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How to recognise a case of measles

Now that measles outbreaks are occurring again, Dr Eithne MacMahon gives an overview on how to detect the most severe of the common childhood diseases

Now that measles outbreaks are occurring again, Dr Eithne MacMahon gives an overview on how to detect the most severe of the common childhood diseases

Prior to the introduction of vaccination in 1968, acute measles killed around 100 children in England and Wales annually and until recent years accounted for a million deaths worldwide each year. Measles could be eliminated by vaccination but because it is so highly infectious, 95% of the population need to receive two doses in order to reduce the pool of susceptible contacts, and halt ongoing transmission.

This was achieved in the UK in the 1990s, but with the decline in mumps, measles and rubella (MMR) vaccination rates in the last decade, measles outbreaks are occurring again with one death reported so far.

Why not have single vaccines?
This suggestion is not backed by evidence. The MMR vaccine has been thoroughly investigated and shown to be very safe, with overwhelming evidence against any autism association. On the other hand the use of single vaccines is an experimental approach, not based on a coherent immunological theory and without evidence on safety and effectiveness. Single vaccines would also put children at increased risk by leaving them vulnerable for longer, and subject them to six injections instead of two. Considering past experience, uptake would undoubtedly be lower if six vaccines were required and risk more measles cases, congenital rubella and mumps-associated deafness.

At-risk groups
Unvaccinated children and adults, and those who have received just a single dose of vaccine, are susceptible. This includes adults born in the UK in the 1970s, people from European countries with sub-optimal coverage, and transient groups and some religious communities with limited access to healthcare. People born before 1970 are likely to have had natural infection and lifelong immunity.

The most vulnerable groups – infants and immunocompromised people – in whom measles is more likely to be severe or fatal, cannot be routinely vaccinated. They depend on high vaccine uptake to keep measles out of circulation. With the current gaps in the herd immunity, it is especially important to ensure that their close contacts are not susceptible.

How cases present
Measles is characterised by a generalised red maculopapular rash lasting three days or more with a fever of 38.3°C or higher, together with cough, coryza or conjunctivitis. The differential diagnosis includes rubella, parvovirus B19, enterovirus, scarlet fever, human herpesvirus 6, human herpesvirus 7, Kawasaki's disease, meningococcaemia, toxic shock syndrome, dengue, HIV, secondary syphilis and drug eruptions.

The key to diagnosing measles is to think of it in the first place – even if cases have not been reported locally. Key information includes age, demographics, vaccination history and exposure history. This is easier in patients who present with a rash, but if you find Koplik's spots you can diagnose measles even before the rash appears.

These bright red spots have a central white speck and occur on the buccal mucosa opposite the second molar teeth in around two-thirds of patients. They appear during the prodrome when patients are generally unwell with fever, malaise and loss of appetite.

The rash first appears behind the ears and on the hairline, spreading across the face and down the neck and trunk to the extremities. Discrete lesions may coalesce in places to appear angry, red and almost furry, like velvet. A runny nose and red eyes are important clues and the patient typically looks miserable. Lymph nodes and spleen may be palpable.

Mortality is highest in infants under 12 months. Complications are more frequent and severe in adults than in children. In uncomplicated cases recovery begins a couple of days after the rash appears.

Measles causes transient immunodeficiency in patients with otherwise normal immunity. Otitis media and laryngotracheobronchitis are common in children under two.

Pneumonia accounts for more than two-thirds of measles-associated deaths and may be due to measles or other viruses or bacteria. Diarrhoea may likewise be due to superinfection. Antibiotics should be considered for possible secondary infections. Hepatitis is not generally a feature of childhood measles but occurs in more than 50% of adults in developed countries.

Post-infectious encephalitis is a not uncommon complication with poor prognosis occurring in one in 1,000 healthy children. Sub-acute sclerosing panencephalitis presents insidiously several years after measles in infancy. In immunocompromised patients, the lack of a rash delays diagnosis of severe, often fatal complications including giant cell pneumonitis and inclusion body encephalitis. Measles in pregnancy can result in intrauterine death and premature delivery.

When you suspect measles
It is important to notify your local health protection unit straightaway. This facilitates appropriate management of contacts and triggers provision of an oral fluid sample collection kit to the patient, parent or GP.

The diagnosis is confirmed by detection of measles IgM in a sample collected between one and six weeks from the onset of the rash, or of measles RNA in earlier samples.

Infection control considerations
Measles is infectious from four days before to four days after the onset of the rash. Patients should not return to school or work for five days from onset of rash.

The virus remains infectious in air and on surfaces for up to two hours and transmission has been demonstrated between sequential patients in a doctor's surgery. It is important to consider who else may have been exposed, and whether post-exposure prophylaxis is needed.

Post-exposure prophylaxis with MMR vaccine
Measles has an average incubation period of 14 days (range six to 19 days) from exposure to onset of rash. Because the vaccine strain replicates and induces antibody protection faster than natural infection, MMR vaccine can be used for post-exposure prophylaxis to susceptible contacts if administered within 72 hours of exposure.

If the vaccination history is doubtful, MMR vaccination is advocated as there are no safety concerns about giving vaccine to immune people.

Vaccination is advisable even after 72 hours as MMR vaccine does not exacerbate the symptoms of natural infection, but will protect against any future exposure to all three infections, bearing in mind that a measles-like illness shortly after vaccination is likely to be just that.

Where the patient has already had one dose of MMR, a second dose may be given routinely at any time three months after the first or, if necessary, as soon as one month later if immediate protection is needed.

Post-exposure prophylaxis in infants
Routine MMR vaccination is not recommended until 12 to 15 months of age as the measles component does not reliably induce immunity in the presence of maternal measles antibody, achieving low rates of seroconversion in children aged less than 12 months.

However, if immediate protection is needed, MMR vaccine can be given from six months of age. Note that this first dose of MMR given before 12 months does not count towards the normal two-dose schedule. Also a second MMR dose under 18 months does not substitute for the pre-school dose.

Human normal immunoglobulin
Contacts who are immunocompromised or pregnant, and infants under nine months who are at particular risk, should be considered for prophylaxis with human normal immunoglobulin within five days of exposure.

Dr Eithne MacMahon is consultant virologist and honorary senior lecturer in infection and immunology, Guy's and St Thomas' NHS Foundation Trust, St Thomas' Hospital, London

Competing interests Dr MacMahon received sponsorship from Aventis Pasteur MSD towards conference attendance in 2002

Measles is infectious from four days before to fours days after the onset of the rash Measles is infectious from four days before to fours days after the onset of the rash

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