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In 1995, Ebola, an especially gruesome and fatal disease, killed hundreds of people in the Democratic Republic of Congo. Ten years later, hundreds more died in Angola of Marburg virus, an equally deadly illness. But for all their notoriety, Ebola and Marburg virus are only two of a group of related illnesses called viral hemorrhagic fevers (VHFs), some of which pose a far greater threat to your health.

Viral hemorrhagic fevers are caused by viruses from four distinct families and range in severity from relatively mild to life-threatening. Although all begin with fever and muscle aches, some viral hemorrhagic fevers progress to far more serious problems, including severe internal and external bleeding (hemorrhage), widespread tissue death (necrosis), and shock.

The origin of Ebola and Marburg viruses isn't known, but the viruses that cause other hemorrhagic fevers are transmitted to humans by animals or insects. In some but not all cases, viral hemorrhagic fevers can then spread from person to person through contact with infected blood or body fluids, including semen. Secondary transmission can also occur through contaminated needles and syringes, a particular problem in developing nations where medical equipment is in short supply and may be reused.

No current treatment can cure viral hemorrhagic fevers, and immunizations exist for only two of the many VHFs. Researchers are working to develop other vaccines, but in the meantime, the best approach is prevention.

The VHF designation includes a broad range of diseases, and signs and symptoms can vary widely, even among members of the same viral family. But VHFs do have some common characteristics, especially their effect on your vascular system — the network of arteries, veins and capillaries that circulates blood throughout your body.

Hemorrhagic fevers make blood vessels more permeable — that is, more likely to leak — causing bleeding that can range from relatively minor to massive. Bleeding may occur under the skin, in internal organs, and from the mouth, eyes, ears and rectum. People with severe bleeding may experience potentially lethal symptoms such as shock and coma, but rarely die of blood loss.

In general, signs and symptoms of most VHFs begin two days to three weeks after you've been exposed to the virus. All begin with fever and muscle aches; many cause vomiting and diarrhea; and all create problems in a number of organ systems, especially your liver, lymphatic system, lungs and sometimes your kidneys.

Problems more specific to diseases within each of the four families of viruses that cause VHFs are listed below.

Arenaviruses
This viral family includes Lassa fever, Argentine hemorrhagic fever, Bolivian hemorrhagic fever, Brazilian hemorrhagic fever and Venezuelan hemorrhagic fever. If you've been exposed to one of these illnesses, you're likely to experience some of the following signs and symptoms:

• Fever, which may be constant or intermittent
• Pain behind your chest wall and in your back
• A sore and inflamed throat with white patches on your tonsils
• Cough
• Abdominal pain
• Vomiting
• Diarrhea
• Conjunctivitis, an inflammation of the transparent membrane (conjunctiva) that covers your eyelids and part of your eyeballs
• Bleeding from your gums or in your stomach, small intestine and other internal organs
• Facial swelling
• Temporary or permanent hearing loss
• Fluid in your lungs (pleural effusion)
• Encephalitis, a life-threatening inflammation of the brain

Bunyaviruses
Included in this group of VHFs are Rift Valley fever, Crimean-Congo hemorrhagic fever, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. Depending on the type of virus involved, signs and symptoms of bunyaviruses may include:

• Fever
• Severe muscle aches, especially in your thighs, hips, back and sometimes in your shoulders
• Nausea and vomiting
• Sore throat
• Headache
• Cough
• Diarrhea
• In the case of Crimean-Congo hemorrhagic fever, sore eyes and sensitivity to light (photophobia); mood swings and agitation; bleeding into the skin and from the gums, rectum and kidneys; and eventual pulmonary failure
• In the case of hantavirus pulmonary syndrome, abnormally low blood pressure (hypotension) and difficulty breathing, possibly leading to respiratory failure

Filoviruses
This viral family consists of the Ebola and Marburg viruses, which are among the most virulent of all known diseases. Signs and symptoms appear within two days to three weeks of infection and initially include those common to other VHFs: fever, intense weakness, muscle pain, vomiting and diarrhea. As the disease progresses, some people also develop:

• A distinctive, hemorrhagic rash
• Bleeding from the nose, mouth, eyes, ears and rectum
• Seizures
• Coma
• Delirium

Depending on the strain, Ebola and Marburg viruses are fatal in 50 percent to 90 percent of  infected people and are almost always fatal in pregnant women.

Flaviviruses
This group includes yellow fever, dengue fever, dengue hemorrhagic fever, Kyasanur Forest disease and Omsk hemorrhagic fever. Yellow fever and dengue fever are probably the most well known VHFs. Dengue is a major public health concern, with 50 million to 100 million cases a year worldwide, including major epidemics at popular tourist destinations.

Signs and symptoms of dengue fever vary with age. Infants and young children usually develop a rash and severe, flu-like symptoms, whereas older children and adults may experience a high fever, severe headache, eye pain, muscle aches and rash. In the hemorrhagic form, an extremely high fever may be accompanied by bleeding, convulsions and circulatory failure.

Both Kyasanur Forest disease and Omsk hemorrhagic fever are biphasic diseases, meaning that the initial signs and symptoms are followed by a brief period of recovery before symptoms reappear. No matter when they occur, signs and symptoms of these two diseases generally include:

• Fever
• Headache
• Pain in your arms and legs
• Severe exhaustion
• A rash on the soft palate of your mouth
• Swollen glands in your neck
• Bleeding in your eyes
• Nosebleeds
• Bleeding in your gastrointestinal tract

Emerging diseases are infections that are appearing for the first time or that are rapidly increasing in incidence and range. Many of these infections have surfaced in the past several decades, including VHFs such as Ebola and hantavirus pulmonary syndrome and illnesses such as HIV/AIDS, Lyme disease and SARS.

Although emerging diseases often seem to have materialized out of thin air, they usually occur because environmental and demographic changes bring already existing germs and humans together. These changes don't occur in isolation, but rather work together in a complex series of interactions. The emergence of hantavirus pulmonary syndrome in the southwestern United States in 1993 is a case in point.

The virus that causes hantavirus pulmonary syndrome (Sin Nombre virus, meaning "No Name virus" in Spanish) spreads to humans through contact with the infected droppings, urine or saliva of deer mice. It's likely that mice in the Four Corners region — an area shared by Colorado, New Mexico, Arizona and Utah where the syndrome first appeared — have carried the virus for hundreds, perhaps thousands, of years. But in 1993, a particular confluence of factors created conditions that allowed the virus to spread to people.

The most important of those factors was a dramatic rise in the mouse population, which burgeoned in 1993 — an unusually mild and rainy year that followed a long drought. Researchers at the Centers for Disease Control and Prevention reported a tenfold increase in mice in the Four Corners area between 1992 and 1993 alone.

But not only did the weather bring more mice into contact with people, it also brought more people into contact with mice. Like the rodent population, the human population had grown: The area was popular with hikers and campers, and more and more vacationers were building second homes in the area. The combination of a large number of infected animals, an increasing number of people and a shrinking habitat is likely to have led to the emergence of a new disease, though one that in all probability was new in name only.

Still, the origins of all emerging disease aren't quite as clear-cut. No one, for instance, yet knows the source of Ebola or of Marburg virus, a slightly less lethal version of Ebola. Although nonhuman primates such as monkeys, chimpanzees and gorillas are susceptible to these diseases and can transmit them to humans, scientists don't think that they're the original source of infection (natural reservoir). Whatever the natural reservoir, it's likely to have harbored the Ebola virus for some time. It's also likely that the virus would have remained hidden had humans not inadvertently come in contact with it.

Human activity is often responsible for the abrupt appearance of a supposedly "new" pathogen. When people alter or encroach on native habitat, especially habitat that's still relatively biodiverse, the stage is set for an emerging disease. Combine this with the mobility and interconnectedness of the modern world, and it becomes clear how an infection that once might have remained isolated in an obscure village can become a worldwide epidemic — sometimes in an astonishingly short period of time. The rapid spread of SARS is a notable example of just how quickly a local illness can become a global problem.

What, where, when and how
Most of the viruses associated with VHFs are zoonotic, which means they reside in an animal or insect host and are dependent on that host for their survival. In general, humans acquire the virus when they come into contact with an infected host, but in some cases, person-to-person transmission can occur after the initial infection. Human epidemics occur sporadically, usually because of a combination of climatic, ecological and social factors.

Arenaviruses
Lassa fever, the greatest public health threat of the arenaviruses, is endemic in West Africa, where it may spread in the following ways:

• Wild rodents. The multimammate rat is the main source of Lassa virus infection. You can acquire the virus from direct contact with infected rat droppings or urine, from touching surfaces or objects or eating food contaminated with these excretions, or by inhaling airborne particles that contain the virus (airborne or aerosol transmission).
• Person-to-person contact. You can contract the virus if you come in contact with the blood, throat secretions or urine of an infected person, especially during the acute fever stage of the illness.
• Sexual contact. Because the virus can be transmitted in semen long after infection, experts recommend that men who have recovered from Lassa fever refrain from sexual activity for at least three months.
• Contaminated needles and syringes. This form of transmission is most likely to occur through an accidental needle stick or in a hospital in a developing nation where equipment may be used on more than one person.

Argentine, Bolivian, Brazilian and Venezuelan hemorrhagic fevers are less pervasive illnesses occurring only in South America. All are carried by rodents. The Bolivian type can also be transmitted from person to person.

Bunyaviruses
The diseases in this group spread by contact with an infected insect or animal:

• Mosquitoes. Rift Valley fever primarily affects animals, especially sheep, cattle, camels and goats, but it can also cause severe disease in humans. It's spread by a number of mosquito species, some of which transmit the virus to their offspring. The infected eggs can survive years of drought, only to release hordes of voracious insects when the rains return. Noninfected mosquitoes pick up the virus when they feed on sick animals and then pass it on to their next victim, perpetuating the cycle of infection. Humans contract the virus through a mosquito bite or through contact with the blood, organs, body fluids and possibly the raw milk of infected animals.
• Ticks. Crimean-Congo hemorrhagic fever, a severe VHF that's often fatal, is endemic in many countries in Africa, Europe and Asia. It's spread by ticks, some of which transmit the virus to their offspring via eggs. More often, ticks acquire the virus when they feed on infected animals, especially cattle, sheep and goats. Humans may become infected from a tick bite or from direct contact with the blood or tissues of sick livestock, especially during the first week after the animal has been exposed to the virus.
• Rodents. Almost all of the 14 or so hantaviruses, which occur worldwide, are spread by rodents, although each virus appears to have a single primary natural reservoir. A few reports have cited other animals as a possible source of infection, but in most cases, you contract the virus when you breathe the aerosolized droppings of infected rodents. The risk is especially high for farm workers, campers and other people who spend time outdoors in endemic areas, but you can also be exposed to the virus when rodents invade your home during cold weather or nest near your house. In some cases, you may contract a hantavirus from a rodent bite.

Filoviruses
The natural reservoir of these diseases isn't known, but once a person is infected, the virus can be transmitted in several ways:

• Person-to-person contact. You can become infected if you're exposed to the blood, body fluids or organs of someone who's severely ill with Ebola or Marburg virus. Most at risk are health care workers and family members caring for a sick relative.
• Contaminated needles and syringes. This form of transmission is most likely to occur in a hospital when equipment is reused or through an accidental needle stick.
• Sexual contact. The Ebola and Marburg viruses can be transmitted through semen, even when the affected person is no longer sick. For that reason, experts recommend refraining from sexual activity for at least three months after recovery.
• Contact with nonhuman primates. In a few instances, researchers have contracted Marburg virus after handling monkeys infected with the disease.

Flaviviruses
In most cases, insects are responsible for the spread of this family of viruses:

• Mosquitoes. Yellow fever and dengue hemorrhagic fever are the most virulent forms of flaviviruses, and both are spread by a particularly efficient species of mosquito: Aedes aegypti. The mosquitoes thrive in and near human habitations where they breed in even the cleanest water. Although most cases of yellow fever and dengue fever occur in South America, Southeast Asia and parts of Africa, a few isolated cases have been reported in the continental United States and in Hawaii. Although these cases are rare, Aedes aegypti is found in a large portion of the United States, increasing the possibility of urban epidemics in this country.
• Ticks. Kyasanur Forest disease and Omsk hemorrhagic fever are far more obscure and isolated illnesses — Omsk hemorrhagic fever, for example, occurs only in parts of western Siberia where it mainly affects muskrat trappers. Although ticks spread both diseases, you can also contract Omsk hemorrhagic fever by handling infected muskrats or by drinking water that contains the virus.

The chance of contracting most VHFs is low. Some are confined to isolated pockets in remote areas where the risk of transmission is slight. And because each virus is usually associated with a specific host species, it's normally restricted to the area where that species lives.

Yet taken together, the viruses that cause VHFs occur over most of the world. And some VHFs, especially Lassa, yellow and dengue hemorrhagic fevers, pose a real threat to people traveling to or living in regions where these diseases are widespread, primarily sub-Saharan Africa and South America.

In general, your risk of contracting any disease abroad depends on your itinerary and activities, the length of your stay and the rate of transmission of a particular disease at the time. If you're visiting a region for a week or two, staying in westernized hotels and taking guided tours, your risk is less than if you're traveling for months and living in a tent. Still, you're at risk of epidemic diseases such as dengue fever even in the best of circumstances, although your risk decreases if there are no outbreaks of the disease during your visit.

In the United States, you're more likely to contract a hantavirus if your region has a large rodent population known to carry the virus. The risk increases if you live in the country or suburbs, you work outdoors, or you spend time in rodent-infested buildings. Still, the risk is generally low. As of May 2006, slightly more than 400 cases of hantavirus pulmonary syndrome had been reported in the United States since 1993. Most occurred in states west of the Mississippi.

On the other hand, hospital workers treating people with VHFs as well as researchers and laboratory personnel who deal with the viruses are at high risk of infection. In Africa, health care workers frequently contract Ebola from patients, and a number of cases of laboratory-acquired infection have been documented worldwide.

See a doctor immediately if you suspect you've been exposed to a VHF virus. If you become ill while traveling in a foreign country, call the U.S. Consulate for a list of doctors. Better yet, find out in advance about medical care in the areas you'll visit, and carry a list of the names, addresses and phone numbers of recommended English-speaking doctors. Your doctor, local or state medical society, the International Association for Medical Assistance to Travellers or the Department of State's Office of Overseas Citizens Services can help provide the information you need. In addition, many experts strongly advise purchasing medical evacuation insurance before you travel.

If you develop signs and symptoms once you return home, consider consulting a doctor who focuses on international medicine or infectious diseases. A specialist may be able to recognize and treat your illness more quickly than a doctor who isn't trained in these areas can.

Diagnosing specific VHFs in the first few days of illness can be difficult. All infected people initially exhibit the same signs and symptoms: fever, muscle aches, headache and extreme fatigue. What's more, these signs and symptoms frequently occur with many diseases that are far more common than VHFs.

To reach an accurate diagnosis, your doctor is likely to ask about your medical and travel history and any exposure to rodents or mosquitoes. Be sure to describe international trips in detail, including the countries you visited and the dates, as well as any contact you may have had with possible sources of infection.

Laboratory tests, usually using a sample of your blood, are needed to confirm a diagnosis of VHF. Because VHF viruses are particularly virulent and contagious, these tests are usually performed in specially designated laboratories using strict precautions.

Although complications of VHFs vary, some problems occur with many of the viruses. They include:

• Uveitis. This eye inflammation affects the uvea, the layer just beneath the white of your eye (sclera). Uveitis causes redness, pain and sensitivity to light, but although it's serious, it rarely leads to permanent blindness if treated promptly and appropriately.
• Pericarditis. This is swelling and irritation of the pericardium, the thin, membranous sac that surrounds your heart. Pericarditis usually causes sharp, stabbing chest pain, shortness of breath and an overall feeling of weakness or fatigue. Mild cases may go away on their own, but severe pericarditis can lead to life-threatening conditions such as cardiac tamponade, which puts pressure on your heart and interferes with its ability to pump blood efficiently.
• Encephalitis. This severe, potentially life-threatening brain inflammation may occur on its own or follow another viral infection. Severe encephalitis can cause respiratory arrest, marked mental impairment, coma and death.
• Orchitis. This acute inflammation of the testicles causes swelling and pain and usually occurs as a result of a viral infection. In some cases, orchitis can lead to infertility.
• Spontaneous abortion. Loss of an unborn child is a frequent complication in pregnant women who develop VHFs.
• Hair loss. Many people who recover from VHFs experience temporary or permanent hair loss.

No specific treatment exists for most VHFs, although the antiviral drug ribavirin may help shorten the course of infection and prevent complications in arenaviruses and bunyaviruses. Whether bleeding complications should be treated with therapies such as clotting factors, platelets and heparin remains a matter of debate.

Supportive care is essential for every person with VHF, no matter what type of virus is involved. This is likely to include measures to:

• Provide adequate fluids and maintain the balance of electrolytes — substances such as sodium, potassium, chloride and calcium, which control the movement of nutrients into cells
• Aid breathing with a mechanical ventilator, if necessary
• If needed, provide kidney dialysis, an artificial way of removing excess fluids and wastes from your body when your kidneys stop working
• Provide therapy for secondary infections and other complications

Preventing VHFs, especially in developing nations, presents enormous challenges. Many of the social, economic and ecological factors that contribute to the sudden appearance and spread of infectious diseases — war, displacement, destruction of habitat, lack of sanitation and proper medical care — are problems that have no easy or quick solutions. For that reason, the best approach in the short-term is to take precautions to protect yourself from infection:

• Get vaccinated. Vaccines exist for yellow fever and Argentine hemorrhagic fever. If you're traveling to areas where these diseases are endemic, consider being vaccinated. The yellow fever vaccine is generally considered safe and effective, although in rare cases, serious side effects can occur. Check with the Centers for Disease Control and Prevention about the status of the countries you're visiting — some require certificates of vaccination for entry. The yellow fever vaccine isn't recommended for children under 9 months of age or for pregnant women, especially during the first trimester.
• Be sure you're mosquito-proof. Take common-sense precautions to protect yourself and your family from mosquitoes, such as wearing light-colored long pants and long-sleeved shirts or better yet, permethrin-coated clothing. Avoid unnecessary activities at dusk and dawn when mosquitoes are most active and apply mosquito repellent with a 20 percent to 25 percent concentration of DEET to your skin and clothing. DEET is a powerful insecticide; use it with care. Apply it in moderation and don't use it on the hands of young children or on infants under 2 months of age. Oil of lemon eucalyptus and other more natural products, including oils of geranium, cedar and lemongrass, may offer protection similar to low concentrations of DEET. If you're staying in tented camps or local hotels, use bed nets and mosquito coils.
• Avoid high-risk areas. Before you travel, obtain a consular information sheet from the Office of Overseas Citizens Services, which provides information on health and security issues. If there's an outbreak or epidemic of infectious disease in an area you're planning to visit, consider revising your itinerary.

At home
Rodent control in and around your home is the primary way to prevent hantavirus infection. These measures can help:

• Keep pet food covered and stored in rodent-proof containers, and don't leave food out overnight.
• Store trash in rodent-proof containers, and clean the containers often.
• Dispose of garbage on a regular basis.
• Make sure doors and windows have tightfitting screens.
• If you use traps inside or outside, be sure to keep children away from them.
• Place woodpiles and stacks of bricks and other materials at least 100 feet from your house.
• Mow your grass closely and keep brush trimmed to within 100 feet from your house.