Viruses are one of the most common infectious agents, causing disease in millions of people on a daily basis. We all get one or two viral infections in a year. The flu and common cold are frequently seen examples of infections caused by viruses. Most of us know that the flu and cold are not serious and rarely lead to death. On the other hand, viruses like HIV cause a more serious viral infection that will eventually progress to AIDS with fatal consequences. Among the most feared of viral infections are those that cause hemorrhagic fever which can lead to a grisly death with internal and external bleeding.
Viral hemorrhagic fever is a disease caused by certain viruses where the bleeding occurs internally and through the orifices (external openings). A fever is a characteristic feature, as it is with most viral infections, and along with the bleeding (hemorrhage) these diseases are therefore known a viral hemorrhagic fever (VHF). There are about 30 viruses that can cause viral hemorrhagic fevers but the focus is often on a few that are either the most dangerous to humans or result in outbreaks, like the 2014 Ebola virus outbreak.
Prior to the 2009 swine flu outbreak, it was always feared that a viral hemorrhagic fever would be the next biological threat to humans on a global scale. The swine flu is caused the H1N1 influenza virus. It did not ultimately be as deadly as initially thought although there is still fear of other types of influenza virus outbreaks. However, in 2014 the spotlight has again fallen on viral hemorrhagic fevers and the potential for a global pandemic given that modern travel patterns and modes of transport mean that people can easily carry these viruses across vast distances in short periods of time.
Dangers of Viral Hemorrhagic Fever
Is viral hemorrhagic fever deadly?
Not all viral hemorrhagic fevers will lead to death in every person who contracts it. Some of the most deadly strains of the Ebola virus do not have a 100% mortality rate, meaning that it will not kill every person who acquires the infections. However, with mortality rates as high as 90%, viral hemorrhagic fevers indeed rank among the most deadly of viral infections. Understanding how the virus causes disease and leads to potentially life-threatening complication is therefore important for every person to know.
How do these viruses cause bleeding?
Viruses are unusual infectious agents when compared to bacteria and fungi. They are minute particles made of an outer coat of protein containing genetic material within it. This genetic material is then injected into host cells which manufacture more of the virus and ultimately kills the cells. Some like HIV have a predilection for only certain cells within the human body, while others will target most cells. Viral hemorrhagic fevers have an affinity for the vascular system which results in its characteristic feature of bleeding (hemorrhage).
In terms of the Ebola virus, it targets mainly the endothelial cells lining the inside of blood vessels, as well as liver cells and certain immune cells. It stimulates certain cells to produce a glycoprotein to suppress immune activity against it thereby evading the immune defenses of the host. As it replicates inside the cells, it eventually causes damage and death to these host cells. This cell damage then triggers inflammation and fever. As a result of the endothelial damage to cells during reproduction the integrity of blood vessels are compromised and it leads to bleeding.
Types of Viruses, Diseases and Location
Viral hemorrhagic fevers are caused by viruses belonging to four families. However, there are other viruses and even some bacteria that can cause hemorrhagic fever although this is rare. In terms of viral hemorrhagic fevers, the families of these viruses can be classified accordingly:
Where in the world are these viruses found?
Contrary to popular belief, not all viral hemorrhagic fevers are native to Africa. These infections can be found across the globe and originate on almost every continent. Modern travel has allowed these infections to be easily carried across oceans and spread among populations that were not previously at risk based on their location. Therefore viral hemorrhagic fevers, especially deadly diseases like Ebola, are a worldwide concern.
- Junin virus: Argentine hemorrhagic fever (South America)
- Lassa virus: Lassa fever (Africa)
- Lymphocytic choriomeningitis virus (LCMV): Lymphocytic choriomeningitis (Americas, Australia, Europe, Japan)
- Machupo virus: Bolivian hemorrhagic fever (South America)
- Guanarito virus: Venezuelan hemorrhagic fever (South America)
- Sabia virus: Brazilian hemorrhagic fever (South America)
- Chapare virus: Chapare hemorrhagic fever (South America)
- Lujo virus: Lujo hemorrhagic fever (Africa)
- Phlebovirus: Rift Valley fever (Africa)
- Nairovirus: Crimean-Congo hemorrhagic fever (Africa, Asia, Europe)
- Hantavirus: Hemorrhagic fever with renal syndrome, Hantavirus pulmonary syndrome (Asia, Europe, worldwide)
- Filovirus: Marburg and Ebola (Africa)
- Flavivirus: Yellow fever (Africa, South America)
- Flavivirus: Dengue hemorrhagic fever (Africa, Americas, Asia)
These are some of the viruses in each family that may cause viral hemorrhagic fever in humans.
Animal and Insect Carriers
Most viruses that cause viral hemorrhagic fever naturally reside in certain animals or insects. These organisms are the natural reservoirs in which the virus resides and replicates. In most instances the animal or insect will not develop the disease despite being a carrier. A person may contract the disease directly from the natural reservoir, usually through a bite, touching its urine or feces, or making contact with its infected tissues. This is known as primary exposure. However, the virus can also be spread from one infected person to another and this is known as secondary exposure.
Here are some of the carriers of the viral hemorrhagic fevers discussed above:
- Rodents: Junin virus, Lassa virus, Lymphocytic choriomeningitis virus, Machupo virus, Guanarito virus, Sabia virus, Chapare virus, Lujo virus, Hantavirus.
- Mosquitoes: Phlebovirus (Rift Valley fever), Flavivirus (yellow fever, dengue)
- Ticks: Nairovirus (Crimean-Congo hemorrhagic fever)
The natural reservoir of some viruses are still unknown. It is believed that the natural reservoir for Ebola and Marburg is the fruit bat but this has not been conclusively established as yet. Similarly the natural reservoir for Chapare virus is thought to be the rodent and usually rodents are the natural reservoir for all arenaviruses.
How long before symptoms appear?
The time between infection and the development of symptoms is known as the incubation period. It varies among the different diseases and even among individuals.
The incubation period for most of the arenaviruses mention above is between 5 days and 2 weeks. For the other viruses it is as follows:
- Phlebovirus: 2 to 5 days
- Nairovirus: 3 to 12 days
- Hantavirus: 9 to 35 days
- Marburg and Ebola: 3 to 16 days
- Yellow fever: 3 to 6 days
- Dengue hemorrhagic fever: Unknown (for dengue it is 3 to 7 days)