What VSPB Does
Researcher in CDC field laboratory during Ebola outbreak in DR Congo in 2007. Photo: Chris Black/WHO.
The Viral Special Pathogens Branch's (VSPB) charter is the study of highly infectious viruses, many of them causing hemorrhagic manifestations in humans. Our daily work involves the investigation of viruses such as Ebola hemorrhagic fever, Marburg hemorrhagic fever, Lassa fever, Rift valley fever virus, Crimean-Congo hemorrhagic fever virus, arenaviruses and hantaviruses, and other recently identified and emerging viral diseases.
Almost all of these viruses are classified as Biosafety Level 4 (BSL-4) pathogens and as such must be handled in special facilities designed to contain them safely. VSPB operates one of the world's few BSL-4 laboratories. In addition, VSPB provides technical and research/diagnostic materials to many international laboratories and collaborators. VSBP staff members are trained to respond to global disease outbreaks and to provide assistance for disease detection and control measures.
Characteristics of the viruses and significance
All of the infectious agents the branch works with are viruses. Each is RNA-coded (often negative-stranded or ambisense in coding strategy), encased in a lipid envelope, and displays some degree of aerosol infectivity in the laboratory. In addition, all of the viruses are vector-borne or zoonotic. Some of them are found in rodents, and some occur in other mammals or arthropods as well. There are no licensed vaccines or therapies for most of the virus diseases investigated by VSPB.
Viruses causing hemorrhagic fever exist in many distant places around the globe, but today's travel patterns can bring any one of them to the United States in a matter of hours. New or previously unidentified viruses continue to appear in countries throughout the world and the following is a partial list of viruses investigated in VSPB that were discovered during the last 20 years: Sabia virus, Guanarito virus, and Sin Nombre virus and other hantaviruses in the Americas; the Ivory Coast species of Ebola virus; the tick-borne flaviviruses in the Middle East; and Nipah virus and SARS virus in Southeast Asia. In addition, several of the viruses handled in VSPB result in sporadic outbreaks in many parts of the world and cause a wide case fatality range (30% to 90%).
Mission of the Viral Special Pathogens Branch
VSPB uses the following disciplines and tools to understand and control these diseases: public health practices, molecular biology, molecular evolution of viruses, clinical diagnostics, clinical medicine, epidemiology, immunology, pathogenesis, comparative biology, ecology, and community education. The Infectious Disease Pathology Branch (IDPB) at CDC assists VSPB with diagnostic pathology and surveillance.
Key Challenges for VSPB
Rift Valley fever virus electron micrograph
The number of known disease-causing viruses has been increasing and this trend is very likely to continue as evidence by the frequency of recent outbreaks (link to recent outbreak data). We anticipate the recognition of new pathogenic Arenaviruses, particularly from the Americas and probably from Africa. Hantavirus pulmonary syndrome is a widespread if uncommon problem in the Americas. In addition, new foci of disease and more virus diseases await discovery and exploration.
The behavior of the known viruses is also unlikely to remain static. They continue to appear in places and in epidemiologic contexts that are unpredictable and novel for us. Recent examples include the extensive role that stockyards have played in Crimean-Congo hemorrhagic fever outbreaks in the Middle East; the appearance of the Reston species of Ebola virus in monkeys in an unexpected part of the world (Texas); the explosive outbreak of hantavirus pulmonary syndrome in Chile; and the first recorded outbreak of Rift Valley fever on the Arabian Peninsula and continued frequent outbreaks of Ebola and Marburg viruses in Africa.
Transmission of the viruses that cause these diseases continues to spark debate and research The viruses which are investigated by VSPB display some capability of infection through small-particle aerosols in the laboratory although, the role of aerosols in the transmission of these continues to raise some uncertainties.
While the aerosol route from the animal reservoirs to humans is well documented and is probably the dominant mode of transmission for some of these viruses (arenaviruses and hantaviruses), some of these viruses can be transmitted person to person after introduction from the zoonotic cycle into the human population, usually through direct contact (handling of infected patients) or by medical procedures in clinical facilities without adequate infection control procedures.
There are considerable clinical and pathogenetic issues surrounding the management of hemorrhagic fever syndromes. Although data are incomplete, it appears that the hemodynamic patterns of dengue fever, hemorrhagic fever with renal syndrome, and hantavirus pulmonary syndrome pursue a course of progressive decrease in cardiac output with rising systemic vascular resistance. This disease course contrasts with the pattern observed in septic shock, in which an intermediate period with low resistance is present. Thus, case management of patients with hemorrhagic fever may need to differ from that of patients with septic shock.
Researcher performing a diagnostic test
Improving the specificity, sensitivity and rapidity of the techniques used for the diagnosis of hemorrhagic fevers has been an area of high priority. Introduction of two special techniques, antigen-detection enzyme-linked immunosorbent assay (ELISA) and IgG /IgM-capture ELISA, have provided sensitive, acute-phase diagnostics for most purposes. The IgM ELISA provides excellent sensitivity and specificity for patients in whom antigen has disappeared or for immunopathologic conditions, such as diseases caused by hantaviruses.
Reverse transcription polymerase chain reaction (RT-PCR) and real-time quantitative PCR assays are used to identify and perform genetic characterization of viruses. RT-PCR also makes viral genetic sequence information immediately available, a characteristic that made the test useful in identifying the Ebola-Zaire species during the outbreak of Ebola hemorrhagic fever in Kikwit, Zaire, in 1995.
The real-time assay allows for rapid and safe diagnosis of high sensitivity to be performed in a manner that allows identification of patients early in the course of the disease. When complemented with antibody assays, the combination allows identification of the vast majority of cases in a rapid fashion. These assays have been utilized in the field in a number of outbreaks to provide on-site diagnosis of the cases in outbreaks to facilitate control efforts.
Campaign to rodent-proof homes to prevent HPS in Ramah, New Mexico.
Prevention of hemorrhagic fevers is often hampered because the reservoirs of the causative agents may be unidentified and can be numerous and difficult to control. This difficulty was evident during CDC's attempts to develop control guidelines for Sin Nombre virus and deer mice in the southwestern United States in 1993 and 1994. Deer mice proved to be prolific and adaptable to a wide range of ecologic settings.
However, solid health communication efforts have been made in a number of areas relevant to hemorrhagic fevers, especially in infection control and diagnostics for health providers. In the case of Lassa fever and hantavirus pulmonary syndrome, large community education campaigns have been mounted by CDC and various collaborators, both domestic and foreign. Such efforts have resulted in increased awareness of these diseases in the affected areas.
Social and Cultural Practices
On the vaccine front, it is evident that a lack of economic incentive hinders the development of vaccines against hemorrhagic fever viruses. Nevertheless, the potential these viruses have for causing illness and death makes it important for us to understand the principles of the immune response and the potential for use of vaccines against them. The success of the Argentine hemorrhagic fever vaccine in preventing the disease in the endemic area in Argentina demonstrates the utility, but also illustrates the financial vulnerability, of the vaccine approach to controlling such locally important diseases.
Although we have made significant progress in detecting these diseases and their causative viruses, as well as the disease manifestations, we have much more to learn about ways to control them. The nature of the outbreaks is dependent on the human cultural practices and ecological parameters that define the natural cycle of the virus in its reservoir hosts. Because such viruses can emerge in new areas and act in unexpected ways, and because new viruses keep appearing, the need to be prepared to respond and study them, control them, and prevent their spread will continue and may become even more critical in the years to come.
|This page last reviewed July 31, 2012|
Content source: Centers for Disease Control and Prevention