Epidemiologyof disasters

David AlexanderUniversity College London

Botulism

Diphteria Salmonella

Cholera

Typhoid

The risk of increased transmission ofdisease after a disaster comes from:-

• the disaster itself (e.g. fecalcontamination of potable water)

• disruption of normal programmesof disease control and prevention

• overcrowding and badhygiene in survivors' camps.

DisasterPerson

to personIn water In food By vector

Cold wave Low Low Low Low

Earthquake Medium Medium Medium Low

Famine High Medium Medium Medium

Fire Low Low Low Low

Flood Medium High Medium High

Heat wave Low Low Low Low

Hurricane Medium High Medium High

Tornado Low Low Low Low

Volcanic eruption Medium Medium Medium Low

Theoretical risk of communicable disease

The main risks comefrom endemic diseases.

Bad response to disease risks:-

• mass vaccination of survivors

• indiscriminate burial or cremation

• sanitary cordons aroundthe affected area

• indiscriminate massdisinfection or disinfestation.

Wenchuan, Sichuan, China, May 2008

Good responses to the problem:

• epidemiological surveillance(but this will increasethe diagnosis rate)

• routine prophyllaxisof health workers.

The values of mortality and morbidity(i.e. dead and injured or infected)

are expressed as:( numerator / denominator )

This means the frequency of ameasured or observed state or event,divided by the total number of people

who are exposed to that stateor event (the population at risk).

A static measure – prevalence rate:the proportion of a given group ofpeople who have a given condition

at a single moment in time.

Period prevalence rate: when aparticular period of time is needed

to count or register all thepeople who have the given condition.

A dynamic measure – incidence rate:the proportion of a group of

people who develop a given conditionover a specified time period.

Non-standardised incidence rate:without reference to population size.

Standardised incidence rate:raw value corrected by• size of the population• (e.g., number of deathsper 10,000 people)

• age-group (e.g.0-4 = infants, 4-15 = children,16+ = adults).

The population is defined as all thepeople who could possibly catch the

disease or have the condition in question.

Outbreak: various cases

Epidemic: many cases

Pandemic: a large, internationalepidemic

• there are no quantitativedefinitions of these terms.

Epidemiological surveillanceshould make use of:-

• existing standardised statistical protocols

• unofficial information from thecommunity (it needs to be verified)

• reports from field workersand their organisations.

In normal times, surveillanceconcentrates on diseases that are:-

• locally endemic

• capable of being controlled

• of public health importance

• monitored under WHOdisease control programmes.

New, post-disaster surveillanceshould be more focussed on

symptoms and conditions that are:-

• directly attributable to the disaster

• capable of being controlled.

The aim of epidemiological surveillance is:-

• to collect data on the risksand incidence of particulardiseases and medical conditions

• to prevent epidemics and restrictthe progress of given pathologies.

The specific objectives ofepidemiological surveillance

• technical: timely identificationto facilitate rapid response

• social: stop rumours, give thepublic a sense of security

• operative: avoid inefficientmeasures of disease prevention.

The surveillance should monitor:-

• diseases that occurduring normal times

• diseases that may be transmittedas a result of the disaster

• rarer diseases that aremonitored under WHO protocols.

Methods of post-disaster surveillance

• open an epidemiologicalobservatory in the disaster area

• receive information and data everyday by phone, fax, email, sitrep, etc.

• create a system of rapidinvestigation of any apparentanomalies in disease transmission.

Data to be recorded

• bacteriologically confirmedcases of disease

• suspected clinical syndrome(i.e. symptoms):- diarrhoea, cough, dermatitis, etc.- diarrhoea with blood, mucus, etc.- fever with diarrhoea, etc.

Disease Incubation

period (days)

Period of communicability

Baccillary dysentery 1-7 ≤28 days

Blenorrhoea 5-12 10 months

Botulism 0.5-1.5 --

Brucellosis 5-21 --

Cholera 0.5-5 about 7 days

Dengue 8-11 --

Diphteria 2-5 ≤28 days

Infectious parotitis 12-26 ≤9 days

Leptospirosis 4-19 --

Meningococcal

meningitis

2-10 rapid

Disease Incubation

period (days)

Period of communicability

Poliomielitis 3-21 ≤42 days

Scarlattina 1-3 10-21 days

Tetanus 4-21 --

Tuberculosis 28-84 some weeks

Typhoid 7-21 variable

Varicella 14-21 ≤27 days

Hepatitis A 15-50 30-50 days

Hepatitis B 45-160 100-160 days

Pertosse 7-21 ≤21 days

Cases of typhoid identifiedtwo days after a flood(a) are an effect of the flood.(b) are not an effect of the flood.

Natural disasters(a) often end with large epidemics

of communicable disease.(b) rarely end with large

epidemics of communicable disease.

When various cases of a communicabledisease are reported in an area that hasrecently been affected by a disaster:(a) the disease has probably been

brought into the area by rescuers.(b) the disease is probably

endemic to the area.

The incidence of certain communicablediseases is internationally notifiable:(a) because people who go into the

disaster area may be disease carriers.(b) because these diseases are

part of international monitoring and control programmes.

After a disaster, mass vaccination:(a) is the only acceptabe response

to the increased risk ofcommunicable disease transmission.

(b) is a waste of time, money and vaccine.

'Morbidity' refers to:(a) the rate of injury or disease.(b) the tendency of survivors to be

clinically depressed, in some casesas a result of injuries received.

Disaster epidemiologists(a) use mass prophyllaxis to try to stop

the progress of communicable diseases.(b) try to stop the progress of

communicable diseases by investigatingthe social and environmental conditionsthat give rise to those diseases.

Epidemiological monitoringafter disasters should include:(a) probable clinical syndromes,

but not apparent symptoms.(b) probable clinical syndromes

and apparent symptoms.

After a disaster:(a) children should be vaccinated

against selected diseases.(b) children absolutely should not be

vaccinated against any diseases.

When vaccines against typhoidand cholera are properly used(a) are perfectly effective.(b) are not perfectly effective.

The incidence rate of a disease is(a) a static measure, while the

prevalence is a dynamic measureof the progress of the disease.

(b) a dynamic measure, while theprevalence rate is a static measureof the progress of the disease.

Disaster epidemiologists:(a) investigate rumours about

the progress of diseases.(b) usually ignore rumours about

the progress of diseases.

[X]

In an area affected by a disaster, therate of diagnosis of diseases and conditions(a) will probably go up during

the emergency phase.(b) will probably go down

during the emergency phase.