Rubeola Virus Growth

The measles virus—or rubeola virus—is a pathogen that causes measles, an infectious disease common in children. Vaccination in industrialized nations has dramatically reduced the incidence of measles, but it still kills an estimated 4 million children each year worldwide, according to the authors of "Microbiology." The measles virus "grows" by replicating or making copies of itself. Like all viruses, it is unable to do so on its own. To replicate or grow it must first find a host and infect a cell.

Transmission

• When children infected with measles cough or sneeze, the microscopic droplets of mucus they expel may contain the measles virus. Outside of a human host, the virus can remain infectious for several hours. The virus enters the body through the respiratory tract, where it infects cells in the lining of the airways. From there, it travels to the lymph nodes and infects certain white blood cells, which transport it throughout the body. There is at present no specific treatment.

Infection

• Embedded in the virus's membrane or envelope are two proteins that trick your cells into granting it entry. The first is a protein called HA, which is the abbreviation for haemagglutinin. HA's shape and structure is such that it can bind to a receptor called SLAM, which is embedded in the membrane of some of your white blood cells. Ordinarily the SLAM receptor performs other functions, but the HA protein matches it like a key fitting into a lock. When HA docks with SLAM it triggers a process that admits the virus into the cell. Once inside, the virus hijacks the cell's machinery to replicate or mass-produce copies of itself.

Time Frame

• In the absence of any limiting factors, the growth of the measles virus is exponential. Each cell infected by a single virion can spawn a large number of new viruses, so the population increases rapidly. Once the immune system becomes alerted to the virus's presence, however, it mounts a counterattack. In practice, then, the viral population grows exponentially only during the early stages of an infection.

Features

• The measles virus is a single-stranded RNA virus, meaning its genetic information is encoded in a single strand of RNA (ribonucleic acid) rather than DNA. It's also negative-sense, meaning its RNA genome doesn't actually directly code for protein. The viral RNA genome actually serves as a template to synthesize a complementary (matching) strand of RNA, which is then translated to make proteins by the host cell's machinery. These proteins pervert the host cell's inbuilt mechanisms to turn them to the task of making more measles viruses. The virus's RNA genome is housed inside a small protein shell called a capsid. The capsid is in turn enclosed by a membrane called an envelope, and the all-important HA protein and fusion protein that enable each virus to infect other cells protrude from the envelope.

Epidemiology

• Epidemiology is in one sense the study of another aspect of viral growth—how a virus (or other disease agent) spreads through the human population. Infection with the virus confers lifetime immunity. Consequently, once the disease has become endemic in a given region following the initial epidemic outbreak, the vast majority of the adult population will have immunity, typically because they survived a bout in childhood. The virus therefore tends to infect each successive generation in early childhood. Those that survive have permanent immunity. The virus does not (so far as is known) have any animal reservoir, so it should in theory be possible to eradicate it entirely with vaccination, although doing so will certainly be challenging.

Evolution

• According to a 2010 report in Virology Journal, the measles virus is believed to share a common ancestor with the rinderpest virus, a pathogen that infects cattle.