Two Americans doctors infected by the recent outbreak of Ebola hemorrhagic fever in Liberia were released from Emory University Hospital in Atlanta on Aug. 21 following a successful treatment with the experimental drug ZMapp.
The doctors, Kent Brantly and Nancy Writebol, were exposed to the fever spreading through Liberia, Guinea, Sierra Leone and Nigeria while working for the charity Samaritan’s Purse. Both received ZMapp before being transported to Atlanta for further treatment.
ZMapp, developed by Mapp Biopharmaceutical Inc. in San Diego, California, includes a cocktail of three different monoclonal mouse antibodies grown in tobacco plants and that bind to Ebola proteins and inhibit their function.
The United States Centers for Disease Control and Prevention (CDC) website lists symptoms of the Ebola virus as fever, internal and external bleeding, weakness, stomach pain and diarrhea. The average incubation time of the virus is 2 to 21 days and is often fatal. Of the recent 1,082 suspected and confirmed cases, the CDC suspects that 624 of these cases were deadly.
David Sanders, an associate professor of biological sciences at Purdue University, conducts research on the viral entry of Ebola and said that it is actually the body’s response rather than the virus itself that causes harm.
“An important point to emphasize it that it is not the virus that actually causes the harm that leads to pathology — it is the immune system’s reaction to the virus that causes the harms,” Sanders said. “So in the case of Ebola what seems to happen is that the innate immune system, which is the early part of the immune system, goes out of control and causes the tissue damage and other symptoms.”
Human cells are ill-equipped to deal with the foreign virus and each person reacts differently to it, so it is not entirely known why some people survive and others do not.
According to Sanders, approved Ebola treatments exploit the ability of viruses to target specific cells in an organism.
“Say you wanted to treat sickle cell anemia. What you need to do is get the genes into the precursors of red blood cells. It turns out that viruses already have that targeting encoded in their structure,” Sanders said. “Pseudotyped viruses, which on the outside have the envelope protein of the virus and the core of the retrovirus […] allows us to deliver the genetic material to a cell that [makes] the virus from which the envelope protein is derived want to enter each cell. Each virus has its own specificity for entry into cells.”
The origins of the virus are still inconclusive, but there is speculation that it is transmitted through a native fruit bat host. Another, more common, mechanism is through the African bushmeat trade in which primates are slaughtered for food and the virus is transmitted to humans through their blood.
Sanders said that all the retroviruses are related — bird retroviruses are related to mouse retroviruses which are related to human retroviruses such as HIV — but that Ebola is most closely related to bird retroviruses.
“Our evidence is that the entry of Ebola is most similar biochemically and structurally to the entry of bird retroviruses,” Sanders said. “So this argues that there was a recent common ancestor between the Ebola virus entry and the bird retroviruses.”
Although it is contained in the United States, the Ebola virus has yet to be fully contained in West Africa. A Liberian doctor, Dr. Abraham Borbor, received one of the few available ZMapp doses, but did not respond to treatment and died on Aug. 25. An additional unrelated outbreak reported in a remote northeastern village in the Democratic Republic of Congo has killed thirteen.