The family Filoviridae contains three genera, Ebolavirus and Marburgvirus, which cause severe disease in humans, and Cuevavirus, which has only been detected as viral RNA in bats in Spain.
The Zaire species of Ebola virus, the causative agent of the 2014 -2016 West African epidemic, is among the most virulent human pathogens known. The case fatality rate in past outbreaks in Central Africa reached 80 to 90%, but the overall fatality rate in West Africa was approximately 40%.
In the past, the Ebola virus was classified as a "hemorrhagic fever virus." However, that term is no longer used because only a small percentage of patients develop significant bleeding, usually occurring in the terminal phase of illness.
Until the 2014 - 2016 epidemic in West Africa, all outbreaks of EVD had occurred in Central Africa or Sudan. The West African epidemic was the largest filovirus outbreak on record. It started in Guinea in late 2013 and was confirmed by the WHO in March 2014. The countries with widespread transmission included Guinea, Liberia, and Sierra Leone. EVD occurred in hundreds of healthcare personnel who were caring for patients. Several patients with EVD (e.g., doctors and nurses infected in West Africa, returning travelers from the region) were treated in hospitals in the US and Europe.
The reservoir host of the Ebola virus is unknown. Evidence is accumulating that various bat species may be a source of infection for humans and wild primates. Person-to-person transmission is associated with direct contact with body fluids from patients with EVD or cadavers of deceased patients. Transmission to healthcare workers may occur when appropriate PPE is not available or properly used, especially when caring for a severely ill patient.
Human infection with the Ebola virus can also occur through contact with wild animals (e.g., hunting, butchering, and preparing meat from infected animals). Almost all data on the pathogenesis of EVD have been obtained from laboratory experiments employing mice, guinea pigs, and nonhuman primates. Case reports and large-scale observational studies of patients in the West African epidemic have provided additional data on pathogenesis consistent with findings in animal studies.
The incubation period of EVD is typically 6 to 12 days but can range from 2 to 21 days. Patients with EVD usually have an abrupt onset of nonspecific signs and symptoms such as fever, malaise, headache, and myalgias. As the illness progresses, vomiting and diarrhea may develop, often leading to significant fluid loss. Patients with worsening disease display hypotension and electrolyte imbalances leading to shock and multiorgan failure, sometimes accompanied by hemorrhage.
For patients with clinical findings consistent with the disease (i.e., fever and severe headache, weakness, muscle pain, vomiting, diarrhea, abdominal pain, or unexplained hemorrhage), healthcare personnel should obtain a careful history to determine if the patient has had a possible exposure. All patients who have or are suspected of having EVD should be promptly isolated. Infection control precautions, including hand hygiene, standard contact, droplet precautions, and appropriate PPE, should be initiated. Hospital infection control staff and the local or state health department should be contacted immediately.
Monitoring for signs and symptoms of EVD should be performed for asymptomatic individuals with exposure. Medical evaluation of symptomatic patients may include laboratory testing. Whether laboratory testing should be performed depends, in part, upon the relative likelihood that a patient was exposed to the virus and the presence of compatible clinical symptoms.
Diagnostic tests for EVD are based on detecting specific RNA sequences by RT-PCR. The Ebola virus is generally detectable in blood samples within three days after the onset of symptoms. Repeat testing may be needed for patients with symptoms for fewer than three days.
EVD should be considered as a differential diagnosis for certain conditions. The differential diagnosis will vary markedly with the clinical and epidemiologic circumstances. For example, travelers from West or Central Africa should be evaluated for illnesses commonly seen in those areas, such as malaria.
Because of its virulence and high infectivity, the Ebola virus is classified as a Category A bioterror agent.
Effective treatment of EVD requires aggressive supportive care to correct volume losses from vomiting and diarrhea, correct electrolyte abnormalities, and prevent shock. Patients may also require evaluation and treatment of concomitant infections. Several investigational antiviral therapies were used to treat patients during the 2014 - 2016 outbreak in West Africa, but their efficacy is unclear, and the availability of these drugs is limited.
EVD is associated with a high risk of fetal death and pregnancy-associated hemorrhage. Decisions regarding obstetrical care must be made on a case-by-case basis.
Early diagnosis and prompt initiation of care increase the likelihood that a patient with EVD will survive. Patients who survive EVD typically show signs of clinical improvement during the second week of illness. After discharge from the hospital, patients should be monitored for at least one year.
To prevent transmission of the Ebola virus, healthcare personnel should follow infection prevention and control recommendations from the CDC and the WHO:
- When caring for a patient with acute illness, precautions should include isolation of hospitalized patients with known or suspected EVD, hand hygiene, the use of standard, contact, and droplet precautions, and the correct use of appropriate PPE.
- For most survivors, only standard precautions are needed for clinical evaluation and care. However, additional precautions are needed for those who present with late-stage manifestations of EVD, such as acute neurological or ocular symptoms.
- Additional strategies to prevent the spread of EVD include carefully monitoring individuals after possible virus exposure, educating patients on reducing the risk of transmission through sexual contact or breastfeeding, and potentially vaccinating high-risk populations.