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Evolving microbes and re-emerging streptococcal disease
Clin Lab Med 22 (2002) 835–848
Evolving microbes and re-emerging streptococcal disease Richard M. Krause, MD Laboratory of Human Bacterial Pathogenesis, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Building 16, Room 202, 16 Center Drive, MSC 6705, Bethesda, MD 20892, USA
Changes in demography and lifestyle and various adverse events are major factors that enhance epidemics [1]. Yet microbes are not idle bystanders. They are constantly evolving. Microbial evolution is driven by mutations and underlying mechanisms of genetic exchange, which also play an important role in the emergence and re-emergence of infectious diseases. The re-emergence of streptococcal disease is considered within this context. Investigators are pursuing molecular studies to explore the possible genetic relationship between the recent re-emerging, virulent group A streptococci (GAS) and streptococci that caused widespread epidemics of severe scarlet fever in an earlier era. Interest in GAS had waned following the widespread use of penicillin to treat GAS pharyngitis and the concomitant prevention of acute rheumatic fever (ARF) and rheumatic heart disease (RHD). Clinicians also found that, in parallel with this important medical advance, cases of GAS pharyngitis were milder than before. But, unexpectedly, a virulent variety of GAS reappeared 15 years ago to cause an increase in the number of cases of streptococcal toxic shock syndrome (STSS) in the United States, Canada, and elsewhere, and in the number of cases of ARF in Salt Lake City and other United States locales. Although penicillin remains the drug of choice to treat GAS infections, resistance is developing to many other antibiotics. For this and other reasons, efforts are under way to construct a synthetic recombinant vaccine based on recently obtained knowledge of the molecular structure of M protein, a major component of GAS that stimulates protective immunity.
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Emerging bacterial infections: genetic aspects Strange as it may seem, microbial evolution is a new idea. Bacterial genetics was not even mentioned in medical schools during the 1940s. At that time, bacteriology was the study of each microbe and the disease it caused. Indeed, in the third edition of Topley and Wilson [2], published in 1946, bacterial genetics is not discussed, although bacterial variation in all its manifestations is described in great length. On the possibility of bacterial mutations, Topley and Wilson write: ‘‘The application to bacteria of terms that have been coined to express changes in form or function occurring in higher plants or animals is not without its dangers; and it is possible that there is little real justification for the use of such a term as mutation, in connection with the variations that bacteria may undergo’’ [2]. Such caution is not surprising for 1946. The mutational origin of bacterial variants was first demonstrated just 3 years earlier in 1943. To be fair, Burnet [3] perceived the occurrence of mutations in a paper he published in 1936 entitled ‘‘Induced Lysogenicity and Mutation of Bacteriophage Within Lysogenic Bacteria,’’ but this work had little impact until some years later. Recently, Lederberg [4] noted the versatile features of bacterial genetics that enhance the facility of microbes to overcome their natural ecological constraints or medical barriers, such as antibiotics and vaccines, and to emerge as something new. These genetic mechanisms include (1) phase variation, whereby silent, archival genetic information reappears as adaptive change (eg, the movement of a promoter from one locus to another, such as the movement of flagella antigens for salmonella); (2) intraclonal processes, such as mutations due to chemical and physical (including ultraviolet) action (bacteria are haploid and therefore there is no delay in gene expression, but prompt natural selection); and (3) interclonal processes, such as promiscuous intra- and interspecies genetic recombination, as well as transformation, transduction-lysogenic conversion, and conjugation. The emergence of new microbes or old editions in new garments stems from genetic evolution. A consequence of this genetic versatility is that microbes can develop new pathogenic vigor, escape population immunity by acquiring new antigens, develop antibiotic or drug resistance, become more transmissible, and escape from the species barrier. Toxigenic Escherichia coli, which can cause bloody diarrhea and the uremic syndrome, and GAS, which can cause toxic shock syndrome and the re-emergence of rheumatic fever, very likely have acquired new vigor as a result of genetic events and evolutionary selection. Although this evolution is poorly understood, the ‘‘new’’ GAS agents likely exploit the changing circumstances of the ecosystem brought about by perturbations in nature and human behavior. The rise and fall of epidemics The historical record shows that epidemics ebb, from a high tide into history’s twilight. AIDS likely will follow the same course in the next
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millennium. Recent research on the rise and decline of epidemics and pandemics embraces multiple disciplines that derive from the population biology of host–microbe associations. Much of what can be learned from the history of past epidemics remains largely speculative, although current theory can be applied to past events to gain a new framework for understanding the historical record. For example, much insight may be obtained about the 1918 influenza pandemic from current efforts to unravel the genome of the influenza virus at that time. In recent years, the evolution of epidemics has become better understood with studies of the molecular and evolutionary epidemiology and the population biology of microbes and the use of mathematical models of ‘‘happenings,’’ a term coined by Sir Ronald Ross for microbial outbreaks [5]. Ross was a pioneer in such mathematical analysis. The history of scarlet fever is a good case for examining factors that influence the rise and fall of epidemics. Scarlet fever was one of the most deadly childhood diseases in the 19th century. Scarlet fever occurs during pharyngitis caused by streptococci that produce certain potent toxins [6]. Why did the pandemic occur, and why has scarlet fever essentially disappeared in the 20th century? Were the particular horrors of the industrial cities of the Victorian era, so well depicted in the novels of Charles Dickens, a cauldron that fomented epidemics of scarlet fever, which abated as living conditions improved? Or, did the streptococci that caused the disease lose the necessary genes for producing the scarlatinal toxins and other factors involved in the pathogenesis of lethal scarlet fever? Even before the antibiotic era, scarlet fever had become much less common, even rare, and a much milder disease. And yet, streptococcal pharyngitis persists as a common childhood illness. A case study: scarlet fever The historical record of scarlet fever in the 19th century is fairly well known. Nineteenth-century clinicians had established excellent diagnostic criteria, and scarlet fever was recognized as a distinct clinical entity, no longer being confused with other childhood exanthems. In general, registries of illness and death were well kept in both Europe and the United States, and they provide reliable data on the incidence of, and mortality from, scarlet fever during this time. As noted, scarlet fever is an infection with GAS, characterized by sore throat (pharyngitis) and a rash that is distinctive and different from rashes associated with measles, rubella, and other childhood diseases. The association of streptococcal pharyngitis and scarlet fever, however, was not established until the first part of the 20th century. For this reason, an analysis of the pandemic must be based on the clinical records of that time. Katz and Morens [7] conclude from a careful review of the literature that scarlet fever most likely had occurred for centuries either as an endemic disease or as localized epidemics. Notably, in the early part of the 19th century,
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a pandemic of often fatal scarlet fever appeared suddenly and swept through Asia, Europe, and the United States (Fig. 1) [7,8]. From 1800 to 1830, family physicians noted a striking increase in mortality not seen by them previously, and fatality rates of up to 30% were often reported. Scarlet fever became the most common fatal infectious childhood disease, more fatal than measles, diphtheria, or pertussis, a fact that is difficult to comprehend today. For a hundred years, pandemic scarlet fever waxed and waned in incidence and severity. ARF and RHD, a late complication of ARF, were very common severe complications. Indeed, special hospitals were established to care for children with ARF and RHD. Despite 100 years of research, the pathogenesis of ARF, RHD, and acute glomerulonephritis (AGN), another major complication of streptococcal infections, remains obscure. Mortality from scarlet fever began to decline about 1880 and, by 1930, clinicians in general remarked on this decline. This change arose even before the availability of antibiotics. Today, scarlet fever is rare and, when it occurs, the disease is not life threatening. Streptococcal infections, without a scarlet fever rash, however, continue to be common during the school year (one to five cases per 1000 students per week) [9,10]. Most cases are treated with antibiotics, but even for those that are not, the risk of a subsequent attack of ARF is low, except in several locales in the United States. In the developing world, including heavily populated countries such as India, GAS
Fig. 1. Boston 1840–1940. Severe streptococcal infections in historical perspective. (Adapted from Katz SL, Morens DM. Severe streptococcal infections in historical perspective. Clin Infect Dis 1992;14:298–307; with permission, The University of Chicago Press, Ó Infections Disease Society of America, all rights reserved.) Depicted in the insert are the recorded deaths in the United States from 1900 to 1960. (Adapted from Quinn RW. Streptococcal infections. In: Evans AS, Feldman HA, editors. Bacterial infections of humans: epidemiology and control. New York: Plenum Medical; 1982. p. 525–52; with permission.)
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pharyngitis is also common and, for many reasons, ARF and RHD remain frequent complications. A research group headed by C.H. Rammelkamp achieved one of the major clinical advances of the 20th century by discovering that adequate treatment of streptococcal pharyngitis prevents subsequent occurrence of ARF and RHD [11]. During the next 10 years, there was an intensive campaign, with strong leadership from the American Heart Association, to prevent ARF by the clinical and bacteriologic detection of GAS pharyngitis, particularly in schoolchildren and military recruits, and prompt treatment with penicillin. By 1975, new cases of ARF and RHD had nearly disappeared in the United States and other developed countries. And yet, the question remains: why does the incidence of streptococcal infections persist, whereas scarlet fever and ARF have mostly disappeared in the United States and other developed countries? Did the earlier streptococci associated with scarlet fever and ARF possess especially virulent characteristics that are lacking in the streptococci isolated from pharyngitis today? Satisfactory answers to these questions have not been obtained, but clues may be forthcoming from current intensive investigations of the bacterial genetics and population and evolutionary epidemiology of GAS. In addition to the genetic evolution of the streptococcal microbe, the 19th century pandemic of scarlet fever surely also resulted from multiple social and demographic factors. The leading factor would have been the intense crowding of populations in the large industrial cities of England, Europe, and the United States. Overwhelming epidemiological evidence, particularly from studies of military recruits confined to close quarters in barracks [12], demonstrates that crowding enhances the spread of streptococcal infections from persons who are infected to those who are susceptible. The fact that the disease was rampant in the crowded tenements of 19th century industrial countries is not surprising. The explosive progress in transportation during the 19th century also likely facilitated the more rapid spread of streptococcal disease than before. As horse carriages and sailing ships gave way to railroads and steamships, millions of travelers, immigrants, soldiers, and sailors traveled more rapidly and farther in less time than ever before. Streptococcal disease, cholera, and other major infections could circle the globe in weeks instead of months and years. The Napoleonic Wars, the Franco-Prussian War, and the United States Civil War undoubtedly also enhanced the spread of streptococcal infections. Massive armies and refugees were traversing continents. Specific mention should be made about milk and food contaminated with GAS, as a major cause of epidemic scarlet fever, pharyngitis, and ARF. The 19th century medical literature includes numerous reports of milk-borne epidemics [13]. And, indeed, one of the first public health investigations undertaken by the staff of The Rockefeller Institute for Medical Research in 1902 concerned the bacterial contamination of the New York milk supply and its relation to the health of children. ‘‘The results of the study were
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shocking and aroused public attention,’’ The Herald reported [14]. This prompted the Board of Health to strengthen the sanitary control of milk. As pasteurization and refrigeration became more widespread in the first half of the 20th century, milk-borne GAS infections became rare. In sum, scarlet fever became a pandemic as a consequence of a virulent streptococcal clone(s) arising through gene mutations or gene transfer; an increase in population density, crowding, slums, and poor nutrition; new modes of transportation that ushered in rapid long-distant travel for individuals and groups; and contaminated milk and milk products. It is likely, although not proven, that the incidence and mortality of scarlet fever subsequently declined because of a loss of streptococcal virulence; population immunity; sanitation and public health measures; improved housing, nutrition, and medical care; and widespread use of antibiotics. All these factors, or any combination of them, that relate to the scarlet fever pandemic are generic and can foster the emergence and re-emergence, and the decline and fall, of other infectious diseases as well. The scarlet fever pandemic is therefore a useful case study.
Group A streptococcus: links with the past? It is tantalizing to speculate that the GAS that caused the lethal pandemic of scarlet fever a century ago may be related to the streptococci that currently cause STSS. Clinicians generally agree that STSS has become more frequent in recent years. Outbreaks of STSS have been well documented in Canada, Europe, and the United States, and scattered cases have been reported in Hong Kong, Japan, and elsewhere in the Far East. STSS begins with a local infection, often a minor puncture wound, which rapidly produces an extensive necrotic lesion that is followed by multiple organ system failure, toxic shock, and death. Even with antibiotic therapy, the death rate from STSS is 15% to 30%, and survivors may be permanently crippled following amputation for extensive and irreversible tissue necrosis. Researchers are intensively investigating the genetic and pathogenic properties of GAS that cause STSS. This research, conducted within the context of population and evolutionary biology, has been reviewed recently [6,15]. The findings are intriguing. For example, the allelic variation of several genes encoding putative virulence factors (scarlet fever toxin, M protein, and other genetic markers) has been related to the increased frequency and severity of STSS. Furthermore, the molecular epidemiology has revealed that the resurgence of streptococcal disease is associated with two distinct GAS clones that have been responsible for a substantial proportion of the episodes of invasive disease. These clones possess the gene encoding exotoxin A, and they represent distinct genotypes expressing serotype M1 and M3 proteins. These data add to the important concept that an outbreak or an epidemic of an infection is due to a microbial clone or cell line.
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Horizontal gene transfer processes are playing a fundamental role in generating diversity in GAS. Several genes encoding putative or proven GAS virulence factors have been implicated in horizontal transfer in natural populations. These include the genes for pyrogenic exotoxins A and C located in bacteriophage and the genes for M protein and streptokinase located in the chromosome. The discovery of significant levels of chromosomal gene recombination among isolates of GAS was unexpected because it was the first time such a finding has been made for a microbe in which DNA transformation is not known to occur. This finding has major implications for understanding how new, unusually virulent cell lines of the species are generated. Extrapolating to disease pathogenesis, such genetic and molecular processes may be driving variation over time in the frequency and severity of disease [6]. Evidence of a connection to the scarlet fever pandemic of the 19th century may emerge from the increasing body of research on the genealogy of GAS, particularly streptococci isolated from patients currently with STSS and from GAS isolated several decades ago from patients with scarlet fever. Whether one or more of the toxins implicated in the pathogenesis of STSS is also a property of the streptococci that caused fatal scarlet fever in the 19th century is not known. This question may be answered in the future by determining the genome of GAS in formalin-preserved pathology tissue from patients who died of scarlet fever in the past century and by using polymerase chain reaction (PCR) analysis and other emerging technologies to determine a genetic link to the current strains that cause STSS today. Other indications of a relationship between scarlet fever and STSS relate to streptococcal immunity. Current efforts to treat STSS include use of human gamma globulin, a good source of antibodies to numerous streptococcal toxins. Similar efforts to develop an antitoxin serum, for scarlet fever, were undertaken prior to the antibiotic era. It was known that an antitoxin serum would not cure the infection. The goal was to neutralize the scarlatinal toxins and to minimize the severe rash and the associated toxemia from which patients died. Several published reports conclusively document that use of antitoxin serum markedly reduced the death rate from scarlet fever. Earlier in the 20th century, bacteriologists also sought to prevent scarlet fever via active immunization using a toxoid vaccine derived from a partially purified preparation of the toxin(s). The vaccine reduced or prevented manifestations of scarlet fever due to subsequent streptococcal infections, including toxemia, and dramatically reduced the death rate from scarlet fever. These studies of both active and passive immunization, summarized in the 1929 and 1936 editions of Topley and Wilson [16,17], were conducted in the late 1920s and early 1930s in locales that had persistent pockets of localized outbreaks of scarlet fever and high mortality from the disease. Such earlier reports on active and passive immunization of scarlet fever are relevant to recent studies of Kaul and associates [18], who have published convincing evidence of successfully treating STSS using pooled
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human gamma globulin. Their studies show a 50% decrease in mortality. Treatment with antibiotics alone cannot prevent the cascade of toxic events initiated by streptococcal toxins, several of which are superantigens. Therefore, treatment must be directed toward neutralizing the toxin(s) and minimizing adverse side effects. The success with gamma globulin treatment also raises the possibility of developing a streptococcal toxoid vaccine to prevent STSS. Although the need for such a vaccine is not immediate, research to prove the feasibility of a vaccine approach would be worthwhile. Because of the unpredictable evolution of microbes and microbial diseases, STSS could become more common. Furthermore, relying on treatment with antibiotics alone for STSS is frequently a case of too little and too late, which raises the possibility of developing a specifically prepared hyperimmune gamma globulin rich in antibodies to the toxins implicated in the pathogenesis of STSS. The re-emergence of acute rheumatic fever in the United States Even before the age of antibiotics, the frequency and severity of ARF were diminishing, and in the last half of the 20th century, pediatricians noted a decrease in the severity of GAS pharyngitis. Stollerman [19] should be credited with calling attention to the evolution of this disease. As noted in 1964, clinical signs and symptoms were milder than in the past. The GAS isolated from the pharynx lacked the characteristics of high virulence of those of the earlier era. The GAS from mild infections possessed a sparse capsule and little M protein. A large capsule and abundant M protein had long been the hallmarks of virulent GAS isolated in earlier years. Stollerman [19] also observed that ARF was much less frequent following mild infections than was the case for severe infections. In the earlier era, 3% of untreated children with GAS pharyngitis developed ARF. Stollerman [19] observed 0.3% or even as little as 0.1%. The reasons for the decline in GAS virulence are a matter of conjecture and speculation, although current genetic studies of recent, less-virulent and earlier virulent strains may reveal the biological factors that led to a decrease in virulence. At about the same time that STSS made its appearance in the 1980s, the prevalence of ARF increased sharply in the Salt Lake City area and other locales in the United States [20] (Fig. 2). As reported by Veasy and colleagues [21], the substantial increase in cases of ARF persisted from 1985 to 1990 and then declined. The prevalence of ARF increased again in 1997 to 1999. During these two peak periods for ARF, M18 was the most frequent M type isolated from patients with pharyngitis and from those who developed ARF. Many of these GAS strains possessed very large capsules and were rich in M protein—characteristics of virulent GAS noted in the past. Ongoing studies of the complete genome of an M18 strain from one of the recent patients in Salt Lake City and of the population and evolutionary
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Fig. 2. Acute rheumatic fever at Primary Children’s Medical Center in Salt Lake City, Utah, 1960–92. (From Denny Jr FW. A 45-year perspective on the streptococcus and rheumatic fever: the Edward Kass Lecture in Infectious Disease History. Clin Infect Dis 1994;19:1110–22; with permission, The University of Chicago Press, Ó Infections Disease Society of America, all rights reserved.)
biology of GAS associated with these outbreaks may yield new clues about the pathogenesis of ARF (J. Musser, personal communication).
Prospects for a vaccine Development of a vaccine to prevent GAS infections already is under way in the United States, supported by the National Institute of Allergy and Infectious Diseases (NIAID) and private industry, and in Australia and Europe. There are four compelling reasons for developing a vaccine for GAS infections. First, many of the technical impediments that previously hindered development of a streptococcal vaccine have now been overcome. GAS protective immunity is due in large part to the type-specific M protein, a major virulence factor. Eighty or more M types of GAS were originally determined by serologic means. The M type of GAS is now most readily identified by rapid sequencing of the gene encoding the M protein. This major technical advance has enhanced research on the epidemiology of GAS and on vaccine development. Because immunity to M protein is type-specific, and because 6, 8, or even 10 different M types of GAS can circulate in a population and cause GAS infection at any time, repeated attacks of GAS infection occur during the
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school year. Also, the prevalent M types can change over time, a fact that has retarded vaccine development. These and other obstacles to vaccine development are being overcome by recent work on the molecular biology of M protein, as summarized in Fig. 3 [22]. As noted in the legend to the figure, the antigenic specificity of
Fig. 3. Proposed model of the M protein from M6 strain D471. The coiled-coil rod region extends approximately 60 nm from cell wall with a short nonhelical domain at the NH2terminus. The proline\glycine-rich region of the molecule is found within the peptidoglycan. The membrane anchor segment extends through the cell membrane with the charged tail extending into the cytoplasm. Data suggest that the membrane anchor may be cleaved shortly after synthesis. The A-, B-, and C-repeat regions are indicated along with those segments containing conserved, variable, and hypervariable epitopes among heterologous M serotypes. (From Fischetti VA. Protection against group A streptococcal infection. In: Stevens DL, Kaplan EL, editors. Streptococcal infections: clinical aspects, microbiology, and molecular pathogenesis. New York: Oxford University Press; 2000; p. 371–89; with permission.)
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M protein is determined by the N-terminal amino acid sequence in the hypervariable region. One strategy to develop a multivalent vaccine employs recombinant techniques to produce complex hybrid proteins containing Nterminal peptides of M protein from four different serotypes. Subsequently, an octavalent preparation was produced. These products are in various stages of vaccine development [23]. Another approach by Fischetti [22] takes advantage of the immunological properties of the conserved region of the M protein that is common to all M types. An epitope of this domain is exposed and stimulates IgA antibodies that react with this common epitope on all M types of GAS. Oral immunization of mice with this conserved region linked to a cholera toxin carrier prevents colonization of mice after nasopharyngeal challenge with multiple M types of GAS. This vaccine is also currently undergoing preliminary studies to prepare for clinical trials. The second compelling reason for developing a vaccine for GAS infections is that widespread use of such a vaccine would prevent ARF and RHD, which continue to burden the populations of developing, nonindustrialized countries. All too frequently in such countries, many children with GAS pharyngitis do not receive adequate treatment and ARF and RHD occur. The prevalence of RHD among school-age children in these countries ranges from 1 to 18 cases per 1000, according to data collected in the 1980s [24]. This information must be updated to formulate effective prevention programs and to document the need for an effective vaccine. If there were an effective vaccine today it could be used promptly to prevent second, third, and fourth attacks of ARF in patients who have had an initial attack. Repeated episodes of ARF lead to an increase in mitral and aortic valve disease. Today thousands of children so afflicted in the developing world must rely on mitral commissurotomy or valve replacement. Currently, repeated attacks of ARF are prevented by oral penicillin daily or intramuscular bicillin monthly. Compliance for either regimen is difficult to maintain in developing countries. Use of a successful vaccine would be a major advance over prophylaxis with penicillin and would streamline and simplify medical care for these patients. The third compelling reason is that GAS are slowly, but surely, developing resistance to more than one antibiotic. Fortunately, GAS have not yet developed resistance to penicillin—and they may not, since penicillin has been used widely for 50 years. But, if GAS became resistant to penicillin, no second line of defense exists that is as effective as penicillin for adequately treating streptococcal pharyngitis. Without this defense, primary and secondary prevention of ARF and RHD would be crippled. Indeed, long-acting penicillin has become a surrogate vaccine for group A streptococcal infections. The fourth reason, although unlikely, is that GAS associated with STSS could become more communicable due to genetic evolution and selection, leading to a higher incidence of STSS. More than 80 years have passed since Rebecca C. Lancefield first demonstrated the protective power of M protein
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antibodies in experimental animals and the antibodies that occur naturally after GAS infection in humans. We can only hope that the molecular wizards of today will match her remarkable discoveries in their efforts to develop a successful vaccine.
A strategy for the future Several years ago, the NIAID and the Centers for Disease Control and Prevention developed complementary strategies to confront the emergence and re-emergence of microbial infections [25,26]. In addition, the Institute of Medicine issued a report on emerging infections [27]. All parties agree that surveillance efforts in the United States and elsewhere must be expanded to ensure that emergence and re-emergence of infectious diseases can be detected at any point in time and in any place. However, surveillance alone is a slender reed that can bend in the storm. Research also is needed to create a strong defense, before or at the time an outbreak of an emerging or re-emerging disease is detected. This research effort must embrace a broad array of biological disciplines as well as interdisciplinary efforts. The survival of microbes, vectors, and intermediate hosts and their adaptation to new habitats need to be better understood. In recent years, Anderson and May [28,29] and others have broadened their research on epidemics to include analysis of population biology as it relates to the dynamics of disease transmission and the evolution of infectious diseases. Recommendations to improve vaccination strategies to minimize persistence of highly contagious diseases, such as measles, is a practical spin-off of this theoretical work. To eliminate measles, for example, new vaccination strategies are likely to be based on such mathematical considerations. In addition, the genetic makeup of microbes and their ability to cause disease must be understood. Research topics include the mechanisms of pathogenesis and the immunological processes that are mobilized by the body to fight microbial invasion and infection. Theobold Smith surely had in mind such a comprehensive strategy when he described, nearly 100 years ago, the mission of the newly created Rockefeller Institute for Medical Research. Smith was an early pioneer in bacteriologic research in the United States and characterized the difference between human and bovine tubercule bacilli [14]. The author recalls SmithÕs description, that the best approach is to study infectious diseases ‘‘from all points of view, for they are the greatest threats to our society.’’ Simon Flexner, the first director of the Institute, realized that this strategy required a firm foundation in the basic sciences of that day—chemistry, physiology, and pathology. The same firm foundation in the basic sciences will be needed to confront the emerging and re-emerging microbes in the 21st century.
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Summary Microbes will evolve and the epidemics they cause will continue to occur in the future as they have in the past. Microbes emerge from the evolutionary stream as a result of genetic events and selective pressures that favor new over old. It is nature’s way. Microbes and vectors swim in the evolutionary stream, and they swim much faster than humans. Bacteria reproduce every 30 minutes and, for them, a millennium is compressed into a fortnight. They are ‘‘fleet afoot,’’ and the pace of research must keep up with them or they will overtake. Microbes were here on Earth 2 billion years before humans arrived, learning every trick of the trade for survival, and they are likely to be here 2 billion years after we depart. Current research on the rise and decline of epidemics is broadly based and includes evolutionary and population genetics of host–microbe relationships. Within this context, the 19th century pandemic of scarlet fever has been described. The possibility is raised that the GAS, which currently cause STSS, possess some of the virulence factors that caused pandemic scarlet fever. Furthermore, the GAS isolated during the recent outbreaks of ARF in certain locales in the United States have the virulence properties of the GAS frequently isolated in the first half of the 20th century. Finally, it is suggested that the strategy to confront emerging infectious diseases should be the study of infectious diseases from all points of view. They remain the greatest threats to our society. References [1] Krause RM, editor. Emerging infections. New York: Academic Press; 1998. [2] Wilson GS, Miles AA. Topley and Wilson’s principles of bacteriology and immunity. 3rd edition. Baltimore: Williams & Wilkins; 1946. p. 288. [3] Burnet FM, Lush D. Induced lysogenicity and mutation of bacteriophage within lysogenic bacteria. Aust J Exp Biol Med Sci 1936;14:27–38. [4] Lederberg J. Infectious agents, hosts in constant flux. ASM News 1998;64:18–22. [5] Ross R. The prevention of malaria (with addendum on the theory of happenings). London: Murray; 1911. [6] Musser JM, Krause RM. The revival of group A streptococcal diseases, with a commentary on staphylococcal toxic shock syndrome. In: Krause RM, editor. Emerging infections. New York: Academic Press; 1998. p. 185–218. [7] Katz SL, Morens DM. Severe streptococcal infections in historical perspective. Clin Infect Dis 1992;14:298–307. [8] Quinn RW. Streptococcal infections. In: Evans AS, Feldman HA, editors. Bacterial infections of humans: epidemiology and control. New York: Plenum Medical; 1982. p. 525–52. [9] Dingle JH, Badger GF, Jordan Jr WS. Illness in the home: a study of 25,000 illnesses in a group of Cleveland families. Cleveland: The Press of Western Reserve University; 1964. p. 97–128. [10] El Kholy A, Sorour AH, Houser HB, et al. A three-year prospective study of streptococcal infections in a population of rural Egyptian school children. J Med Microbiol 1973;6: 101–10. [11] Denny FW, Wannamaker LW, Brink WR, et al. Prevention of rheumatic fever. Treatment of the preceding streptococcic infection. JAMA 1950;143:151–3.
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[12] Wannamaker LW. The epidemiology of streptococcal infections. In: McCarty M, editor. Streptococcal infections. New York: Columbia University Press; 1954. p. 157–75. [13] Scott HH. Some notable epidemics. London: Edward Arnold & Company; 1934. [14] Corner GW. A history of the Rockefeller Institute, 1901–1953: origins and growth. New York: The Rockefeller Institute Press; 1964. p. 47. [15] Low DE, Schwartz B, McGeer A. The reemergence of severe group A streptococcal disease: an evolutionary perspective. In: Scheld WM, Armstrong D, Hughes JM, editors. Emerging infections. Washington (DC): ASM Press; 1998. p. 93–123. [16] Topley WWC, Wilson GS. The principles of bacteriology and immunity. 1st edition. London: Edward Arnold & Company; 1929. p. 958–9. [17] Topley WWC, Wilson GS. The principles of bacteriology and immunity. 2nd edition. London: Edward Arnold & Company; 1936. p. 1167–8. [18] Kaul R, McGeer A, Norrby–Teglund MK, et al. Intravenous immunoglobulin therapy for streptococcal toxic shock syndrome—a comparative observational study. Clin Infect Dis 1999;28:800–7. [19] Stollerman GH. The epidemiology of primary and secondary rheumatic fever. In: Uhr JW, editor. The Streptococcus, rheumatic fever and glomerulonephritis. Baltimore: Williams & Wilkins; 1964. p. 311–29. [20] Denny Jr FW. A 45-year perspective on the streptococcus and rheumatic fever: the Edward Kass Lecture in Infectious Disease History. Clin Infect Dis 1994;19:1110–22. [21] Veasy LG, Tani LY, Hill HR. Persistence of acute rheumatic fever in the intermountain area of the United States. J Pediatr 1994;124:9–16. [22] Fischetti VA. Protection against group A streptococcal infection. In: Stevens DL, Kaplan EL, editors. Streptococcal infections: clinical aspects, microbiology, and molecular pathogenesis. New York: Oxford University Press; 2000. p. 371–89. [23] Dale JB. Multivalent group A streptococcal vaccines. In: Stevens DL, Kaplan EL, editors. Streptococcal infections: clinical aspects, microbiology, and molecular pathogenesis. New York: Oxford University Press; 2000. p. 390–401. [24] Michaud C, Trejo-Gutierrez J, Cruz C, et al. Rheumatic heart disease. In: Jamison DT, Mosley WH, Measham AR, et al, editors. Disease control priorities in developing countries. New York: Oxford University Press; 1993. p. 221–32. [25] Centers for Disease Control and Prevention (US). Addressing emerging infectious disease threats: a prevention strategy for the United States. Atlanta: US Department of Health and Human Services, Public Health Service; 1994. [26] National Institute of Allergy and Infectious Diseases (US). Report of the task force on microbiology and infectious diseases. National Institutes of Health, Publication no. 92– 3320. Bethesda (MD): US Department of Health and Human Services, Public Health Service; 1992. [27] Institute of Medicine (US). Emerging infections: microbial threats to health in the United States. Washington (DC): National Academy Press; 1992. [28] Anderson RM. Analytical therapy of epidemics. In: Krause RM, editor. Emerging infections. New York: Academic Press; 1998. p. 23–50. [29] Anderson RM, May RM. Infectious diseases of humans—dynamics and control. Oxford: Oxford University Press; 1991.
Evolving microbes and re-emerging streptococcal disease Richard M. Krause, MD Laboratory of Human Bacterial Pathogenesis, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Building 16, Room 202, 16 Center Drive, MSC 6705, Bethesda, MD 20892, USA
Changes in demography and lifestyle and various adverse events are major factors that enhance epidemics [1]. Yet microbes are not idle bystanders. They are constantly evolving. Microbial evolution is driven by mutations and underlying mechanisms of genetic exchange, which also play an important role in the emergence and re-emergence of infectious diseases. The re-emergence of streptococcal disease is considered within this context. Investigators are pursuing molecular studies to explore the possible genetic relationship between the recent re-emerging, virulent group A streptococci (GAS) and streptococci that caused widespread epidemics of severe scarlet fever in an earlier era. Interest in GAS had waned following the widespread use of penicillin to treat GAS pharyngitis and the concomitant prevention of acute rheumatic fever (ARF) and rheumatic heart disease (RHD). Clinicians also found that, in parallel with this important medical advance, cases of GAS pharyngitis were milder than before. But, unexpectedly, a virulent variety of GAS reappeared 15 years ago to cause an increase in the number of cases of streptococcal toxic shock syndrome (STSS) in the United States, Canada, and elsewhere, and in the number of cases of ARF in Salt Lake City and other United States locales. Although penicillin remains the drug of choice to treat GAS infections, resistance is developing to many other antibiotics. For this and other reasons, efforts are under way to construct a synthetic recombinant vaccine based on recently obtained knowledge of the molecular structure of M protein, a major component of GAS that stimulates protective immunity.
E-mail address: [email protected] (R.M. Krause). 0272-2712/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 2 7 2 - 2 7 1 2 ( 0 2 ) 0 0 0 2 7 - 6
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Emerging bacterial infections: genetic aspects Strange as it may seem, microbial evolution is a new idea. Bacterial genetics was not even mentioned in medical schools during the 1940s. At that time, bacteriology was the study of each microbe and the disease it caused. Indeed, in the third edition of Topley and Wilson [2], published in 1946, bacterial genetics is not discussed, although bacterial variation in all its manifestations is described in great length. On the possibility of bacterial mutations, Topley and Wilson write: ‘‘The application to bacteria of terms that have been coined to express changes in form or function occurring in higher plants or animals is not without its dangers; and it is possible that there is little real justification for the use of such a term as mutation, in connection with the variations that bacteria may undergo’’ [2]. Such caution is not surprising for 1946. The mutational origin of bacterial variants was first demonstrated just 3 years earlier in 1943. To be fair, Burnet [3] perceived the occurrence of mutations in a paper he published in 1936 entitled ‘‘Induced Lysogenicity and Mutation of Bacteriophage Within Lysogenic Bacteria,’’ but this work had little impact until some years later. Recently, Lederberg [4] noted the versatile features of bacterial genetics that enhance the facility of microbes to overcome their natural ecological constraints or medical barriers, such as antibiotics and vaccines, and to emerge as something new. These genetic mechanisms include (1) phase variation, whereby silent, archival genetic information reappears as adaptive change (eg, the movement of a promoter from one locus to another, such as the movement of flagella antigens for salmonella); (2) intraclonal processes, such as mutations due to chemical and physical (including ultraviolet) action (bacteria are haploid and therefore there is no delay in gene expression, but prompt natural selection); and (3) interclonal processes, such as promiscuous intra- and interspecies genetic recombination, as well as transformation, transduction-lysogenic conversion, and conjugation. The emergence of new microbes or old editions in new garments stems from genetic evolution. A consequence of this genetic versatility is that microbes can develop new pathogenic vigor, escape population immunity by acquiring new antigens, develop antibiotic or drug resistance, become more transmissible, and escape from the species barrier. Toxigenic Escherichia coli, which can cause bloody diarrhea and the uremic syndrome, and GAS, which can cause toxic shock syndrome and the re-emergence of rheumatic fever, very likely have acquired new vigor as a result of genetic events and evolutionary selection. Although this evolution is poorly understood, the ‘‘new’’ GAS agents likely exploit the changing circumstances of the ecosystem brought about by perturbations in nature and human behavior. The rise and fall of epidemics The historical record shows that epidemics ebb, from a high tide into history’s twilight. AIDS likely will follow the same course in the next
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millennium. Recent research on the rise and decline of epidemics and pandemics embraces multiple disciplines that derive from the population biology of host–microbe associations. Much of what can be learned from the history of past epidemics remains largely speculative, although current theory can be applied to past events to gain a new framework for understanding the historical record. For example, much insight may be obtained about the 1918 influenza pandemic from current efforts to unravel the genome of the influenza virus at that time. In recent years, the evolution of epidemics has become better understood with studies of the molecular and evolutionary epidemiology and the population biology of microbes and the use of mathematical models of ‘‘happenings,’’ a term coined by Sir Ronald Ross for microbial outbreaks [5]. Ross was a pioneer in such mathematical analysis. The history of scarlet fever is a good case for examining factors that influence the rise and fall of epidemics. Scarlet fever was one of the most deadly childhood diseases in the 19th century. Scarlet fever occurs during pharyngitis caused by streptococci that produce certain potent toxins [6]. Why did the pandemic occur, and why has scarlet fever essentially disappeared in the 20th century? Were the particular horrors of the industrial cities of the Victorian era, so well depicted in the novels of Charles Dickens, a cauldron that fomented epidemics of scarlet fever, which abated as living conditions improved? Or, did the streptococci that caused the disease lose the necessary genes for producing the scarlatinal toxins and other factors involved in the pathogenesis of lethal scarlet fever? Even before the antibiotic era, scarlet fever had become much less common, even rare, and a much milder disease. And yet, streptococcal pharyngitis persists as a common childhood illness. A case study: scarlet fever The historical record of scarlet fever in the 19th century is fairly well known. Nineteenth-century clinicians had established excellent diagnostic criteria, and scarlet fever was recognized as a distinct clinical entity, no longer being confused with other childhood exanthems. In general, registries of illness and death were well kept in both Europe and the United States, and they provide reliable data on the incidence of, and mortality from, scarlet fever during this time. As noted, scarlet fever is an infection with GAS, characterized by sore throat (pharyngitis) and a rash that is distinctive and different from rashes associated with measles, rubella, and other childhood diseases. The association of streptococcal pharyngitis and scarlet fever, however, was not established until the first part of the 20th century. For this reason, an analysis of the pandemic must be based on the clinical records of that time. Katz and Morens [7] conclude from a careful review of the literature that scarlet fever most likely had occurred for centuries either as an endemic disease or as localized epidemics. Notably, in the early part of the 19th century,
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a pandemic of often fatal scarlet fever appeared suddenly and swept through Asia, Europe, and the United States (Fig. 1) [7,8]. From 1800 to 1830, family physicians noted a striking increase in mortality not seen by them previously, and fatality rates of up to 30% were often reported. Scarlet fever became the most common fatal infectious childhood disease, more fatal than measles, diphtheria, or pertussis, a fact that is difficult to comprehend today. For a hundred years, pandemic scarlet fever waxed and waned in incidence and severity. ARF and RHD, a late complication of ARF, were very common severe complications. Indeed, special hospitals were established to care for children with ARF and RHD. Despite 100 years of research, the pathogenesis of ARF, RHD, and acute glomerulonephritis (AGN), another major complication of streptococcal infections, remains obscure. Mortality from scarlet fever began to decline about 1880 and, by 1930, clinicians in general remarked on this decline. This change arose even before the availability of antibiotics. Today, scarlet fever is rare and, when it occurs, the disease is not life threatening. Streptococcal infections, without a scarlet fever rash, however, continue to be common during the school year (one to five cases per 1000 students per week) [9,10]. Most cases are treated with antibiotics, but even for those that are not, the risk of a subsequent attack of ARF is low, except in several locales in the United States. In the developing world, including heavily populated countries such as India, GAS
Fig. 1. Boston 1840–1940. Severe streptococcal infections in historical perspective. (Adapted from Katz SL, Morens DM. Severe streptococcal infections in historical perspective. Clin Infect Dis 1992;14:298–307; with permission, The University of Chicago Press, Ó Infections Disease Society of America, all rights reserved.) Depicted in the insert are the recorded deaths in the United States from 1900 to 1960. (Adapted from Quinn RW. Streptococcal infections. In: Evans AS, Feldman HA, editors. Bacterial infections of humans: epidemiology and control. New York: Plenum Medical; 1982. p. 525–52; with permission.)
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pharyngitis is also common and, for many reasons, ARF and RHD remain frequent complications. A research group headed by C.H. Rammelkamp achieved one of the major clinical advances of the 20th century by discovering that adequate treatment of streptococcal pharyngitis prevents subsequent occurrence of ARF and RHD [11]. During the next 10 years, there was an intensive campaign, with strong leadership from the American Heart Association, to prevent ARF by the clinical and bacteriologic detection of GAS pharyngitis, particularly in schoolchildren and military recruits, and prompt treatment with penicillin. By 1975, new cases of ARF and RHD had nearly disappeared in the United States and other developed countries. And yet, the question remains: why does the incidence of streptococcal infections persist, whereas scarlet fever and ARF have mostly disappeared in the United States and other developed countries? Did the earlier streptococci associated with scarlet fever and ARF possess especially virulent characteristics that are lacking in the streptococci isolated from pharyngitis today? Satisfactory answers to these questions have not been obtained, but clues may be forthcoming from current intensive investigations of the bacterial genetics and population and evolutionary epidemiology of GAS. In addition to the genetic evolution of the streptococcal microbe, the 19th century pandemic of scarlet fever surely also resulted from multiple social and demographic factors. The leading factor would have been the intense crowding of populations in the large industrial cities of England, Europe, and the United States. Overwhelming epidemiological evidence, particularly from studies of military recruits confined to close quarters in barracks [12], demonstrates that crowding enhances the spread of streptococcal infections from persons who are infected to those who are susceptible. The fact that the disease was rampant in the crowded tenements of 19th century industrial countries is not surprising. The explosive progress in transportation during the 19th century also likely facilitated the more rapid spread of streptococcal disease than before. As horse carriages and sailing ships gave way to railroads and steamships, millions of travelers, immigrants, soldiers, and sailors traveled more rapidly and farther in less time than ever before. Streptococcal disease, cholera, and other major infections could circle the globe in weeks instead of months and years. The Napoleonic Wars, the Franco-Prussian War, and the United States Civil War undoubtedly also enhanced the spread of streptococcal infections. Massive armies and refugees were traversing continents. Specific mention should be made about milk and food contaminated with GAS, as a major cause of epidemic scarlet fever, pharyngitis, and ARF. The 19th century medical literature includes numerous reports of milk-borne epidemics [13]. And, indeed, one of the first public health investigations undertaken by the staff of The Rockefeller Institute for Medical Research in 1902 concerned the bacterial contamination of the New York milk supply and its relation to the health of children. ‘‘The results of the study were
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shocking and aroused public attention,’’ The Herald reported [14]. This prompted the Board of Health to strengthen the sanitary control of milk. As pasteurization and refrigeration became more widespread in the first half of the 20th century, milk-borne GAS infections became rare. In sum, scarlet fever became a pandemic as a consequence of a virulent streptococcal clone(s) arising through gene mutations or gene transfer; an increase in population density, crowding, slums, and poor nutrition; new modes of transportation that ushered in rapid long-distant travel for individuals and groups; and contaminated milk and milk products. It is likely, although not proven, that the incidence and mortality of scarlet fever subsequently declined because of a loss of streptococcal virulence; population immunity; sanitation and public health measures; improved housing, nutrition, and medical care; and widespread use of antibiotics. All these factors, or any combination of them, that relate to the scarlet fever pandemic are generic and can foster the emergence and re-emergence, and the decline and fall, of other infectious diseases as well. The scarlet fever pandemic is therefore a useful case study.
Group A streptococcus: links with the past? It is tantalizing to speculate that the GAS that caused the lethal pandemic of scarlet fever a century ago may be related to the streptococci that currently cause STSS. Clinicians generally agree that STSS has become more frequent in recent years. Outbreaks of STSS have been well documented in Canada, Europe, and the United States, and scattered cases have been reported in Hong Kong, Japan, and elsewhere in the Far East. STSS begins with a local infection, often a minor puncture wound, which rapidly produces an extensive necrotic lesion that is followed by multiple organ system failure, toxic shock, and death. Even with antibiotic therapy, the death rate from STSS is 15% to 30%, and survivors may be permanently crippled following amputation for extensive and irreversible tissue necrosis. Researchers are intensively investigating the genetic and pathogenic properties of GAS that cause STSS. This research, conducted within the context of population and evolutionary biology, has been reviewed recently [6,15]. The findings are intriguing. For example, the allelic variation of several genes encoding putative virulence factors (scarlet fever toxin, M protein, and other genetic markers) has been related to the increased frequency and severity of STSS. Furthermore, the molecular epidemiology has revealed that the resurgence of streptococcal disease is associated with two distinct GAS clones that have been responsible for a substantial proportion of the episodes of invasive disease. These clones possess the gene encoding exotoxin A, and they represent distinct genotypes expressing serotype M1 and M3 proteins. These data add to the important concept that an outbreak or an epidemic of an infection is due to a microbial clone or cell line.
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Horizontal gene transfer processes are playing a fundamental role in generating diversity in GAS. Several genes encoding putative or proven GAS virulence factors have been implicated in horizontal transfer in natural populations. These include the genes for pyrogenic exotoxins A and C located in bacteriophage and the genes for M protein and streptokinase located in the chromosome. The discovery of significant levels of chromosomal gene recombination among isolates of GAS was unexpected because it was the first time such a finding has been made for a microbe in which DNA transformation is not known to occur. This finding has major implications for understanding how new, unusually virulent cell lines of the species are generated. Extrapolating to disease pathogenesis, such genetic and molecular processes may be driving variation over time in the frequency and severity of disease [6]. Evidence of a connection to the scarlet fever pandemic of the 19th century may emerge from the increasing body of research on the genealogy of GAS, particularly streptococci isolated from patients currently with STSS and from GAS isolated several decades ago from patients with scarlet fever. Whether one or more of the toxins implicated in the pathogenesis of STSS is also a property of the streptococci that caused fatal scarlet fever in the 19th century is not known. This question may be answered in the future by determining the genome of GAS in formalin-preserved pathology tissue from patients who died of scarlet fever in the past century and by using polymerase chain reaction (PCR) analysis and other emerging technologies to determine a genetic link to the current strains that cause STSS today. Other indications of a relationship between scarlet fever and STSS relate to streptococcal immunity. Current efforts to treat STSS include use of human gamma globulin, a good source of antibodies to numerous streptococcal toxins. Similar efforts to develop an antitoxin serum, for scarlet fever, were undertaken prior to the antibiotic era. It was known that an antitoxin serum would not cure the infection. The goal was to neutralize the scarlatinal toxins and to minimize the severe rash and the associated toxemia from which patients died. Several published reports conclusively document that use of antitoxin serum markedly reduced the death rate from scarlet fever. Earlier in the 20th century, bacteriologists also sought to prevent scarlet fever via active immunization using a toxoid vaccine derived from a partially purified preparation of the toxin(s). The vaccine reduced or prevented manifestations of scarlet fever due to subsequent streptococcal infections, including toxemia, and dramatically reduced the death rate from scarlet fever. These studies of both active and passive immunization, summarized in the 1929 and 1936 editions of Topley and Wilson [16,17], were conducted in the late 1920s and early 1930s in locales that had persistent pockets of localized outbreaks of scarlet fever and high mortality from the disease. Such earlier reports on active and passive immunization of scarlet fever are relevant to recent studies of Kaul and associates [18], who have published convincing evidence of successfully treating STSS using pooled
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human gamma globulin. Their studies show a 50% decrease in mortality. Treatment with antibiotics alone cannot prevent the cascade of toxic events initiated by streptococcal toxins, several of which are superantigens. Therefore, treatment must be directed toward neutralizing the toxin(s) and minimizing adverse side effects. The success with gamma globulin treatment also raises the possibility of developing a streptococcal toxoid vaccine to prevent STSS. Although the need for such a vaccine is not immediate, research to prove the feasibility of a vaccine approach would be worthwhile. Because of the unpredictable evolution of microbes and microbial diseases, STSS could become more common. Furthermore, relying on treatment with antibiotics alone for STSS is frequently a case of too little and too late, which raises the possibility of developing a specifically prepared hyperimmune gamma globulin rich in antibodies to the toxins implicated in the pathogenesis of STSS. The re-emergence of acute rheumatic fever in the United States Even before the age of antibiotics, the frequency and severity of ARF were diminishing, and in the last half of the 20th century, pediatricians noted a decrease in the severity of GAS pharyngitis. Stollerman [19] should be credited with calling attention to the evolution of this disease. As noted in 1964, clinical signs and symptoms were milder than in the past. The GAS isolated from the pharynx lacked the characteristics of high virulence of those of the earlier era. The GAS from mild infections possessed a sparse capsule and little M protein. A large capsule and abundant M protein had long been the hallmarks of virulent GAS isolated in earlier years. Stollerman [19] also observed that ARF was much less frequent following mild infections than was the case for severe infections. In the earlier era, 3% of untreated children with GAS pharyngitis developed ARF. Stollerman [19] observed 0.3% or even as little as 0.1%. The reasons for the decline in GAS virulence are a matter of conjecture and speculation, although current genetic studies of recent, less-virulent and earlier virulent strains may reveal the biological factors that led to a decrease in virulence. At about the same time that STSS made its appearance in the 1980s, the prevalence of ARF increased sharply in the Salt Lake City area and other locales in the United States [20] (Fig. 2). As reported by Veasy and colleagues [21], the substantial increase in cases of ARF persisted from 1985 to 1990 and then declined. The prevalence of ARF increased again in 1997 to 1999. During these two peak periods for ARF, M18 was the most frequent M type isolated from patients with pharyngitis and from those who developed ARF. Many of these GAS strains possessed very large capsules and were rich in M protein—characteristics of virulent GAS noted in the past. Ongoing studies of the complete genome of an M18 strain from one of the recent patients in Salt Lake City and of the population and evolutionary
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Fig. 2. Acute rheumatic fever at Primary Children’s Medical Center in Salt Lake City, Utah, 1960–92. (From Denny Jr FW. A 45-year perspective on the streptococcus and rheumatic fever: the Edward Kass Lecture in Infectious Disease History. Clin Infect Dis 1994;19:1110–22; with permission, The University of Chicago Press, Ó Infections Disease Society of America, all rights reserved.)
biology of GAS associated with these outbreaks may yield new clues about the pathogenesis of ARF (J. Musser, personal communication).
Prospects for a vaccine Development of a vaccine to prevent GAS infections already is under way in the United States, supported by the National Institute of Allergy and Infectious Diseases (NIAID) and private industry, and in Australia and Europe. There are four compelling reasons for developing a vaccine for GAS infections. First, many of the technical impediments that previously hindered development of a streptococcal vaccine have now been overcome. GAS protective immunity is due in large part to the type-specific M protein, a major virulence factor. Eighty or more M types of GAS were originally determined by serologic means. The M type of GAS is now most readily identified by rapid sequencing of the gene encoding the M protein. This major technical advance has enhanced research on the epidemiology of GAS and on vaccine development. Because immunity to M protein is type-specific, and because 6, 8, or even 10 different M types of GAS can circulate in a population and cause GAS infection at any time, repeated attacks of GAS infection occur during the
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school year. Also, the prevalent M types can change over time, a fact that has retarded vaccine development. These and other obstacles to vaccine development are being overcome by recent work on the molecular biology of M protein, as summarized in Fig. 3 [22]. As noted in the legend to the figure, the antigenic specificity of
Fig. 3. Proposed model of the M protein from M6 strain D471. The coiled-coil rod region extends approximately 60 nm from cell wall with a short nonhelical domain at the NH2terminus. The proline\glycine-rich region of the molecule is found within the peptidoglycan. The membrane anchor segment extends through the cell membrane with the charged tail extending into the cytoplasm. Data suggest that the membrane anchor may be cleaved shortly after synthesis. The A-, B-, and C-repeat regions are indicated along with those segments containing conserved, variable, and hypervariable epitopes among heterologous M serotypes. (From Fischetti VA. Protection against group A streptococcal infection. In: Stevens DL, Kaplan EL, editors. Streptococcal infections: clinical aspects, microbiology, and molecular pathogenesis. New York: Oxford University Press; 2000; p. 371–89; with permission.)
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M protein is determined by the N-terminal amino acid sequence in the hypervariable region. One strategy to develop a multivalent vaccine employs recombinant techniques to produce complex hybrid proteins containing Nterminal peptides of M protein from four different serotypes. Subsequently, an octavalent preparation was produced. These products are in various stages of vaccine development [23]. Another approach by Fischetti [22] takes advantage of the immunological properties of the conserved region of the M protein that is common to all M types. An epitope of this domain is exposed and stimulates IgA antibodies that react with this common epitope on all M types of GAS. Oral immunization of mice with this conserved region linked to a cholera toxin carrier prevents colonization of mice after nasopharyngeal challenge with multiple M types of GAS. This vaccine is also currently undergoing preliminary studies to prepare for clinical trials. The second compelling reason for developing a vaccine for GAS infections is that widespread use of such a vaccine would prevent ARF and RHD, which continue to burden the populations of developing, nonindustrialized countries. All too frequently in such countries, many children with GAS pharyngitis do not receive adequate treatment and ARF and RHD occur. The prevalence of RHD among school-age children in these countries ranges from 1 to 18 cases per 1000, according to data collected in the 1980s [24]. This information must be updated to formulate effective prevention programs and to document the need for an effective vaccine. If there were an effective vaccine today it could be used promptly to prevent second, third, and fourth attacks of ARF in patients who have had an initial attack. Repeated episodes of ARF lead to an increase in mitral and aortic valve disease. Today thousands of children so afflicted in the developing world must rely on mitral commissurotomy or valve replacement. Currently, repeated attacks of ARF are prevented by oral penicillin daily or intramuscular bicillin monthly. Compliance for either regimen is difficult to maintain in developing countries. Use of a successful vaccine would be a major advance over prophylaxis with penicillin and would streamline and simplify medical care for these patients. The third compelling reason is that GAS are slowly, but surely, developing resistance to more than one antibiotic. Fortunately, GAS have not yet developed resistance to penicillin—and they may not, since penicillin has been used widely for 50 years. But, if GAS became resistant to penicillin, no second line of defense exists that is as effective as penicillin for adequately treating streptococcal pharyngitis. Without this defense, primary and secondary prevention of ARF and RHD would be crippled. Indeed, long-acting penicillin has become a surrogate vaccine for group A streptococcal infections. The fourth reason, although unlikely, is that GAS associated with STSS could become more communicable due to genetic evolution and selection, leading to a higher incidence of STSS. More than 80 years have passed since Rebecca C. Lancefield first demonstrated the protective power of M protein
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antibodies in experimental animals and the antibodies that occur naturally after GAS infection in humans. We can only hope that the molecular wizards of today will match her remarkable discoveries in their efforts to develop a successful vaccine.
A strategy for the future Several years ago, the NIAID and the Centers for Disease Control and Prevention developed complementary strategies to confront the emergence and re-emergence of microbial infections [25,26]. In addition, the Institute of Medicine issued a report on emerging infections [27]. All parties agree that surveillance efforts in the United States and elsewhere must be expanded to ensure that emergence and re-emergence of infectious diseases can be detected at any point in time and in any place. However, surveillance alone is a slender reed that can bend in the storm. Research also is needed to create a strong defense, before or at the time an outbreak of an emerging or re-emerging disease is detected. This research effort must embrace a broad array of biological disciplines as well as interdisciplinary efforts. The survival of microbes, vectors, and intermediate hosts and their adaptation to new habitats need to be better understood. In recent years, Anderson and May [28,29] and others have broadened their research on epidemics to include analysis of population biology as it relates to the dynamics of disease transmission and the evolution of infectious diseases. Recommendations to improve vaccination strategies to minimize persistence of highly contagious diseases, such as measles, is a practical spin-off of this theoretical work. To eliminate measles, for example, new vaccination strategies are likely to be based on such mathematical considerations. In addition, the genetic makeup of microbes and their ability to cause disease must be understood. Research topics include the mechanisms of pathogenesis and the immunological processes that are mobilized by the body to fight microbial invasion and infection. Theobold Smith surely had in mind such a comprehensive strategy when he described, nearly 100 years ago, the mission of the newly created Rockefeller Institute for Medical Research. Smith was an early pioneer in bacteriologic research in the United States and characterized the difference between human and bovine tubercule bacilli [14]. The author recalls SmithÕs description, that the best approach is to study infectious diseases ‘‘from all points of view, for they are the greatest threats to our society.’’ Simon Flexner, the first director of the Institute, realized that this strategy required a firm foundation in the basic sciences of that day—chemistry, physiology, and pathology. The same firm foundation in the basic sciences will be needed to confront the emerging and re-emerging microbes in the 21st century.
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Summary Microbes will evolve and the epidemics they cause will continue to occur in the future as they have in the past. Microbes emerge from the evolutionary stream as a result of genetic events and selective pressures that favor new over old. It is nature’s way. Microbes and vectors swim in the evolutionary stream, and they swim much faster than humans. Bacteria reproduce every 30 minutes and, for them, a millennium is compressed into a fortnight. They are ‘‘fleet afoot,’’ and the pace of research must keep up with them or they will overtake. Microbes were here on Earth 2 billion years before humans arrived, learning every trick of the trade for survival, and they are likely to be here 2 billion years after we depart. Current research on the rise and decline of epidemics is broadly based and includes evolutionary and population genetics of host–microbe relationships. Within this context, the 19th century pandemic of scarlet fever has been described. The possibility is raised that the GAS, which currently cause STSS, possess some of the virulence factors that caused pandemic scarlet fever. Furthermore, the GAS isolated during the recent outbreaks of ARF in certain locales in the United States have the virulence properties of the GAS frequently isolated in the first half of the 20th century. Finally, it is suggested that the strategy to confront emerging infectious diseases should be the study of infectious diseases from all points of view. They remain the greatest threats to our society. References [1] Krause RM, editor. Emerging infections. New York: Academic Press; 1998. [2] Wilson GS, Miles AA. Topley and Wilson’s principles of bacteriology and immunity. 3rd edition. Baltimore: Williams & Wilkins; 1946. p. 288. [3] Burnet FM, Lush D. Induced lysogenicity and mutation of bacteriophage within lysogenic bacteria. Aust J Exp Biol Med Sci 1936;14:27–38. [4] Lederberg J. Infectious agents, hosts in constant flux. ASM News 1998;64:18–22. [5] Ross R. The prevention of malaria (with addendum on the theory of happenings). London: Murray; 1911. [6] Musser JM, Krause RM. The revival of group A streptococcal diseases, with a commentary on staphylococcal toxic shock syndrome. In: Krause RM, editor. Emerging infections. New York: Academic Press; 1998. p. 185–218. [7] Katz SL, Morens DM. Severe streptococcal infections in historical perspective. Clin Infect Dis 1992;14:298–307. [8] Quinn RW. Streptococcal infections. In: Evans AS, Feldman HA, editors. Bacterial infections of humans: epidemiology and control. New York: Plenum Medical; 1982. p. 525–52. [9] Dingle JH, Badger GF, Jordan Jr WS. Illness in the home: a study of 25,000 illnesses in a group of Cleveland families. Cleveland: The Press of Western Reserve University; 1964. p. 97–128. [10] El Kholy A, Sorour AH, Houser HB, et al. A three-year prospective study of streptococcal infections in a population of rural Egyptian school children. J Med Microbiol 1973;6: 101–10. [11] Denny FW, Wannamaker LW, Brink WR, et al. Prevention of rheumatic fever. Treatment of the preceding streptococcic infection. JAMA 1950;143:151–3.
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[12] Wannamaker LW. The epidemiology of streptococcal infections. In: McCarty M, editor. Streptococcal infections. New York: Columbia University Press; 1954. p. 157–75. [13] Scott HH. Some notable epidemics. London: Edward Arnold & Company; 1934. [14] Corner GW. A history of the Rockefeller Institute, 1901–1953: origins and growth. New York: The Rockefeller Institute Press; 1964. p. 47. [15] Low DE, Schwartz B, McGeer A. The reemergence of severe group A streptococcal disease: an evolutionary perspective. In: Scheld WM, Armstrong D, Hughes JM, editors. Emerging infections. Washington (DC): ASM Press; 1998. p. 93–123. [16] Topley WWC, Wilson GS. The principles of bacteriology and immunity. 1st edition. London: Edward Arnold & Company; 1929. p. 958–9. [17] Topley WWC, Wilson GS. The principles of bacteriology and immunity. 2nd edition. London: Edward Arnold & Company; 1936. p. 1167–8. [18] Kaul R, McGeer A, Norrby–Teglund MK, et al. Intravenous immunoglobulin therapy for streptococcal toxic shock syndrome—a comparative observational study. Clin Infect Dis 1999;28:800–7. [19] Stollerman GH. The epidemiology of primary and secondary rheumatic fever. In: Uhr JW, editor. The Streptococcus, rheumatic fever and glomerulonephritis. Baltimore: Williams & Wilkins; 1964. p. 311–29. [20] Denny Jr FW. A 45-year perspective on the streptococcus and rheumatic fever: the Edward Kass Lecture in Infectious Disease History. Clin Infect Dis 1994;19:1110–22. [21] Veasy LG, Tani LY, Hill HR. Persistence of acute rheumatic fever in the intermountain area of the United States. J Pediatr 1994;124:9–16. [22] Fischetti VA. Protection against group A streptococcal infection. In: Stevens DL, Kaplan EL, editors. Streptococcal infections: clinical aspects, microbiology, and molecular pathogenesis. New York: Oxford University Press; 2000. p. 371–89. [23] Dale JB. Multivalent group A streptococcal vaccines. In: Stevens DL, Kaplan EL, editors. Streptococcal infections: clinical aspects, microbiology, and molecular pathogenesis. New York: Oxford University Press; 2000. p. 390–401. [24] Michaud C, Trejo-Gutierrez J, Cruz C, et al. Rheumatic heart disease. In: Jamison DT, Mosley WH, Measham AR, et al, editors. Disease control priorities in developing countries. New York: Oxford University Press; 1993. p. 221–32. [25] Centers for Disease Control and Prevention (US). Addressing emerging infectious disease threats: a prevention strategy for the United States. Atlanta: US Department of Health and Human Services, Public Health Service; 1994. [26] National Institute of Allergy and Infectious Diseases (US). Report of the task force on microbiology and infectious diseases. National Institutes of Health, Publication no. 92– 3320. Bethesda (MD): US Department of Health and Human Services, Public Health Service; 1992. [27] Institute of Medicine (US). Emerging infections: microbial threats to health in the United States. Washington (DC): National Academy Press; 1992. [28] Anderson RM. Analytical therapy of epidemics. In: Krause RM, editor. Emerging infections. New York: Academic Press; 1998. p. 23–50. [29] Anderson RM, May RM. Infectious diseases of humans—dynamics and control. Oxford: Oxford University Press; 1991.