Hemorrhagic fevers

Hemorrhagic fevers are caused by viruses that exist throughout the world butare most common in tropical areas. Early symptoms, such as muscle aches and fever, can progress either to mild illness or to a more serious, potentially fatal disease. In severe cases, a prominent symptom is bleeding (hemorrhaging) from orifices, such as the nose or vagina, and from internal organs.

These viruses also exist in insects, spiders and other animals that become infected but do not die and often have no symptoms. The viruses are transmittedto humans either directly by one of these infected animals or by an intervening species such as mosquitoes.

Hemorrhagic fevers are generally linked to specific locations. If many peoplereside in one of these areas, the number of cases may be high. Fatality rates are variable. In cases of dengue hemorrhagic fever-dengue shock syndrome, 1-5% of those afflicted die. 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. Not all cases progress to the very serious symptom of bleeding. The severity of bleeding rangesfrom pinpoint hemorrhages under the skin surface to distinct bleeding from body orifices. Depending on the particular virus, a wide range of other symptoms may occur, including abdominal pain, apathy, appetite loss, blurry vision,chills, coma, convulsions, coughing, delirium, diarrhea,disorientation, dizziness, fever, gastrointestinal upset, headache, jaundice, joint pain, low back pain, malaise, muscle aches, nausea, rash, runny nose, seizures, sore throat, stupor, swelling or flushing of the face, or vomiting.

The viruses that cause hemorrhagic fevers are found most commonly in tropicallocations; however, some are found in cooler climates. The disease is typically harbored by rodents, ticks, or mosquitoes, but person-to-person transmission can also occur in health care settings or through sexual contact.

Ebola, caused by a member of the Filoviridae family of viruses, is most commonly found in Africa, particularly the Republic of the Congo and Sudan. Ebolais fatal in 30-90% of cases. Another, rarer filovirus, the Marburg virus, isfound in sub-Saharan Africa. It is not known which animals harbor filoviruses.

Argentinian, Brazilian, Bolivian, and Venezuelan hemorrhagic fevers are caused by viruses of the Arenaviridae family. Lassa fever, which occurs in west Africa, also arises from an arenavirus. Infected rodents shed virus particles in their urine and saliva, which humans may inhale or otherwise contact. Untreated, South American hemorrhagic fevers have a 10-30% fatality rate. Lassa fever has an overall fatality rate lower than 2%, but hospitals may encounter 20% fatality rates, treating typically the most serious of cases.

Yellow fever and dengue fever are caused by the Flaviviridae family of viruses. Yellow fever occurs in tropical areas of the Americas and Africa and is transmitted from monkeys to humans by mosquitoes. Mosquitoes also transmit thedengue virus. Dengue fever is most common in southeast Asia and areas of theAmericas.

Rift Valley fever is caused by the phlebovirus, a member of the Bunyaviridaefamily found in sub-Saharan Africa and the Nile delta. Natural reservoirs arewild and domestic animals, and transmission occurs through contact with infected animals or through mosquito bites. Death, which occurs in fewer than 3% of cases, is attributable to massive liver damage.

Crimean-Congo hemorrhagic fever is caused by nairovirus (another member of the Bunyaviridae family) and occurs in central and southern Africa, Asia, Eurasia, and the Middle East. The virus is found in hares, birds, ticks, and domestic animals and may be transmitted by ticks or by contact with infected animals. Death rates range from 10% in southern Russia to 50% in parts of Asia.

Hemorrhagic fever with renal (kidney) syndrome is caused by other members ofthe Bunyaviridae family: the hantaviruses: Hantaan, Seoul, Puumala, and Dobrava. Hantaan virus occurs in northern Asia, the Far East, and the Balkans; Seoul virus is found worldwide; Puumala virus is found in Scandinavia andnorthern Europe; while Dobrava virus occurs in the Balkans. Wild rodents transmit the virus via their excrement or body fluids or through direct contact.Death rates range from 0-10%.

Since the hemorrhagic fevers share symptoms with many other diseases, positive identification relies on evidence of the viruses in the bloodstream--such as detection of antigens and antibodies, or finding the virus itself in the body.

Lassa fever, and possibly other hemorrhagic fevers, respond to ribavirin, anantiviral medication. However, most of the hemorrhagic fever viruses can onlybe treated with supportive care. Such care centers around maintaining correct fluid and electrolyte balances in the body and protecting the patient against secondary infections. Heparin and vitamin K administration, coagulation factor replacement, and blood transfusions may lessen or stop bleeding in somecases.

Permanent disability can occur with some types of hemorrhagic fever. About 10% of severely ill Rift Valley fever victims suffer retina damage and may be permanently blind, and 25% of South American hemorrhagic fever victims sufferpotentially permanent deafness.

Proper treatment is vital. For individuals who survive hemorrhagic fevers, prolonged convalescence is usually inevitable. However, survivors seem to gainlifelong immunity against the virus that made them ill.

To prevent hemorrhagic fevers, attempts have been made in urban and settled areas to destroy mosquito and rodent populations. In places where such measures are impossible, individuals can use insect repellents, mosquito netting, and other methods to minimize exposure.

Vaccines have been developed against yellow fever, Argentinean hemorrhagic fever, and Crimean-Congo hemorrhagic fever. Vaccines against other hemorrhagicfevers are being researched.

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