Where do viruses come from?
Measles is a common infection in children and is spread by the respiratory route, but where did the virus come from? The disease is characterized by a prodromal (initial) illness of fever, coryza, cough, and conjunctivitis followed by appearance of a generalized maculopapular rash. Measles virus (MeV) infects approximately 30 million people annually, with a mortality of 197,000, mainly in developing countries. In the prevaccine era, more than 90% of 15-year-old children had a history of measles. Measles remains a major cause of mortality in children, particularly in areas with inadequate vaccination and medical care.
MeV infection can confer lifelong immunity, and there is no animal reservoir or evidence of latent or common persistent infection except for a rare condition called subacute sclerosing panencephalitis (SSPE). Therefore, maintenance of MeV in a population requires constant supply of susceptible individuals. If the population is too small to establish continuous transmission, the virus can be eliminated. Mathematical analysis shows that an immunologically-naïve population of 250,000-500,000 is needed to maintain MeV. This is approximately the population of the earliest urban civilizations in ancient Middle Eastern river valleys around 3000-2500 BCE. Historically, the first scientific description of measles-like syndrome was provided by Abu Becr, known as Rhazes, in the 9th century. However, smallpox was accurately described by Galen in the 2nd second century whereas measles was not. Epidemics identified as measles were recorded in the 11th and 12th centuries.
MeV is a member of the genus Morbillivirus, which belongs to the family Paramyxoviridae. In addition to MeV, Morbillivirus includes dolphin and porpoise morbillivirus, canine distemper virus, phocid distemper virus, peste des petits ruminants virus, and rinderpest virus (RPV). Genetically and antigenetically, MeV is most closely related to RPV, which is a pathogen of cattle. MeV is assumed to have evolved in an environment where cattle and humans lived in close proximity. MeV probably evolved after commencement of livestock farming in the early centers of civilization in the Middle East. The speculation agrees with the mathematical analysis mentioned above.
Molecular clock analysis can estimate the age of ancestors in evolutionary history by phylogenetic patterns. The basic approach to estimating molecular dates is to measure the genetic distance between species and use a calibration rate (the number of genetic changes expected per unit time) to convert the genetic distance to time. Pomeroy et al. showed that “Time to the Most Recent Common Ancestor” of the current MeV circulating worldwide is recent, i.e., within the last century (around 1943). Nevertheless, the time when MeV was introduced to human populations has not been investigated. In this study, the researchers performed molecular clock analysis on MeV to determine the time of divergence from RPV, suggesting that MeV emerged in humans in the 11th to 12th centuries.
Origin of measles virus: divergence from rinderpest virus between the 11th and 12th centuries. Virology Journal 2010, (7):52 doi:10.1186/1743-422X-7-52
Measles, caused by measles virus (MeV), is a common infection in children. MeV is a member of the genus Morbillivirus and is most closely related to rinderpest virus (RPV), which is a pathogen of cattle. MeV is thought to have evolved in an environment where cattle and humans lived in close proximity. Understanding the evolutionary history of MeV could answer questions related to divergence times of MeV and RPV. We investigated divergence times using relaxed clock Bayesian phylogenetics. Our estimates reveal that MeV had an evolutionary rate of 6.0 – 6.5×10-4 substitutions/site/year. It was concluded that the divergence time of the most recent common ancestor of current MeV was the early 20th century. And, divergence between MeV and RPV occurred around the 11th to 12th centuries. The result was unexpected because emergence of MeV was previously considered to have occurred in the prehistoric age. MeV may have originated from virus of non-human species and caused emerging infectious diseases around the 11th to 12th centuries. In such cases, investigating measles would give important information about the course of emerging infectious diseases.