An image of the measles virus taken with an electron microscope.

An image of the measles virus taken with an electron microscope.

Sanofi Pasteur

What does measles actually do?

The United States is now experiencing what promises to be one of the worst outbreaks of measles since the virus was declared eliminated from the country in 2000.  It began in early January at Disneyland Resort in Anaheim, California, and has since spread to 14 states and infected 84 people, according to the U.S. Centers for Disease Control and Prevention (CDC). Measles, caused by a paramyxovirus from the genus Morbillivirus, is one of the most contagious diseases in the world, infecting more than 90% of susceptible hosts that come in contact with an afflicted individual. In the absence of widespread vaccination, the average person with measles will infect an average of 12 to 18 other people; in contrast, Ebola is typically transmitted to 1.5 to 2.5 people. Children, in particular, are more likely to experience complications as a result of a measles infection. Although the overall mortality rate for children who get measles is only between 0.1% and 0.2%, as many as one out of every 20 children will also develop pneumonia. The disease symptoms can be managed with common anti-inflammatory drugs, hydration, and rest, but like many other viral illnesses, there is no cure and antibiotics will have no effect. Death rates are much higher in developing countries.

What does measles do to the immune system?

Measles virus is spread from person to person through the air in coughed-out aerosolized droplets that are inhaled. The virus typically first comes in contact with host lung tissue, where it infects immune cells called macrophages and dendritic cells, which serve as an early defense and warning system. From there, the infected cells migrate to the lymph nodes where they transfer the virus to B and T cells. A surface protein on these white blood cells, known as CD150, serves as the virus’s point of entry during this critical step. The infected B and T cells then migrate throughout the body releasing virus particles into the blood. The spleen, lymph nodes, liver, thymus, skin, and lungs are eventual destinations for the virus. In rare instances (about one in 1000 cases), the virus can cross the blood-brain barrier and cause dangerous swelling of the brain; infection of lung cells causes a hacking cough that keeps the virus circulating in the population.

Why does measles cause a rash?

One of the classic presentations of a measles infection is a rash characterized by flat red blotches that starts on the face and moves down the body all the way to the feet over a few days. The rash is a symptom of inflammation occurring in the skin. As the virus travels in the blood, it infects capillaries in the skin. Immune cells detect the infection and respond by releasing chemicals such as nitric oxide and histamines, which destroy the viral invaders and call other immune cells into action. These same chemicals, however, cause swelling and damage to host cells, resulting in the often itchy skin rash, which usually occurs concurrently with a fever that can reach as high as 40°C. 

Why is measles back?

Measles has actually been back. Last year was one of the worst years in recent history for the United States: CDC reported 644 cases from 23 separate outbreaks during 2014; between 2001 and 2013, no single year saw more than 250 cases. (Because measles was declared “eliminated” in the United States in 2000, outbreaks have been triggered by virus “imported” from other countries, which then finds an unvaccinated person.) With 84 people infected already in 2015, things are not off to the best start.

Number of measles infections each year from 2001 through 2014.

Number of measles infections each year from 2001 through 2014.

CDC

Part of the reason for the resurgence is a rise in the number of parents who refuse to vaccinate their children against the virus. Despite an overwhelming amount of scientific and medical evidence demonstrating both the safety and efficacy of the measles vaccine, some parents refuse to vaccinate their children for reasons of personal belief. The problem is compounded because many like-minded parents are geographically clustered. For the entire state of California, the vaccine refusal rate is only 2.6%, indicating that most citizens are cognizant of the benefits of immunization. However, a recent study of pediatric health records revealed that in some areas the vaccine refusal rate is as high as 13.5%. Other factors such as a lack of time, education, or money can cause parents to inadvertently fall behind on their children’s immunizations.

Disneyland provided an ideal starting point for an epidemic. In such a crowded atmosphere, a single infected individual might come in contact with hundreds or thousands of people in a day. Even if 90% of people are immunized, the virus’s high infectivity will allow it to spread through the population by jumping between susceptible individuals.

How can I protect my children from getting measles?

Get them vaccinated.