Hemorrhagic Fevers

Overview :

Although hemorrhagic fevers are regarded as emerging diseases, they probably have existed for many years. This designation isn't meant to imply that they are newly developing, but rather that human exposure to the causative viruses is increasing to the point of concern.

These viruses are maintained in nature in arthropod (insects, spiders and other invertebrates with external hard skeletons) or animal populations—so-called disease reservoirs. Individuals within these populations become infected with a virus but do not die from it. In many cases, they don't even develop symptoms. Then the viruses are transmitted from a reservoir population to humans by vectors—either members of the reservoir population or an intervening species, such as mosquitoes.

Hemorrhagic fevers are generally either endemic or linked to specific locations. If many people reside in an endemic area, the number of cases may soar. For example, dengue fever, a type of hemorrhagic fever, affects approximately 100 million people annually. A large percentage of those infected live in densely populated southeast Asia; an area in which the disease vector, a mosquito, thrives. Some hemorrhagic fevers are exceedingly rare, because people very infrequently encounter the virus. Marburg hemorrhagic fever, which has affected fewer than 40 people since its discovery in 1967, provides one such example. Fatality rates are also variable. In cases of dengue hemorrhagic fever-dengue shock syndrome, 1-5% of the victims perish. On the other end of the spectrum is Ebola, an African hemorrhagic fever, that kills 30-90% of those infected.

The onset of hemorrhagic fevers may be sudden or gradual, but all of them are linked by the potential for hemorrhaging. However, not all cases progress to this very serious symptom. Hemorrhaging may be attributable to the destruction of blood coagulating factors or to increased permeability of body tissues. The severity of bleeding ranges from petechiae, which are pin-point hemorrhages under the skin surface, to distinct bleeding from such body orifices as the nose or vagina.