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PIDPR 5/21/04 11:21 AM Page i PIDPR 5/21/04 11:21 AM Page i Lecture Notes: Epidemiology and Public Health Medicine PIDPR 5/21/04 11:21 AM Page ii PIDPR 5/21/04 11:21 AM Page iii Lecture Notes Epidemiology and Public Health Medicine Richard Farmer MB, PhD, FFPH, FFPM Professor of Epidemiology Postgraduate Medical School University of Surrey Stirling House Surrey Research Park Guildford Surrey, UK Ross Lawrenson MRCGP, FAFPHM, MD Dean of Medicine & Professor of Primary Health Care Postgraduate Medical School University of Surrey Stirling House Surrey Research Park Guildford Surrey, UK Fifth Edition PIDPR 5/21/04 11:21 AM Page iv © 2004 by Blackwell Publishing Ltd Blackwell Publishing, Inc., 350 Main Street, Malden, Massachusetts 02148-5020, USA Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Blackwell Publishing Asia Pty Ltd, 550 Swanston Street, Carlton, Victoria 3053, Australia The right of the Authors to be identified as the Authors of this Work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher First published in 1977 under the title Lecture Notes on Epidemiology and Community Medicine Second edition 1983 Third edition 1991 Fourth edition 1996 Reprinited 1997, 1998 Fifith edition 2004 Library of Congress Cataloging-in-Publication Data Farmer, R D T Lecture notes on epidemiology and public health medicine / Richard D.T Farmer, Ross Lawrenson — 5th ed p ; cm Includes bibliographical references and index ISBN 1-4051-0674-3 Epidemiology Public health [DNLM: Epidemiologic Methods Health Services Preventive Medicine WA 950 F234L 2004] I Title: Epidemiology and public health medicine II Lawrenson, Ross III Title RA651.F375 2004 614.4 — dc22 2004000864 ISBN 1-4051-0674-3 A catalogue record for this title is available from the British Library Set in 8/12 Stone Serif by SNP Best-set Typesetter Ltd., Hong Kong Printed and bound in India by Replika Press Pvt Ltd Commissioning Editor: Vicki Noyes Editorial Assistant: Nic Ulyatt Production Editor: Fiona Pattison Production Controller: Kate Charman For further information on Blackwell Publishing, visit our website: http://www.blackwellpublishing.com PIDPR 5/21/04 11:21 AM Page v Contents Preface, vi List of Abbreviations, viii Part Epidemiology General principles, ‘Cause’ and ‘risk’ and types of epidemiological study, Descriptive studies, 14 Surveys, survey methods and bias, 24 Cohort studies, 32 Case–control studies, 38 Intervention studies, 45 Health information and sources of data, 51 Indices of health and disease, and standardization of rates, 63 10 Medical demography, 69 11 Evidence-based medicine, 82 Part Prevention and Control of Disease 12 General principles, 91 13 Health promotion and health education, 96 14 Control of infectious disease, 103 15 Immunization, 114 16 Environmental health, 127 17 Screening, 133 Part Health Services 18 19 20 21 History and principles, 143 The National Health Service, 153 Health targets, 162 Evaluation of health services, 173 Appendices: Further Reading and Useful Websites Appendix 1: Suggested further reading, 181 Appendix 2: Useful websites, 182 Index, 183 v PIDPR 5/25/04 2:22 PM Page vi Preface The UK Government is committed to improving the nation’s health and reducing health inequalities Whilst the provision of health care is in a state of constant change it is important to remember that the key objective is to maintain and improve the health of the population This was recognized by Derek Wanless in his report Securing Good Health for the Whole Population published on 25th February 2004 This document focused on prevention and the wider determinants of health To prevent disease and improve health it is essential to understand why diseases arise; and conversely why, in many cases, they not To this it is necessary to study the distribution and natural history of diseases in populations and to identify the agents responsible; effective strategies can then be planned In the same way that the provision of health care should be evidence based, the introduction of new preventive strategies should be rigorously evaluated and researched The application of evidencebased medicine is applicable to both clinical and public health practice In the past the importance of public health medicine and the related basic medical sciences, in particular medical statistics and sociology applied to medicine, was not emphasized in the undergraduate medical education This relative neglect changed in the 1990s with the GMC's recommendation on undergraduate medicine Tomorrow’s Doctors This publication recommended that the theme of public health medicine should figure prominently in the undergraduate curriculum, encompassing health promotion and illness prevention, assessment and targeting of population needs and awareness of environmental and social factors in disease This explicit and forceful advocacy for the discipline from a body as influential as the GMC undoubtedly gave added momentum to the development of medical education Similar changes emphasising the importance of disease prevention and the need to ensure that health care vi is relevant effective and efficient are evident within the NHS in the UK as in many other countries This is exemplified in the NHS plan The New NHS; modern, dependable (1997) This new edition of Lecture Notes: Epidemiology and Public Health Medicine, as before, covers the basic tools required for the practice of epidemiology and preventive health The chapters in the first section of the book outline the principles of epidemiology and lead the reader to some classic examples from the medical literature A new chapter has been included on the practice of evidencebased medicine The second section of the book covers the areas of prevention and control of disease — in particular the chapter on health promotion has been updated to reflect the advances that have occurred over the last eight years The chapter on occupational health has been dropped from this edition The final section has been updated to reflect the changes in the provision of health care Change is now a constant in the health services and the shift between central control and devolution of responsibility will continue to ebb and flow At the time of writing we are seeing more devolution of responsibility and the primary care trusts have a tremendous opportunity to deliver health services that are truly responsive to patient needs We should also recognise the successes brought about through the introduction of health targets — the incidence of heart disease is falling; the mortality from breast and cervical cancer has fallen as screening for these diseases has increased; and many infectious diseases, for practical purposes, have been eliminated We still have many challenges — obesity and diabetes are increasing rapidly, alcohol abuse has been recognized as a growing social problem and the spread of sexually transmitted disease and HIV still poses challenges We hope readers will find that this new edition continues to provide a basic structure to under- PIDPR 5/21/04 11:21 AM Page vii Preface standing epidemiology and public health and that many of our readers will be encouraged to delve deeper into the subject Acknowledgements We are greatly indebted to Dr Peter English of the Health Protection Agency for his help and support in the updating of the chapters on infectious diseases and immunization We must also recognise the contribution of Emeritus Professor David Miller who was the co-author of the first four editions of this book We would also like to thank Mrs Pat Robertson, our PA at the University, for her help and support Richard Farmer Ross Lawrenson vii PIDPR 5/21/04 11:21 AM Page viii List of Abbreviations AHA AIDS BCG BMA CCDC CDSC CEHO CHAI DHA DoH DTP EBM FHSA GMC GPRD HEA HES Hib HIV HPA HSE ICD IHD IPV ITT MMR MRC NHS NHSME NICE OPCS OPV PCT PHLS PMR RAWP RCT RHA SARS SMR STD WHO viii Area Health Authority acquired immune deficiency syndrome bacille Calmette—Guérin (vaccine) British Medical Association Consultant in Communicable Disease Control Communicable Disease Surveillance Centre Chief Environmental Health Officer Commission for Healthcare Audit and Inspection District Health Authority Department of Health diphtheria/tetanus/pertussis (vaccine) evidence-based medicine Family Health Service Authority General Medical Council General Practic Research Database Health Education Authority hospital episode statistics haemophilus influenzae type b (vaccination) human immunodeficiency virus Health Protection Agency Health and Safety Executive International Classification of Diseases ischaemic heart disease injected polio vaccine intention to treat measles/mumps/rubella (vaccine) Medical Research Council National Health Service National Health Service Management Executive National Institute for Clinical Excellence Office of Population Censuses and Surveys oral polio vaccine primary care trust Public Health Laboratory Service perinatal mortality rates Resource Allocation Working Party randomized controlled trial Regional Health Authority severe acute respiratory syndrome standardized mortality ratio sexually transmitted disease World Health Organization PID1 5/21/04 11:22 AM Page Part Epidemiology PID10 5/21/04 11:36 AM Page 81 Medical demography Chapter 10 Thus, while a large proportion of fetal and perinatal mortality is difficult to prevent, much can be done to reduce rates by appropriate antenatal and postnatal care and advice Summary • Every industrialized nation has low mortality compared with non-industrialized countries Further substantial decline in mortality in industrialized countries is unlikely because the major causes of death are associated with old age • There is great potential for further substantial reduction in mortality in Asia, Africa and Latin America This will be achieved by control of the major infective diseases, especially gastrointestinal and respiratory infections in children and AIDS • The principal factors acting against any quick reduction in mortality in developing countries are malnutrition, illiteracy and poverty • Industrialization is inversely related to changes in fertility Four explanations for this can be adduced as follows: • in urban societies children are not an economic asset; • as the infant death rate declines, the proportion of children who survive to adulthood increases and the number of births required to attain a desired family size is smaller; • in urban societies, there are greater opportunities for women outside the domestic environment, and being committed to child rearing restricts a woman’s activities; and • in educated societies, the influence of secular rationality is stronger which allows readier acceptance of contraception 81 PID11 5/21/04 11:37 AM Page 82 Chapter 11 Evidence-based medicine Introduction One of the guiding principles behind the new NHS is that health care should be based on evidence The idea of evidence-based practice was introduced into the UK relatively recently The first workshop was organized by Professor David Sackett at Oxford in 1995 Medicine has not always been evidence based — that is why evidence-based medicine (EBM) has been regarded as a new discipline Traditionally the teaching of medicine has been based on an apprenticeship-type system Undergraduate students observed their teachers as they practised, learnt from them and emulated them This same tradition was continued in postgraduate training EBM is the application of population-based research to the care of patients Increasingly patients and those responsible for paying for health care expect evidence that the treatments offered are of proven benefit They want to know if a drug is prescribed that it not only lowers the cholesterol level or reduces the blood pressure but that it also prolongs life or reduces the risk of heart attack or stroke Many illnesses or diseases have a range of treatments, all of which are effective Some treatments are of unproven value and others although effective can cause harm For other conditions there are no effective treatment When evaluating interventions in an environment of limited resources it is desirable to compare the relative bene82 fits and risks of available treatments (including no treatment) Benefits and harm can be assessed in terms of either cost, or some measure of health outcome Much medical practice is based on anecdotal evidence and ‘experience’ which may be unreliable and biased EBM promotes the concept that clinical (and health care) decision-making should be based on the best patient/populationbased studies It uses a hierarchy of evidence, with the highest quality normally coming from randomized controlled trials (RCTs) These are not always possible and in such circumstances evidence from other studies is used Hierarchy of evidence • • • • • Systematic review of randomized trials Randomized controlled trial Cohort studies Case–control studies Case series or case reports EBM came about because of the revolution in information technology The rapid growth in the availability of electronic databases of the medical literature allowed original research papers to be identified and retrieved rapidly This can be done without leaving the consulting room Before the advent of computerized databases many doctors PID11 5/21/04 11:37 AM Page 83 Evidence-based medicine Chapter 11 relied on medical textbooks, supplemented by browsing the journals that crossed their door or picking up new ideas from observing the practice of consultant colleagues Unfortunately the textbooks are usually out of date and many of the journals were little more than medical newspapers, which published articles that were newsworthy rather than of scientific merit Consultant practice was followed because it was believed that consultants were better informed of new developments and better equipped to appraise new studies critically Whilst this was often true, there can be much variation in consultant practice and this can lead to uncertainty in decision-making Practising EBM EBM is used by clinicians to help their decisionmaking It utilizes a structured approach, involving five key steps These include taking a clinical scenario and from this identifying the key questions that are needed for the management of the patient That question should then be formulated in such a way that it can be answered through use of the medical literature The structured question produces key words which are used to help formulate the search strategy that identifies the relevant papers The papers then have to be critically appraised and the evidence synthesized and used to help clinical decision-making Structured approach to EBM • • • • • Clinical scenario Structured question Search for the relevant papers Critically appraise the evidence Use evidence to help decision-making structure their questions (P is for population, I for intervention, C for comparison and O for outcome.) P—How are a group of patients similar to the one in the clinical scenario described? Patients included in the published studies may differ in some respects to the patient or clinical scenario encountered Key factors such as age, gender, diagnosis, ethnicity and so forth need to be considered I—Which main intervention, treatment (or exposure or prognostic marker) is being considered? C—What is the main alternative with which this manoeuvre or treatment should be compared? Many studies on new therapies are compared against placebo This is required by the licensing authorities to demonstrate efficacy but clinicians want to know whether the new treatment is more effective than the treatment usually offered O—What can be accomplished with the patient — e.g increased length of life, shorter hospital stay, less pain, etc? The outcome must be measurable There are objective measures of health or disease that can be used to assess one treatment or group against another Searching for the evidence The key search terms are found from the structure of the question, particularly the intervention/exposure of interest and the outcome of interest The search strategy should be as sensitive as possible — it should identify all the relevant papers to help answer the clinical question Specificity is obtained by combining search terms and by using the relevant filters for the different types of question (For example, a filter can be developed that restricts the search to randomized trials.) Critically appraising the evidence Forming structured questions Forming an answerable question from a clinical scenario is the first step in practising EBM It is a discipline that requires practice Practitioners of EBM often use the acronym PICO to help them Once a relevant paper has been identified it is important to be able to appraise it critically This is done by asking three simple questions • How valid is the study? Is it well designed and carried out in an appropriate population? 83 PID11 5/21/04 11:37 AM Page 84 Chapter 11 Evidence-based medicine • What are the results of the study This often entails unpicking the results and presenting them in a way that is more relevant to the question • Will the results of this study affect practice, i.e is it clinically relevant? It is important to be aware that a statistically significant finding is not necessarily clinically relevant noted that if 20 characteristics are looked at, then by chance (at the 5% level) a significant difference between the groups is likely to be found in at least one The larger the study the more likely the groups are to be similar Thus big studies are to be preferred This will also help avoid the problem of a Type or a Type error (see below) Drop-out rates Randomized controlled trials The principal form of evidence when considering whether a treatment works or whether an exposure causes a particular outcome is an RCT Hence it is important to understand the principal components of an RCT The study should be appraised critically to see whether it has been well conducted and can be believed Points to look for in appraising an RCT • • • • • • • Randomization Characterization of the groups Drop-out rates Intention to treat analysis Blinding Sample size Results Were all patients who entered the trial properly accounted for and attributed at its conclusion? If patients are lost to follow-up it may be that those patients who left the study had a different outcome to those who were included in the final analysis For example, if the outcome of interest is death, patients lost to follow-up may have had a higher death rate than those who are followed up throughout the study This leads to an underestimate of mortality in the groups studied Similarly, if the drop-out rate between groups is different bias may be introduced A common reason for a difference in drop-out rates is that one treatment causes more side-effects or is ineffective Ideally all patients should receive the treatment to which they were randomized, be followed up and their outcomes noted Intention to treat analysis Randomization The method of randomization can introduce bias and influence the generalizability of the findings It is therefore important to know how the randomization was carried out in order to be able to assess whether this is likely to have influenced the results It is particularly important to check that the staff involved in recruiting subjects to the study were not also responsible for the randomization Characterization of the groups In the paper there should be a table showing the characteristics of the treatment groups being compared Sometimes by chance, particularly in small studies, the groups may be unequal (e.g more men in one group) and this can cause bias It should be 84 In reality there are always some patients who not receive the treatment to which they were randomized By analysing the results using intention to treat analysis any bias due to unplanned drop-outs or cross-overs will be avoided (A cross-over is when a patient is allocated to treatment A but actually receives treatment B.) If on intention to treat analysis there is still a treatment effect then this is likely to be a true effect Analysis by actual treatment group is also usually worthwhile and is more likely to show a statistically significant difference between groups With placebo-controlled trials it has been shown that compliant patients who take their placebo have a better outcome (up to 30% better) than the non-compliant patients Including dropouts in the placebo group can introduce bias Again, intention to treat analysis removes this bias PID11 5/21/04 11:37 AM Page 85 Evidence-based medicine Chapter 11 Blinding In a single blind randomized trial the patient is unaware which treatment they are receiving This is important when the assessment of the outcome of interest is subjective — such as pain, anxiety, etc In a drug trial this can be achieved by giving one group of patients a placebo In a double-blind trial both the patient and the investigator should not know which treatment they received This then removes possible bias in both the reporting and recording of the outcome of interest least the 95% level to avoid a Type error • The value of the power desired — this is usually set at the 80% level because missing a small but true difference is less important than identifying a spurious positive association Greater power requires a bigger sample size and will incur additional costs The results Sample size Small studies can sometimes be misleading A Type error occurs when a study concludes that two treatments are different when in fact they are not If a study is repeated 20 times, on one occasion by chance a statistical difference (at the 5% level) will be detected This difference occurs by chance but if the results of only this one study are published it will give a biased impression that the treatment investigated is worthwhile when in fact it is not A Type error is when the study concludes that the treatment groups are not different when in fact they are In this case, unless a big enough study has been carried out the difference will not be detected — a Type error It may be that although there is a true difference between the two interventions, the size of this difference is small and may not be clinically relevant Thus a Type error is often not considered such a serious problem as a Type error Conclude groups are not different Conclude groups are different • An estimate of the response rate in one of the groups — if the outcome of interest is a rare event, then a larger sample size will be needed • Level of statistical significance — this is usually at Groups are not different Groups are different Correct decision Type error Type error Correct decision When designing a study it is important to ensure that Type and Type error is avoided The size of the sample required is determined by: • Difference in response rates to be determined — this should be a clinically significant difference The results of an RCT are usually presented as a relative risk (see p 11) Relative risk (RR) is the absolute risk in the treated group divided by the absolute risk in the untreated group (or vice versa) Another way the results can be presented is by estimating the number needed to treat (NNT) This takes into account not only the RR but also the absolute risk in the two groups being investigated The NNT is calculated by taking the reciprocal of the absolute risk reduction (ARR) It indicates how many people would have to be treated with A as compared to B in order to prevent one additional outcome of interest For example, imagine 2000 patients with mild hypertension are randomly allocated to treatment or placebo At the end of the year patients in the placebo group have had a stroke and only in the treated group have suffered a stroke The RR for the treated group if 0.5 Thus the treatment produces a 50% reduction in the number of strokes However the NNT in this example is 500 Five hundred people will have to be treated for year (and carry any risks associated with that treatment) for one patient to benefit NNTs are probably a more relevant index to be used for clinical practice although there is evidence that decision makers are more likely to alter their practice when presented with the RR Critical appraisal of systematic reviews A systematic review involves identification of all the relevant primary papers in human populations 85 PID11 5/21/04 11:37 AM Page 86 Chapter 11 Evidence-based medicine that deal with a focused question These papers are then appraised critically to identify their strengths and weaknesses Finally a summary of the evidence is reported Well done systematic reviews are increasingly being accepted as the highest form of evidence in the hierarchy of evidence The ‘gold standard’ for reviews are those done to the criteria set down by the Cochrane Collaboration — a group of researchers and clinicians interested in undertaking systematic reviews of randomized trials Since a systematic review is a retrospective look at published papers, it is important to make the process rigorous and well defined to prevent bias and thus distortion of the findings Points to look for in appraising a systematic review • • • • Criteria for inclusion Sensitivity of the search Method of selection Validity of the studies Criteria for inclusion A systematic review should have clearly defined criteria for the inclusion of studies This usually includes the type of study (for therapy questions ideally an RCT), the populations included in the studies, the treatments or exposures and relevant outcomes The criteria should not be so restrictive that important studies are likely to be missed Ideally the authors should list all trials reviewed with a reject log and reasons for exclusion Sensitivity of the search The systematic review should demonstrate that a sensitive search strategy was adopted Ideally it will include: • The words that were used in the interrogation of the medical databases, which terms were combined and which intersected • The time period over which papers could be included • The databases and other sources that have been 86 searched, e.g Medline, Cochrane, Embase, Cinahl, etc • Details of secondary references, i.e the references cited by the papers that were retrieved from the original search • Studies published in languages other than English Researchers undertaking systematic reviews are often tempted to exclude these papers because of the cost of having them translated (as well as the delay) However this may mean that some perfectly valid studies that deal with the question are not included This again can introduce bias • Grey literature Studies with negative findings may be difficult to get published If only studies with positive results are published then the published papers will give a positive result The results of unpublished studies (the grey literature) if relevant should be included They can be obtained by contacting researchers known to be active in the field of interest Also drug companies often have unpublished studies which they may release to researchers Once all the papers have been collected then they should be appraised critically and an evaluation of the overall findings made Meta-analysis A meta-analysis is a particular type of systematic review that uses quantitative methods to combine the results of several independent studies considered by the analyst to be combinable The overall results are weighted by the size of the contributing studies This means that the larger studies will have the main influence on the outcome The results of a meta-analysis can be presented in a tabular or graphical form Challenges to evidence-based practice There is a word of warning about the rational or scientific approach to medicine Firstly there is a mismatch between the needs of patients and the research agenda that provides the evidence Research agendas are set by those with the funds — particularly the research councils and the PID11 5/21/04 11:37 AM Page 87 Evidence-based medicine Chapter 11 pharmaceutical industry The Medical Research Council has been criticized for the lack of involvement of patients in setting research agendas Rather, it is a panel of scientists with a particular view of what research is needed that commissions studies Their viewpoint has a major influence on the type of research that is funded The pharmaceutical industry, which provides more than 60% of the funds for medical research in the UK, naturally has a different perspective They want to find medications that will alleviate patients’ problems and will lead to commercial success Investigations of behavioural or population-based interventions have a much harder time attracting funds Consequently there is more evidence on the effectiveness of interventions that involve drug treatment than other modalities Studies on new drugs often exclude specific groups for ethical or safety reasons For example, there are few studies amongst women of childbearing age, children and the elderly Practising EBM can be difficult for obstetricians, paediatricians and geriatricians A consequence of excluding certain groups from RCTs is well illustrated by the recommendations for the use of lipid-lowering therapy It has been suggested that only those who are at high risk of heart disease and who are under 75 years of age should be treated Part of the reason for the age cut-off is that the trials of lipid-lowering therapy excluded older patients (above 70 or 75 years of age) This was not because these patients are unlikely to benefit but because the likelihood of side-effects and adverse events are higher in older people Consequently it makes sense to try to avoid including these patients in the trials Unfortunately there is then no direct evidence from the RCTs of the outcomes in older patients The second problem is that EBM often clashes with clinical experience and does not take account of the context in which practice takes place It has been shown that the occurrence of an adverse event when treating a patient was one of the biggest barriers to following treatment suggested by published studies Doctors have always had the clinical freedom to make judgements about the best course for individual patients, balancing their knowledge of the patient, their clinical experience and the evidence from the literature Increasingly with the publication and dissemination of protocols and service frameworks the ability to balance experience with evidence-based practice is being eroded Thirdly there is the issue of the balance between the patient’s experience and understanding and that of the health service Clinicians are encouraged to respect the autonomy of patients and to take into account their views and experience when offering them treatment But often these conflict with evidence from the published literature Which should take precedence? Strengths and weaknesses of EBM EBM has a number of strengths and weaknesses Its practice requires basic skills in searching databases of the medical literature, skills in epidemiology to help appraise the relevant papers and skills in statistics to help interpret the results Strengths • Helps clinicians in their decision-making • Helps ensure consistency of care offered to patients • Develops skills in critical appraisal • Helps clinicians keep up to date • Helps in the development of evidence-based guidelines Weaknesses • Development of the evidence base has been biased • Ignores the benefit of clinical experience • Does not take into account patient choice • Can be time consuming and requires the acquisition of basic skills 87 PID11 5/21/04 11:37 AM Page 88 PID12 5/21/04 11:38 AM Page 89 Part Prevention and Control of Disease PID12 5/21/04 11:38 AM Page 90 PID12 5/21/04 11:38 AM Page 91 Chapter 12 General principles Introduction The health of a population depends both on the provision of health care for the sick and on public health services to promote health and prevent the spread of disease Until the middle of the 19th century the state accepted little responsibility for health The first attempts to improve public health in the UK involved legislation, beginning with the Nuisances Act of 1846, which aimed to remove sewage and offal from the streets This was followed by a series of further public health acts At the beginning of the 20th century the Government turned its attention to personal medical care David Lloyd George introduced insurance-based health care for workers in 1911 The majority of health care, however, was still obtained privately, or through friendly societies and charitable institutions The state took no major role as a provider of health care until the inception of the NHS in 1948, which promised access to free health services for all Today, the Secretary of State for Health is responsible to Parliament for the work of the Department of Health (DoH) whose aim is to improve the health and well-being of people in England (see DoH website at http://www.doh.gov.uk/) Separate arrangements are in place for Scotland, Wales and Northern Ireland Earlier generations tended to accept ill health and premature death as unavoidable hazards of human existence Over time people have come to expect a long and healthy life Nowadays if illness occurs it is assumed that modern medicine can or ought to be able to restore the sufferer to normal health These changed expectations have been brought about to a large extent by the publicity given to the more dramatic advances in medical knowledge and treatments and by the evident success of modern medicines in reducing mortality, particularly during infancy and childhood The public also feels a sense of ownership of the health service and expects ready access to it when needed Although it is true that during the past 50 years the scope and effectiveness of medical treatments have been extended greatly, it is also true that many of the diseases which commonly affect humans are self-limiting and that medical treatment does little to alter their natural course Furthermore, few of the diseases that result in death or major disability can be cured The main impact of modern medicine has tended to be to allow people to live longer and more comfortably with their diseases The public often fails to appreciate this For many of the major diseases, it is both logical and desirable to take steps where possible to prevent their occurrence Even if a treatment eventually becomes available, a strategy of prevention would usually be more cost effective in improving both public and personal health In future, it is hoped that medical research and practice will give greater attention to the means whereby health can be promoted and diseases prevented Historically, infec91 PID12 5/21/04 11:38 AM Page 92 Chapter 12 General principles tious diseases were the major causes of morbidity and mortality, particularly in children and young adults Their control over the past 150 years owes more to social and economic progress than it does to specific medical intervention Preventive programmes during this period have included such measures as improvements in sanitation, water supply, the quantity and quality of food and the quality of housing, safer conditions in the workplace and raised standards of personal hygiene All of these carry obvious and immediate benefits other than those purely related to health: they make life more comfortable and pleasant with little or no restriction on personal freedom Most of the changes were at community level and were the result of legislation rather than action by individuals This made them comparatively easy to institute By contrast, some of the more recent advances in the control and prevention of communicable diseases, such as the elimination of diphtheria and poliomyelitis in many countries and the worldwide eradication of smallpox, required mainly medical action (immunization) and thus can rightly be claimed as major medical achievements The benefits of environmental improvements, as well as of specific immunization, however, will be sustained only by continued vigilance Much modern preventive medicine is directed to this end In the past, the presence of a disease in the community served as a constant reminder of its nature and consequences In societies dependent upon distant memories of childhood infections such as measles, whooping cough, polio and tuberculosis, continuing public education is essential to sustain preventive activities With the exception of smallpox the causal organisms have not been eradicated from human populations Thus the diseases can recur The virtual elimination of the older lifethreatening infectious diseases has brought the non-infectious illnesses into greater prominence In modern times, despite the emergence of new infectious disease such as legionnaires’ disease, HIV and severe acute respiratory syndrome (SARS), it is cardiovascular disease, malignancies, degenerative conditions (such as arthritis) and other chronic illnesses which occur amongst older people that are the major health problems The prevention of many 92 of these diseases is more complicated than the control of infectious diseases and therefore progress is more difficult to achieve There has been a proportional increase in accidents as a cause of morbidity The problems of prevention of chronic diseases centre around their natural history, the difficulty in identifying aetiological agents and the fact that many have multiple causes Moreover, they are generally characterized by having a long latent period between exposure to the aetiological agent and the appearance of symptoms In many cases, the symptoms have an insidious onset and by the time they are of sufficient severity to cause the affected individual to seek medical attention, irreparable damage has been done Prevention of these diseases often depends on actions by the individual, rather than passively enjoying improvements in the environment brought about by the actions of others It demands modification of personal behaviour in such matters as the use of tobacco and alcohol, diet and exercise at a time in life when the risks of contracting the disease in question are seen as remote It is also a fact that, even for common diseases, the absolute risks for the individual are indeed relatively small In these circumstances, campaigns to persuade people to change their lifestyle require great skill and patience sustained over long periods of time These lifestyle changes also need to be complemented by public policies that promote health by, for example, the taxation of tobacco and alcohol products, the subsidizing of food production and the provision of public recreational facilities These all require a political will to be implemented Despite the difficulties, prevention remains an important aspiration and progress is being made in some of these diseases (e.g in reduction of cancer mortality), both by action at a political level and by persuading people to change their lifestyle and habits The interaction between the social and physical environment and health has also been much more widely recognized in the last 30 years by national and international bodies such as the World Health Organization (WHO) It has led to the concept of the promotion of a healthy environment and lifestyle being adopted in a number of cities Acknowledgement that employment, housing, bal- PID12 5/21/04 11:38 AM Page 93 General principles Chapter 12 anced diets and a social and economic environment that promotes health are all important in improving the quality of people’s lives and increasing the length of life has meant that both government and local policies which affect social factors have to take into account the long-term consequences to health Principles of prevention Disease is the result of a harmful interaction between the host (humans), a pathogenic agent and the environment (Fig 12.1) Agent, host and environment form a dynamic system in which, in the healthy individual, the balance normally favours the host Thus, if the agent is locally absent or contained, or its capacity to cause disease is matched by the host’s protective mechanisms, or the environment inhibits the spread of the agent, health is maintained Disease or injury occurs when the balance is disturbed, for example owing to changes in the pathogenicity of an agent, changes in environmental conditions that favour the survival and transmission of the agent to humans, or the breakdown or absence of human normal defence mechanisms The control and prevention of disease depends on effective intervention in the relationship between agent, host and environment to ensure that the balance remains in the human’s favour, or, if disease does occur, to ensure that its Affects presence and survival of agents Environmental conditions: Physical Biological Social Affects exposure of humans to agents Agent properties: Microbial Chemical Physical Psychological progress is rapidly arrested or reversed or its consequences minimized Useful preventive action does not necessarily require knowledge of the cause of a disease There are many examples of effective prevention that preceded discovery of the agent or complete understanding of the causal mechanism For example, in the 18th century, Lind (Fig 12.2) and Blane demonstrated that scurvy in the crews of ships could be prevented by the consumption of adequate amounts of citrus fruit; this was long before vitamin C was discovered In the 19th century John Snow (Fig 12.3) showed that cholera was transmitted by drinking water polluted by sewage His findings led to the elimination of cholera by the provision of pure water supplies many decades before the isolation of the causal organism In this century, Doll et al (see Chapter 5) demonstrated that those who stop smoking cigarettes substantially reduce their risk of contracting lung cancer, though the carcinogenic agent in tobacco smoke has yet to be identified In general, however, a full and accurate understanding of the causes Affects human capacity to resist diseases Human protective mechanism: Immunity natural (non-specific) acquired (specific) Behaviour Figure 12.1 Interactions of agent, host and environment, causing disease Figure 12.2 James Lind (1716–94) author of the treatise on scurvy 93 PID12 5/21/04 11:38 AM Page 94 Chapter 12 General principles Strategies related to the agent If the agent can be identified, it may be possible to remove or destroy it at source For example, by ceasing to use asbestos as an insulating material, the incidence of mesothelioma has been reduced; the control of bovine tuberculosis in humans was achieved by eradication of the disease from milking herds and pasteurization of milk Strategies related to the environment Figure 12.3 John Snow (1813–58) epidemiologist who studied the transmission of cholera of diseases and of the factors that determine the balance between agent, host and environment is helpful in order to construct appropriately directed preventive and control programmes Epidemiological studies are used to identify the causal agents and those elements in the environment or in people’s behaviour and personal characteristics that are key determinants of the natural history of disease Intervention strategies Based on the knowledge gained from epidemiological studies three main types of intervention strategy may be adopted Intervention strategies Strategies related to: • Agent • Environment • Humans 94 These include attention to general environmental factors such as standards of housing, nutrition, working conditions, water supplies, sewage disposal and the control of environmental pollution Environmental measures directed at the specific causes of individual diseases are also important and people may be protected from potentially injurious agents by the construction of barriers between them and the source of harm Examples of such measures include the prevention of transmission of food-borne infection by hygienic food production methods; elimination of vectors, for example action to prevent the spread of malaria or yellow fever by mosquito control; and the use of machine guards in industry to reduce the risk of accidents Strategies related to humans There are three strategies involving individuals • The enhancement of general or specific resistance to disease, i.e by improved nutrition or immunization • The modification of personal behaviour, i.e by encouraging people to adopt healthier lifestyles by not smoking, moderating alcohol intake, improving diet, avoiding obesity, exercising regularly, etc • The use of screening to detect predisposing conditions or the early stages of disease when action can be taken to prevent its onset or control its progress, for example tuberculin testing for tuberculosis, blood pressure measurement to identify hypertension, or mammography for breast cancer detection PID12 5/21/04 11:38 AM Page 95 General principles Chapter 12 Preventive action Action is usually classified as follows Action • Primary prevention: prevents disease starting • Secondary prevention: detects disease early • Tertiary prevention: damage limitation Primary prevention This aims to prevent a disease process from starting It often calls for strategies directed at the removal or destruction of agents but can also include environmental control, immunization, health promotion and health education Secondary prevention This aims to detect disease at the earliest possible stage and to institute measures to cure or prevent its further progression Screening programmes backed by effective interventions are the most important examples of secondary prevention Tertiary prevention This is concerned with ‘damage limitation’ in peo- ple with manifest disease by modifying continuing risk factors such as smoking and by the implementation of effective rehabilitation High-risk individual vs population strategy Where a choice of strategy exists, the planning of a preventive programme should take account of certain practical considerations The most desirable approach is one that gives the greatest benefit to the largest number of people In some instances, this may mean that the most effective strategy is to target high-risk individuals Such programmes, whilst of benefit to individuals, may little to reduce the overall burden of disease in a population Sometimes a population-based approach which confers a smaller benefit on a large number of individuals may yield greater dividends The population strategy has the advantage that there is no need to identify a high-risk group Everyone is targeted Interventions that are simple and require minimal cooperation from individuals are usually the most successful Economic factors must also be considered when deciding on the most appropriate intervention strategy Each of these strategies for prevention is considered in detail in the chapters that follow 95 ... 000 10 0 000 Under 50 000 Rural 11 8 11 2 11 4 99 84 11 7 87 12 6 15 1 10 9 10 9 11 0 10 6 98 10 9 95 11 0 99 98 99 88 10 7 90 98 98 10 0 95 90 96 94 10 1 11 6 96 99 89 71 79 82 85 95 11 1 76 12 4 67 56 analysis of data... Periodic changes 10 18 71 18 91 1 911 19 31 Years 19 51 19 71 Figure 3.2 Tuberculosis mortality in England and Wales, 18 71 19 71 (logarithmic scale) (Reproduced with permission from Prevention and Health:... Chemotherapy and BCG vaccination 600 400 200 18 55 18 75 18 95 19 15 19 35 19 55 19 65 Years Figure 3 .1 Tuberculosis mortality in England and Wales, 18 55 19 65 (arithmetic scale) 15 5/ 21/ 04 11 :25 AM Page 16 Chapter