Effect of Ebola Progression on Transmission and Control in Liberia

West Africa is overwhelmed by the most devastating Ebola epidemic known to date. It continues to increase exponentially, with the fastest rate of spread in Liberia (1). By August 2014, the number of cases in Liberia exceeded the capacity of all Ebola treatment units (2, 3). Because this public health crisis shows no signs of improvement and the risk for Ebola spreading beyond West Africa continues to mount, it is an international imperative to determine effective approaches to stem transmission of this virus. Early Ebola symptoms include sudden high fever, muscle pain, and severe headache followed by pharyngitis, abdominal pain, and maculopapular rash (4), whereas the late phase is marked by vomiting, diarrhea, hemorrhagic diathesis, and multiorgan dysfunction (4). Ebola is primarily transmitted through direct contact with infected bodily fluids and contaminated materials. Therefore, close contacts of patients with Ebola, such as family members, health care workers, and those preparing bodies for burial, are at high risk for infection (5). In the current absence of pharmaceutical prophylaxis and treatment (6), control strategies rely on 1) active case ascertainment and isolation, 2) identification of patients’ contacts with monitoring of them for 21 days, and 3) identification of Ebola deaths for hygienic burial (7). However, the implementation of these approaches is falling short because of the rapid spread of the outbreak combined with the limited resources available. The risk for Ebola transmission increases with the viral load of infected individuals, which for nonsurvivors is greatest in the later stages of illness and immediately after death (8). In combination with their viral load, the number of close contacts of patients determines transmissibility of the virus (9, 10). Both the viral load and number of contacts of a patient with Ebola may change during the infectious period (for example, ante- and postmortem contacts); thus, the contribution of these factors to disease transmission to other patients may vary with disease progression (also known as age-of-infection), defined as the number of days since exposure (11). This distinction is clinically relevant because the viral load of survivors peaks 4 days after symptom onset and then rapidly declines, whereas viral load of nonsurvivors continues to rise. In addition, among nonsurvivors, the mean viral load throughout the infection period is 100-fold higher than that among survivors (8, 12). To our knowledge, previous Ebola transmission models have not considered the effect of disease progression and case fatality on transmission. We present the first Ebola transmission model that distinguishes between survivors and nonsurvivors and incorporates disease progression to evaluate Ebola transmission and the effectiveness of targeted control measures. Our model integrates epidemiologic and clinical data on Ebola viral load, daily infection incidence, and case fatality together with primary data on contact mixing patterns collected from patients with Ebola in Montserrado County, Liberia. We used this model to evaluate the distribution of secondary cases resulting from infected individuals as disease progresses, differentiating between survivors and nonsurvivors. We then evaluated the potential effect of case isolation and social behavior change through contact reduction for controlling Ebola transmission in Liberia.