Measles
Measles is a highly contagious, vaccine-preventable infectious disease caused by measles virus. Other names include morbilli, rubeola, 9-day measles, red measles, and English measles.
Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days. Initial symptoms typically include fever, often greater than, cough, runny nose, and inflamed eyes. Small white spots known as Koplik spots may form inside the mouth two or three days after the start of symptoms. A red, flat rash which usually starts on the face and then spreads to the rest of the body typically begins three to five days after the start of symptoms. Common complications include diarrhea, middle ear infection, and pneumonia. These occur in part due to measles-induced immunosuppression. Less commonly, seizures, blindness, or inflammation of the brain may occur.
Measles is an airborne disease which spreads easily from one person to the next through the coughs and sneezes of infected people. It may also be spread through direct contact with mouth or nasal secretions. It is extremely contagious: nine out of ten people who are not immune and share living space with an infected person will be infected. Furthermore, measles's reproductive number estimates vary beyond the frequently cited range of 12 to 18, with a 2017 review giving a range of 3.7 to 203.3. People are infectious to others from four days before to four days after the start of the rash. While often regarded as a childhood illness, it can affect people of any age. Most people do not get the disease more than once. Testing for the measles virus in suspected cases is important for public health efforts. Measles is not known to occur in other animals.
Once a person has become infected, no specific treatment is available, although supportive care may improve outcomes. Such care may include oral rehydration solution, healthy food, and medications to control the fever. Antibiotics should be prescribed if secondary bacterial infections such as ear infections or pneumonia occur. Vitamin A supplementation is also recommended for children under the age of 5. Among cases reported in the U.S. between 1985 and 1992, death occurred in 0.2% of cases, but may be up to 10% in people with malnutrition. Most of those who die from the infection are less than five years old.
The measles vaccine is safe and very effective at preventing the disease and is often delivered in combination with other vaccines. Due to the ease with which measles is transmitted from person to person in a community, more than 95% of the community must be vaccinated in order to achieve herd immunity. Vaccination resulted in an 80% decrease in deaths from measles between 2000 and 2017, with about 85% of children worldwide having received their first dose as of 2017. Measles affects about 20 million people a year, primarily in the developing areas of Africa and Asia. It is one of the leading vaccine-preventable disease causes of death. In 1980, 2.6 million people died from measles, and in 1990, 545,000 died due to the disease; by 2014, global vaccination programs had reduced the number of deaths from measles to 73,000. Despite these trends, rates of disease and deaths increased from 2017 on due to a decrease in vaccination.
Signs and symptoms
Symptoms begin 7-23 days after exposure with a prodrome of fever, malaise, and cough. The fever with measles increases in a stepwise fashion and peaks at -. After the two to four days of prodrome, Koplik spots appear inside the cheeks opposite the molars as clusters of white lesions on reddened areas. They are pathognomonic for measles, but are present for only a short time and therefore are not always seen. The classic symptoms include a fever, cough, coryza, and conjunctivitis, referred to as "the three C's", and a maculopapular rash. Diarrhea may develop after the prodrome and can last for up to a month.The characteristic measles rash is classically described as a generalized red maculopapular rash that begins three to five days after the prodrome; on average, 14 days after exposure, but as few as 7 or as many as 21 days post-exposure. The rash starts on the back of the ears or on the face and thereafter spreads to the rest of the body. It is caused by the cellular and humoral immune system's clearing of infected skin cells, as is the conjunctivitis. Measles conjunctivitis often also causes photophobia. The rash is said to "stain", changing color from red to dark brown, before disappearing.
Uncomplicated cases of measles typically improve within days of rash onset and resolve within 7–10 days. The virus is contagious from four days before to four days after the rash appears.
Because development of the rash and conjunctivitis requires a functional immune system, immunocompromised people may not be diagnosed as readily.
People who have been vaccinated against measles but have incomplete protective immunity for any reason may develop modified measles. Modified measles is characterized by a prolonged incubation period and milder symptoms. People with modified measles may have morbilliform rash without the characteristic high fever and mild respiratory symptoms. They are also less contagious, and have lower viral loads. Atypical measles is distinct from modified measles and was reported in recipients of the inactivated measles vaccine used between 1963 and 1967. Atypical measles is characterized by a rash that first appears on the arms and legs, rather than on the head, and can be petechial instead of morbilliform; high fever; and severe pneumonia.
Complications
Complications of measles are relatively common, affecting approximately 30% of cases. Even in previously healthy children, measles can cause serious illness requiring hospitalization. Some complications are caused directly by the virus, while others are caused by viral suppression of the immune system. This phenomenon, known as "immune amnesia", increases the risk of secondary bacterial infections; two months after recovery there is an 11–73% decrease in the number of antibodies against other bacteria and viruses. Population studies from prior to the introduction of the measles vaccine suggest that immune amnesia typically lasts 2–3 years. Primate studies suggest that immune amnesia in measles is effected by replacement of memory lymphocytes with ones that are specific to measles virus, since they are destroyed after being infected by the virus. This creates lasting immunity to measles re-infection, but decreases immunity to other pathogens.The most severe direct complications from measles are pneumonia, encephalitis, and blindness. One to three out of every 1,000 children who become infected with measles will die from respiratory and neurological complications. Complications that are directly related to the virus in the respiratory system include viral pneumonia, pneumonitis, and viral laryngotracheobronchitis. Giant-cell pneumonia and pneumonitis occur most often in young children and immunocompromised people. The death rate in the 1920s was around 30% for measles pneumonia.
Measles encephalitis may occur at any point in the course of the disease. Acute post-infectious measles encephalitis occurs during the first week of infection. Measles-inclusion body encephalitis occurs 1-6 months after the acute infection. Subacute sclerosing panencephalitis, a progressive and fatal form of encephalitis, may occur years after the acute infection, and affects about 1 in 600 unvaccinated infants under 15 months who contract measles.
Corneal ulceration can lead to corneal scarring or perforation, causing blindness, and is more severe in those with vitamin A deficiency.
Common secondary infections include infectious diarrhea, bacterial pneumonia, and otitis media.
Those under 5 and over 20 are at higher risk for complications, especially adults. Immunocompromising conditions, including pregnancy, malnutrition, hematologic cancer, HIV/AIDS, and use of immunosuppressive medication also increase risk for complications. Between 1987 and 2000, the case fatality rate across the United States was three deaths per 1,000 cases attributable to measles, or 0.3%. In underdeveloped nations with high rates of malnutrition and poor healthcare, fatality rates have been as high as 28%. In immunocompromised persons the fatality rate is approximately 30%.
Measles infection in pregnancy is not directly teratogenic, but can lead to intrauterine growth restriction and preterm birth, as well as mortality, which ranges from 5-30% depending on endemicity of the virus.
Cause
Measles is caused by the measles virus, a single-stranded, non-segmented, negative-sense, enveloped RNA virus of the genus Morbillivirus within the family Paramyxoviridae. It is related most closely to rinderpest, a cattle virus eradicated in 2001, and canine distemper, a mammalian disease that causes neurologic deterioration. There are 24 strains of measles virus divided into eight clades designated A-H.The virus is one of the most contagious human pathogens and is spread by coughing and sneezing via close personal contact or direct contact with secretions. It remains infectious for up to two hours via suspended respiratory droplets. It is not easily spread by fomites, because the virus is inactivated within a few hours by ultraviolet light and heat. It is also inactivated by trypsin, acidic environments, and ether. Measles is so contagious that if one person has it, 90% of non-immune people who have close contact with them will also become infected. Humans are the only natural hosts of the virus, and no other animal reservoirs are known to exist, although mountain gorillas are believed to be susceptible to the disease.
Risk factors for measles virus infection include immunodeficiency caused by HIV/AIDS, immunosuppression following receipt of an organ or a stem cell transplant, alkylating agents, or corticosteroid therapy, regardless of immunization status; travel to areas where measles commonly occurs or contact with travelers from such an area; and the loss of passive, inherited antibodies before the age of routine immunization.