IIDefinitions

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This section provides the definitions needed for outbreak confirmation and investigation.

A. Case definitions for measles surveillance and outbreak investigation

Clinical case definition of suspected measles

Any person in whom a clinician suspects measles

OR

Any person with:

  • fever, and
  • generalized maculopapular (i.e. non-vesicular) rash, and
  • cough, coryza (i.e. runny nose) or conjunctivitis (i.e. red eyes).

Laboratory criteria for diagnosis

  • Presence of measles-specific IgM antibodies is the recommended method for measles diagnosis1 (see Annex 1 for more details).

Case classification

  • Clinically confirmed measles: Any person meeting the clinical case definition of suspected measles and cannot be discarded.2
  • Laboratory-confirmed: A case that meets the clinical case definition and has laboratory-confirmation of measles virus infection.
  • Epidemiologically confirmed: A case that meets the clinical case definition and is linked epidemiologically3 to a laboratory-confirmed or another epidemiologically-confirmed case.
  • Discarded: A suspected case that has been subjected to serology or virus identification but has not met any of the laboratory criteria for diagnosis, or a suspected case which has been linked epidemiologically to a confirmed case of another communicable disease (e.g. rubella).

It is important to note that during an epidemic, most cases are clinically confirmed.

A measles-related death is a death in an individual with confirmed (clinically, laboratory, or epidemiologically) measles in which death occurs within 30 days of rash onset and is not due to other unrelated causes e.g. a trauma or chronic disease.

B. Definition of a measles outbreak

The definition of an outbreak will vary according to the phase of measles control. This guide recommends two different definitions for measles outbreaks depending on whether the country has conducted nationwide catch-up SIA’s or not. These definitions are intended to provide general guidance only, and an assessment of the situation should be done on a case-by-case basis to determine if an outbreak is suspected or confirmed.

1. For countries (or states/provinces of large countries) that have completed nationwide catch-up measles SIAs

A suspected outbreak of measles is defined as the occurrence of five or more reported suspected cases of measles in one month per 100 000 population living in a geographical area (e.g. district/block) (17).

A confirmed measles outbreak is defined as the occurrence of three or more confirmed measles cases (at least two of which should be laboratory-confirmed; IgM positive) in a health facility/district/block (approximate catchment population of 100 000) in a month.

2. For countries (or states/provinces of large countries) that have not yet completed nationwide catch-up SIAs

A suspected measles outbreak is defined as “an increase in the expected number of suspected measles cases being reported in a specific geographical area”.

An increase in the number of suspected measles cases means a substantial increase in incidence compared to non-epidemic years, or incidence similar to the incidence in an epidemic year. For example, Table 1 shows the attack rates for the urban area of Niamey in the Republic of the Niger during the first calendar week of 2002–2004. In this example, it appears that 2004 was an epidemic year. However, when available, data for several weeks of the year and over at least five years should be taken into account to determine if a measles outbreak is suspected.

Table 1. Attack rates per 100 000 population in Niamey, Niger, first calendar week 2001 – 2004.

Table 1

Attack rates per 100 000 population in Niamey, Niger, first calendar week 2001 – 2004.

A specific geographical area includes the catchment area of a health centre or a sector of a large city. In measles endemic regions, localized outbreaks occur within larger communities, with each local community experiencing outbreaks every 3–5 years. Hence, figures collected at regional and national level may not detect localized outbreaks.

If information from previous years is unavailable, the situation should be followed for the coming weeks. In this case, an ascending epidemic curve with an increasing number of cases over 3–4 weeks above the number seasonally expected should be considered as a suspected outbreak.

A confirmed measles outbreak is defined as the occurrence of three or more confirmed measles cases (at least two of which should be laboratory-confirmed; IgM positive) in a health facility/district/block (approximate catchment population of 100 000) in a month.

If laboratory confirmation is not possible, an outbreak may be documented through a sustained and progressive rise in clinically-confirmed cases over a three-week period. In addition, hospitalization of a proportion of the suspected cases due to measles-related complications (e.g. diarrhea or respiratory infections such as pneumonia or croup), can be used to distinguish measles from outbreaks of other infections with fever and rash, where rates of hospitalization and related complications are usually much lower.

Footnotes

1

Measles virus identification in a case that meets the clinical case definition of suspected measles is also a confirmation of a measles virus infection.

2

A suspected case in which laboratory investigations have equivocal results for measles-specific IgM antibodies after being tested twice, should be classified as a clinically confirmed measles case.

3

Epidemiological linkage: a case that meets the clinical case definition and has contact with a laboratory-confirmed measles case whose rash onset was within the preceding 21 days, or a case that lives in the same district or adjacent districts where a measles outbreak has been laboratory-confirmed and transmission is plausible.