Iceberg concept of disease occurrence

Methods to measure prevalence in a population

EPIDEMIOLOGIST

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Sub-clinical cases

Disease free group

Distribution of Heard

1

2

3

4

5

Diagnosed and controlledDiagnosed

but not controlled

Misdiagnosed casesPopula

tion at risk

Risk free group

•The marginal cost of subclinical disease per infectious unit is usually greater than the marginal cost of clinical disease per infectious unit.•This means that to prevent animal from non infectious units to a burden of one infectious unit is higher than the cost of increasing an already high infectious burden by one unit.

E.g.- Relationship between somatic cell count and decrease in milk production in case of mastitis

• Iceberg model depends upon

1. Agent-host-environment relationship2. Natural history of

disease(Progression of disease process in an individual over time)

VirulencePathogenisity

Resisatncesucceptibility

TemperatureMoisture etc.

Infectious diseases• Diseases with vast base- 1. Bovine paratuberculosis, Human tuberculosis,

Bovine lukemia virus, Mastitis (sub-clinical cases)

2. Jhones disease, prion disease, HIV, rabies (long ip)

3. Rickettsiosis (low specific and sensitive diagnostic tests)

• Diseases with visible tip-1. Tetanus, Anthrax, Clostridial infection (less sub-

clinical cases)2 Staphylococcal food poisoning, pasteurellosis,

influenza (short ip)

Non- infectious diseases• Mostly metabolic diseases• Diabetes mellitus, hypothyroidism,

hypertension etc(mostly in dog), milk fever (in cattle)

How can we detect sub-clinical cases

SCREENING……..

Definition• It is the search for unrecognized

disease/sub-clinical disease in a population by means of rapid test

• Early detection of disease(prescriptive screening)

• Whole population(mass screening)• Target population(strategic

screening)• Rapid and reliable test(high

specificity and sensitivity)• Economical in term of manpower and

materials• Precise and repeatable• Scientifically accurate

Mistakes made by epidemiologist

1. Giving importance to ill animals only

2. No clinical signs no disease3. Only a single agent is cause of

infection4. False interpretation of clinical

intervention5. Dependency on clinical records

Uses –• For detection of sub-clinical and in

apparent cases• Treatment of in apparent cases • Control of diseases with more

number of sub-clinical cases• To have a detailed knowledge

regarding natural history of diseases

Methods to measure prevalence in a population

http://healthpowerforminorities.com/

• Prevalence is defined as number of instances of disease or related attributes in a known population, at a designated time, without distinction between old and new cases.

• Probability of having infection

1. Point prevalence- Amount of disease in a population at a particular point of time

• Cross sectional/snap shot of the disease

Number of individuals having a disease at a particular point of time

Population Size• Study of chronic disease

2. Period prevalence- Specified period of time(beginning +

number of new cases during that period)• Period prevalence = Number of cases that occurred in a

given period(old + new)

Study Population Size• Longitudinal type study

• Life time prevalence- Number of individuals having diseases at least part of their life

How to determine Prevalence

• By doing survey• Survey may be of 2 types1. Screening Survey2. Questionnaire Survey

Screening Survey• Survey is done using screening• Rapid, simple, inexpensensive• Can be used to detect disease on a

large scale• Includes serological, biochemical,

haematological, physical examination • Helps in quantitative estimation of

prevalence

Questionnaire Survey• Contains a series of questions which

are answered by the respondent either in written format or through verbal communiucation

• Easy to perform, quick and less man power needed

• But not reliable many times

Reference • Concepts of Epidemiology: An integrated introduction

to the ideas, theories, principles and methods of epidemiology by Raj S. Bhopal

• http://www.vetmed.wsu.edu/courses-jmgay/EpiMod2.htm

• Veterinary Epidemiology by Michael Thrushfield• Veterinary medicine and human health By CW Schwabe• https://en.wikipedia.org/wiki/Prevalence• http://health.knowledgeblog.org/2011/07/22/basic-sta

tistics-for-epidemiology/• Principles of infectious disease epidemiology/

http://health.mo.gov/training/epi/Mod4StudentOutline.pdf

Thank You