Measles

Measles

Measles is an infectious disease caused by a virus of the paramyxovirus group. It infects only man and the infection results in life-long immunity to the disease. It is one of several exanthematous (rash-producing) diseases of childhood, the others being rubella (German measles), chicken pox, and the now rare scarlet fever. The disease is particularly common in both pre-school and young school children.

The measles virus mainly infects mucous membranes of the respiratory tract and the skin. The symptoms include high fever, headache, hacking cough, conjunctivitis, and a rash that usually begins inside the mouth on the buccal mucosa as white spots, (called Koplik's spots) and progresses to a red rash that spreads to face, neck, trunk and extremities. The incubation period varies but is usually 10 to 12 days until symptoms appear. Four to five days before the onset of the rash, the child has fever or malaise and then may develop a sore throat and cough. The duration of the rash is usually five days. The child is infectious throughout the prodromal (early) period and for up to four days after the first appearance of the rash. The virus is highly contagious and is transmitted through respiratory droplets or though direct contact. Measles is also sometimes called rubeola or the nine-day measles.

Although certain complications can arise, in the vast majority of cases, children make a full recovery from measles. Acute local complications can occur if there is a secondary infection, for example pneumonia due to bacteria such as staphylococci , Streptococcus pyogene, pneumococci, or caused by the virus itself. Also, ear infections and secondary bacterial otitis media can seriously aggravate the disease. Central nervous system (CNS) complications include post-measles encephalitis, which occurs about 10 days after the illness with a significant mortality rate. Also, sub-acute sclerosing panencephalitis (SSPE), a rare fatal complication, presents several years after the original measles infection. Because hemorrhagic skin lesions, viraemia, and severe respiratory tract infection are particularly likely in malnourished infants, measles is still frequently a life-threatening infection in Africa and other underdeveloped regions of the world. The microbiological diagnosis of measles is not normally required because the symptoms are characteristic. However, if an acute CNS complication is suspected, paired sera are usually sent for the estimation of complement fixing antibodies to measles. If SSPE is suspected, the measles antibody titres in the CSF (determining the level of antibodies present) are also estimated.

Epidemiological studies have shown that there is a good correlation between the size of a population and the number of cases of measles. A population of at least 500,000 is required to provide sufficient susceptible individuals (i.e. births) to maintain the virus within the population. Below that level, the virus will eventually die out unless it is re-introduced from an outside source. On the geological time-scale, man has evolved recently and has only existed in large populations in comparatively modern times. In the past, when human beings lived in small populations, it is concluded that the measles virus could not exist in its present form. It may have had another strategy of infection such as to persist in some form and infect the occasional susceptible passer-by, but this remains unproven. It has been suggested that the modern measles virus evolved from an ancestral animal virus, which is also common to the modern canine distemper and the cattle disease rinderpest. This theory is based on the similarities between these viruses , and on the fact that these animals have been commensal (living in close proximity) with man since his nomadic days. The ancestral virus is thought to have evolved into the modern measles virus when changes in the social behavior of man gave rise to populations large enough to maintain infection. This evolutionary event would have occurred within the last 6000 years when the river valley civilizations of the Tigris and Euphrates were established. To our knowledge, measles was first described as a disease in ninth century when a Persian physician, Rhazes, was the first to differentiate between measles and smallpox . The physician Rhazes also made the observation that the fever accompanying the disease is a bodily defense and not the disease itself. His writings on the subject were translated into English and published in 1847.

The measles virus itself was first discovered in 1930, and John F. Enders of the Children's Hospital in Boston successfully isolated the measles virus in 1954. Enders then began looking for an attenuated strain, which might be suitable for a live-virus vaccine . A successful immunization program for measles was begun soon after. Today measles is controlled in the United States with a vaccination that confers immunity against measles, mumps , and rubella and is commonly called the MMR vaccine. Following a series of measles epidemics occurring in the teenage population, a second MMR shot is now sometimes required by many school-age children as it was found that one vaccination appeared not to confer life-long immunity.

In October 1978, the Department of Health, Education, and Welfare announced their intention of eliminating the measles virus from the U.S.A. This idea was inspired by the apparently successful global elimination of smallpox by the World Health Organization vaccination program, which recorded its last smallpox case in 1977.

Death from measles due to respiratory or neurological causes occurs in about 1 out of every 1000 cases and encephalitis also occurs at this frequency, with survivors of the latter often having permanent brain damage. Measles virus meets all the currently held criteria for successful elimination. It only multiplies in man; there is a good live vaccine (95 % effective) and only one sero-type of the virus is known. Usually measles virus causes an acute infection but, rarely (1 out of every million cases), the virus persists and reappears some 2-6 years causing SSPE. However, measles virus can only be recovered with difficulty from infected tissue and SSPE is a non-transmissible disease. To successfully eliminate measles, it would be necessary to achieve a high immunization level, especially in children.

See also Antibody-antigen, biochemical and molecular reactions; History of immunology; History of public health; Immunity, active, passive and delayed; Immunology; Varicella; Viruses and responses to viral infection