Ebola virus disease (EVD), formerly known as Ebola hemorrhagic fever, is a severe, often fatal illness in humans. The virus is transmitted to people from wild animals and spreads in the human population through human-to-human transmission. The average Ebola virus disease case fatality rate is around 50%. Case fatality rates have varied from 25% to 90% in past outbreaks.
The natural reservoir host of the Ebola virus remains unknown. On the basis of evidence and the nature of similar viruses, researchers believe that the virus is animal-borne and that bats are the most likely reservoir. Four of the five identified Ebola virus species are known to cause disease in humans:
Ebola virus (Zaire ebolavirus)
Sudan virus (Sudan ebolavirus)
Taï Forest virus (Taï Forest ebolavirus)
Bundibugyo virus (Bundibugyo ebolavirus)
The fifth, Reston virus (Reston ebolavirus), has caused disease in other primates but not in humans. The natural reservoir host of these filoviruses has not been identified, although fruit bats of the Pteropodidae family are suspected to be natural Ebola virus hosts. Researchers from the University of Makeni, the National Institutes of Health, the Centers for Disease Control and Prevention, Tulane University and others found that Ebola viruses circulate in certain species of bats in West Africa without causing them illness—including in some species from the genera Hypsignathus, Epomops and Myonycteris. Their findings point to fruit bats as likely natural viral reservoirs for ebolaviruses.
Ebola virus disease primarily affects humans and other primates, including gorillas, chimpanzees and monkeys. Ebola is introduced into the human population through close contact with the blood, secretions, organs or other bodily fluids of infected animals. In Africa, infection has been documented through the handling of infected chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines found ill or dead or in the rainforest.
Once the Ebola virus enters the human population, human-to-human transmission can continue through close contact with infected blood and other body fluids of symptomatic patients or corpses infected with Ebola virus. This includes direct contact through broken skin or mucous membranes by contact with blood, secretions, organs or other bodily fluids of infected people, and with surfaces and materials (e.g. bedding, clothing) contaminated with these fluids.
Health workers have frequently been infected while treating patients with suspected or confirmed Ebola virus infection. This has occurred through close contact with patients when infection control precautions are not strictly practiced. Burial ceremonies in which mourners have direct contact with the body of the deceased person can also play an important role in the transmission of Ebola. Men who recovered from the disease can still transmit the virus through their semen for nearly 2 months after recovery from illness.
The initial symptoms of Ebola include sudden onset of fever, fatigue, muscle pain, headache and sore throat. This is followed by vomiting, diarrhea, rash, symptoms of impaired kidney and liver function, and in some cases, both internal and external bleeding. Laboratory findings include low white blood cell and platelet counts and elevated liver enzymes.
Treatment is symptomatic and supportive. No licensed treatments or vaccines are currently available for Ebola virus disease. Possible treatments evaluated and used in some past outbreaks include blood transfusions from recovered patients, experimental vaccines and antiviral drugs. Currently, three candidate vaccines are being tested for safety in clinical trials. Monoclonal antibody therapies look promising, but more research is needed. The most effective means of prevention is through good infection control practices in hospitals.
Ebola virus disease continues to pose a threat in Africa, where sporadic outbreaks occur. While knowledge surrounding the disease has expanded greatly in recent decades, more research is still needed for licensed vaccines and treatments. Continued surveillance, community engagement and rapidly addressing outbreaks are crucial to reduce human impact. Understanding animal reservoirs and transmission remains a priority to help prevent future emergences of this deadly pathogen.