One reason for the manifestation of more severe Dengue courses is the increasing mobility of the populace. This leads to the phenomenon of people overcoming one Dengue infection, then traveling to an area where a different Dengue serotype is endemic. What happens if they are infected again is known as an "antibody-dependent enhancement of viral infection" or ADE, see p. 399). Antibodies from the first infection attach to the viruses of the fresh infection, which are, however, not neutralized, but instead allowed entrance into cells via Fc receptors, resulting in DHF and DSS. Children still carrying antibodies from their mother during the first year of life can also experience these severe infection courses due to the same mechanism.
Diagnosis. A flavivirus infection always involves viremia (transmission by bloodsucking arthropods!). The viruses can be isolated from blood by inoculating cell cultures or newborn mice. In autopsies of fatal cases they can be isolated from liver tissue. The viruses are labile by nature and identification can take time, for which reason the diagnostic focus is on serology (titer rise or IgM detection).
Epidemiology and prevention. A cycle of infection involving a vertebrate host (mammals, birds) and a transmitting vector (bloodsucking mosquitoes and flies, ticks) has developed for most flavivirus infections. The cycles are efficient for the virus and relatively harmless for the host. The vertebrate host frequently shows few signs of disease and recovers from the infection after a brief viremia. During this period, the bloodsucking vector is infected, which thereafter remains a lifelong salivary secretor and thus infectious. In ticks, transovarian transmission of the virus is also possible. The human host is a dead end for the virus, not a normal component of the cycle. Exceptions to this are Dengue fever and urban yellow fever.
Humans are the only known main hosts for the Dengue virus. There are two forms of yellow fever: rural or jungle ("sylvatic") yellow fever with a monkey-mosquito-monkey (sometimes human) cycle and urban yellow fever with humans as the main hosts and Aedes mosquitoes as the transmitting vectors. This form is on the upswing due to increasing numbers of breeding places (e.g., empty tin cans in garbage piles) for the vector. Another "new" (more accurately: revived) infectious disease is the West Nile viral infection,
observed for the first time in the USA (New York) in 1999, apparently introduced into the area by migrating birds. It is still not known why the geographic distribution of the virus or infected birds changed.
Vaccines are available against yellow fever (live vaccine) and European tickborne encephalitis (dead vaccine).
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