Ebola is the common term for a group of viruses belonging to genus Ebolavirus, family Filoviridae, which cause Ebola hemorrhagic fever. The disease can be deadly and encompasses a range of symptoms including vomiting, diarrhea, changes in skin color, general body pain, internal and external bleeding, and fever. Mortality rates are generally high, ranging from 50% – 100%, with the cause of death usually due to hypovolemic shock or Multiple organ dysfunction syndrome.
The virus is named after the Ebola River in the African nation-state of the Democratic Republic of the Congo (formerly Zaire), near the site of the first outbreaks. The Democratic Republic of Congo has been the site of four recent outbreaks, including one in May 2005.
Ebola is believed to be a zoonotic virus, although despite considerable effort by the World Health Organization, no animal reservoir capable of sustaining the virus between outbreaks has been identified. One possible candidate reservoir is the fruit bat. Another is the dog.
Because Ebola is lethal and since no approved vaccine or treatment is available, Ebola is classified as a Biosafety Level 4 agent, as well as a Category A Bioterrorism agent and a select agent by the CDC.
The symptoms of Ebola are rather similar to that of the Marburg virus, which is also in the family Filoviridae.
Size and Shape
Electron micrographs of members of Ebolavirus show them to have the characteristic thread-like structure of a filovirus. The virions are variable in shape and may appear as a “U”, “6”, coiled, circular, or branched shape, however, laboratory purification techniques, such as centrifugation, may contribute to the various shapes seen. Virions are generally 80 nm in diameter. They are variable in length, and can be up to 1400 nm long. On average however, the length of a typical Ebola virus is closer to 1000 nm. In the center of the virion is a structure called nucleocapsid, which is formed by the helically wound viral genomic RNA complexed with the proteins NP, VP35, VP30 and L. It has a diameter of 40 – 50 nm and contains a central channel of 20 – 30 nm in diameter. Virally encoded glycoprotein (GP) spikes 10 nm long and 10 nm apart are present on the outer viral envelope of the virion, which is derived from the host cell membrane. Between envelope and nucleocapsid, in the so called matrix space, the viral proteins VP40 and VP24 are located.
Each virion contains one molecule of linear, single-stranded, negative-sense RNA, totalling 18900 nucleotides in length. The 3′ terminus is not polyadenylated and the 5′ end is not capped. It codes for seven structural proteins and one non-structural protein. The gene order is 3′ – leader – NP – VP35 – VP40 – GP/sGP – VP30 – VP24 – L – trailer – 5′; with the leader and trailer being non-transcribed regions which carry important signals to control transcription, replication and packaging of the viral genome into new virions. The genomic material by itself is not infectious, because viral proteins, among them the RNA-dependent RNA polymerase, are necessary to transcribe the viral genome into mRNAs, as well as for replication of the viral genome.
Ebola Haemorrhagic Fever
Symptoms are varied and often appear suddenly. Initial symptoms include: high fever (at least 38.8°C/101°F), severe headache, muscle, joint, or abdominal pain, severe weakness and exhaustion, sore throat, nausea, and dizziness. Before an outbreak is suspected, these early symptoms are easily mistaken for malaria, typhoid fever, dysentery, influenza, or various bacterial infections, which are all far more common.
Ebola goes on to cause diarrhea, dark or bloody stool, vomiting blood, red eyes from swollen blood vessels, red spots on the skin from subcutaneous bleeding, maculopapular rash, purpura, and bleeding internally and externally from any orifice, including from the nose, mouth, rectum, genitals or needle puncture sites.
Other secondary symptoms include hypotension (less than 90mm Hg), hypovolemia, tachycardia, severe organ damage (especially the kidneys, spleen, and liver) as a result of disseminated systemic necrosis, and proteinuria. The span of time from onset of symptoms to death (usually due to hypovolemic shock and/or multiple organ failure) is usually between 7 and 14 days. By the second week of infection, patients will either defervesce (the fever will lessen) or undergo systemic multiorgan failure.
Among humans, the virus is transmitted by direct contact with infected body fluids, or to a lesser extent, skin or mucus membrane contact. The incubation period can be anywhere from 2 to 21 days, but is generally between 5 and 10 days.
Although airborne transmission between monkeys has been demonstrated in a laboratory, there is very limited evidence for human-to-human airborne transmission in any reported epidemics. Nurse Mayinga might represent the only possible case. The means by which she contracted the virus remain uncertain.
So far all epidemics of Ebola have occurred in sub-optimal hospital conditions, where practices of basic hygiene and sanitation are often either luxuries or unknown to caretakers and where disposable needles and autoclaves are unavailable or too expensive. In modern hospitals with disposable needles and knowledge of basic hygiene and barrier nursing techniques, Ebola rarely spreads on such a large scale.
In the early stages, Ebola may not be highly contagious. Contact with someone in early stages may not even transmit the disease. As the illness progresses, bodily fluids from diarrhea, vomiting, and bleeding represent an extreme biohazard. Due to lack of proper equipment and hygienic practices, large scale epidemics occur mostly in poor, isolated areas without modern hospitals and/or well-educated medical staff. Many areas where the infectious reservoir exists have just these characteristics. In such environments all that can be done is to immediately cease all needle sharing or use without adequate sterilization procedures, to isolate patients, and to observe strict barrier nursing procedures with the use of a medical rated disposable face mask, gloves, goggles, and a gown at all times. This should be strictly enforced for all medical personnel and visitors.
Treatment is primarily supportive and includes minimizing invasive procedures, balancing electrolytes, replacing lost coagulation factors to help stop bleeding, maintaining oxygen and blood levels, and treating any complicating infections. Despite some initial anecdotal evidence, blood serum from Ebola survivors has been shown to be ineffective in treating the virus. Interferon is also thought to be ineffective. In monkeys, administration of an inhibitor of coagulation (rNAPc2) has shown some benefit, protecting 33% of infected animals from a usually 100% (for monkeys) lethal infection. In early 2006, scientists at USAMRIID announced a 75% recovery rate after infecting four rhesus monkeys with Ebola virus and administering antisense drugs.
Vaccines have been produced for both Ebola and Marburg that were 100% effective in protecting a group of monkeys from the disease. These vaccines are based on either a recombinant Vesicular stomatitis virus or a recombinant Adenovirus carrying the Ebola spike protein on its surface. Early human vaccine efforts, like the one at NIAID in 2003, have so far not reported any successes.
Despite numerous studies, the wildlife reservoir of Ebolavirus has not been identified. Between 1976 and 1998, from 30,000 mammals, birds, reptiles, amphibians and arthropods sampled from outbreak regions, no Ebolavirus was detected apart from some genetic material found in six rodents (Mus setulosus and Praomys species) and a shrew (Sylvisorex ollula) collected from the Central African Republic in 1998. Ebolavirus was detected in the carcasses of gorillas, chimpanzees and duikers during outbreaks in 2001 and 2003 (the carcasses were the source of the initial human infections) but the high mortality from infection in these species precludes them from acting as reservoirs.
Plants, arthropods and birds have also been considered as reservoirs, however bats are considered the most likely candidate. Bats were known to reside in the cotton factory in which the index cases for the 1976 and 1979 outbreaks were employed and have also been implicated in Marburg infections in 1975 and 1980. Of 24 plant species and 19 vertebrate species experimentally inoculated with Ebolavirus, only bats became infected. The absence of clinical signs in these bats is characteristic of a reservoir species. In 2002-03, a survey of 1,030 animals from Gabon and the Republic of the Congo including 679 bats found Ebolavirus RNA in 13 fruit bats (Hyspignathus monstrosus, Epomops franquetti and Myonycteris torquata). Bats are also known to be the reservoirs for a number of related viruses including Nipah virus, Hendra virus and lyssaviruses.
Ebola as a Weapon
Ebola is classified as a Category A Biological terrorism agent by the CDC as well as being considered a select agent that has the “potential to pose a severe threat to public health and safety”. Ebola was considered in biological warfare research at both Fort Detrick in the United States and Biopreparat in the Soviet Union during the Cold War.
Ebola shows potential as a biological weapon because of its lethality but due to its relatively short incubation period it may be more difficult to spread since it may kill its victim before it has a chance to be transmitted. As a result, some developers have considered breeding it with other agents such as smallpox to create so-called chimera viruses.
As a terrorist weapon, Ebola has been considered by members of Japan’s Aum Shinrikyo cult, whose leader, Shoko Asahara led about 40 members to Zaire in 1992 under the guise of offering medical aid to Ebola victims in what was presumably an attempt to acquire a sample of the virus.
Popular Description and Representation
Ebola and Marburg have served as a rich source of ideas and plotlines for many forms of entertainment. The infatuation with the virus is likely due to the high mortality rate of its victims, its mysterious nature, and its tendency to cause gruesome bleeding from body orifices.
Much of the representation of the Ebola virus in fiction and the media is considered exaggerated or myth. Many of the stories about Ebola in Preston’s book The Hot Zone are refuted in the book Level 4: Virus Hunters of the CDC by Joseph B. McCormick, an employee of the CDC at the time of the early outbreaks. One pervasive myth follows that the virus kills so fast that it has little time to spread. Victims die very soon after contact with the virus. In reality, the incubation time is usually about a week. The average time from onset of early symptoms to death varies in the range 3-21 days, with a mean of 10.1. Although this would prevent the transmission of the virus to many people, it is still enough time for some people to catch the disease.
Another myth states that the symptoms of the virus are horrifying beyond belief. Victims of Ebola suffer from squirting blood, liquefying flesh, zombie-like faces and dramatic projectile bloody vomiting, at times, from even recently deceased. In actual fact, only a fraction of Ebola victims have severe bleeding that would be even somewhat dramatic to witness. Approximately 10% of patients suffer some bleeding, but this is often internal or subtle, such as bleeding from the gums. Ebola symptoms are usually limited to extreme exhaustion, vomiting, diarrhea, abdominal pain, a high fever, headaches and other body pains.
The following is an excerpt from an interview with Philippe Calain, M.D. Chief Epidemiologist, CDC Special Pathogens Branch, Kikwit 1996:
“At the end of the disease the patient does not look, from the outside, as horrible as you can read in some books. They are not melting. They are not full of blood. They’re in shock, muscular shock. They are not unconscious, but you would say ‘obtunded’, dull, quiet, very tired. Very few were hemorrhaging. Hemorrhage is not the main symptom. Less than half of the patients had some kind of hemorrhage. But the ones that had bled, died.”