Flaviviruses belong to the Togaviridae and include several species that can cause different diseases – including tick-borne encephalitis, St. Louis encephalitis, Japanese encephalitis, and Murray-Valley encephalitis, as well as yellow fever and dengue fever.
What are flaviviruses?
Flavivirus is not a single pathogen; instead, the term describes a genus of viruses that can cause various diseases in humans. Flaviviruses belong to the Togaviridae, formerly known as ARBO-B viruses. The abbreviation stands for the English term arthropod-borne virus and refers to viruses that have a similar infection mechanism but need not be otherwise related and do not necessarily share other common features. As is common with viruses, the genetic material of the pathogen is contained in an outer envelope that has no organelles of its own. Viruses do not have their own metabolism, but depend on a host in whose biological processes they interfere. In the case of flaviviruses, human cells, among others, serve as hosts. Ticks, mosquitoes, and similar insects can transmit the virus.
Occurrence, distribution, and characteristics
Flaviviruses average 50 nm in size and have a slightly different spherical envelope depending on the type of virus. For example, yellow fever virus, which is also a flavivirus, is approximately 22-38 nm in diameter and can pass from one person to another with the help of mosquitoes. When flaviviruses are transmitted, the pathogens enter the mosquitoes or ticks when they suck blood. If they subsequently bite or bite another person, the viruses can also infect the new organism. To do this, the virus injects its genetic material into human cells, which serve as its host. The genetic information is stored in the form of ribonucleic acid (RNA). At the molecular level, RNA differs only slightly from deoxyribonucleic acid (DNA). The virus then induces its host cell to make copies of itself, thereby causing it to replicate. The process of replication can differ depending on the type of virus. The various flaviviruses differ not only in terms of their geographic occurrence, but also in terms of the carrier used to move them from one host to the next. Tick-borne encephalitis, as its name suggests, is usually due to flaviviruses that enter the human body through ticks, whereas in St. Louis encephalitis, mosquitoes transmit the flavivirus. Japanese encephalitis is common in (South) East Asia and is transmitted to humans by Culex mosquitoes, especially from pigs and birds. Children in particular contract this form of encephalitis, which can be accompanied by fever, muscle and limb pain, and chills. Mosquitoes also act as vectors in Murray Valley encephalitis. This rare form of encephalitis is found, for example, in Australia, where it is particularly prevalent in the north of the country. The dengue virus is also transmitted by mosquitoes and is responsible for dengue fever. It is most common in Southeast Asia, and virology distinguishes between different types of the virus. Type III and type IV of dengue virus is known to cause hemorrhagic fever, especially in the bodies of children.
Diseases and symptoms
Flaviviruses can cause various diseases in the human body. Encephalitis is an inflammation of the brain that can manifest itself in a variety of symptoms. Typical signs of the disease include fever, convulsions, mental abnormalities and clouding of consciousness. In addition, encephalitis can lead to neurological focal syndromes in which specific functional systems are affected. The nature of the resulting disorder depends on which part of the brain is affected by the focus of inflammation. Encephalitis may also spread to the meninges and spinal cord, and less commonly to the roots of the spinal nerves. In some cases, encephalitis results in permanent damage or death. Yellow fever also results from infection with a flavivirus. The most characteristic feature of the disease is a combination of fever and jaundice. Other possible symptoms include circulatory problems, bleeding, liver and kidney disorders. The fever usually occurs in episodes.After the first episode of fever, one to two days may pass during which the patient suffers from no acute symptoms before the infection triggers another episode of fever. The relapses result from the phases of viral replication. Dengue fever, which also results from flavivirus infection, is, like yellow fever, a subtropical or tropical disease. In addition to fever, common symptoms include muscle pain, joint pain, and headache, as well as swelling of the lymph nodes and a skin rash similar to that of measles. The fever usually begins 5-8 days after the actual infection, often taking the course of a saddle curve: Thus, the peaks in the fever curve can be distinguished from each other by a slight drop in body temperature.
