hemorrhagic fevers

any of a variety of highly fatal viral diseases that are characterized by massive external or internal bleeding or bleeding into the skin. Other symptoms vary by the type of viral hemorrhagic fever but often include fever, malaise, muscle aches, vomiting, and shock. Most viral hemorrhagic fevers are geographically restricted because they are transmitted by specific animal or insect hosts (reservoirs) that occupy narrow and sometimes localized ecological niches. Viral hemorrhagic fevers are caused by viruses of four families: Flaviviridae, Arenaviridae, Bunyaviridae, and Filoviridae.

The most common viral hemorrhagic fevers are dengue and yellow fever, caused by related mosquitoborne flaviviruses. In the late 18th century, yellow fever epidemics in American coastal cities caused widespread panic, but the disease currently occurs only in developing countries of Africa and South America. It is the only major viral hemorrhagic fever for which an effective preventive vaccine exists and is widely used. Most cases of dengue, seen in tropical areas, are mild and influenza-like, but all four dengue viruses may produce dengue hemorrhagic fever or its severe form, dengue shock syndrome. Unlike yellow fever, which affects the liver and causes severe bleeding, dengue involves the liver only minimally and tends to induce only minor bleeding that is rarely fatal. However, if dengue shock syndrome occurs, patients may die when fluids and electrolytes in their vascular compartments shift into tissues, collapsing the blood volume and precipitating low blood pressure and shock. Dengue is unique among the fatal hemorrhagic fevers in that even severe cases can be effectively treated with simple fluid administration.

The arenaviruses are highly adapted to specific rodent hosts, which may become silently infected and excrete the virus in feces, urine, and saliva. However, when humans come into contact with food or soil contaminated by these rodent excreta, disease may result. The arenaviruses cause the diseases Lassa fever (occurring in Africa), Argentine hemorrhagic fever, Bolivian hemorrhagic fever, Brazilian hemorrhagic fever, and Venezuelan hemorrhagic fever.

Hantaviruses, Rift Valley fever virus (genus phlebovirus), and Crimean-Congo hemorrhagic fever virus (genus nairovirus) belong to the family Bunyaviridae. The hantaviruses, like the arenaviruses, are spread to humans by rodent contact. Hantaviruses cause Korean hemorrhagic fever and hantavirus pulmonary syndrome, which is highly fatal owing to accumulation of fluid in the lungs but features only minor hemorrhagic manifestations. Rift Valley fever, a mosquitoborne disease that is fatal in sheep and cattle, occurs in East and Southern Africa and the Middle East. Most people who contract Rift Valley fever survive, but a minority develop fatal hemorrhagic fevers, encephalitis, or severe eye disease. Crimean-Congo hemorrhagic fever, found in East and Southern Africa, the Middle East, and Russia, is a tickborne disease of cattle and other farm animals that is occasionally transmitted to humans.

The filoviruses, seen in Central and East Africa, include Ebola virus and Marburg virus. These are among the most highly fatal of the hemorrhagic fevers; some strains of Ebola cause death in up to 90 percent of victims. The filoviruses may also cause disease in primates. Marburg virus was discovered when it was transported with imported monkeys to Marburg, Germany, and caused a fatal outbreak. The origin of filovirus epidemics remains unclear; however, the virus has been found in the Old World fruit bat Rousettus aegypticus, which lives in areas throughout sub-Saharan Africa. Scientists suspect that these bats may be responsible for outbreaks of Marburg disease.


David Morens