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Measles--coming soon to a playground near you? A measles outbreak in Christchurch serves as a reminder of the dangers of this serious viral illness.

The February outbreak of measles in Canterbury is a reminder we should not be complacent about "childhood" diseases and vaccination programmes. In developed nations, about one in 1000 people die from measles. This rate is much higher in developing nations, following disasters, and for the very young or immunocompromised who cannot be vaccinated.

Measles is transmitted via droplet infection into the respiratory tract, and incubates for an average of 14 days. In the prodromal (early) stage, the virus triggers a range of symptoms as the immune system responds to the infection: malaise, anorexia, swollen glands, fever, headache and aching muscles. Specific to the measles in this early stage is a cough, coryza (runny nose and sneezing), and conjunctivitis. Koplik's spots may also appear inside the mouth, on the mucosa of the lips and cheeks. (1) One to two days later, a maculopapular skin rash begins behind the ears and spreads from the face to the trunk and limbs.

The measles virus enters body cells via two key receptors. The first is attached to lymphocytes and other immune cells, including those in the respiratory tract. The second is found in epithelial cells and keratinocytes-immune responses here cause the measles rash. (1) Immuno compromised people with measles may not develop a rash.

Measles is highly contagious. New virus particles shed directly into the mucus of the respiratory tract to be coughed onto the next victim or into the environment where they survive for about one hour. Very little of the virus is needed for transmission. (1) The average number of infections a person spreads while they are infected with a virus, in a susceptible population, is the basic reproductive ratio (R0). A virus with a high R0 is very difficult to contain. Measles has an R0 of 12 to 18-each person infected will, on average, pass that on to 12-18 others (although there have been individual reports of "super-spreaders" infecting up to 200 people). (1)

The virus initially infects immune cells in the lungs, which are then taken up by the lymphatic system. It undergoes massive replication in local lymphatic tissue and enters the blood to spread through the body. (2) Because the virus targets immune cells, numbers of B- and T-lymphocytes fall rapidly. This increases susceptibility to secondary infections, even two years after the attack, and increases childhood mortality rates from other causes following a measles outbreak. (3)

Common measles complications include ear infection and diarrhoea. Pneumonia causes two thirds of measles-related deaths, especially where hospital care is not available. In New Zealand, about one in 15 people with measles require hospitalisation. (4) Measles during pregnancy increases the risk of miscarriage, prematurity and maternal death. In one in 1000 measles cases, the virus crosses into the central nervous system, causing acute disseminated encephalomyelitis-autoimmune destruction of myelin produces permanent motor, sensory and intellectual changes and sometimes death. (1)

In infants and immunocompromised patients, measles can also cause measles inclusion body encephalitis up to a year after infection, with focal seizures, visual or hearing loss and mental status changes. The condition is rapid and usually fatal.j Rarely, measles develops into a latent infection that, after some years, causes subacute sclerosing panencephalitis. This is always fatal, with slow degenerating nervous function leading to death within 12 to 36 months. (1)

References

(1) Laksono, B., de Vries, R. D., McQuaid, S., Duprex, W. P., & de Swart, R. L. (2016). Measles virus host invasion and pathogenesis. Viruses, 8(8), 210.

(2) Lemon, K., de Vries, R. D., Mesman, A. W., McQuaid, S., van Amerongen, G., Yuksel, S., Ludlow, M.,... de Swart, R. L. (2011). Early target cells of measles virus after aerosol infection of non-human primates. PLoS Pathogens. Retrieved from https://journals.plos.org/plospathogens/ article?id=10.1371/journal.ppat.1001263

(3) Laksono, B., de Vries, R. D., Verburgh, R. J., Visser, E. G., de Jong, A., Fraaij, P., Ruijs, W.,... de Swart, R. L. (2018). Studies into the mechanism of measles-associated immune suppression during a measles outbreak in the Netherlands. Nature Communications, 9, 4944.

(4) Immunsation Advisory Centre. (2017). Measles. Retrieved from www.immune.org.nz/diseases/measles

Caption: Measles is highly contagious and increases susceptibility to secondary infection.
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Title Annotation:science shorts
Author:Casey, Georgina
Publication:Kai Tiaki: Nursing New Zealand
Article Type:Report
Geographic Code:8NEWZ
Date:Apr 1, 2019
Words:696
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