- Email: [email protected]
Biological aspects of infectious disease
1941
PUBLIC HEALTH BOOK R E V I E W S
Biological Aspects of Infectious Disease. By F. M. BURNEr,M.D., Asst. Director, Walter and Eliza Hall Institute, Melbourne. London: Cambridge UniversityPress. 1940. Pp. iii+308. Price 15s. net. In 1883 the Island of Krakatoa in the East Indies was apparently completely sterilised of living organisms by intense volcanic activity, Within 60 years it has again become covered with dense tropical forest. First of all the lifeless volcanic ash was colonised by air-borne spores of bacteria, primitive plants, mosses and ferns; then in succession appeared grasses and shrubs from sea-borne seeds. Twenty-three years after the eruption a forest of coconuts, palms, fig-trees and others had developed along the shore and was gradually spreading inland. In another half-century, excluding further catastrophe, Krakatoa will be completely covered with tropical rain-forest. This succession of different types of vegetation is seen in all such denuded areas left to uncontrolled natural processes. In every case the succession goes on to a final state characteristic of the region, the " climax," determined mainly by the climate and nature of the soil. Whatever the nature of the climax, once reached it remains essentially constant and is likely to change only as a result of long period climatic conditions. A related series of changes in the animal population will occur and with the establishment of the climax state of vegetation, animal species will adjust themselves to an uneasy equilibrium. In a constant environment a reasonable approach to a stable balance will be maintained. Assuming that a biological approach gives a better starting point for the professional study of human infectious disease than a purely medical one, Dr. F. M. BurneR, in his " Biological Aspects of Infectious Disease" enlarges on his leading theme of infectious disease as a manifestation of the interaction of living things. While the development of an approximately balanced condition between contending species is a characteristic of the relation between host and parasites, most parasites are restricted to one host species, or at most, to a small number of related species; and the main problem that a parasitic species has to solve if it is to survive is to manage the transfer of its offspring from one individual host to another. By whatever method a parasite passes from host to host an increased density of susceptible population will facilitate its spread from infected to uninfected individuals. Beyond a limiting point the density of susceptibles will result in constant reinfection, more severe symptoms and a higher mortality. This leads to a restoration of balance to that density of population optimum for the survival of the species. In the case of man, city life appears essential to civilisation. While efficient sewerage and water supply, together with ordinary decent cleanliness, have rid us in temperate climes of typhoid, dysentery, cholera, plague, typhus and malaria, we have not been able--and perhaps never will be able--to block the spread of those diseases which are spread by droplet infection. If anything, the greater size and mobility of modern city populations has probably increased the opportunities
for the spread of these infections. Further than this, the extent and speed of modern travel between all civilised countries has more or less converted them into a single closed environment in the biological sense. The infectious diseases of Europe, North America, Australia and New Zealand are almost identical. There is now a real opportunity for those parasitic microbes, whose attack we cannot prevent, to reach the state of relative equilibrium postulated as the normal climax of interaction between organisms. For many diseases that seems to have been accomplished. The commoner diseases of childhood--measles, chickenpox, diphtheria, whooping-cough, and infantile paralysis, have their epidemic prevalence and kill their proportion of children; but the. proportion of deaths is small, and remains relativel); constant for each disease from decade to decade. ~he organisms responsible continue to survive, yet their activity rarely causes any perceptible interference with the general life of the community, however tragic the result in individual experiences. All these childhood diseases are followed by relative or complete immunity--they affect childhood precisely because they are followed by immunity. Measles is cited as an example of one manner in which man and an agent of infectious disease have become adapted to one another without serious damage to the host species and with steady survival of the virus species concerned. On introduction to a city or country the epidemic spreads progressively, 12 or 14 days between successive crops of infections, and, under ordinary city conditions, will not diminish greatly until a high proporton of susceptible children are infected. As each passes through his infection he becomes immune and the germ finds progressively greater difficulty in spreading. Eventually measles diseappears from the community-but in the meantime the virus is spreading through other communities.~ In about two years' time another crop of susceptible children has appeared and sooner or later the measles virus re-enters the community and the cycle is repeated. Dr. Burnet's view of infectious disease is that of a conflict between man and his parasites, which, in a constant environment, would tend to result in a virtual equilibrium, a climax state in which both species would survive indefinitely. Man, however, lives in an environment constantly being changed by his own activities, and few of his diseases have attained such an equilibrium. The evoIution of many of the commoner disease-producing bacteria has probably, in many cases, been by the over-stepping of their bounds, and by the effort to gain a living at the expense of the adjacent living cells of bacteria not actually growing in living tissues but in immediate contact with them. Among the commoner human diseases caused by bacteria, a great many are due to organisms identical with, or at least very closely related to, those found normally in these situations, e.g., the normal inhabitants of the throat are closely similar to the pathogenic bacteria which produce infections of the throat and adjacent tissues, while the organisms of typhoid fever and dysentery are of the B . coil stock. These are examples of bacteria which normally, being mere scavengers, have developed a capacity to attack living tissues adjacent to their normal habitat. Once 63
PUBLIC HEALTH having developed such powers they may extend their harmful activity to other parts of the body, but most commonly there is a marked predilection for one site of attack. Some bacteria specialise on a parasitic existence, and, in general, the more a bacterium becomes specialised for a parasitic existence, the more infectious is the disease which results from its activity. As a rule it is found that the bacteria responsible for the infectious diseases are more difficult to grow on artificial media than the less specialised ones like the staphylococcus or the colon bacillus. Certain specialised bacilli have so concentrated on a parasitic existence that they can be found only in diseased tissues. Of the four causing human disease, namely, the tubercle bacillus, Bacillus leprae, Gonococcus, and Sp. pallida, two cannot be made to grow on any artificial medium. Where two organisms have developed a host-parasite relationship the survival of the parasite species is best served not by the destruction of the host but by the developments of a balanced condition in which sufficient of the substance of the host is consumed to allow the parasite's growth and multiplication, but not sufficient to kill the host. Long established endemic diseases are, in normal circumstances, mild diseases of low death rate. If any undue advantage is given to the parasite, such as under-nutrition or over-crowding of the host species, severe and fatal outbreaks may occur which, by diminishing the numbers of the host wilt tend to rectify the circumstances which gave rise to them. If both host and parasite are to survive, a mild, rather long-lasting infection which does no serious damage to the host and provides adequate opportunity for the parasite to be transfeCred to other hosts is the most advantageous relationship for both. Such a relationship is established by mutual adaptation of the two species concerned. Highly fatal diseases in nature nearly always represent the effect of first contact with a micro-organism which has developed its parasitic habit in some different host. On introduction into a community a new disease will avoid some, though many escape with a trivial illness or no apparent sickness at all. While infants may be highly susceptible, children are less affected than adults, and young adults of from 15 to 35 years provide the greatest number of cases, and usually the highest fatality rate. Having passed through the epidemic stage it will smoulder through the country. A new susceptible population of young children gradually accumulates amongst whom the next outbreak of the disease occurs. As the years go on, the new virus, unless it completely dies out, takes its place amongst the epidemic diseases of the country, producing a crop of infections amongst young children every year or two, and producing a definite illness in an occasional adult. The greater the ease with which the infection spreads the younger will be the age of maximal incidence. The converse is probably true, namely, that any infection with a maximal incidence in infants or young children is widely endemic in the community. Space does not permit of reference to any more of these generalisations made by Dr. Burnet and his application of them to determine whether any outbreak is that of a new or old infection. His hypothesis 64
JANUARY regarding the influenza epidemic of 1918, which must be the last extract from this most fascinating book, is that of the development of an antigenieally new strain of organism which therefore resulted in what was, in effect, a new disease. During the years before 1918 the influenza virus was prevalent in various parts of the world. Most individuals were being infected from time to time and as a result developed a considerable degree of immunity against strains whose antigenic structure resembled those which had infected them. As a result of the war, men from every country of the globe were crowded into Europe under conditions highly favourable to the spread of respiratory infections. There was every opportunity for any type of new variatio~ in the influenza virus, which might have appeared in Europe or elsewhere, to make itself felt. Somewhere a strain appeared which was almost entirely different antigenically from any which had been current previously, and to which, therefore, the whole human race was susceptible, so that influenza became a new disease. Simply as a result of the exuberance of its multiplication it gave rise to further antigenic types differing more or less from the first new form, with the result that influenza appeared in successive waves, each of which failed to immunise against the succeeding ones.
Practical Child Psychotherapy. By Dr. CURTBO~NHEIM with a foreword by Prof. H. FINKELSTEIN. London John Bale Medical Publications, Ltd. 1939. pp. 177. Price 10s. 6d. net. In this short work the author has sought to embody the results of his ten years' experience as a psychotherapist at the Clinic for Nervous and Difficult Children at the Kaiser and Kaiserin Friedrich Kinderkrankenhaus at Berlin. Though not entirely a personal commentary, the book is to a certain extent an exposition of the author's views regarding treatment, and it is therefore not very remarkable that, just as he admits that in the earlier days of psychotherapy enthusiasm for the young science outran discretion in its use, in his exposition he is perhaps somewhat optimistic regarding the number of conditions for which this treatment may be used beneficially. It is obvious that the work is intended largely for the general practitioner. Dr. Boenheim divides psychotherapy broadly into " major" and " minor" degrees, and he contends that " minor" psychotherapy can generally be applied by the patient's practitioner, while the" major" degree is in the province of the practising psychotherapist. This being the case, it might be thought that the author would make a clear differentiation between treatment which may be classified as of " major " or " minor" degrees respectively, but this he seldom does, and the inference is that most cases should be dealt with by the practitioner in the first instance. The book is divided into a general part, which deals historically with the present position of psychotherapy and with the indications for its use, and a special part, which discusses disorders of the alimentary, urinary, and sexual organs, nervous habits, motor disorders, and other syndromes in which psychotherapy has been found helpful. The general part is rather concise, but the author does manage to convey a good idea of the
PUBLIC HEALTH BOOK R E V I E W S
Biological Aspects of Infectious Disease. By F. M. BURNEr,M.D., Asst. Director, Walter and Eliza Hall Institute, Melbourne. London: Cambridge UniversityPress. 1940. Pp. iii+308. Price 15s. net. In 1883 the Island of Krakatoa in the East Indies was apparently completely sterilised of living organisms by intense volcanic activity, Within 60 years it has again become covered with dense tropical forest. First of all the lifeless volcanic ash was colonised by air-borne spores of bacteria, primitive plants, mosses and ferns; then in succession appeared grasses and shrubs from sea-borne seeds. Twenty-three years after the eruption a forest of coconuts, palms, fig-trees and others had developed along the shore and was gradually spreading inland. In another half-century, excluding further catastrophe, Krakatoa will be completely covered with tropical rain-forest. This succession of different types of vegetation is seen in all such denuded areas left to uncontrolled natural processes. In every case the succession goes on to a final state characteristic of the region, the " climax," determined mainly by the climate and nature of the soil. Whatever the nature of the climax, once reached it remains essentially constant and is likely to change only as a result of long period climatic conditions. A related series of changes in the animal population will occur and with the establishment of the climax state of vegetation, animal species will adjust themselves to an uneasy equilibrium. In a constant environment a reasonable approach to a stable balance will be maintained. Assuming that a biological approach gives a better starting point for the professional study of human infectious disease than a purely medical one, Dr. F. M. BurneR, in his " Biological Aspects of Infectious Disease" enlarges on his leading theme of infectious disease as a manifestation of the interaction of living things. While the development of an approximately balanced condition between contending species is a characteristic of the relation between host and parasites, most parasites are restricted to one host species, or at most, to a small number of related species; and the main problem that a parasitic species has to solve if it is to survive is to manage the transfer of its offspring from one individual host to another. By whatever method a parasite passes from host to host an increased density of susceptible population will facilitate its spread from infected to uninfected individuals. Beyond a limiting point the density of susceptibles will result in constant reinfection, more severe symptoms and a higher mortality. This leads to a restoration of balance to that density of population optimum for the survival of the species. In the case of man, city life appears essential to civilisation. While efficient sewerage and water supply, together with ordinary decent cleanliness, have rid us in temperate climes of typhoid, dysentery, cholera, plague, typhus and malaria, we have not been able--and perhaps never will be able--to block the spread of those diseases which are spread by droplet infection. If anything, the greater size and mobility of modern city populations has probably increased the opportunities
for the spread of these infections. Further than this, the extent and speed of modern travel between all civilised countries has more or less converted them into a single closed environment in the biological sense. The infectious diseases of Europe, North America, Australia and New Zealand are almost identical. There is now a real opportunity for those parasitic microbes, whose attack we cannot prevent, to reach the state of relative equilibrium postulated as the normal climax of interaction between organisms. For many diseases that seems to have been accomplished. The commoner diseases of childhood--measles, chickenpox, diphtheria, whooping-cough, and infantile paralysis, have their epidemic prevalence and kill their proportion of children; but the. proportion of deaths is small, and remains relativel); constant for each disease from decade to decade. ~he organisms responsible continue to survive, yet their activity rarely causes any perceptible interference with the general life of the community, however tragic the result in individual experiences. All these childhood diseases are followed by relative or complete immunity--they affect childhood precisely because they are followed by immunity. Measles is cited as an example of one manner in which man and an agent of infectious disease have become adapted to one another without serious damage to the host species and with steady survival of the virus species concerned. On introduction to a city or country the epidemic spreads progressively, 12 or 14 days between successive crops of infections, and, under ordinary city conditions, will not diminish greatly until a high proporton of susceptible children are infected. As each passes through his infection he becomes immune and the germ finds progressively greater difficulty in spreading. Eventually measles diseappears from the community-but in the meantime the virus is spreading through other communities.~ In about two years' time another crop of susceptible children has appeared and sooner or later the measles virus re-enters the community and the cycle is repeated. Dr. Burnet's view of infectious disease is that of a conflict between man and his parasites, which, in a constant environment, would tend to result in a virtual equilibrium, a climax state in which both species would survive indefinitely. Man, however, lives in an environment constantly being changed by his own activities, and few of his diseases have attained such an equilibrium. The evoIution of many of the commoner disease-producing bacteria has probably, in many cases, been by the over-stepping of their bounds, and by the effort to gain a living at the expense of the adjacent living cells of bacteria not actually growing in living tissues but in immediate contact with them. Among the commoner human diseases caused by bacteria, a great many are due to organisms identical with, or at least very closely related to, those found normally in these situations, e.g., the normal inhabitants of the throat are closely similar to the pathogenic bacteria which produce infections of the throat and adjacent tissues, while the organisms of typhoid fever and dysentery are of the B . coil stock. These are examples of bacteria which normally, being mere scavengers, have developed a capacity to attack living tissues adjacent to their normal habitat. Once 63
PUBLIC HEALTH having developed such powers they may extend their harmful activity to other parts of the body, but most commonly there is a marked predilection for one site of attack. Some bacteria specialise on a parasitic existence, and, in general, the more a bacterium becomes specialised for a parasitic existence, the more infectious is the disease which results from its activity. As a rule it is found that the bacteria responsible for the infectious diseases are more difficult to grow on artificial media than the less specialised ones like the staphylococcus or the colon bacillus. Certain specialised bacilli have so concentrated on a parasitic existence that they can be found only in diseased tissues. Of the four causing human disease, namely, the tubercle bacillus, Bacillus leprae, Gonococcus, and Sp. pallida, two cannot be made to grow on any artificial medium. Where two organisms have developed a host-parasite relationship the survival of the parasite species is best served not by the destruction of the host but by the developments of a balanced condition in which sufficient of the substance of the host is consumed to allow the parasite's growth and multiplication, but not sufficient to kill the host. Long established endemic diseases are, in normal circumstances, mild diseases of low death rate. If any undue advantage is given to the parasite, such as under-nutrition or over-crowding of the host species, severe and fatal outbreaks may occur which, by diminishing the numbers of the host wilt tend to rectify the circumstances which gave rise to them. If both host and parasite are to survive, a mild, rather long-lasting infection which does no serious damage to the host and provides adequate opportunity for the parasite to be transfeCred to other hosts is the most advantageous relationship for both. Such a relationship is established by mutual adaptation of the two species concerned. Highly fatal diseases in nature nearly always represent the effect of first contact with a micro-organism which has developed its parasitic habit in some different host. On introduction into a community a new disease will avoid some, though many escape with a trivial illness or no apparent sickness at all. While infants may be highly susceptible, children are less affected than adults, and young adults of from 15 to 35 years provide the greatest number of cases, and usually the highest fatality rate. Having passed through the epidemic stage it will smoulder through the country. A new susceptible population of young children gradually accumulates amongst whom the next outbreak of the disease occurs. As the years go on, the new virus, unless it completely dies out, takes its place amongst the epidemic diseases of the country, producing a crop of infections amongst young children every year or two, and producing a definite illness in an occasional adult. The greater the ease with which the infection spreads the younger will be the age of maximal incidence. The converse is probably true, namely, that any infection with a maximal incidence in infants or young children is widely endemic in the community. Space does not permit of reference to any more of these generalisations made by Dr. Burnet and his application of them to determine whether any outbreak is that of a new or old infection. His hypothesis 64
JANUARY regarding the influenza epidemic of 1918, which must be the last extract from this most fascinating book, is that of the development of an antigenieally new strain of organism which therefore resulted in what was, in effect, a new disease. During the years before 1918 the influenza virus was prevalent in various parts of the world. Most individuals were being infected from time to time and as a result developed a considerable degree of immunity against strains whose antigenic structure resembled those which had infected them. As a result of the war, men from every country of the globe were crowded into Europe under conditions highly favourable to the spread of respiratory infections. There was every opportunity for any type of new variatio~ in the influenza virus, which might have appeared in Europe or elsewhere, to make itself felt. Somewhere a strain appeared which was almost entirely different antigenically from any which had been current previously, and to which, therefore, the whole human race was susceptible, so that influenza became a new disease. Simply as a result of the exuberance of its multiplication it gave rise to further antigenic types differing more or less from the first new form, with the result that influenza appeared in successive waves, each of which failed to immunise against the succeeding ones.
Practical Child Psychotherapy. By Dr. CURTBO~NHEIM with a foreword by Prof. H. FINKELSTEIN. London John Bale Medical Publications, Ltd. 1939. pp. 177. Price 10s. 6d. net. In this short work the author has sought to embody the results of his ten years' experience as a psychotherapist at the Clinic for Nervous and Difficult Children at the Kaiser and Kaiserin Friedrich Kinderkrankenhaus at Berlin. Though not entirely a personal commentary, the book is to a certain extent an exposition of the author's views regarding treatment, and it is therefore not very remarkable that, just as he admits that in the earlier days of psychotherapy enthusiasm for the young science outran discretion in its use, in his exposition he is perhaps somewhat optimistic regarding the number of conditions for which this treatment may be used beneficially. It is obvious that the work is intended largely for the general practitioner. Dr. Boenheim divides psychotherapy broadly into " major" and " minor" degrees, and he contends that " minor" psychotherapy can generally be applied by the patient's practitioner, while the" major" degree is in the province of the practising psychotherapist. This being the case, it might be thought that the author would make a clear differentiation between treatment which may be classified as of " major " or " minor" degrees respectively, but this he seldom does, and the inference is that most cases should be dealt with by the practitioner in the first instance. The book is divided into a general part, which deals historically with the present position of psychotherapy and with the indications for its use, and a special part, which discusses disorders of the alimentary, urinary, and sexual organs, nervous habits, motor disorders, and other syndromes in which psychotherapy has been found helpful. The general part is rather concise, but the author does manage to convey a good idea of the