Measles Case Definition

Measles Basics

Child with measles
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With measles cases rising, one unintended consequence is that some kids get caught up in the measles quarantines or worries about measles outbreaks just because they have a fever and a rash and have a possible or suspected case of measles.

In addition to understanding classic measles symptoms, it is important to know how the CDC defines a case of measles to help make sure kids with measles are quickly identified and quarantined.

Measles Case Definition

A clinical case definition of measles is described as a child or adult with:

  1. a generalized rash that lasts three or more days
  2. a fever at or above 101°F (38.3°C)
  3. other classic measles symptoms, including cough, coryza (runny nose), or conjunctivitis (red eyes)

What about vaccination status?

You would expect that a diagnosis of measles would be more highly suspected in a person who is unvaccinated or only partially vaccinated. Surprisingly, vaccination status is not part of the case definition for measles.

Laboratory Confirmed Measles Case

Unlike many other infections that can be simply diagnosed based on their clinical symptoms, such as chicken pox, roseola, and fifth disease, etc., because a case of measles means extensive work at outbreak control, it is very important that the measles diagnosis be accurate and confirmed.

According to the CDC, a measles case can be confirmed by one or more of the following laboratory criteria:

  • a positive serologic test for measles immunoglobulin M antibody (IgM)
  • a significant increase (usually at least a fourfold increase) in measles antibody levels (IgG), but that means you have to do the test twice to see the increase in antibody levels
  • isolation of the measles virus from nose or throat swabs, nasal aspirates, throat washes, or urine, typically by PCR testing or viral culture, by a state public health laboratory or the CDC

    Testing for measles IgM and IgG together is most commonly recommended.

    Keep in mind that 30% of measles IgM tests can give a false negative result if they are done within the first three days of the onset of the rash, so a repeat test should be done if measles is still suspected.

    Measles IgM and IgG levels can also be affected by a recent measles vaccination, so be sure to remind your pediatrician if your child was recently vaccinated and the doctors suspects your child has measles.

    A child can have a confirmed measles case without testing if he meets the measles case definition and his case is linked to another confirmed case.

    Is It Measles?

    In addition to confirmed and laboratory confirmed cases, other terms used to describe measles cases include:

    • suspected measles case - any illness with a fever and a rash
    • probable measles case - a measles case that meets the measles case definition, has lab testing that was noncontributory or wasn't done, and is not linked to another confirmed case
    • internationally imported case - a measles case that is linked to exposure to someone who was outside the United States during at least part of the time that they would have been exposed to measles and their measles rash started within 21 days of entering the United States
    • import-linked cases - are linked to an internationally imported case
    • endemic case - a measles case that can't be linked to international travel and for which there is evidence of an endemic chain of transmission or a chain of continuous measles virus transmission for 12 or more months within the United States

    Measles was declared eliminated in the United States in 2000. However, it was just the endemic spread of measles that was eliminated. Until measles is eliminated worldwide, or measles vaccination rates get higher, we will continue to have internationally imported cases and measles outbreaks, like we have been seeing in recent years.

    Sources

    Centers for Disease Control and Prevention. Manual for the surveillance of vaccine-preventable diseases. Centers for Disease Control and Prevention, Atlanta, GA, 2008.

    National Notifiable Diseases Surveillance System (NNDSS). CSTE Position Statement. Measles / Rubeola 2013 Case Definition.

    WHO-recommended standards for surveillance of selected vaccine-preventable diseases. WHO-recommended surveillance standard of measles. Accessed February 2016.

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