Emerging Viral Diseases Why We Need to Worry About Bats, Camels, and Airplanes
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Chapter 16 Emerging Viral Diseases Why We Need to Worry about Bats, Camels, and Airplanes James W. Le Duc1, Neal Nathanson2 1Galveston National Laboratory, University of Texas Medical Branch, Galveston, TX, USA; 2Department of Microbiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA Chapter Outline 1. How Do New Viral Diseases Emerge? 215 3.1 Human Ecology 225 1.1 Discovery of the Etiology of an Existing Disease 215 3.2 Deliberate Introduction of a Virus New to a 1.2 Increase in Disease Caused by an Existing Virus 216 Specific Population 226 1.3 Accumulation of Susceptible Hosts and 3.3 Xenotransplantation 226 Viral Reemergence 217 4. How Are Emergent Viruses Identified? 226 1.4 Virus New to a Specific Population 217 4.1 Classic Methods of Virus Discovery 227 2. Zoonotic Infections as a Source of Emerging Viral Diseases 217 4.2 The Henle–Koch Postulates 228 2.1 Dead-end Hosts 217 4.3 Methods for Detection of Viruses that Are Difficult 2.2 Limited Spread among Humans 218 to Grow in Cell Culture 228 2.3 Crossing the Species Barrier 219 4.4 Computer Modeling of Emerging Infections 229 2.4 The Species Barrier and Host Defenses 225 5. Reprise 229 3. Why Viral Diseases Are Emerging at an Increasing Further Reading 229 Frequency 225 One of the most dramatic aspects of virology is the emer- disease. An example is La Crosse virus, a mosquito- gence of new virus diseases, which often receives widespread transmitted bunyavirus that was first isolated from a fatal attention from the scientific community and the lay public. case of encephalitis in 1964. The isolation of the causal Considering that the discipline of animal virology was estab- agent and the development of serological tests made it pos- lished over 100 years ago, it may seem surprising that new sible to distinguish La Crosse encephalitis from the rubric virus diseases are still being discovered. How this happens is of “arbovirus encephalitis, etiology unknown.” Since that the subject of this chapter. time, about 100 cases have been reported annually, without any significant increase since the 1970s. It appears that the 1. HOW DO NEW VIRAL DISEASES emergence of this “new” disease reflected only the new- found ability to identify this etiologic entity, rather than any EMERGE? true change in its occurrence. There are many recent books and reviews (see Further Hantavirus pulmonary syndrome is another example of Reading) that list the plethora of determinants that can lead the “emergence” of an existing but previously unrecognized to the emergence of infectious diseases (Table 1). In this disease. In 1993, in the four corners area of the southwestern chapter, we concentrate on those determinants that relate United States, there occurred a small outbreak of cases of to viral pathogenesis and deal only briefly with the many acute pulmonary illness with a high mortality. Epidemio- societal and environmental factors that can be instrumental logic and laboratory investigation rapidly identified the in disease emergence. causal agent, a previously unknown hantavirus, now named Sin Nombre virus (SNV). SNV is an indigenous virus of 1.1 Discovery of the Etiology of an deer mice (Peromyscus maniculatus) that are persistently infected and excrete the virus. Apparently deer mice pro- Existing Disease duce virus-infected excreta and, when they infest human In some instances, the “emergence” of a viral disease repre- dwellings, aerosolized fomites can result in occasional sents the first identification of the cause of a well-recognized human infections. The 1993 outbreak is thought to reflect Viral Pathogenesis. http://dx.doi.org/10.1016/B978-0-12-800964-2.00016-1 Copyright © 2016 Elsevier Ltd. All rights reserved. 215 216 PART | III Emergence and Control of Viral Infections a transient rise in deer mouse populations associated with TABLE 1 Some of the Factors that Lead to Emergence an unusual crop of pine nuts, a major food source for these or Reemergence of Viral Diseases rodents. The recognition of SNV soon led to the discov- Factors Leading to ery of other heretofore unrecognized hantaviruses in North, Emergence Determinant Central and South America, many of which also cause seri- ous human disease. Economic and social Population growth, density, development distribution Environmental changes such as 1.2 Increase in Disease Caused by an deforestation, dam building, global Existing Virus warming On occasion, a virus that is already widespread in a popu- Increased global travel lation can emerge as a cause of epidemic or endemic dis- Increased international commerce ease, due to an increase in the ratio of cases to infections. Agribusiness, food processing, Such an increase can be caused by either an increase in distribution host susceptibility or enhancement of the virulence of the virus. Although counterintuitive, there are some dramatic Poverty Inadequate public health systems instances of such phenomena. Open defecation Increase in host susceptibility. Poliomyelitis first Lack of safe water appeared as a cause of summer outbreaks of acute infantile paralysis in Sweden and the United States late in the nine- Societal breakdown Civil chaos teenth century (Figure 1). Isolated cases of infantile paralysis War had been recorded in prior centuries, and an Eygptian tomb Human factors Sexual activity painting indicates that poliomyelitis probably occurred in early recorded history. Why then did poliomyelitis emerge Substance abuse abruptly as an epidemic disease? When personal hygiene Biological factors Natural mutation and public health were primitive, poliovirus circulated as Antimicrobial resistance a readily transmitted enterovirus, and most infants were infected while they still carried maternal antibodies (up to Immunosuppression 9–12 months of age). Under these circumstances, the virus produced immunizing infections of the enteric tract, but pas- sively acquired circulating antibodies prevented invasion of 28,000 28,000 8,000 7,000 6,000 5,000 4,000 3,000 Annual reported cases 2,000 1,000 0 1885 1890 1895 1900 1905 1910 1915 Year FIGURE 1 The appearance of epidemic poliomyelitis in the United States, 1885–1916. The graph is based on reported cases (mainly paralytic) during an era when reporting was estimated at about 50%. Data from Lavinder CH, Freeman SW, Frost WH. Epidemiologic studies of poliomyelitis in New York City and the northeastern United States during the year 1916. Washington: United States Public Health Service (1918). Emerging Viral Diseases Chapter | 16 217 the spinal cord. With the improvement of personal hygiene and permissive vectors. In an unusual event, WNV was in the late-nineteenth century, infections were delayed until imported into New York City, probably by the introduction 1–3 years of age, after the waning of maternal antibodies. of infected vector mosquitoes that were inadvertent passen- Infections now occurred in susceptible children, resulting in gers on a flight from the Middle East, where the virus is outbreaks of infantile paralysis. This reconstruction is sup- enzootic. This hypothesis was supported by the finding that ported by seroepidemiological studies conducted in North the genomic sequence of the New York isolates was closely Africa in the 1950s, when epidemic poliomyelitis first related to the sequence of contemporary isolates from Israel. emerged in this region. Some American mosquito species were competent vectors Increase in viral virulence. Viruses may undergo sud- for WNV, and certain avian species such as American crows den increases in virulence resulting in emergence of dra- were highly susceptible. As a result, West Nile encephalitis matic outbreaks. An outbreak of lethal avian influenza emerged as a significant disease new to the United States. in Pennsylvania in 1983 is one documented example. In Over a period of several years WNV spread across the con- eastern Pennsylvania, avian influenza appeared in chicken tinent, finally reaching the west coast and many areas in farms early in 1983, but the virus was relatively innocuous Latin America. and most infections were mild. However, in the fall of that Chikungunya virus (CHIKV), another mosquito-borne year a fatal influenza pandemic spread rapidly through the arbovirus, has been endemic for many years in regions of same farms. When viruses from the spring and fall were Africa and Asia where it periodically caused outbreaks of a compared, it appeared that both isolates had almost identical febrile illness associated with severe arthritis and arthralgia. genomes. The fall virus had acquired a single point muta- In 2005–2006, CHIKV caused a large epidemic on the tion in the viral hemagglutinin that facilitated the cleavage island of Reunion in the Indian Ocean, apparently asso- of the hemagglutinin. The virus could now replicate outside ciated with a mutation that allowed the virus to be more the respiratory tract, markedly increasing its virulence (dis- efficiently transmitted by vector mosquitoes. Chikungunya cussed in Chapter 7, Patterns of infection). This point muta- subsequently spread to India and elsewhere in Asia, fol- tion led to the emergence of an overwhelming epizootic, lowed by the Americas. As of November 2014, transmission which was only controlled by a widespread slaughter pro- of CHIKV had been documented in 40 countries or territo- gram involving millions of birds. Similar outbreaks of avian ries in the Caribbean, Central, South and North America, influenza have occurred subsequently in other countries. resulting in nearly one million suspected cases each year. It appears that CHIKV may become established in the New 1.3 Accumulation of Susceptible Hosts World, where virtually the entire human population cur- rently lacks immunity. and Viral Reemergence A virus that is endemic in a population may “fade out” 2. ZOONOTIC INFECTIONS AS A SOURCE and disappear, because the number of susceptibles has fallen below the critical level required for perpetuation in OF EMERGING VIRAL DISEASES that population. If the population is somewhat isolated, Zoonotic infections of animals that can be transmitted to the virus may remain absent for many years.