The Measles Virus The Measles Virus, also called Rubeola, is a highly contagious acute viral disease that can result in serious complications and death. The virus comes from the genus Morbillivirus and belongs to the Polymyxovirus Family.
The Virus and Replication • • • •
The virions are enveloped and enclose a helical nucleocapsid containing negative single-stranded RNA. The HN protein binds to glycoprotein receptors on the host cell. The F protein mediates fusion with the cell’s plasma membrane. Once the virus enters the cell, transcription, translation, and replication begins.
P protein is a phosphoprotein L is a polymerase subunit M is a matrix protein NP is the nucleocapsid protein
Replication The negative single-stranded RNA is transcribed into six subgenomic monocistronic mRNA’s. After the mRNA’s are transcribed by the transcriptase, they are translated into viral proteins. When the concentration of the NP builds up in the cell, the virus switches from transcription to replication. First, the (-)RNA strand is transcribed by ignoring all stop sequeces and making a full length of complementary (+)RNA. The resulting (+)RNA is not mRNA but a replication template for making new (–)RNA. The new (–)RNA associates with the NP and transcriptase to form the nucleocapsid and then the M, F, and HN proteins migrate to the surface of the plasma membrane. The virions than exit the cell by budding off the cell membrane, leaving them enveloped. The F protein must be cleaved into two subunits in order to make the virus infectious. An activated protein evades the immune system by causing the infected cell to fuse with non-infected cells. Many fused cells are referred to as syncytia or multi-nucleated giant cells. Syntycia are one way to diagnose paramyxoviruses in the laboratory.
Dynamics of Transmission • Virus resides in mucus in the nose and throat of the infected person. • Direct Transmission: Droplets from coughing, sneezing, or breathing directly to a non infected person via the upper respiratory system and the conjunctiva. • Indirect Transmission: Non infected person touching an infected surface and putting hands on mouth or nose. • Virus can remain alive and active on a surface for up to two hours. • Virus resides in back of the throat and lungs where it multiplies.
Dynamics of Transmission • Incubation Period: 8 to 12 days between infection and the onset of symptoms. • Infected person is contagious from approximately 4 days before the onset of the rash until approximately 4 days after the rash disappears. • Highly contagious: 90% of infected person’s contacts who are susceptible will get the infection. • In the Third World: Greatest infection rates in those under 2 years old with malnutrition, an important contributing factor.
Symptoms • • • • •
Cough Coryza (runny nose) Conjunctivitis (itchy, watery red eyes) Fever - 101°F or higher Red maculopapular rash (little spots and bumps) usually starts at the head and moves down the trunk of the body • Koplik’s spots inside the cheek of the mouth
Complications of Measles Symptoms Approximately 20% of people with measles symptoms will develop one or more complication. These measles complications are more common in children under 5 years of age and adults over 20 years of age. – – – – – – – –
Respiratory conditions (croup, bronchitis, bronchiolitis) Eye conditions (corneal ulceration, keratitis, or blindness) Diarrhea Ear infections (otitis media) Pneumonia Encelphilitis Seizures Death
Diagnostic Tests When suspected of having measles: • Doctor will perform a physical exam and measles can be diagnosed from the signs and symptoms. • To ensure proper diagnosis or if an atypical for measles is suspected: – Blood tests for the measles antibodies: If antibody titer rises by 4 fold between acute and convalescence phase or if the measles specific IgM is found. – Throat culture to look for the measles virus itself.
Number & Percentage of Reported Measles Cases Among U.S. Residents
Number of Reported Measles Cases
Measles Globally According to the WHO (World Heath Organization) data, global deaths due to measles fell from 48%, from 871,000 in 1999 to an estimated 454,000 in 2004. The largest reduction occurred in sub-Saharan Africa, the region with the highest burden of the disease, where estimated measles cases and deaths dropped by 60%. These statistics make measles one of the single leading causes of death, among children in most developing countries â€“ more than HIV, TB, and malnutrition â€“ despite the availability of a safe, effective and relatively inexpensive vaccine for more than 40 years.
Although ongoing measles transmission was declared eliminated in the United States in 2000 and in the WHO Region of the Americas in 2002, approximately 20 million cases of measles occur each year worldwide.
Treatment & Prevention Treatment • No current treatment that can kill the measles virus • Supportive care: Providing relief of measles symptoms as the body fights the disease • Supportive care includes: • • • •
IV fluids Control of fever or pain Antibiotics to treat secondary bacterial infections In developing countries, supportive care also includes 2 days worth of Vitamin A medication to reduce the risk of blindness and death
Treatment & Prevention Measles complications can be dangerous â€˘ 1 out of 1000 measles patients will develop inflammation of the brain. â€˘ 1 out of 1000 will die.
Treatment & Prevention • Prior to 1963, measles was an expected life event. More than half of the population had measles by the time they were 6 years old and 90% had the measles by the time they were 15 years old. • Prevention includes: • Vaccination: measles vaccine contained in the MMR vaccine. • MMR Vaccine: live, weakened combination vaccine against measles, mumps, and rubella. • Measles vaccine made by: obtaining measles virus from back of throat of infected person and growing inside a chick embryo cell.
Treatment & Prevention • Developed immunity lasts a lifetime in 95% of children. • Second dose of vaccine recommended for the remaining 5%. • Booster recommended for those who develop an immune response • Herd immunity: Critical to prevention of the spread of measles.