Epidemiology





Ebola, named after a river in central Africa where it was first seen, is a member of the filovirus family, with the similar Marburg virus being the other member of this class. There are currently four identified strains – Zaire, Sudan, Ivory Coast, and Reston – of which the first three are fatal to humans. As the nomenclature implies, Ebola is endemic to central Africa. The virus has caused outbreaks in Sudan, Ivory Coast, Gabon, Zaire (now the Democratic Republic of the Congo, Congo, and in South Africa. The first recorded outbreak occurred in 1976, and many more have followed. There have also been outbreaks of the Reston strain among primates in both the U.S. and the Philippines.


Reservoirs

The natural reservoir and route of transmission to humans is still unknown for Ebola. Despite extensive surveys of hundreds of species of animals, insects and plants by PCR analysis, there is no definitive answer. Some possible reservoirs include:


However, when many of these species are infected in a laboratory, the virus proves fatal, thus negating them as potential reservoirs. Indeed, ecological studies of primates in central Africa have shown that Ebola has decimated their populations in recent years. An interesting theory, supported by recent experiments, holds that under certain conditions of immune deficiency, such as that given by HIV, SIV, or other similar diseases, an animal would be able to live with the infection for long periods of time, thus acting as a reservoir for the virus.

Many experts have put forth a hypothesis that the appearance and increase of outbreaks is directly linked to deforestation and rainfall. As more and more of the dense jungle is cleared for farmland, this raises the possibility that humans will come into contact with previously un-encountered species that have lived in ecological isolation for prolonged periods of time. These unknown species include both the reservoir and the Ebola virus itself.

Transmission and Prevention

Once Ebola has initially spread into a human, there are several transmission routes. The virus can spread through both first and second degree contact with saliva, blood, vomit, diarrhea, and other bodily secretions, and it has been shown that under certain conditions, aerosol particles containing the virus can form and disseminate within a closed space. However, this is not considered a major transmission route in human epidemics. Corpses also serve as a route of transmission, and this is a significant problem in many parts of Africa where funerary traditions involve veneration of a corpse through physical contact. Due to this high potential for infection and the extremely high mortality rate, Ebola must be studied in a level 4 biosafety lab.


Much of the attention that is given to Ebola is based upon the dramatic progression of the disease and the mortality rate of 70-90%. Because the disease is so virulent and the onset is so fast (2-21 days), there is no carrier state, or immune state for that matter. This rapid lethality is perhaps the diseases greatest weakness, as the disease usually kills it hosts before they can either move to far, or infect a large number of victims.

Since the reservoir is unknown, it is not exactly clear what factors put a person at risk for Ebola. Once an outbreak has begun however, healthcare workers are in the greatest risk group, as they must come into contact with those individuals who are infected. To control the spread of Ebola, it is recommended that standard barrier procedures be followed and that all equipment be sterilized. This is often a problem in areas where the virus appears, as many medical facilities in rural settings cannot afford to follow these procedures, allowing, for example, the re-use of needles. This type of activity, resulting in a nosocomial outbreak, may amplify the epidemic greatly, as well as severely strain the healthcare resources of an area as primary providers become infected as well. It is also important to dispose of cadavers promptly, as the virus can spread from these as well.













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This page last updated: 14 April 2004.