Enteric Infections and Immunity

INFECTIOUS AGENTS AND PATHOGENESIS Series Editors: Mauro Bendinelli, University of Pisa

Herman Friedman, University of South Florida

COXSACKIEVIRUSES A General Update

Edited by Mauro Bendinelli and Herman Friedman

DNA TUMOR VIRUSES Oncogenic Mechanisms

Edited by Giuseppe Barbanti-Brodano, Mauro Bendinelli, and Herman Friedman

ENTERIC INFECTIONS AND IMMUNITY Edited by Lois J. Paradise, Mauro Bendinelli, and Herman Friedman

FUNGAL INFECTIONS AND IMMUNE RESPONSES Edited by Juneann W. Murphy, Herman Friedman, and Mauro Bendinelli

MYCOBACTERIUM TUBERCULOSIS Interactions with the Immune System

Edited by Mauro Bendinelli and Herman Friedman

NEUROPATHOGENIC VIRUSES AND IMMUNITY Edited by Steven Specter, Mauro Bendinelli, and Herman Friedman

PSEUDOMONAS AERUGINOSA AS AN OPPORTUNISTIC PATHOGEN Edited by Mario Campa, Mauro Bendinelli, and Herman Friedman

PULMONARY INFECTIONS AND IMMUNITY Edited by Herman Chmel, Mauro Bendinelli, and Herman Friedman

VIRUS-INDUCED IMMUNOSUPPRESSION Edited by Steven Specter, Mauro Bendinelli, and Herman Friedman

A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately upon publication. Volumes are billed only upon actual shipment. For further information please contact the publisher.

Enteric Infections and Immunity

Edited by

Lois J. Paradise University of South Florida College of Medicine Tampa, Florida

Mauro Bendinelli University of Pisa Pisa, Italy

and

Herman Friedman University of South Florida College of Medicine Tampa, Florida

Springer Science+Business Media, LLC

Library of Congress Cataloging in Publication Data

On file

ISBN 978-1-4899-0315-0 ISBN 978-1-4899-0313-6 (eBook) DOI 10.1007/978-1-4899-0313-6

© 1996 Springer Science+Business Media New York Originally published by Plenum Press, New York in 1996 Softcover reprint of the hardcover 1st edition 1996

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All rights reserved

No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

Contributors

DAVID W. K. ACHESON • Division of Geographic Medicine and Infec­tious Diseases, Tupper Research Institute, New England Medical Cen­ter, Boston, Massachusetts 02111

SERGIO ARIAS-NEGRETE • Institute of Parasitology, McGill University, Ste-Anne de Bellevue, Quebec H9X 3V9, Canada

MERLIN S. BERGDOLL • Food Research Institute, University of Wis­consin, Madison, Wisconsin 53706

KRlS CHADEE • Institute of Parasitology, McGill University, Ste-Anne de Bellevue, Quebec H9X 3V9, Canada

SHUNZO CHIBA • Department of Pediatrics, Sapporo Medical Univer­sity, School of Medicine, Sapporo, Hokkaido 060, Japan

WITOLD CIEPLAK, JR. • Laboratory of Intracellular Parasites, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, Montana 59840

MICHAEL S. DONNENBERG • Division of Infectious Diseases, Univer­sity of Maryland School of Medicine, R. A. Cowley Shock Trauma Center, Baltimore, Maryland 2120l

TOBY K. EISENSTEIN • Department of Microbiology and Immunol­ogy, Temple University, School of Medicine, Philadelphia, Pennsylvania 19140

ROBERT M. GENTA • Departments of Pathology, Medicine, and Micro­biology and Immunology, Baylor College of Medicine, and Center for Infectious Diseases, University of Texas School of Public Health, Houston, Texas 77030

MARTIN F. HEYWORTH • Department of Veterans Mfairs Medical and Regional Office Center, Fargo, North Dakota 58lO2; and Department of Internal Medicine, University of North Dakota, Grand Forks, North Dakota 58203

v

vi CONTRIBUTORS

DUAN HUANG • Department of Microbiology and Immunology, Tem­ple University, School of Medicine, Philadelphia, Pennsylvania 19140

GERALD T. KEUSCH • Division of Geographic Medicine and Infectious Diseases, Tupper Research Institute, New England Medical Center, Boston, Massachusetts 02111

HIROSHI KIYONO • The Immunobiology Vaccine Center, Departments of Oral Biology and Microbiology, University of Alabama at Birming­ham, Birmingham, Alabama 35294-2170; and Department of Mucosal Immunology, Research Institute for Microbial Disease, Osaka Univer­sity, Osaka 565, Japan

MICHAEL E. KONKEL • Department of Microbiology, Washington State University, Pullman, Washington 99164-4233

DAVID M. LYERLY • TechLab, Inc., Corporate Research Center, Blacks­burg, Virginia 24060

JERRYR. McGHEE • The ImmunobiologyVaccine Center, Departments of Oral Biology and Microbiology, University of Alabama at Birming­ham, Birmingham, Alabama 35294-2170; and Department of Mucosal Immunology, Research Institute for Microbial Disease, Osaka Univer­sity, Osaka 565, Japan

JOHN J. MEKAlANOS • Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115

LOIS J. PARADISE • Department of Medical Microbiology and Immu­nology, University of South Florida College of Medicine, Tampa, Florida 33612

DAVID W. PASCUAL • Veterinary Molecular Biology, Montana State University, Bozeman, Montana 59717-0360

RONALD P. RABINOWITZ • Division of Infectious Diseases, University of Maryland School of Medicine, R. A. Cowley Shock Trauma Center, Baltimore, Maryland 21201

MARTIN G. SCHWACHA • Department of Microbiology and Immunol­ogy, Temple University, School of Medicine, Philadelphia, Pennsylvania 19140

ROGER L. VAN TASSELL • Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

MATIHEW K. WALDOR • Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115

TRACY D. WILKINS • Fralin Biotechnology Center, Virginia Poly­technic Institute and State University, Blacksburg, Virginia 24061

KENNETH H. WILSON • Infectious Diseases Section, VA Medical Cen­ter, and Duke University, Durham, North Carolina 27705

Preface

As we approach the end of this millennium, enteric diseases remain impor­tant public health problems. In many parts of the world, sanitary measures have advanced little over the last century, although some of the governments in those areas are striving to improve facilities for sanitation and to educate their people in proper handling of food, water, sewage, and other modes of transmission of pathogenic microbes. Even in highly developed countries, outbreaks of diarrheal diseases occur today. Globally, the annual morbidity from enteric infections is estimated at several billion and deaths at several million per year. In this volume, descriptions of some of these diseases, of immunity that results from them, of clinical studies that promote under­standing of individual and community immunity, of molecular factors of pathogenesis, and/or of advances in vaccine development have been pro­vided by leading researchers. At present, the application of molecular methods is enhancing the identification of protective antigens of many microorganisms. In addition, new methods for design and delivery of vac­cines are being devised. Perhaps then more effective tools for reducing at least some of these diseases will be available within the next decade.

vii

Lois J. Paradise Herman Friedman Mauro Bendinelli

Contents

Introduction ............................................ xv

LOIS J. PARADISE

1. Indigenous Microorganisms as a Host Defense 1

KENNETH H. WILSON

1. Introduction ........................................ 1 2. Composition of Intestinal Biota. . . . . . . . . . . . . . . . . . . . . . . . 2 3. Molecular Approaches to Determine Composition of

the Biota ........................................... 3 4. Role of the Host in Determining the Composition of

the Biota ........................................... 3 5. Colonic Biota as a Host Defense ....................... 6 6. Control Mechanisms ................................. 8 7. Overall Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

References .......................................... 11

2. Mucosal Immunity: Molecular and Cellular Aspects of Immune Protection to Enteric Infections ........................... 15

DAVID W. PASCUAL, HIROSHI KIYONO, AND JERRY R. McGHEE

1. Introduction ........................................ 15 2. The Gastrointestinal Immune System. . . . . . . . . . . . . . . . . . . 17 3. Antigen-Specific Immune Responses in the GI Tract 26

ix

x CONTENTS

4. Summary........................................... 29 References .......................................... 30

3. Vibrio cholerae: Molecular Pathogenesis, Immune Response, and Vaccine Development ................................ 37

MATTHEW K. WALDOR AND JOHN J. MEKALANOS

1. Introduction ........................................ 37 2. Serologic and Biotypic Classification ................... 38 3. Pathogenesis of Cholera .............................. 39 4. Cholera Toxin and Other Toxins ...................... 39 5. Colonization Factors ................................. 41 6. Regulation of Virulence Gene Expression . . . . . . . . . . . . . . . 44 7. Host Susceptibility ................................... 45 8. Immune Response ................................... 46 9. Vaccine Development ................................ 48

References .......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4. Immunity to Salmonella Infections ........................ 57

TOBY K. EISENSTEIN, DUAN HUANG, AND MARTIN G. SCHWACHA

l. Introduction ........................................ 57 2. Animal Models ...................................... 58 3. Killed Vaccines and Humoral Immunity in Murine Models

of Typhoid Fever .................................... 59 4. The Concept of Cellular Immunity and Immunity to

Salmonella Infection ................................. 61 5. Genetic Determinants of Resistance to Salmonella ....... 62 6. The Influence of Genetically Determined Innate

Resistance or Susceptibility to Salmonella and Mechanisms of Acquired Host Resistance .......................... 64

7. Implications of Mouse Studies for Human Vaccination and Host Defense to Salmonella ........................... 65

8. Further Studies on Mechanisms of Host Resistance to Salmonella Infection ................................. 65

9. Cytokine Responses and Host Resistance to Salmonella ... 68 10. Immune Responses to Oral Infection. . . . . . . . . . . . . . . . . . . 69

CONTENTS xi

11. Summary........................................... 71 References .......................................... 73

5. Shigella Infection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

GERALD T. KEUSCH AND DAVID W. K. ACHESON

1. Introduction ........................................ 79 2. Epidemiology ....................................... 80 3. Clinical Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 4. Pathogenesis ........................................ 83 5. Immune Responses .................................. 89

References .......................................... 94

6. Escherichia coli .......................................... 101

RONALD P. RABINOWITZ AND MICHAEL S. DONNENBERG

1. Introduction ........................................ 101 2. Enterotoxigenic E. coli. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3. Enteroinvasive E. coli ................................. 107 4. Enteropathogenic E. coli .............................. 108 5. Enterohemorrhagic E. coli. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6. Enteroaggregative E. coli .............................. 117 7. Diffusely Adherent E. coli ............................. 119 8. Summary ........................................... 120

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

7. Molecular Pathogenesis of Campylobacter jejuni Enteritis 133

MICHAEL E. KONKEL AND WITOLD CIEPLAK, JR.

1. Introduction ........................................ 133 2. Microbiology of C. jejuni .............................. 134 3. Pathologic and Epidemiologic Aspects of C. jejuni Enteritis 134 4. Immune Response to C. jejuni Infection ................ 135 5. Pathogenic Mechanisms of C. jejuni .................... 137 6. Concluding Remarks ................................. 142

References .......................................... 142

xu CONTENTS

8. Clostridia and Bacteroides in Enteric Infections 149

DAVID M. D:ERLY, ROGER L. VAN TASSELL, AND TRACY D. WILKINS

1. Introduction ........................................ 149 2. Pseudomembranous Colitis and Antibiotic-Associated

Diarrhea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 3. Clostridium perfringens Food Poisoning .................. 156 4. Enteritis Necroticans (Pigbel) ......................... 160 5. Iota Toxin-Mediated Enterotoxemia. . . . . . . . . . . . . . . . . . . . 162 6. Bacteroides fragilis Enterotoxin. . . . . . . . . . . . . . . . . . . . . . . . . . 163 7. Conclusions ................. . . . . . . . . . . . . . . . . . . . . . . . . 165

References .......................................... 166

9. The Staphylococcal Toxins in Human Disease. . . . . . . . . . . . . .. 169

MERLIN S. BERGDOLL

1. Introduction ........................................ 169 2. The Staphylococcal Enterotoxins ...................... 169 3. Staphylococcal Toxic Shock Syndrome Toxin ............ 174 4. Exfoliatin or Epidermolytic Toxin . . . . . . . . . . . . . . . . . . . . .. 177 5. Unknown Staphylococcal Toxins ....................... 178 6. Staphylococcal Toxins as Superantigens . . . . . . . . . . . . . . . . . 178 7. Epilogue............................................ 182

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

10. Immunological Aspects of Viral Gastroenteritis. . . . . . . . . . . . . . 187

SHUNZO CHIBA

1. Viral Gastroenteritis and Its Etiological Agents. . . . . . . . . . . 187 2. Immunity against Rotavirus Gastroenteritis .............. 188 3. Immunity against Gastroenteritis Caused by Norwalk Virus

and Human Calicivirus ............................... 199 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 200

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

CONTENTS xiii

11. Immunopathogenesis of Entamoeba histolytica 207

SERGIO ARIAS-NEGRETE AND KRIS CHADEE

1. The Life Cycle of E. histolytica ......................... 207 2. Pathogenesis of E. histolytica ........................... 208 3. Pathogenesis of Invasive Amebiasis. . . . . . . . . . . . . . . . . . . .. 214 4. Immune Response in Amebiasis ....................... 214 5. Summary........................................... 221

References .......................................... 221

12. Giardia Infections ....................................... 227

MARTIN F. HEYWORTH

1. Giardia Organisms and Species ........................ 227 2. Clinical Features and Pathophysiology of Giardiasis ...... 229 3. Epidemiology of Giardiasis ............................ 230 4. Immunological Responses to Giardia Trophozoites ....... 231 5. Diagnosis of Giardiasis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 233 6. Treatment of Giardiasis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

References .......................................... 234

13. The Immune Regulation of Intestinal Helminthiases

ROBERT M. GENTA

239

1. Introduction ........................................ 239 2. Biological Features of Intestinal Helminths. . . . . . . . . . . . . . 240 3. Systemic Immune Responses to Intestinal Helminths ..... 242 4. Intestinal Responses to Helminthic Parasites ............ 246 5. Do Immune Responses against Parasites Induce Resistance

in Humans? ......................................... 249 6. Evasion of the Host Immune Responses ................ 251 7. Conclusions ......................................... 252

References ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

Index...................................................... 257

Introduction LOIS J. PARADISE

Gastrointestinal diseases are both a bane of the developing areas of the world and a disturbing problem in industrialized regions, despite greater emphasis on sanitary measures to promote safe food-handling and water distribution. Diarrheal disease is the most common acute infectious disease in the world and is responsible for the considerable mortality in developing countries. (1)

In these countries, continuing sanitation problems and failure to prevent exposure of babies to enteric pathogens result in acute, life-threatening and/ or fatal diseases. If the individual survives acute diarrheal disease, chronic disease may persist for the remainder oflife. In the aged population, acute infectious enteritis is again a major cause of death. In the United States and other developed countries, many niches for traditional enteric organ­isms remain, although sanitation generally is better and inspection of food and water resources aimed at reducing the risk of exposure to infection is more common. In addition, outbreaks occur with newly recognized enteric pathogens. Complacency, undoubtedly, is another factor. Because many people believe that food and water supplies are entirely safe today, they may be careless about precautions such as cooking food sufficiently to destroy pathogens. In the industrialized world, both the incidence and mortality are higher among infants and young children and the very elderly. (2)

Microorganisms associated with human diarrhea include various spe­cies of bacteria, fungi, viruses, and protozoa. Some helminths also cause diarrheal disease. The acquisition of diarrheal diseases depends on both broad and specific aspects of the characteristics of individuals and on their socioeconomic environment and activities. (3) Age is a determining factor for susceptibility to infection by some organisms. With some enteric pathogens,

LOIS J. PARADISE • Department of Medical Microbiology and Immunology, University of South Florida College of Medicine, Tampa, Florida 33612.

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xvi LOIS J. PARADISE

very young individuals are particularly at risk; for other pathogens, the elderly are most at risk; and with still other pathogens, both age groups are readily affected. Regardless of age, severely immunocompromised patients, such as those with acquired immunodeficiency syndrome (AIDS), are partic­ularly susceptible to secondary infectious diseases, including diarrheal dis­eases. Immunocompromised patients not only are more susceptible to some microbial species that are indigenous organisms for immunocompetent populations, but also are at risk for infections caused by microorganisms once considered environmental microbes unable to initiate human disease. A number of these agents cause enteric diseases. In addition to host-specific factors (who), when and where exposure takes place are epidemiologically important. (3) In tropical and temperate zones, seasons of higher incidence of enteric diseases differ, occurring more in summer in the former and in winter in the latter areas. However, this may be an oversimplification since other climatic conditions, such as rainfall, may introduce variations into this pattern. The incidence of diarrheal disease also differs with the particular species of microorganisms present in the population and environment of a given location.

As might be expected, mechanisms that affect the intestine and result in diarrhea vary with the organism or group of organisms. Diarrhea is in­creased frequency and/ or fluidity of bowel movements. (4) Discharges may be watery and contain no evidence of an inflammatory response to the infection. This picture is found with organisms that produce enterotoxins or other chemicals capable of affecting intestinal epithelial cells directly and thereby alter their secretory functions. Watery, noninflammatory diarrheas occur with cholera, rotaviral disease, and Clostridium difficile and enterotox­igenic Escherichia coli infections, among others. With microorganisms such as shigellae, Salmonella enteritidis, enteroinvasive E. coli, and Entamoeba histo­lytica, invasion of epithelial cells and/or the effects of cytotoxins produced by the microorganisms on the epithelium result in inflammatory diarrheal disease. Some gastrointestinal pathogens penetrate more deeply into tissues than others. Salmonella typhi produces a systemic febrile disease and is a member of this group. With the last two groups of agents, inflammatory diarrhea is the norm. Leukocytes appear in the stool, indicating that poly­morphonuclear leukocytes have been recruited to the area and have es­caped into the intestinal lumen. This does not occur with watery diarrhea. With helminthic infections, mast cells and their mediators may play roles in diarrhea production. Studies of various bacterial, viral, protozoan, and helminthic organisms have demonstrated that they also interfere with nor­mal intestinal motility and the resulting dysfunction promotes diarrhea. (4)

Understanding of the pathogenesis of some enteric diseases has ad­vanced markedly at the cellular and molecular levels within the last couple of

INTRODUCTION xvii

decades. For enteric microorganisms more recently recognized as being pathogenic, knowledge has not developed to an equivalent state, but techni­cal advances of the last decade should allow faster progress toward under­standing than was possible in the past. In the chapters that follow, the state of knowledge concerning mechanisms of action of toxins, virulence factors such as adhesions, nutritional requirements, mechanisms of invasion, ef­fects on eukaryotic cell signaling, and stimulation of the inflammatory response by induction and the actions of cytokines is discussed for each group of organisms. The host response to cholera and cholera toxin (CT) was the first to be described and provided a model for studying similar aspects of some other enteric diseases. In Chapter 3, the molecular charac­teristics of CT and virulence factors of V cholerae that enable colonization of the intestines and molecular aspects of genetic control of these factors are delineated. For E. coli strains associated with diarrheal diseases, more is known about enterotoxigenic (ETEC) and enteropathogenic (EPEC) strains than for more recen tly described virulent strains (Chapter 6) , but progress is escalating. ETEC strains cause a diarrheal disease similar to, but less severe than, cholera. CT enhances intracellular adenylate cyclase activity; ETEC enterotoxin increases guanylate cyclase activity. The net result in each case is increased secretion of water and electrolytes, but the two differ in electro­lytes affected and the extent of the responses induced. In EPEC diarrhea, toxins produced are cytotoxins that directly affect intestinal epithelial cells. The result is an inflammatory diarrhea rather than a watery, noninflamma­tory one. In its pathogenesis, the disease resembles shigellosis rather than cholera. Other E. coli strains produce hemorrhagic diarrheas. It is obvious that within this one species, various mechanisms of action on the human gastrointestinal tract have developed. It is important to note, however, that most strains of E. coli are not pathogenic and many are members of the human indigenous, intestinal microflora and as such undoubtedly partici­pate in protecting the host from infection with pathogens (Chapter 1).

Genetic con trol of the various virulence factors in strains of one species or among groups of pathogenic species may be plasmid or chromosomal in origin or both. Identification of genetic bases for formation and/ or release of components related to virulence will enable performance of experi­ments, e.g., using deletion mutants or transfectants, to discover whether they also are protective antigens. When immune responses that protect against infectious challenge have been identified, vaccines can be designed from the identified components. The vaccines can be tested in model systems not just for protection against challenge inoculations of the microorganisms, but also for molecular analysis of the protection afforded. These results can be compared to findings in some humans challenged after vaccination and others immunized by naturally acquired disease. Successful and useful vac-

xviii LOIS J. PARADISE

cines are available for few of the enteric pathogens. Using molecular tech­niques to study these aspects of active immunization, it should be possible to prepare vaccines that are highly effective and yet have few side effects.

Both clinical symptoms and immune responses are results of the mo­lecular and cellular interactions that pathogenesis comprises. Studies that determine the interrelationships of these aspects eventually will point out likely routes for design and development of optimally safe and protective vaccines for these diseases. The fact that understanding of the pathogenesis and immune responses of the enteric diseases is incomplete has hindered vaccine development, as implied above. For many years humoral immunity was the focus for evaluation of vaccines. In time, cellular immune functions were observed to be involved in some cases. Still more recently, mucosal immunity has been in the forefront. Although knowledge of the existence, functions, and capabilities of the mucosal immune system has advanced markedly during the past few decades (Chapter 2), the relative importance of humoral and mucosal antibody responses as well as cellular immune mechanisms in protecting against primary infection or reinfection with en teric agen ts is only partly understood. This is described for several diseases in the chapters of this volume. For example, the pathogenic mechanisms of rotavirus and the Norwalk group of viral diarrheas (Chapter 10) have been considerably less well clarified than for many bacterial diseases. There are several reasons for this, but one is the lack of certainty that animals serve as adequate models for human studies. In addition, it has not always been certain that clinical studies have been dealing with primary infection rather than reinfection. For other diseases, controversies exist because of differing observations that cannot be resolved within the prevailing theories. Increas­ing recognition that mucosal and humoral antibodies may have synergistic effects on intestinal immunity and that cellular immunity may further en­hance protection may lead to investigations that will provide solution. (5)

In summary, the potential for effective vaccine development has im­proved greatly. Conditions that continue to inhibit immediate development include: (1) identification of protective antigen(s) for many organisms is still in progress; (2) understanding of those immune functions that are active in protection is incomplete; and (3) some effective, molecularly derived vac­cines require modification to eliminate deleterious side effects. Progress is being made in these same areas, e.g., (1) specific epitopes of protective antigens have been isolated for some organisms and are being tested for efficacy; (2) the proposition that interactions among mucosal, humoral, and cellular immune functions provide the gastrointestinal protective barrier, not just one or the other, is being examined; and (3) investigators are examining purified antigenic molecules to modify their toxic properties.

INTRODUCTION xix

REFERENCES

1. Walsh, J. S., and Warren, K. S., 1979, Selective primary health care, N. Eng!.]. Med. 301: 967-974.

2. Bennett, R. G., and Greenough, W. B., III, 1993, Approach to acute diarrhea in the elderly, Gastroenterol. Clin. North Am. 22:517-533.

3. Guerrant, R. L., 1990, Principles and syndromes of enteric infection, in: Principles and Practice of Infectious Diseases, 3rd ed. (G. L. Mandell, R. G. Douglas, Jr., and J. E. Bennett, eds.), Churchill Livingstone, Edinburgh, pp. 837-851.

4. O'Loughlin, E. v., Scott, R. B., and Gall, D. G., 1991, Pathophysiology of infectious diarrhea: Changes in intestinal structure and function,]. Pediatr. Gastroenterol. Nutr. 12:5-20.

5. Monath, T. P., and Neutra, M. R., 1994, Introduction to Symposium on Mucosal Immunity: Protection Against Pathogens, Am.]. Trop. Hyg.50(Suppl.):1-2.