the organism
• increases exposure dose and infects more animals.
NATURAL HISTORY OF DISEASE
Normal
Risk factors
Disease
Death
recover
disabled
NATURAL HISTORY OF DISEASE
Stage of susceptibility Stage of preclinical disease Stage of clinical disease Stage of disability
ICEBERG PRINCIPLE OF DISEASE
ICEBERG (PHENOMENON) PRINCIPLE
ICEBERGPHENOMENON
deathdeath
disabilitydisability
ClinicalClinical
ClinicalClinical
Pre-clinicalPre-clinical
SusceptibilitySusceptibility
HealthyHealthy
•Clinical•Sub-clinical
Outcome
Host susceptibility
Exposure
Agent source
Steps in the Disease Process
CONCEPT OF CAUSATION
The basis for most epidemiological investigations
To identify causal relationships and potential risk factors
A framework for identifying causes of infectious disease
KOCH’S POSTULATES
The agent has to be present in every ca se of the disease.
The agent has to be isolated and grown in pure culture.
The agent has to cause disease when in oculated into a susceptible animal and
the agent must then be able to be recov ered from that animal and identified.
CAUSATION OF DISEASE
The agent Is present when the disease exists Is absent when the disease does not exist The agent can be isolated in pure culture
and results in disease when it is given to exposed animals
Exposure Occurs before the disease occurs
CAUSATION OF DISEASE
Consistency The disease is reproducible in different populati
ons at different times Strength of statistical association
The results are not due to chance -Dose response
Increase in exposure leads to increase in disease
SOURCES OF INFECTIOUS DISEASE
Environment Live Animals / Dead animals Feed and Water Animal products -Animal by products Reservoir ( wild animals, insects) Fomites (clothing, equipment, vehicle
s) Vectors (insects)
EXPOSURE
• Initial introduction into the population• Transmission within the population• Direct transmission• Horizontal• Vertical
• Indirect transmission• Marketing systems
• Exposure dose of disease agent• Route of exposure• Animal density
HOST SUSCEPTIBILITY
Species, breed, strain Age Sex Genetics Animal management and husbandry
INFECTIOUS DISEASE
Three terms are used to describe an infectious disease according to the various outcomes that many occur after exposure to the causative agent and their population based definitions are given below Infectivity Pathogenicity Virulence
INFECTIVITY
the percentage (or proportion) of individuals exposed to a particular agent who become infected
No of infected following exposure
Total of population at exposureInfectivity =
PATHOGENICITY
the percentage of infected individuals who develop clinical disease due to the particular agent
No of clinically affected following exposure
Total of infected at exposure
Pathogenicity =
VIRULENCE
the percentage of individuals with clinical disease who become serious ill or die
No of severe (fatal) cases following exposure
Total of clinically infected cases at exposure
Virulence =
Mode of Transmission
• Direct Transmission– Direct contact– Droplet spread
• Indirect– Air borne– Vehicle borne (food water)– Vector borne (arthropods: ticks, mosquitoes)
• Influenza: droplet spread, vehicle borne• Salmonella: vehicle borne, direct contact• TB: air borne• Cutaneous Anthrax: direct contact• Pneunonic Plague: air borne
TYPE OF EPIDEMIOLOGY (STUDY DESIGN)
Descriptive epidemiology survey: time, place, person Case report, case series
Analytical epidemiology (risk factors) Cross-sectional Cohort Case-control
Experimental epidemiology Randomized control trial Clinical trial Community trial
Epidemiology study
Distribution
Risk factors
Analytic study
Descriptive study
DESCRIPTIVE EPIDEMIOLOGY
Epidemiology
Distribution
Risk factors
Time
Place
Person
Analytic study
Descriptive study
Etiology
DESCRIPTIVE EPIDEMIOLOGY
What (How much): occurred Who: animals or humans When: time Where: place
DESCRIPTIVE EPIDEMIOLOGY
Detection of individual case Detection of outbreaks Measuring the impact of disease Understand the nature of a disease Understand the way that disease
spreads and is distributed
DESCRIPTIVE EPIDEMIOLOGY
Generate hypotheses and ideas for further research
Evaluation of prevention and control measures
Support planning activities for animal health program
BASIC MEASURES AND TOOLS OF DESCRIPTIVE EPIDEMIOLO
GY
Data collection classification / organization summarizing presentation
INCIDENCE
the number of NEW cases that devel op over a certain time period.
INCIDENCE RATE
No. of new cases of a disease occurring in the population
during a specified period of time
No. of persons who are at risk of developing the disease
during that period of time
x100
PREVALENCE
the number of existing cases including old and new cases that have developed
at some point during a time period.
No. of cases of a disease present in the population
at a specified time
No. of persons in the population at that specified time
Prevalence rate
x100
INCIDENCE AND PREVALENCE
EXAMPLE
Question Type of measure
Do you currently have asthma? Point prevalenceHave you had asthma during the last 2
years?Period
prevalenceHave you ever had asthma? Cumulative
incidence
No of clinically ill
Population
Morbidity rate =
No of infectedPopulation
Infection rate =
No of deathsPopulation
Mortality rate =
No of deathsNo of clinically ill
Case fatality rate =
ANALYTICAL EPIDEMIOLOGY
How: adjust policy and response Why: prevent and control
CROSS-SECTIONAL
A random sample of individuals from a population is taken at a point in time
Surveys to collect data
CROSS-SECTIONAL
Advantages: quick to conduct and cost is moderate
compared with other study designs. Disadvantages:
cannot provide information on the incidence of disease in a population only an estimate of prevalence
Difficult to investigate cause and effect relationships
COHORT
Comparing disease incidence over time between groups
Prospective cohort Non-disease case Expose and non-expose
Retrospective cohort Disease case Evaluated for evidence of exposure to the
agent
Cohort
COHORT
Advantages: monitored over time for disease occurrence estimates of the absolute incidence of
disease in exposed and non-exposed Disadvantages:
long follow-up period case of rare diseases large groups are
necessary Losses to follow-up expensive
CASE-CONTROL
Comparing the frequency of past exposure between cases who develop the disease (or other outcome of interest) and controls chosen to reflect the frequency of exposure in the underlying population at risk
CASE-CONTROL
Advantages: an efficient method for studying rare
diseases subjects have experienced the outcome of
interest at the start of the study quick to run and cheaper than other study
Disadvantages: Can not provide information on the disease
incidence in a population Reliant on the quality of past records or
recollection of study participants Difficult to ensure an unbiased selection of
the control group
