4th National Symposium on Biosafety

Biosafety and Emerging Infections: Key Issues in the Prevention and Control of Viral Hemorrhagic Fevers

Clarence J.Peters, MD
Special Pathogens Branch/Division of Viral and Rickettsial Diseases
National Center for Infectious Diseases/Centers for Disease Control and Prevention
1600 Clifton Road
Atlanta, GA 30333
404-639-1511

Introduction

This review describes the phenomenon of emerging infections and some of the biosafety considerations associated with the study of an increasing number of new and recently recognized viruses. Factors influencing the emergence of pathogenic microorganisms are described, using the hemorrhagic fever viruses as examples. Particular attention is given to the outbreak of Ebola virus hemorrhagic fever in Zaire in 1995.

Approximately one dozen hemorrhagic fever viruses have been identified to date, most of which are biosafety level-4 (BSL-4) agents or would be BSL-4 agents if vaccines were not available for them (Table 1). A key feature shared by these pathogens is that they cause human disease characterized by diffuse vascular damage and increased vascular permeability (Table 2). The agents that cause these hemorrhagic fever syndromes are small RNA viruses with a lipid envelope; all are basically zoonotic viruses and most are infectious via the aerosol route. These viruses persist in nature, not in humans, but they make occasional incursions into human populations. They belong to several families: Arenaviridae, which includes agents such as Machupo and Lassa viruses; Bunyaviridae, which includes hantaviruses, nairoviruses (e.g., Crimean-Congo hemorrhagic fever virus), and phleboviruses (e.g., Rift Valley fever virus); Filoviridae (e.g., Ebola virus); and Flaviviridae, which includes yellow fevervirus and some of the tick-borne viruses (e.g., Kyasanur Forest and Omsk hemorrhagic fever viruses).

Hemorrhagic Fevers and Their Virus Families
ARENAVIRUS
Old World
	Lassa Fever	Lassa
American
	Argentine HF	Junin Virus
	Bolivian HF	Machupo Virus
	Venezuaelan HF	Guanarito Virus
	Brazilian HF	Sabia Virus
BUNYAVIRIDAE
	Phelobovirus	Rift Valley Fever
	Nairovirus	Crimean Congo HF
	Hantavirus	HF with Renal Syndrome
		Hantavirus Pulmonary Syndrome
FILOVIRUS
	Marburg HF
	Ebola HF
FLAVIVIRUS
	Yellow Fever
	Dengue HF
	KFD and Omsk HF

Viral Hemorrhagic Fever: Clinical Definition
Acute infection that begins with fever, myalgia, malaise and progresses to prostration Evidence of vascular dysregulation and increased vascular permeability Multisystem involvement Hemorrhage indicates extent of small vessel involvement but not necessarily large in volume Shock, encephalopathy, extensive hemorrhage poor prognosis

The public health threat posed by emerging infections is vividly illustrated by reports of viral hemorrhagic fevers in various regions of the world from 1993 to 1995 (Table 3). In 1993, Rift Valley fever reappeared in Egypt after a 13-year absence. Usually confined to the sub-Saharan countries of Africa, Rift Valley fever emerged in Egypt in 1977 and caused a major epidemic, but then disappeared from this region after 1980 – until the outbreak in 1993. Similarly, Bolivian hemorrhagic fever reemerged in 1993, the first report of this disease since 1975. Later in 1993, our group was integrally involved in the discovery of Sin Nombre virus, a previously unrecognized pathogen that causes hantavirus pulmonary syndrome (HPS). This critical finding led to a series of related studies which in 1993-1994 identified at least three more new hantaviruses, including two in the United States and one in Brazil, that also cause HPS. Recent reports of similar viruses in Paraguay and in several regions in Argentina provide further evidence that hantavirus disease will be a continuing public health problem, with a broad geographic range in the Americas.

VIRAL HEMORRHAGIC FEVERS: 1993-1995
1993	Rift Valley fever	Epidemic in Egypt
1993	Bolivian HF	First case since 1975
1993	HPS	Discovery
1994	HPS	New viruses found
1994	Bolivian HF	Family epidemic
1994	Sabia virus	Laboratory infection, US
1994	Lassa fever	Case imported Lagos
1994	CCHF	UAE epidemic stockyards
1994	Ebola virus	New subtype in Ivory Coast
1995	Ebola virus	Epidemic in Zaire

In 1994, Bolivian hemorrhagic fever recurred with an outbreak in which the index patient infected six family members, all of whom died. The circumstances of this familial outbreak strongly suggested that aerosol transmission of the causative agent, Machupo virus, had occurred. All the hemorrhagic fever viruses discussed here have some degree of aerosol infectivity, but they cannot be spread from person to person by aerosol with ease. Apparently, aerosols are not generated in sufficient strength in external secretions of patients. This mode of transmission was therefore unexpected with regard to these cases of Bolivian hemorrhagic fever.

An unrelated incident that same year involving the recently discovered Sabia virus (an arenavirus, as is Machupo virus) showed yet again that arenaviruses are aerosol infectious and extremely hazardous (Table 4). A virologist in a U.S. research laboratory became infected with Sabia virus after performing a procedure with a high-speed centrifuge to clarify tissue culture containing the virus. During this procedure, a small amount of tissue culture fluid leaked from a hairline crack in one of the centrifuge bottles into a centrifuge chamber. Aerosol transmission of the virus occurred either during the centrifugation process or after the centrifuge was opened and the spill was cleaned up. The researcher was subsequently hospitalized, treated with the antiviral drug ribavirin, and recovered. This incident demonstrates both the potential risk of conducting laboratory research on hazardous microorganisms and the need for strict education in and adherence to well-established safety procedures.

Again in 1994, a single case of Lassa fever caused widespread concern because it was imported into Lagos, Nigeria. This is the first example of this disease, which has potential for human-to-human transmission, occurring in one of the large, crowded African cities, where the likelihood for an explosive epidemic is considerably greater than in the rural West African villages where Lassa and closely related viruses usually occur. In the United Arab Emirates, Crimean-Congo hemorrhagic fever virus caused several outbreaks among workers and other individuals in stockyards. Human disease caused by this virus typically occurs in rural settings; however, when large numbers of livestock are brought together and they become infested with ticks, poorly understood infection dynamics may arise that contribute to the spread of disease among workers in stockyards and even among people who work with the pelts from the hides from these animals. Finally, in 1994 a new species of Ebola virus appeared in the Ivory Coast, and in 1995 a large epidemic of Ebola virus hemorrhagic fever occurred in Zaire (see below).

Laboratory Exposure to Sabia Virus, United States, 1994
Aug 1994: tube developed crack during high-speed centrifugation Scientist did not open rotor in biosafety cabinet Surgical mask was used instead of respirator Accident not reported, no medical surveillance 8 days later: fever self-attributed to malaria Ribavirin Rx begun 3 days later with fever, leukopenia Prompt clinical and virological response to IV ribavirin

Emerging Infections: A Temporal Perspective

These recent examples of viral hemorrhagic fever offer some insights into the phenomenon of emerging infections and the type of public health infrastructure and international networking needed for responding effectively to these problems. The issue of emerging infections has received considerable attention in recent years, even though new infectious diseases have been documented throughout modern history. Much of the current emphasis on emerging infections derives from the Institute of Medicine (IOM) report published in 1992. The report, developed by a panel of distinguished scientists and physicians under the leadership of Nobel laureate Joshua Lederberg and Robert Shope, describes in detail the concept of emergence, reviews the factors that contribute to emergence of infectious diseases, and outlines a series of recommendations for addressing these diseases. After the release of the IOM report, the Centers for Disease Control and Prevention (CDC) assembled a team to write a plan for responding to the problem of emerging infections. In 1994, CDC issued its plan, Addressing Emerging Infectious Disease Threats: A Prevention Strategy for the United States, which established a number of program objectives in the areas of surveillance, applied research, prevention and control, and infrastructure. Implementation of this plan is currently a major priority of CDC and the National Center for Infectious Diseases (NCID).

The publication of the IOM report and the CDC plan, and the ongoing work associated with the recommendations contained in those reports, touch on a number of important issues, one of the more interesting of which involves the perspective of timing (Table 5). We are often preoccupied with the exigencies of everyday life and may not take the long view either retrospectively or prospectively. Consider what has occurred historically, for example, since 1492 on this continent. Settlers and immigrants to North America brought with them "new" infectious diseases, including smallpox, yellow fever, measles and others, that decimated Native American populations and in many cases continue to plague our amalgamated U.S. population. Several infections that could be treated effectively with antibiotics in the 1970s (e.g., tuberculosis, enterococcus, Staphylococcus aureus) are now resistant to such drugs. Past and present experience with zoonotic diseases has shown that shifts in host species play an important role in the emergence of infectious diseases in humans (e.g., human immunodeficiency virus [HIV]). This temporal perspective allows us to consider the concept of emerging infections in an evolutionary context. Emerging infections are not a transient phenomenon that can be resolved with simple, short-term solutions. Rather, they represent a constant and powerful force in nature, a product of the ongoing interplay between human and animal populations and their environments. As observed by Dr. Lederberg in 1994, "Subjecting ourselves to the iron law of evolution is not likely to be to our taste, especially in a 50-year time period."

WHAT IS THE TEMPORAL PERSPECTIVE OF ...
A politician? A voter? A ball player? A team manager? A scientist? A granting agency? Our perspective and planning on emerging viruses?	Next election? Next TV program? Next play or game? Next season? Duration of the grant? Next budget cycle? Current epidemic? Last epidemic? Next grant?

Is the threat from these emerging diseases over? Recent trends suggest the answer is no. , for example, Figure 1shows several leading causes of death for young adults (e.g., diabetes, cancer, injuries) plotted against HIV infection rates for the period from 1982 to 1993. Since the incubation period for HIV is about 10 years and the mortality among infected persons is virtually 100%, the death rate for HIV-related disease can be projected over the next decade. These data provide a dramatic example of how a virus – in this case a zoonotic agent that changed reservoirs – imported from another continent can in a relatively short period become a significant public health problem.

Factors of Emergence: A Probability Equation

Activities that can influence virus circulation in nature are occurring all the time (Table 6) and, with the proper perspective, can be gleaned from the press. For instance, a recent issue of the New York Times contained three articles describing events related to emergence. The first deals with the recent introduction of four ladybug species into the United States and the resultant displacement of one native species of lady bugs. This illustrates how new species are constantly being imported into North America and perturbing the natural order. In terms of transmission of zoonotic diseases, the effect of the introduction of Aedes albopictus into the United States remains to be seen – it may become an alternate vector for eastern equine encephalitis virus in Florida or La Crosse virus. In the second example, the Department of the Interior's effort to have black-footed ferrets reintroduced into the wild has encountered a possible roadblock: plague has been introduced from the Old World into the Americas and now occurs among ground squirrels, and ferrets are susceptible to plague. Therefore, it may not be possible to reintroduce black-footed ferrets into their former habitat, because when this species attacks their natural prey they may be threatened once again by plague. The third example concerns a report that the Aum Shinrikyo followers, the same group that released nerve gas in the Tokyo subway, had developed plans involving the use of biological weapons. Not surprisingly, there are reports of some groups attempting to obtain Ebola virus from Zaire to add to their arsenal. These brief examples from a single issue of a newspaper demonstrate the potential for a myriad of activities to affect the probability that infectious agents will emerge.

FACTORS IN EMERGENCE
Microbial evolution: mutation, recombination, transfer Environment: weather, land use, water Social: economic, warfare, population, urbanization Health care: antibiotics, immunosuppression, devices Food: production and distribution Behavior:sex, drugs, travel, child care Public health infrastructure

Another aspect of the probability equation is the manner in which certain viruses reproduce. RNA viruses (which include the hemorrhagic fever viruses listed in Table 1) are notoriously unpredictable in their genomic replication. These viruses have a poorly faithful copy mechanism and lack a proofreading activity. This is commonly reflected in the variable plaque size seen with these viruses. Consequently, the progeny of one virus from a cell may carry several mutations, and by the time a plaque is formed a wide range of genotypes may have been produced. Selective forces are strong and mutants can be stabilized or lost depending on the milieu. For this reason, one of our recent paradigms is that certain organisms, particularly RNA viruses, are capable of rapid mutation and adaptation. This paradigm was not evident years ago when the U.S. Army was attempting to develop dengue vaccines. There was widespread acceptance of the theory that products of each virus spread from the initiator were genetically identical, and the approach to vaccine development relied on plaquecloning to develop stable accumulations of mutants. However, this was the wrong paradigm these vaccines had an enormous propensity for reversion, and this is a major reason there are no acceptable dengue vaccines today. If a single predominate genotype from a stable population is added to a substrate (e.g., a cell line or an animal), a similar spectrum of genotypes will usually be produced because of mutation and natural selection. If a different genotype is added to the same substrate, initially there may be some variation in the viral products, but usually they will drift back to the original distribution because of selection. If the selective pressures are altered (e.g., by addition of a clone to a another substrate), the distribution will be different This occurred during the development of a chikungunya vaccine. The cell type was changed from the master seed to the production seed, and the vaccine partially reverted.

The uncertainty associated with probabilistic scientific approaches is counterintuitive to the deterministic view commonly held by the public and which is so prominent in the political process. We could compare this to the classic physics model of an atom with electrons revolving around a nucleus (Figure 2) and the quantum mechanics approach; the Schrodinger equation describes the probability we will encounter an electron at a particular place around the atomic nucleus, and the Heisenberg principle assures us we will not be able to pinpoint the location and momentum of the electron. The rapid production of RNA virus mutations leads to a high degree of uncertainty, particularly if there should be a change in the cell or animal substrate for replication and consequently a change in selective pressures. Despite these complexities, the principle of probability as it applies to emergence can be distilled down to a simple equation: variable viruses, multiple ecologic niches (e.g., African cities), and global travel and transport together equate to opportunities for viruses with different properties to emerge (Figure 3). In this context, the speed of global travel should be measured in terms of incubation periods rather than days, and this is becoming an increasingly important issue. For example, the long incubation period of HIV was a major factor in the spread of this virus worldwide. In contrast, some viruses with short incubation periods, such as Lassa virus, would require rapid travel (e.g., to major cities via large airports) for extensive dissemination in the general population.

One other essential aspect of this phenomenon of emerging infections concerns the public health infrastructure and our collective commitment to conducting the necessary research on these viruses. In 1990, I gave a lecture in which I presented Table 7 as an example of the direction that viral hemorrhagic fever control would take in the shortterm future. Today, with the exception of PCRbased assays, there is nothing substantially new to add to this approach. Unmet research needs remain largely "unmet". Of course, one could argue that my forecasts were wrong, but there have been no successful improved alternates. This lack of progress during the past 6 years is in large part due to the fact that the only two BSL4 laboratories in the United States those operated by the U.S. Army and by CDC are not being used at capacity. The Army's program has been cut back markedly in recent years because of both budgetary and personnel decreases in the Department of Defense. CDC's program has been curtailed at a time when our small group has been occupied with responding to recent public health emergencies. If these two laboratories were staffed at an adequate level, there would enough BSL4 laboratory space to meet most of the existing demand. On a larger scale, if we are to be better prepared to meet the challenges of controlling emerging infections, there needs to be a more concerted, longtermcommitment to the strategy outlined in CDC's report, particularly in the areas of international surveillance, applied research, communication and education, and strengthening of the public health system.

TRENDS IN VIRAL HEMORRHAGIC FEVER CONTROL
SURVEILLANCE/PREDICTION Remote Sensing (RVF/Hantaan) Rapid Automated ELISA for Vector Infection (RVF, CCHF) Rapid Automated ELISA for IgG/IgM antibodies in sentinels (RVF, CCHF) Detection Methods for fixed tissues (RVF, CCHF, Ebola/ Marburg, Yellow Fever, Dengue)
PREVENTION Ecological Strategies (RVF) New Vaccine Candidates Argentine HF (conventional, live attenuated) Rift Vally Fever (mutagenized, MP-12)
THERAPY Early diagnosis (Lassa, RVF, CCHF, Hantaan, Ebola/Marburg, Yellow Fever) Ribavirin Therapy (RVF, CCHF, Hantaan)
UNMET RESEARCH NEEDS Ways to control Lassa Fever, Hantaan Understand filovirus reservoir Role of Hantaan in chronic disease

Filovirus Hemorrhagic Fevers

Since 1967, only 14 episodes (sporadic cases or outbreaks) involving filoviruses have been documented (Table 8). Very little is known about filoviruses, which include Marburg and Ebola viruses. These have the potential for aerosol transmission and for causing severe human disease, but their natural reservoir(s) remains unknown. Filoviruses are almost certainly zoonotic agents, and they are not spread from person to person continuously nor are they maintained as a latent infection in primates. Marburg virus was the first filovirus to be discovered; the virus was imported into Marburg, Germany, with African green monkeys in 1967. Ebola virus was first recognized in 1976, when it caused major epidemics in Sudan and Zaire. Currently, there are four known genetically distinct Ebola viruses, called "subtypes," each named for the geographic area where it was first recognized: Zaire, Sudan, Ivory Coast, and Reston. With the exception of Reston virus subtype, these Ebola subtypes are from the forested regions of central Africa.

DOCUMENTED FILOVIRUS INFECTIONS
VIRUS	YEAR	LOCATION	CASES	% MORT
Marburg	1967	Europe	31	23
Marburg	1975	Zimbabwe	3	33
Ebola (Zaire)	1976	Northern Zaire	318	88
Ebola (Sudan)	1976	Southern Sudan	284	53
Ebola (Sudan)	1976	England	1	0
Ebola (Zaire)	1977	Western Zaire	1	100
Ebola (Sudan)	1979	Southern Sudan	34	65
Marburg	1980	Kenya	2	50
Marburg	1987	Kenya	1	100
Ebola (Reston)	1989	Philippines/USA	4	0
Ebola (Reston)	1992	Philippines/Italy	0	0
Ebola (CdI))	1994	Ivory Coast	1	0
Ebola (Zaire)	1995	Southern Zaire	318	77
Ebola (?)	1996	Eastern Liberia	1	0
Ebola (?)	1996	Northern Gabon	37	57

In 1989, the U.S. Army research laboratory in Frederick, Maryland (where I was stationed at the time), had closed out its Ebola virus research program. After looking for Ebola and Marburg viruses in Africa for several years, we had recovered only a single isolate of Marburg virus and decided it was time to pursue other projects. To enable the laboratory to retain some diagnostic capability for filoviruses, all the reagents were stored in a freezer. Later that year, our laboratory recovered an Ebola virus isolate from monkeys housed in a quarantine facility located about 40 miles away in Reston, Virginia. In collaboration with CDC, we set about to control an outbreak of filovirus disease among monkeys in the quarantine facility. It turned out that this particular strain of Ebola virus is less pathogenic for monkeys than are other strains. The Reston subtype of Ebola virus kills only about 85% of rhesus and macaque monkeys, whereas the Zaire subtype kills virtually all monkeys it infects. Moreover, unlike the Reston virus, the Zaire subtype is also highly pathogenic for humans. In 1976, the Zaire strain infected 318 people in northern Zaire, and 88% of these patients died. The Reston subtype infected only four persons during the outbreak. All were animal handlers and ony one had a recognized parenteral exposure. None of these persons became overtly ill.

The Reston outbreak served as an important wake-up call for the U.S. Army and CDC research groups. Among other things, it demonstrated the need for better diagnostic tools. Dr. Tom Ksiazek developed a highly sensitive antigen-detection enzyme-linked immunosorbent assay (ELISA) capable of detecting all the known Ebola virus subtypes within 4 hours. There was also a need for an improved serologic test, since the indirect fluorescent antibody (IFA) test for filovirus infection was nonspecific and unreliable. Figure 4 shows the IFA test results of a young veterinarian with no history of exposure to Ebola virus. Like many IFA results, his titers have no interpretable pattern and the results are highly misleading. Because of the unreliability of the IFA test, previous Ebola virus research based on the results of this method are invalid or at least suspect. To address this problem, our group succeeded in developing more specific ELISA-based serologic tests, the use of which was crucial during our response effort in Zaire in 1995.

Ebola Outbreak: Kikwit, Zaire

In May 1995, we received an urgent call from the American embassy in Kinshasa, Zaire, informing us that a mysterious disease had reportedly caused hundreds to thousands of deaths in the city of Kikwit. A Zairian physician suspected the disease might be Ebola virus hemorrhagic fever and forwarded a number of specimens to the Institute of Tropical Medicine in Antwerp. However, their hazardous virus laboratory has been closed for years. We obtained the serum specimens, tested them by the antigen-detection ELISA developed in 1989, and found that the patients were infected with Ebola virus; these findings were later confirmed by virus isolation. A few of these specimens were not strongly positive by the antigen-detection ELISA and required PCR analysis for confirmation. The syringes that were used to draw these specimens in Kikwit were tested at CDC several months later and Ebola virus was isolated from them, indicating that this virus will persist in the environment if it is contained in protein solution and buffer.

Kikwit is located approximately 250 miles east of Kinshasa, a distance that can be covered in 30 to 45 minutes by air or in about 15 hours by driving. When our team arrived in Kikwit as part of a multinational response group in May 1995, the situation was dire. Many of the patients who had died were buried in the yard behind the town's main hospital. Inside the hospital, there was a lack of medical supplies, containment gear, adequate sanitation, and other necessities – essentially, the hospital had shut down. Similar conditions had contributed to the nosocomial spread of Ebola virus infection during previous epidemics in Africa. Although the use of contaminated needles had played an important role in previous outbreaks, this was not as great a factor in the Kikwit epidemic. The CDC team led by Dr. Pierre Rollin took immediate steps to rehabilitate the hospital on the expectation that this epidemic would follow the pattern observed during previous Ebola outbreaks. Transmission of Ebola virus requires close contact with patients or their body fluids, and we believed that the institution of stringent safety and quarantine measures in the hospital would make it possible to bring the epidemic under control in a way that might not occur if patients remained in their homes in this large African city. The members of the team cleaned out dirty needles and syringes, removed cadavers, recruited people to work in the hospital, and trained them in the use of containment gear that had been shipped in to help control the epidemic.

Another contributing factor to the spread of infection involved the local customs for preparing corpses for burial. Relatives and friends of the deceased followed traditional preparation and burial rituals that require considerable contact with the cadaver. To minimize the risk of infection among local residents, Red Cross workers collected cadavers in body bags and then decontaminated the bags with hypochlorite solution.

Because it was vital to ascertain the extent of the outbreak, the response team also immediately began an exhaustive epdemiologic investigation to identify all case-patients and their contacts and to determine the origin of the epidemic. This investigation traced the epidemic back to a case of hemorrhagic fever in a charcoal worker in January 1995. The infection was spread from this charcoal worker to residents in the town. When these patients sought medical attention, the infection was introduced into two local hospitals, where nosocomial spread resulted in marked amplification of the epidemic.

As was true for previous epidemics of Ebola virus hemorrhagic fever, the basic safety and infection-control measures were successful in bringing the epidemic in Kikwit under control. The outbreak began to wane in late May and early June; the last reported case-patient had onset of illness on June 21. Overall, our preliminary data indicate that there were 315 case-patients; 244 (77%) died. Patients ranged in age from 2 months to 71 years (mean age 37 years, median age 36 years); 26 patients were younger than 17 years of age. The sex ratio was 166 females, 149 males. Ninety (30%) cases occurred among health care workers, importantly only one of these patients was infected after the rehabilitation of the hospital and the introduction of safety precautions (e.g., the wearing of masks, gowns, and gloves). Most of the cases occurred in locations throughout Kikwit, although several cases were identified in surrounding villages. Evidence of virus transmission was found in only one or two of these villages. One patient traveled to Kinshasa and was hospitalized there; the patient's illness was recognized as possible Ebola virus hemorrhagic fever, barrier precautions were instituted, and no secondary cases were reported.

One of the interesting findings of the epidemiologic investigation was that two patients appeared to be the source of infection for as many as 50 other patients. This "super spreader" phenomenon has been observed before in outbreaks of hemorrhagic fever and other infectious diseases. Interhuman transmission in Kikwit was probably through close personal contact. This pattern documented in a survey of case-households in Kikwit that was conducted by Scott Dowell and his colleagues to ascertain the types of contact that had occurred between the primary case and any secondary cases in the household. Approximately 16% of the household members became infected; most of these patients were adults. The survey results are supportive of a pattern in which the close contact of the case-patients from the hospital was primarily with the adult spouse who cared for that patient. These findings also suggest that the strategy for encouraging patients to seek treatment at the hospital should have an effect in slowing transmission.

With regard to laboratory diagnostic methods, the antigen-detection ELISA proved to be a highly specific and reliable method. In addition, Dr. Sherif Zaki's molecular pathology group in the Division of Viral and Rickettsial Diseases at CDC developed a new technique based on immunohistochemistry that detects Ebola virus antigen in skin biopsy samples. The skin biopsy samples are easy and relatively safe to obtain, and they can be fixed in formalin and shipped internationally without worrying about dry ice or liquid nitrogen to preserve them and without having to declare the samples as infectious material. The availability of this method provides us with a safe, relatively simple method for conducting laboratory-based surveillance for Ebola virus hemorrhagic fever in Africa and possibly for other infectious diseases elsewhere.

After it was clear that the epidemic in Kikwit had been contained, we began another crucial phase of the investigation: to identify the source of the outbreak, the natural reservoir of Ebola-Zaire virus. Until we have this information we will be unable to identify risk behaviors and environmental conditions that may have led to its reemergence. This information would enable us to make predictions about the occurrence of these outbreaks and to develop prevention and control strategies. For several reasons, including the fact that the virus grows firly well in mammalian cells and poorly in amphibian, reptile and mosquito cells, it was decided that we should concentrate on obtaining samples from mammalian species. We have collected serum and tissue specimens from approximately 3,000 vertebrates and about 30,000 arthropods, and we are collaborating with a number of different organizations, particularly the U.S. Army Medical Research Institute on Infectious Diseases, on the analysis of these specimens. Because these kinds of investigations are difficult and require a considerable amount of laboratory testing, it will be at least a year before this work is completed.

Summary

Emerging infections have posed a serious threat to human health for centuries and undoubtably will continue to so in the years ahead. The emergence of infectious agents is dependent on a variety of diverse factors in nature and in society as well as of our environment. Among the most lethal of the emerging microbial threats are the viral hemorrhagic fevers, particularly in the form of filovirus disease. Our understanding of these viruses is poor, and consequently we have a very limited capability to predict how and when these agents will emerge. The hazardous nature of these viruses, their potential for aerosol transmission, and the lack of vaccines and therapeutic drugs compound the risk involved in dealing with these agents in clinical and laboratory settings and epidemiologic investigations. The proper application of biosafety principles in each of these settings is critical to our efforts to address these extraordinary viruses and the diseases they cause.

Office of Health and Safety, Centers for Disease Control and Prevention, 1600 Clifton Road N.E., Mail Stop F05 Atlanta, Georgia 30333, USA Last Modified: 1/2/97
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