Special Issue ,' Infectious Disease as an Evolutionary Paradigm Joshua Lederberg Sackler Foundation Scholar, Rockefeller University, New York, New York, USA The basic principles of genetics and evolution apply equally to human hosts and to emerging infections, in which foodborne outbreaks play an important and growing role. However, we are dealing with a very complicated coevolutionary process in which infectious agent outcomes range from mutual annihilation to mutual integration and resynthesis of a new species. In our race against microbial evolution, new molecular biology tools will help us study the past; education and a global public health perspective will help us deal better with the future. Life expectancy in the United States from 1900 to the present (Figure 1) shows an overall steady rise, reflecting improved health conditions in general, the result of advances in medical science, hygiene, personal care, health technolo- gies, and public health administrations. The rise decelerates asymptotically to a near plateau from the 1950s to the 197Os, reflecting an epidemic of coronary disease, which we do not yet fully understand. Improvements in medical care, attention to life style, or indiscriminate use of aspirin may all be responsible for the subsequent decrease in deaths from coronary disease. Up to the 194Os, the rising curve is jagged, reflecting sporadic infectious disease outbreaks, especially the Spanish influenza outbreak of 1918. Whether the life expectancy curve continues to rise smoothly or whether it has some jagged declines depends on what we do about transmission of infectious disease, including foodborne disease. When plotted another way (Figure 21, both the absolute number of deaths from infectious disease and the proportion of total deaths attributable to infectious disease also show steady amelioration from 1900 almost to the present. The 1918 Spanish influenza pandemic may be a prototype for future emerging infections. Address for m-respondence: Joshua Ledertmg. Rockefeller University, 1230 York Avenue, New York. NY 10021-6399, USA; fax: 212-327-8651; e-mail: Lederber&nail.xckefeller.edu. This text is also available online - URL: ftp://ftp.cdc.gov/pub/EID/vol3nc4fasciiflederber.txt bttp:Nwww.cdc.govlncidod/EID/eid.htm Vol. 3, No. 4, October-December 1997 417 Figure 1. Life expectancy in the United States, at birth, 20th century. 1.000 r I : .' i 0" ,m ,710 ,920 ,930 19.0 1950 1960 1970 1760 177 J . I 1 - I -I 0 50 40 30 20 IO 0 YUr ime - Pcram----- Figure 2. Trends in infectious diseases mortality, 1900-1992. Source: CDC, unpub. data. EmergingInfectious Lkeases Specia 1 Issue Although minimized as not much more than a bad cold, influenza took a terrible toll in 1918, especially on young people (Figure 31. Somewhat older persons may have been protected by immunity from prior exposure to related strains of influenza. The disease, with rapid onset of fulminating pneumonic symptoms, killed 20 to 25 million persons worldwide. The infectious agent was not available for study at that time. However, very recently the Armed Forces Institute of Pathology recovered with PCR technology genetic fragments of the 1918 influenza virus (1). Less than 10% of the entire genome has been recovered to date, but recovery of complete sequences is likely. Although the target genes have not yet provided a clue as to why the 1918 influenza was so devastating, they demonstrate the enormous potential of today's molecular biology tools. *a I I ,692 / -nmu / 1' Figure 3. Pneumonia and influenza mortality, by age, in certain epidemic years. (Reprinted with permission of W. Paul Glezen and Epidemiologic Reviews. Emerg- ing Infections: Pandemic Influenza. Epi Rev 1996;18:66). These tools will enable us to better study paleovirology and paleomicrobiology. We are accustomed to stereotyping historical disease outbreaks as if we really knew what they were, but we really know very little detail about their genetic features. For example, we talk about the great historic plagues as if they indeed were Yersinia or cholera or malaria. We should look forward to finding out about the 14th century black death, if it was indeed Yersinia pestis. Although clinically unmistakable, that is not to say it was caused by the identical genotype of present Yersinia strains. We need to look ahead as well as back. In this century, emerging and reemerging infections have stimulated flurries of interest, but in general we have been complacent about infectious diseases ever since the introduction of antibiotics. The effect of antibiotics on acute infections and tuberculosis as well as the effect of polio vaccination led to a national, almost worldwide, redirection of attention to chronic and constitutional diseases. However, the HIV pandemic in the early 1980s caught us off guard, reminding us that there are many more infectious agents in the world. It is fortuitous that retroviruses had already been studied from the perspective of cancer etiology; otherwise, we would have had no scientific platform whatsoever for coping with HIV and AIDS. The Committee on International Science Engineering and Technology provided an inter- agency review setting out a policy framework for the United States' global response to infectious disease (Table 1). The policy provides a worldwide mantle for surveillance and monitor- ing, remedial measures, development of new drugs, vaccines, and treatment modalities. The global outlook is necessary, even if for purely selfish reasons, because to infectious agents the world is indivisible, with no national boundaries. Our thinking has been impoverished in terms of budget allocations for dealing with health on an international basis. We are engaged in a type of race, enmeshing our ecologic circumstances with evolutionary changes in our predatory competitors. To our advantage, we have wonderful new technology; we have rising life expectancy curves. To our disadvantage, we have crowding; we have social, political, economic, and hygienic stratification. We have crowded together a hotbed of opportunity for infectious agents to spread over a significant part of the population. Affluent and mobile people are ready, willing, and able to carry afflictions all over the world within 24 hours' notice. This condensation, stratification, and mobility is unique, defining us as a very different species from what we were 100 years ago. We are enabled by a different set of technologies. But despite many potential defenses-vaccines, antibiotics, diagnostic tools-we are intrinsically more vulnerable than before, at least in terms of pandemic and communicable diseases. We could imaginably adapt in a Darwinian fashion, but the odds are stacked against us. We cannot compete with microorganisms whose populations are measured in exponents of lo", Emerging Infectious Diseases 418 Vol. 3, NO. 4, October-December 1997 ._`._ .--_~ "-1 _"-.- _._._ .^.._ ,.". _ .i..--. - Special Issue Table 1. Examples of pathogenic microbes and infectious diseases recognized since 1973 (2) Year 1973 1975 1976 1977 1977 1977 1977 1980 1981 1982 1982 1982 1983 1983 1985 1986 1988 1988 1989 1989 1991 1991 1991 1992 1992 1993 1993 1994 1995 Microbe Rotavirus Parvovirus B19 Cryptosporidium Ebola virus Legionella Hantaan virus Campylobacter jejuni Human T-lymphotropic virus I (HTLV-1) Toxic producing strains of Staphylococcus aureus Escherkhia coli 0157:H7 HTLV-II Born&a burgdorferi Human immunodeficiency virus(HIV1 Helicobacterpylori Enterocytozoon bieneusi Cyclospora cayetanensis Human herpes-virus-6 (HI-IV-61 Hepatitis E Ehrlichia chafeensis Hepatitis C Guanarito virus Encephalitozoon hellem New species of Babesia Vibrio cholerae 0139 Bartonella henselae Sin Nombre virus Encephalitozoon cuniculi Sabia virus Virus Major cause of infantile diarrhea worldwide Virus Aplastic crisis in chronic hemolytic anemia Parasite Acute and chronic diarrhea parvum Virus Ebola hemorrhagic fever Bacteria Legionnaires' disease pneumophila Virus Hemorrhagic fever with renal syndrome (HRFS) Bacteria Enteric pathogens distributed globally Virus T-cell lymphoma-leukemia Bacteria Toxic shock syndrome(tampon use) Bacteria Virus Bacteria Virus Bacteria Parasite Parasite Virus Virus Bacteria Virus Virus Parasite Parasite Bacteria Bacteria Virus Parasite Virus HHV-8 Virus Disease Hemorrhagic colitis; hemolytic uremic syndrome Hairy cell leukemia Lyme disease Acquired immunodeficiency syndrome (AIDS) Peptic ulcer disease Persistent diarrhea Persistent diarrhea Roseola subitum Enterically transmitted non-A, non-B hepatitis Human ehrlichiosis Parenterally transmitted non-A, non-B liver infection Venezuelan hemorrhagic fever Conjunctivitis, disseminated disease Atypical babesiosis New strain associated with epidemic cholera Cat-scratch disease; bacillary angiomatosis Adult respiratory distress syndrome Disseminated disease Brazilian hemorrhagic fever Associated with Kaposi sarcoma in AIDS patients lo", 1016 over periods of days. Darwinian natural selection has led to the evolution of our species but at a terrible cost. If we were to rely strictly on biologic selection to respond to the selective factors of infectious disease, the population would fluctuate from billions down to perhaps millions before slowly rising again. Therefore, our evolutionary capability may be dismissed as almost totally inconsequential. In the race against microbial genes, our best weapon is our wits, not natural selection on our genes. New mechanisms of genetic plasticity of one microbe species or another are uncovered almost daily. Spontaneous mutation is just the begin- ning. We are also dealing with very large populations, living in a sea of mutagenic influences (e.g., sunlight). Haploid microbes can immediately express their genetic variations. They have a wide range of repair mechanisms, themselves subject to genetic control. Some strains are highly mutable by not repairing their DNA; others are relatively more stable. They are extraordinarily flexible in responding to environ- mental stresses (e.g., pathogens' responses to antibodies, saprophytes' responses to new environments). Mechanisms proliferate whereby bacteria and viruses exchange genetic material quite promiscuously. Plasmids now spread throughout the microbial world (3). They can cross the boundaries of yeast and bacteria. Lateral transfer is very important in the evolution of microorganisms. Their pathogenic- ity, their toxicity, their antibiotic resistance do not rely exclusively on evolution within a single clonal proliferation. We have a very powerful theoretical basis whereby the application of selective pressure (e.g., antibiotics in food animals) will result in drug resistance carried by plasmids, or patho- gens attacking humans. It is not easy to get direct Vol. 3, No. 4, October-December 1997 419 Emerging infectious Diseases and immediate epidemiologic evidence, but the foundations for these phenomena exist and must be taken into account in the development of policies. We have barely begun to study the responses of microorganisms under stress, although we have examples where root mecha- nisms of adaptive mutability are themselves responses to stress. In recent experiments, bacterial restriction systems are more permissive of the introduction of foreign DNA, possibly letting down their guard in response to "mutate or die" circumstances. This does not reflect bacterial intelligence-that they know exactly what mutations they should undergo in response to environmental situations. Their intrinsic mutability and capacity to exchange genetic information without knowing what it is going to be is not a constant; it is certainly under genetic control and in some circumstances varies with the stress under which the microbes are placed. Evolution is more or less proportionate to the degree of genetic divergence among the different branches of the three-tiered tree of life, with the archaeal branch, the eubacterial branch, and the eukaryotes (Figure 4). The tree illustrates the small territory occupied by humans in the overall world of biodiversity. It shows mitochondria right BACTERIA ARCHAEA wmR. \ EUCARYA Special Issue next to Escherichia coli. Bacterial invasion of a primitive eukaryote 2-112 to 3 billion years ago, synchronized with the development of primitive green oxygen-generating plants, conferred a selective advantage to complexes that could use oxygen in respiration. Our ancestors were once invaded by an oxidative-capable bacterium that we now call a mitochondrium and that is present in every cell of every body and almost every species of eukaryote. We did not evolve in a monotonous treelike development; we are also the resynthesis of components of genetic development that diverged as far as the bacteria and were reincorporated into the mitochondrial part of our overall genome. Another example of lateral transfer is the symbiosis that resulted from chloroplast invasion of green plants. The outcome ofencounters between mutually antagonistic organisms is intrinsically unpredict- able. The 1918 influenza outbreak killed half percent of the human population; but because the consequences were to either kill the host or leave the host immune, the virus died out totally, leaving no trace in our genomes, as far as we know. Historic serology on survivors has found memory cells and antibodies against HlNl, the serotype of the resurrected 1918 virus. Unlike the influenza virus, which left no known genetic imprint, 400 to 500 retroviruses are integrated into our human genome. The full phylogeny of these encounters is unknown, but many of these viruses may precede the separation of homo sapiens from the rest of the hominid line. Infectious agent outcomes range from mutual annihilation to mutual integration and resynthe- sis of a new species. Much has been made of the fact that zoonoses are often more lethal to humans than to their original host, but this phenomenon cannot necessarily be generalized. Most zoonoses do not affect humans adversely. Some are equally capable in a new host. We tend to pay most attention, however, to those, such as yellow fever, for which we have not genetically or serologically adapted and which cause severe disease. Figure 4. The three-domain tree of life based on small- subunit rRNA sequences. Reprinted with permission of Norman R. Pace and ASM News. ASM News 1996;62(9):464. Canine distemper provides an example of a quasihereditary adaptation. In the Serengeti, the disease migrated from village dogs to jackals, which shared prey and had contact with lions. About one-fourth of the preserve's 4,000 lions died of canine distemper (4) but the survivors are immune and will pass immunoglobulin, to their offspring. The cubs' maternal immunity will Emerginghktious Dktx.5~ 420 Vol. 3, No. 4, October-December 1997 likely mitigate infection and permit a new probably progress in the next 20 years, it is equilibrium, not because of genetic adaptation paradoxical that we know more about hemoglo- but because of the preimmunized host. This is bin S than any other molecular disease. The also the most plausible explanation for how entire concept of genetic determination of protein savage the polio virus has been as a paralytic structure has been based on these early infection of young people. It may also apply to observations, yet we are still searching with limited hepatitis, where cleaner is not always better if it success for ways to put it to therapeutic use. means we do not have the "street smarts" to Biotechnology may enable other forms of respond to new infectious challenges. These genetic intervention through which homo nongenetic adaptations between parasite and sapiens could conceivably bypass natural selec- host complicate our outcome expectations. tion and random variation. In the absence of Short-term shifts in equilibrium can give alternatives, we might speculate about these ferocious but temporary advantages to a virus. kinds of "aversive therapies" as a last resort to Long-term outcomes are most stable when they save our species. involve some degree of mutual accommodation, The ultimate origin of life is still the subject of with both surviving longer. New short-term many theories, as is the origin of viruses (Table deviants, however, can disrupt this equilibrium. 2). Each virus is different. We know nothing of The final outcome of the HIV pandemic cannot be virus phylogenies and cannot even substantiate predicted. More strains with longer latency may the distinctions of the several hundred catego- be taking over, mitigating the disease. However, ries. We do not know their origin, only that they deviant strains could counteract this effect by interact with host genomes in many ways. overcoming immunity and rapidly proliferating, Particles could come out of any genome, become with earlier and more lethal consequences. free-living (i.e., independent, autonomously We should also consider somatic evolution, a replicating units in host cells), reenter a host Darwinian process that occurs with every infection. genome as retroviruses and possibly others do, In the clonal selection model of immunogenesis (51, and repeat the cycle dozens of times. But no one an apparently random production ofimmunoglobu- can give a single example or claim to have lin variants, both by reassortment of parts and by significant knowledge ofhow any particular virus localized mutagenesis, gives rise to candidate evolved, thus presenting a scientific challenge for antibodies, which then proliferate in response to the next 20 or 30 years. matching epitopes. We do not understand the We are dealing with more than just predation details of how a given epitope enhances stepwise and competition. We are dealing with a very improvements in affinity and productivity of antibodies at various stages. The process may be more complicated than we realize; so may Table 2. The origin of viruses Darwinian evolution. Viruses are genomic fragments-that can replicate Despite the prior arguments against relying only in the context of an intact living cell. They cannot on host or genotype evolution as a response to therefore be primitive antecedents of cells. infection, historically we have done so and now Within a given species, viruses may have emerged have "scars of experience." A notable example is as genetic fragments or reduced versions from malaria, wherein the Duf@ mutation against chromosomes, plasmids, or RNA of Plasmodium uiuax is the only host defense with 1) the host or related species no deleterious consequences, The thalassemias, 2) distant species GGPD deficiency, and hemoglobin S are all 3) larger parasites of the same or different hosts hemopoietic modifications that thwart the 4) further evolution and genetic interchange plasmodia; but in homozygotes, they themselves among existing viruses cause disease. In the evolution of our species, for Once established, they may then cycle back into every child spared an early death because a the genome ofthe host as an integrated episome; there hemoglobin S mutation impeded Plasmodium they may have genetic functions or in principle might development, another will succumb to sickle cell reemerge as new viruses. disease unless we can intervene. Specific These cycles have some substantiation in the remedies do not exist. Although somatic gene world of bacterial viruses; but we have no clear data on therapy is an interesting possibility, one that will the provenience of plant or animal viruses. Special Issue Vol. 3, No. 4, October-December 1997 421 Emerging infectious Diseases complicated coevolutionary process, involving Table 3. Genetic evolution merger, union, bifurcation, and reemergence of Microbes (bacteria, viruses, fungi, protozoa) new species (Table 3). Divergent phenomena can Rapid and incessant occur in any binary association, with unpredict- Huge population sizes lOI'+ and generation able outcomes. We have hundreds of retroviruses times in minutes vs. years in our genome and no knowledge of how they got there. As to HIV, we have no evidence as yet that Intraclonal process it has ever entered anyone's germ line genome: DNA replication-may be error-prone-in sea we really do not know whether it ever enters of mutagens sunlight; unshielded chemicals, germ cells. The outcomes of even that interaction incl. natural products could be much more complicated than the purely RNA replication-intrinsically unedited, >W parasite/host relationships we are accustomed to. swarm species Innovative technologies for dealing with Haploid: immediate manifestation, but partial microbial threats have the potential for fascinat- recessives not accumulated contra multicopy ing therapeutic opportunities (Table 4). Some, plasmids like bacteriophage, have been set aside as Amplification laboratory curiosities. Nothing is more exciting Site-directed inversions and transpositions: than unraveling the details of pathogenesis. phase variation Having the full genomes of half a dozen parasitic ?? Other specifically evolved mechanisms: organisms opens up new opportunities for genome quadrant duplication; silencing therapeutic invention in ways that we could not Interclonal process have dreamed of even 5 years ago, which will lead Promiscuous recombination-not all to many more technologies. In food microbiology, mechanisms are known we should keep in mind the probiotic as well as Conjugation-dozens of species the adversarial and pathogenetic opportunities Viral transduction and lysogenic integration: in our alimentary tracts. universal The Committee on International Science Classical: phage-borne toxins in Engineering and Technology report (2) provides C. diphtheriae some recommendations (Table 5). We need a Plasmid interchange (by any of above) and global perspective. We need to invest in public integration health, especially food microbiology, not just Toxins of B. anthracis medical care, in dealing with disease. It is PLSteU :: lieat dttenuation: plasmid 16SS; important to prevent foodborne disease through chemically induced sensible monitoring, standards of cleanliness, RNA viral reassortment; ?? and and consumer and food-handler education and recombination? not just care for its victims. Transgressive-across all boundaries Today we emphasize individual rights over Artificial gene splicing community needs more than we did 50 to 75 years Bacteria and viruses have picked up host ago. Restraining the rights and freedoms of genes (antigenic masking?) individuals is a far greater sin than allowing the Interkingdom: P. tumefaciens and plants, infection of others. The restraints placed on E. coli and yeast Typhoid Mary might not be acceptable today, Vegetable and mineral! oligonucleotides when some would prefer to give her unlimited and yeast. rein to infect others, with litigation their only Host-parasite coevolution recourse. In the triumph of individual rights, the Coadaptation to mutualism or accentuation public health perspective has had an uphill of virulence? struggle in recent pandemics. Jury is still out (May and Anderson). Many Education, however, is a universally accepted zoonotic convergences. countermeasure, especially important in Probably divergent phenomena, with short- foodborne diseases. Food safety programs should term flareups and Pyrrhic victories, atop more specifically target food handlers, examining long-term trend to coadaptation. their hands to determine if they are carriers, to ensure they are complying with basic sanitation. Special Issue Emerging Infectious Diseases ._ 422 Vol. 3, No. 4, October-December 10.": ,~ ~.. Specia 1 Issue Table 4. Technologies to address microbial threats Antibacterial chemotherapy Potentially unlimited capability; bacterial metabolism and genetic structure notably different from human genome sequencing pointing to bacterial vulnerabilities Economic-structural factors-public expectation for unachievable bargains in safety assurance, cost of development, and ultimate pricing Dilemmas of regulation of (ab)use Resurgent interest in bacteriophage and other biologically oriented approaches Antiviral chemotherapy Much more difficult program, inherently Gross underinvestment New approaches: antisense, ribozymes, targeted D/RNA cleavers Problematics of sequence-selective targets Vaccines Gross underinvestment; other structural problems as above Liability/indemnification Vaccination as service to the herd New approaches: hot biotechnology is coming along especially live attenuated: but testing dilemmas Safety issues about use of human cells lines; adjuvants Immunoglobulins and their progeny Phage display and diversification: biosynthetic antibody Passive immunization for therapy Biologic response modifiers New world of interleukins, cell growth factors so far just scratching surface Interaction with pathogenesis Intersection with somatic gene therapy Technologies for diagnosis and monitoring Etiologic agents and control Host polymorphisms and sensitivities Homely technologies needed Simple, effective face-masks Palatable water-disinfectants Home-use diagnostics of contamination Table 5. CISET' recommendations for addressing global infectious disease threats 1. Concerted global and domestic surveillance and diagnosis of disease outbreaks and endemic occurrence. This must entail the installation of sophisticated laboratory capabilities at many centers now lacking them. 2. Vector management and monitoring and enforcement of safe water and food supplies; and personal hygiene (e.g., Operation Clean Hands). 3. Public and professional education. 4. Scientific research on causes ofdisease, pathogenic mechanisms, bodily defenses, vaccines, and antibiotics. 5. Cultivation of the technical fruits of such research, with the full involvement of the pharmaceutical industry and a public understanding of the regulatory and incentive structures needed to optimize the outcomes. *Committee on International Science, Engineering and Technology Policy of the National Science and Technology Council. We typically do this only after an outbreak. Perhaps we should have further debate on the social context for constraints and persuasion to contain the spread of infectious agents. References 1. 2. 3. 4. 5. Taubenberger JK, Reid AH, Frafit AE, Bijwaard KE, Fanning TG. Initial genetic characterization of the 1918 "Spanish" influenza virus. Science 1997;275: 1793- 6. NSTC-CISET Working Group on Emerging and Reemerging Infectious Diseases. Infectious disease-a global health threat. Washington@C): The Group;1996. Lederberg J. Plasmid (1952-1997). Plasmid. In press 1997. Roelke-Parker ME, Munson L, Packer C, Kock R, Cleaveland S, Carpenter M, et al. A canine distemper virus epidemic in Serengeti lions (Panthera lea). Nature 1996;379:441-5. Lederberg J. The ontogeny of the clonal selection theory of antibody formation: reflections on Darwin and Ehrlich. Ann N Y Acad Sci 1988;546: 175-87. Vol. 3, No. 4, October-December 1997 423 Emerging Infectious Diseases