With outbreaks of measles occurring across the country, the largest in Texas with 279 cases including 36 hospitalizations and one death as of March 18, 2025, measles is on everyone’s minds. The first article in this series described clinical suspicion of measles and how to make a measles diagnosis. In the second article, we discussed who should receive a measles vaccine, both in the routine vaccination schedule and during outbreak settings. In this last installment, we describe how measles virus impacts the body and complications that may arise from a measles infection.
Measles virus is transmitted person-to-person via respiratory droplets and aerosol particles that are breathed, coughed, or sneezed into the air by an infected person up to four days before the onset of rash and until about four days afterwards. The virus can survive in the air for about two hours, so direct close contact with an infected person is not necessary for transmission. Measles virus enters mucosal tissues of the eyes, nose, and mouth where it infects epithelial cells and immune cells including lymphocytes, dendritic cells, and macrophages. The immune cells bring the virus to local lymph nodes and initiate an immune response, which is both necessary for the elimination of the virus from the body but also responsible for the clinical manifestations that begin 10-14 days later.
Measles is known to increase susceptibility to other infections, acutely resulting in serious measles-bacterial coinfections and causing long-term infection susceptibility. Notably measles-induced immune suppression affects the response to both new infections and immune memory to previous infections and vaccinations. This is a primary reason that mortality due a variety of infectious diseases, not just measles virus, dropped after the introduction of the measles vaccine.
A common complication of measles infection is pneumonia, which can be caused by measles virus itself (called giant cell pneumonitis) or a secondary bacterial infection due to measles-induced immune suppression. Other complications include laryngotracheobronchitis (croup), otitis media, and diarrhea.
Keratoconjunctivitis is an ocular complication of measles infection that can result in blindness. In the pre-vaccination era, measles was thought to be a major contributor to blindness in children. Today, this is more likely to occur in children who are malnourished, particularly those with vitamin A deficiency or those who are immunocompromised.
Measles infection is also known to cause short- and long-term neurologic complications that manifest most commonly as various forms of encephalitis.
- Primary measles encephalitis occurs in about one-to-three cases per 1,000 patients and mortality is 10-15 percent.
- Acute post-measles encephalitis (or acute disseminated encephalomyelitis [ADEM]) is caused by immune-mediated inflammation in the brain. The incidence is about one per 1,000 patients and mortality is five percent in children and 25 percent in adults.
- Measles inclusion body encephalitis is a long-term complication that can arise up to a year after acute measles and typically presents with altered mental status, motor deficits, and seizures. Mortality is about 75 percent.
- Subacute sclerosing panencephalitis (SSPE) is the most severe neurologic complication that invariably progresses to coma and death. SSPE occurs in one per 25,000 measles cases overall but one in 5,500 infections in children less than a year old.
Vaccination is the best way to prevent acute measles infection and avoid these serious complications. Measles was declared eliminated from the United States in 2000, but millions of cases continue to occur worldwide. Occasionally international travelers bring measles back to the U.S., which used to result in either no or limited spread due to community immunity (otherwise known as herd immunity). However, undervaccinated communities like the one in Gaines County, Texas are susceptible to measles outbreaks and unvaccinated children are most at risk.
