When is an Epidemic an Epidemic?*

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Pe r s p e c t i v e

When is an Epidemic an Epidemic?*

Manfred S. Green MD PhD

1,2

, Tiberio Swartz MD MPH

1,2

, Elana Mayshar JD

, Boaz Lev MD

Alex Leventhal MD MPH

3,4

, Paul E. Slater MD MPH

and Joshua Shemer MD

2,3

Israel Center for Disease Control, Ministry of Health, Tel Hashomer, Israel

Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv, Israel

Israel Ministry of Health

Hebrew University-Hadassah Medical School, Jerusalem, Israel

Key words:

epidemic, outbreak, risk, infectious diseases, West Nile fever

Abstract

Background:

The large number of cases of West Nile

fever diagnosed in Israel in 2000 once again brought into focus

the confusion that frequently accompanies the use of the term

``epidemic.''

Objectives:

To examine the different definitions of the

term ``epidemic'' and to propose ways in which it can be used

to both improve communication among professionals and

provide the public with a better sense of the associated risks.

Methods:

The literature was reviewed for the various

definitions of the terms ``epidemic'' and ``outbreak.'' Sources

included popular and medical dictionaries, ancient documents,

epidemiology texts, legal texts, and the medical literature.

Results:

The term epidemic is variously defined. The

broad definition given by epidemiologists ± namely, more

disease than is anticipated by previous experience ± is less

meaningful to the general public. In some ways it conflicts with

the definitions found in the popular literature, which generally

imply danger to the public and a very large number of victims.

Conclusions:

The interpretation of the term epidemic may

vary according to the context in which it is used. For risk

communication, we suggest that every effort be made to add

descriptive terms that characterize the epidemic.

IMAJ 2002;4:3-6

The occurrence of more than 400 laboratory-confirmed cases of

West Nile fever in Israel in 2000 once again raised the question

as to whether and when an increase in the incidence of a

disease should be described officially as an epidemic. Inter-

pretation of the term ``epidemic'' could depend on the context in

which it is used. Epidemiologists use it in its most general form

and define an epidemic as follows: ``An epidemic is the

occurrence in a community or region of cases of an illness,

specified health behavior, or other health-related events clearly

in excess of normal expectancy; the community or region, and

the time period in which cases occur, are specified precisely'' [1].

The definition does not specify a minimum number of cases.

The area covered by an epidemic may be limited to a small area

such as a school classroom, or it may extend to include many

countries. Epidemics may also last from a few hours to many

years.

The term ``outbreak'' is considered by epidemiologists to be a

synonym for ``epidemic.'' In the professional literature these

terms are used interchangeably, and sometimes even together.

There are scientific articles in which the title refers to an

epidemic, while in the body of the paper the episode is called

an outbreak. Occasionally, the term ``epidemic outbreak'' is

used, such as in ``an epidemic outbreak of hepatitis A among

homosexual men'' [2]. Associated terms include ``endemic,''

which describes a situation where cases of the disease are

constantly occurring in the population; ``hyperendemic,'' where

there is a constantly high incidence of disease in the

population; and ``pandemic,'' where the increased incidence of

the disease is of global proportions. It should be stressed that

in addition to infectious diseases, contemporary use of the term

``epidemic'' can be used to describe a relative excess of cases

under a wide variety of conditions. These include non-

communicable diseases such as breast cancer, physical condi-

tions such as obesity, and behavioral conditions such as

smoking and alcohol consumption. The term ``epidemic curve''

refers to a graph showing the cases of a disease by time of

occurrence and demonstrates the trends in the incidence of

disease with time. It is a basic tool for identifying the emergence

of epidemics of both infectious and non-infectious diseases.

For infectious disease epidemics, sub-categories have been

defined according to the mode of transmission of the disease. In

a common-source epidemic, such as that originating from

contaminated food, there is a sharp increase in the number of

cases, and there may be a secondary wave caused by

The opinions expressed in this article are those of the authors alone and do

not purport to be those of the Ministry of Health.

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subsequent person-to-person spread. A purely person-to-

person spread epidemic is typified classically by influenza,

where droplet spread results in an initial slow accumulation of

cases followed by a more rapid increase in incidence. Epidemics

can also result from contact with infected vectors. West Nile

fever falls into this category, where the virus is transmitted by

infected mosquitoes to reservoir hosts such as crows or geese

that in turn infect other mosquitoes, which may transmit the

disease to ``incidental'' hosts such as humans and horses.

Risk communication

Other than for the simple monitoring of disease activity, the

term ``epidemic'' is used to communicate risk [3,4]. The origin of

the word ``epidemic'' comes from the Greek epi meaning ``upon''

and demos meaning ``people,'' which together implies ``that

which falls upon populations'' [1]. The general population may

perceive the term ``epidemic'' somewhat differently from that

intended by epidemiologists. The Internet site dictionary.com

defines an epidemic as something that spreads rapidly and

extensively by infection and affects many individuals in an area

or population at the same time. The Oxford Dictionary [5] gives

a non-technical definition of an epidemic as a widespread

occurrence of a disease in a community at a particular time. In

the Encyclopedia Britannica [6], an epidemic is described as an

occurrence of disease that is temporarily of high prevalence,

and the Encarta Dictionary [7] describes it as an outbreak of a

disease that spreads more quickly and more extensively among

a group of people than would normally be expected.

The Hebrew translation of epidemic is magefa. The earliest

biblical reference to this term appears in the book of Numbers

(B'midbar), which recounts that during the wandering of the

Jews in the Sinai desert, 14,700 people died from a ``magefa'' [8].

The English translation of ``magefa'' describes a ``plague'' or

``pestilence,'' connoting a dangerous and contagious disease [8].

The Mishna (the commentary on Jewish law), completed 3,000

years ago, provides a definition surprisingly similar to that of

the epidemiologists: ``What is considered to be a pestilence? [A

situation where] in a city that can provide 500 foot-soldiers,

three persons die over a three day period'' [9]. One of the

modern translations of ``magefa'' is ``a severe infectious disease''

[10], quite inconsistent with the contemporary professional

epidemiologist's definition of an epidemic.

The variety of definitions of ``epidemic'' highlights the

problems associated with risk communication and the need

for consistency in terminology. This is particularly important,

since epidemics tend to be emotionally charged events. An

analogy can be drawn with clinical medicine, where there is

need to describe the condition of a patient to his or her family

and sometimes the media. Usually terms such as mild,

moderate, severe or critical are used. Such a classification has

limited medical value but is a simplified way of conveying to the

lay public the level of risk of an adverse outcome for that

patient. Thus, when an increased incidence of disease is

described as an ``outbreak,'' the general public may view this

as being less serious than if the term ``epidemic'' is used, even

though the epidemiologists may contend that the words are

synonymous. In fact, Chambers Concise Dictionary [11] defines

an epidemic as a ``a widespread outbreak,'' and even Stedman's

Medical Dictionary [12] defines an ``outbreak'' as a ``localized

epidemic.''

Administrative aspects

In contrast to the epidemiologists' very general definition of an

``epidemic,'' the term has been defined quantitatively for certain

diseases. This is of importance to legislators and adminis-

trators. For example, in January 2000, influenza activity in Britain

was discussed in the House of Lords. The opposition asked for

the officially accepted definition of an epidemic. Lord Hunt of

Kings Heath replied that [while] the epidemiologic definition of

an epidemic is an increase in the occurrence of a disease above

its baseline level, administrative definitions can be set for

different diseases in which an arbitrary threshold is selected

above which the term ``epidemic'' is applied'' [13].

In fact, in 1996, the UK Department of Health introduced an

administrative definition of an epidemic of influenza. An

epidemic is declared if the rate of consultations for flu-like

symptoms in a sample of reporting by general practitioners

exceeds 400 per 100,000 population in one week [14]. In the

United States, an epidemic of influenza is confirmed when

death rates from pneumonia and influenza exceed a threshold

determined by the Centers for Disease Control and Prevention.

This threshold is based on data from the vital statistics offices of

121 cities for death certificates in which pneumonia was stated

as the underlying cause of death, or influenza was listed

anywhere on the certificate. The proportion of deaths due to

influenza or pneumonia was computed and a regression model

was used to produce a seasonal baseline. An increase in deaths

of 1.645 standard deviations above the seasonal baseline (5% of

all) is defined as the epidemic threshold [15].

In some countries where meningococcal meningitis is

hyperendemic, it is necessary to rapidly institute special

preventive measures (such as mass vaccination) in the event

of an epidemic. Moore et al. [16] used meningitis rates from

active surveillance data in Burkina Faso for detecting meningitis

epidemics. They estimated that a threshold of 15 cases per

100,000 per week in a specified study region averaged over two

weeks was sufficiently sensitive and specific in detecting

epidemics exceeding 100 cases per 100,000 per year. This was

suggested as the ``epidemic threshold.''

Legal aspects

Under what circumstances should the health authorities

formally declare an epidemic? Firstly it should be clear that

the use of the term ``epidemic'' by health authorities is primarily

intended in the professional sense. It implies that an unusual

event has occurred and is usually accompanied by a description

of the measures that will be taken to control the epidemic.

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M.S. Green et al.

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These include the defined cooperation between different local

and national authorities and allocation of special resources

available for disease control. It is important to distinguish

between the situation whereby the health authorities simply

describe an increased incidence of disease as an epidemic and

the much rarer event where a disease is declared to be

particularly dangerous to the health of the population. In the

latter situation, the purpose of the declaration is to enable the

health authorities and others to take extraordinary measures

such as house-to-house visits and quarantine.

This is delineated in a note to the British Public Health Act of

1936, on which the Israeli Public Health Ordinance 1940 is

based. Here, epidemic diseases are defined as ``those which

prevail among a large portion of a country, rage for a certain

time, and then gradually diminish and disappear, to return

again at periods more or less remote.'' It is further stated: ``It is

essential to the medical notion of all epidemic diseases, that it

should be dependent on some common and widely extended

cause, of a temporary in contradistinction to a persistent

nature'' [17]. In Palestine under the British mandate, Public

Health Ordinance No. 40 of 1940, clause 20, deals with

``emergency powers.'' It states that ``when any part of Palestine

appears to be threatened with or is affected by a formidable

epidemic, endemic or infectious disease, that is to say, plague,

cholera, yellow fever, smallpox, typhus fever, or other disease

declared by the High Commissioner by notice in the Gazette to

be a formidable infectious disease, the High Commissioner may

by notice as aforesaid declare that the health of the public is

seriously threatened by the threat or existence of such disease

in Palestine or in neighbouring territories.'' This ordinance

remains in effect in Israel, with the Minister of Health

authorized, in place of the High Commissioner, to make the

declaration necessary for the implementation of extraordinary

measures. The Minister of Health must publish an official

declaration regarding the existence of a serious threat to the

public health, without necessarily using the term ``epidemic.''

Definition of an epidemic when a baseline

is lacking

There are situations where the data are inadequate to

determine a baseline or reference by which to decide whether

or not there has been an exceptional increase in incidence.

Clearly, for a truly ``new'' disease, any number of cases may

signify an epidemic. For example, after the first few cases of

AIDS were diagnosed, it would have made sense to label that an

epidemic. A somewhat different situation exists when new

diagnostic methods become available. In this case, there is an

apparent increase in the incidence of the disease due to the

increased ability to diagnose it. This question was raised

regarding West Nile fever in Israel, where only recently was the

laboratory test for serology introduced for routine diagnosis. In

such a situation, various surrogate indicators are used to

identify the occurrence of an epidemic. For example, for West

Nile fever, the incidence of encephalitis was compared with that

of previous years. In any event, where a historical baseline is

lacking, a clear temporal increase in the incidence of the disease

should be sufficient to declare an ``epidemic'' in order to recruit

the resources necessary for controlling the event.

Conclusion

The word ``epidemic'' is an emotionally charged term. It means

different things to different people, and professionals using the

term may have an intended meaning quite different from the

public's perception of the word. It may also imply different

things when translated into other languages. We believe there is

a need to define a more understandable official lexicon for

``epidemics,'' particularly when attempting to communicate risk

to the population. While recognizing that epidemiologists will

continue to use the words ``outbreak'' and ``epidemic'' inter-

changeably, for risk communication the term ``outbreak'' should

be used according to its standard general dictionary definition

as being a more limited type of epidemic.

Furthermore, descriptive words should be used to better

define the epidemic. The nature of the spread, number of cases,

case-fatality, and area affected should be included. Thus, the

disease should be described as contagious or non-contagious,

with mild, moderate or severe clinical manifestations. The size

of the epidemic could be described as small, medium or large,

depending on the percentage of the population affected. Finally,

the distribution of the epidemic should be classified as

widespread or localized. As an example, the West Nile fever

epidemic in Israel in 2000 could be described as a medium-

sized, relatively widespread epidemic of a non-contagious

disease with generally mild to moderate clinical manifestations

and a significant death rate only in very elderly people. While

this approach will require us to expand our terminology in order

to be more specific, it should make risk communication much

more effective.

References

1. Last JM, ed. A Dictionary of Epidemiology. New York: Oxford

University Press, 1995.

2. Christenson B, Brostrom C, Bottiger M, Hermanson J, Weiland O,

Ryd G, Berg JV, Sjoblom R. An epidemic outbreak of hepatitis A

among homosexual men in Stockholm. Hepatitis A, a special hazard

for the male homosexual subpopulation in Sweden. Am J Epidemiol

1982;116:599±607.

3. Glanz K, Yang H. Communicating about risk of infectious diseases.

JAMA 1996;275:253±6.

4. Griffin RJ, Dunwoody S, Zabala F. Public reliance on risk commu-

nication channels in the wake of a cryptosporidium outbreak.

Risk Anal 1998;18:367±75.

5. The New Shorter Oxford Dictionary. 4th edn. New York: Oxford

University Press, 1993.

6. Encyclopedia Brittanica, New York.

7. Encarta World English Dictionary. Soukhanov A, ed. New York: St

Martins Press, 1999.

8. Numbers 17:11-15. The Holy Scriptures. Jerusalem: Koren Publish-

ers, 1984:181.

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When is an Epidemic an Epidemic?

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9. Taanith 3:4. In: Blackman P, ed. Mishnayoth, Volume 2. New York:

Judaica Press, 1963:423.

10. Even Shoshan English-Hebrew Dictionary. Jerusalem: Kiryat-Sefer,

1997.

11. The Chambers Dictionary. Schwarz CE, ed. New York: W & R

Chambers Ltd, 1998.

12. Stedman's Medical Dictionary, 26th edn. Baltimore: Williams &

Wilkins, 1995.

13. Lords Hansard Text for 24 Jan 2000. London: HM Government

Printers, 2000.

14. Fleming DM. Weekly returns service of the Royal College of General

Practitioners. Commun Dis Public Health 1999;2:96±100.

15. Brammer TL, Izurieta HS, Fukuda K, Schmeltz LM, Regnery HL, Hall

HE, Cox NJ. Surveillance for influenza ± United States, 1994-95,

1995-96, and 1996-7 seasons. Morb Mortal Wkly Rep CDC Surveill

Summ 2000;49:13±28.

16. Moore PS, Plikaytis BD, Bolan GA, Oxtoby MJ, Yada A, Zoubga A,

Reingold AL, Broome CV. Detection of meningitis epidemics in

Africa: a population-based analysis. Int J Epidemiol 1992;21:155±62.

17. Lumley's The Public Health Acts Annotated. London: Butterworth,

1952.

Correspondence:

Dr. M.S. Green, Israel Center for Disease Control,

Gertner Institute, Sheba Medical Center, Tel-Hashomer 52621, Israel.

Phone: (972-3) 534-9595

Fax: (972-3) 534-9881

email m.green@trendline.co.il

C a p s u l e

Screening reluctant genes on the double

Genome sequencing projects have revealed thousands of

genes of unknown functions. For the budding yeast Sacchar-

omyces cerevisiae, large-scale gene deletion analysis has shown

that >80% of the approximately 6,200 predicted or known

yeast genes are not required for viability. Thus, many genes

and pathways of eukaryotic cells may be functionally

redundant, or may not show easily recognizable phenotypes

if perturbed. To address this problem, Tong et al. developed

an automated method for systematic construction of double

mutants called synthetic genetic array (SGA) analysis. A yeast

strain that carries a mutation in the "query" gene is linked to a

selectable marker and crossed to members of a collection of

haploid deletion strains in which almost every non-essential

gene in the yeast genome is represented. If a double mutant

cannot be produced or grows much more slowly than normal,

it is an indication that there may be a functional interaction

between the two genes. Putative interactions that are

identified through this technology can then be readily

confirmed by tetrad analysis. Eight query genes involved in

cytoskeletal organization, DNA repair, or unknown functions

were analyzed, resulting in the construction of a network

identifying 291 putative genetic interactions involving 204

genes.

Science 2001;294:2364

Dr.

Ruth Lang

passed away in November 2001 after a long illness. Dr. Lang,

a member of our Editorial Board, headed both the Unit of Infectious Diseases and

The Travelers' Clinic at Meir Hospital in Kfar Saba. The editors and Editorial Board

extend their condolences to Dr. Lang's family.

M.S. Green et al.

IMA

Vol 4

January 2002

Perspective