What are viral hemorrhagic fevers?
Viral hemorrhagic fevers (VHFs) refer to a group of illnesses caused by several families of viruses. In general, the term "viral hemorrhagic fever" is used to describe a severe condition in which multiple organ systems in the body are affected. Typically, the vascular (blood vessel) system is damaged, and the body loses its ability to control bleeding. Some types of hemorrhagic fever viruses cause relatively mild illnesses; others cause severe, life-threatening disease.
How are hemorrhagic fever viruses grouped?VHFs are caused by viruses of four distinct families:
- Arenaviruses (Lassa fever and New World arenaviruses, such as Argentine hemorrhagic fever and Bolivian hemorrhagic fever)
- Filoviruses (Ebola hemorrhagic fever and Marburg hemorrhagic fever)
- Bunyaviruses (Crimean-Congo hemorrhagic fever, Rift Valley fever, hantavirus pulmonary syndrome, and hemorrhagic fever with renal syndrome)
- Flaviviruses (dengue, yellow fever, Omsk hemorrhagic fever, and Kyasanur Forest disease)
These virus families share a number of features:
- They are all RNA viruses, and all are covered in a fatty coating, making them susceptible to detergents and disinfectants.
- The viruses live and breed within an animal or insect, called the natural reservoir or host.
- The viruses are restricted to the geographic areas where their host species live.
- Humans are not the natural reservoir for any of these viruses. Humans are infected when they come into contact with infected hosts. However, with the exceptions of Rift Valley fever, most hantaviruses, and all the flaviviruses, if humans do become infected, they can spread the virus to one another.
- Human cases or outbreaks of hemorrhagic fevers caused by these viruses rarely occur. An outbreak cannot be easily predicted.
- With a few exceptions, there is no cure or established drug treatment for VHFs.
In rare cases, other viral and bacterial infections can cause a hemorrhagic fever; scrub typhus is an example.
What carries viruses that cause viral hemorrhagic fevers?
Most viruses associated with VHFs naturally exist in an animal (primarily rodents) or insect host and are therefore termed zoonotic. The viruses reproduce and are maintained within their host, usually without causing illness. They are totally dependent on their hosts for reproduction and overall survival. Examples of reservoir hosts include the mulitmammate rat, cotton rat, deer mouse, house mouse, and other field rodents. Ticks and mosquitoes serve as insect vectors - insects that can spread the virus - for some of the illnesses. The hosts of some viruses, such as Ebola and Marburg viruses, are unknown.
Where are cases of viral hemorrhagic fever found?
Taken together, the viruses that cause VHFs are distributed over much of the globe. However, because each virus is associated with particular host species, the virus and the disease it causes are usually seen only where the host species live(s). Some hosts live in limited areas, restricting the risk of getting VHFs caused by these viruses to those areas. Other hosts may be found throughout entire continents. Examples include the group of rodents that carries the viruses responsible for hantavirus pulmonary syndrome (HPS) in North and South America, or the different group of rodents that carries the viruses that cause hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia. A few hosts are distributed nearly worldwide, such as the common rat. It can carry Seoul virus, a cause of HFRS; therefore, humans can get HFRS anywhere where the common rat is found.
People usually become infected only in areas where the host lives, but there are exceptions. In rare instances, people become infected by a host that has been exported from its native habitat. For example, the first outbreaks of Marburg hemorrhagic fever, in Marburg and Frankfurt, Germany, and in Yugoslavia, occurred when laboratory workers handled imported monkeys infected with Marburg virus.
Occasionally, a person becomes infected in an area where the virus occurs naturally and then travels elsewhere. If the virus is a type that can be spread by person-to-person contact, the traveler could infect other people. For instance, in 1996, a medical professional treating patients with Ebola hemorrhagic fever (Ebola HF) in Gabon unknowingly became infected. When he later traveled to South Africa and was treated for Ebola HF in a hospital, the virus was spread to a nurse. She became ill and died. Because more and more people travel each year, outbreaks of these diseases are becoming an increasing threat in places where they rarely, if ever, have been seen before.
If a VHF were to occur in people not known to have traveled to an affected area, the possibility of a bioterrorist attack would be considered.
How are hemorrhagic fever viruses spread?
Viruses causing hemorrhagic fever are first transmitted (spread) to humans when humans interact with the reservoir hosts or the host environment. For example, the viruses carried in rodent hosts are transmitted to humans through contact with urine, feces, saliva, or other body excretions from the infected rodents. The viruses transmitted by insects are usually spread when the vector mosquito or tick bites a human, or when a human crushes a tick. However, some of the insect vectors may first spread the virus to animals (livestock, for example). Humans then become infected when they care for or slaughter the infected animals.
Some viruses that cause VHF can spread from one person to another, once the first person has become infected. Ebola, Marburg, Lassa, and Crimean-Congo hemorrhagic fever viruses are examples. This type of transmission of the virus can occur directly, through close contact with infected people or their body fluids. It can also occur indirectly, through contact with objects contaminated with infected body fluids. For example, contaminated syringes and needles have played an important role in spreading infection in outbreaks of Ebola hemorrhagic fever and Lassa fever.
The possibility that some VHFs could be spread through the air is unlikely, but it cannot be ruled out completely. It is known, for example, that the viruses can be spread by way of air to lab personnel during certain lab procedures, but the specific conditions needed for aerosol transmission to occur would be difficult for terrorists to create outside a lab setting.
What are the symptoms of viral hemorrhagic fever illnesses?
Specific signs and symptoms vary by the type of VHF, but they often begin with high fever, fatigue, dizziness, muscle aches, loss of strength, and exhaustion. People with severe cases of VHF show signs of bleeding under the skin, in internal organs, or from the mouth, eyes, or ears. However, although they may bleed from many sites around the body, people rarely die because of blood loss. Severely-ill people may also experience shock, coma, delirium, or seizures. Some types of VHF are associated with kidney failure.
How soon after infection do symptoms appear?
Depending on the virus, symptoms can appear between 2 and 30 days following exposure.
Are there medical tests to determine whether a person has been exposed to a hemorrhagic fever virus?
How are patients with viral hemorrhagic fever treated?
Patients receive supportive medical therapy, but generally speaking, there is no other treatment or cure for VHFs. Ribavirin, an anti-viral drug, has been effective in treating some people with Lassa fever or hemorrhagic fever with renal syndrome.
How can cases of viral hemorrhagic fever be prevented and controlled?
With the exception of yellow fever and Argentine hemorrhagic fever, for which vaccines have been developed, no vaccines exist that can protect against these diseases. Therefore, in places where VHFs occur naturally, prevention efforts focus on avoiding contact with animal and insect hosts. If prevention methods fail and a case of VHF were to occur, precautions would be taken to prevent spread from person to person, if the virus can be transmitted in this way. (Only some VHFs can be spread from person to person.)
Hantavirus is the only VHF that naturally occurs in the eastern United States. It is a rare disease that occurs in people exposed to mouse urine and droppings contaminated with the virus. Most cases of hantavirus have occurred among people who have cleaned hunting cabins and other dwellings without wearing appropriate masks.
For those hemorrhagic fever viruses that can be transmitted from one person to another, avoiding close physical contact with infected people and their body fluids is the most important way of controlling the spread of disease. Proper infection control practices, such as isolating infected patients and wearing protective clothing when treating them, would be taken by health-care facilities and health professionals if a case of VHF occurred in New York City.
What has New York City done to address the threat of viral hemorrhagic fevers?
Many federal, State, and City agencies-including the New York City Health Department-have been working together for several years to prepare for the detection and response to a bioterrorist event in New York City. In cooperation with other emergency response agencies, the New York City Health Department has established a comprehensive surveillance system to improve our ability to detect and respond to the release of a biological agent.
As part of this plan, the New York City Health Department is continuously looking for any indication of bioterrorism in the City. The New York City Health Department regularly asks all health-care providers in New York City to be alert for any unusual disease clusters, and to immediately report such occurrences to us. Additionally, several surveillance systems are in place to quickly detect an increase in unusual illnesses, including monitoring of 911-ambulance calls and emergency department visits. The response plan includes coordinating with OEM and other City, State, and federal agencies; alerting hospitals and the medical care community; communicating with the public; and ensuring that appropriate medical care and prevention services are provided.
Hantavirus and the related Bunyavirus, which causes Crimean-Congo hemorrhagic fever, cannot be grown in laboratories. Therefore, it is unlikely that either virus could be used as a biological weapon.
The New York Cith Health Department has worked closely with the New York City hospital community to prepare for the challenges that the City would face if a biological agent were released intentionally. If one or more cases of VHF occurred in New York City, the partnership between the New York City Health Department, the Office of Emergency Management, and the hospital community would ensure that patients receive the necessary medical care and that the outbreak would be contained.
Last Updated: December 4, 2012