An outbreak of Ebola hemorrhagic fever that began in early July 2012 has involved at least 36 individuals and 16 deaths. So far the disease has been confined to a rural region west of Kampala, the capital of Uganda. The subject of Richard Preston’s scary The Hot Zone, Ebola virus is newsworthy because it can be highly lethal and inspires scenarios of pandemics. An examination of the virus and the disease should temper our fear of this pathogen.
Ebola virus was first isolated in 1976 during an outbreak near the Ebola River in northern Democratic Republic of the Congo. There are five distinct Ebola virus species: Zaire, Sudan, Cote d’Ivoire, Bundibugyo, and Reston. Ebola virus Sudan has been isolated from individuals in the Ugandan outbreak. This Ebola virus species, along with Zaire and Bundibugyo, has been associated with outbreaks of highly lethal disease in Africa. Since the discovery of Ebola virus there have been 1,850 documented cases and more than 1,200 deaths.
Humans are first infected with Ebola virus through the handling of infected chimpanzees, gorillas, forest antelopes, and porcupines. These infected individuals then transmit the virus to others via contact with blood, secretions, organs, or other contaminated body fluids. During Ebola virus outbreaks, it is common for family members to transmit the virus to one another. Consistent with this characteristic, nine of the 16 individuals who died during the Ugandan outbreak were members of one household. The virus is also transmitted to healthcare workers who treat infected patients, especially when proper infection control procedures are not followed.
Although non-human primates are known to transmit Ebola virus to humans, they are not believed to be the reservoir of the virus, which probably inhabits the rainforests of Africa and the Western Pacific. There is good evidence that bats might harbor the virus, then pass it on to nonhuman primates and then to humans. Studies are currently underway to positively identify the reservoir of the virus, which will help in efforts to control infection.
It is important to note that during Ebola virus outbreaks, infection can be limited by proper infection control measures, such as isolation and barrier nursing techniques. The good news is that the virus does not appear to spread by the respiratory route. This means that you may walk through an infected town without fear of inhaling the virus and acquiring the disease.
Infection with Ebola virus begins with flu-like symptoms of high fever, weakness, muscle pain, headache, and sore throat. Vomiting, diarrhea, rash, liver and kidney dysfunction, and internal and external bleeding—hemorrhagic fever—subsequently occur. The incubation period for the illness is about 21 days.
Is Ebola virus really so scary? The World Health Organization states that the case fatality ratio of Ebolavirus is between 25 percent and 90 percent. However, the results of several serological surveys (antibody tests of blood serum used to establish the pervasiveness of a disease) have shown that many individuals have antibodies against Zaire Ebola virus—purportedly the most lethal. The results of one study revealed antibodies in 10 percent of individuals in non-epidemic regions of Africa.
A similar seroprevalence rate (9.5 percent) was reported in villages near Kikwit in the Democratic Republic of Congo, where an outbreak occurred in 1995. In addition, a 13.2 percent seroprevalence was detected in the Aka Pygmy population of Central African Republic. No Ebola hemorrhagic fever cases were reported in these areas.
A more recent study examined sera from 4,349 individuals in 220 villages in Gabon. Antibodies against Zaire Ebola virus were detected in 15.3 percent of those tested, with the highest levels in forested regions. The authors believe that the seropositive individuals had mild or asymptomatic Ebola virus infection. These findings indicate that the fatality rates of Zaire Ebola virus that are quoted widely are likely to be vast overestimates. Why the infection is more lethal during outbreak conditions is not known.
Although research is ongoing to identify antivirals to treat Ebola virus infection and vaccines to prevent infection, none are yet available. Patients receive supportive therapy, such as intravenous fluids and maintaining oxygen levels in the blood.
Could Ebola virus spread from Uganda to other parts of the world? Because of the long incubation period, it is possible that an individual could become infected and then travel elsewhere before symptoms of infection were evident. This scenario took place in 2007 when Marburg virus, which is in the same family as Ebola virus, was brought to Colorado by a tourist. The patient had traveled to Uganda in December 2007 and visited a python cave, which houses thousands of bats—some of which might be reservoirs for Marburg virus. Two weeks after visiting the cave, she became ill and returned to the U.S., where she was treated in a hospital and released.
After hearing in July that another tourist who had visited the same python cave had died of Marburg fever, she returned to the hospital. Samples taken during the January hospital visit were then examined, and last month—one year after her illness—proved positive for Marburg virus. The patient has recovered, and none of the hospital workers who had contact with the patient became infected. Her traveling companion to Uganda, who cared for her during her illness, was healthy.
It is clearly important to identify all contacts of Ebola patients in Uganda and observe them for 21 days to ensure that they have not been infected. But history tells us that Ebola virus infection rarely spreads outside a limited geographical area. The reasons for this limitation are unknown but provide the rest of the world with confidence that they will avoid infection.
The Ugandan outbreak of Ebola hemorrhagic fever will not be over until 42 days—two incubation periods—have elapsed after the last known case. Unfortunately for the inhabitants of Uganda, that time is well in the future, as new cases continue to be identified. However even our limited knowledge of Ebola virus and the disease it causes indicates that the rest of the world need not worry about pandemic scenarios, as the virus is likely to remain confined to Uganda.
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Vincent Racaniello is a professor of microbiology at Columbia University Medical Center. He has done laboratory research on viruses for 37 years and writes the Virology Blog.