8. The problem of what constitutes resistance or susceptibility to this infection are as far from solution as they are in most other respiratory diseases, and the attempts to explain the reasons for epidemics have been as futile as they are for meningitis and many other respiratory epidemics.
9. It would not appear that the immunological reaction against this infection has been discovered, but the possibility of its being of an antitoxic nature opens an interesting field for investigation.
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THE PATHOLOGY OF EPIDEMIC INFLUENZA
By Oskar Klotz, M. D., C. M.
The discussion to be entered into in this report will be limited to an experience dealing with epidemic influenza as it was met with in the emergency Military Hospital in Pittsburgh. We shall largely confine our attention to the observations which came directly under our supervision, and in as much as this investigation was continued during the epidemic as it swept over this district, the intensive study was limited to a time period of about five weeks. During this period much material was collected, which since then, has taken us a considerable time to analyze. We have thought it more valuable to restrict our discussion to this material in that it illustrates the pathological lesions as they occurred during the acute stage of the disease. We have not entered upon a discussion of the sequelæ or the chronic lesions which are not uncommonly found following in the wake of an acute epidemic nor do we deal with the lesions arising in cases of sporadic influenza, such as are always with us. As is so well illustrated in the literature, there is probably no disease which has so many late complications and sequelæ as influenza. The investigations upon the protean lesions have been fully reported in numerous papers during the intervals between epidemics. A comprehensive bibliography upon influenza will be found at the end of the extensive report by Leichtenstern (1905). There is very much less accurate information available upon the actual lesions present during the acute disease when present in epidemic or pandemic form, than upon the many clinical complications in various systems and organs. In fact, our knowledge of the pathology of influenza lies more largely in the field of associated lesions such as the late events in the bronchi, the sinuses of the head, abscesses, meningitis and other conditions, rather to be viewed as complications than as portions of the disease. There are relatively few thorough pathological analyses of the influenza lesions as they are found in the acute epidemic disease.
A fair literature has already appeared upon epidemic influenza from the many countries and regions over which the present pandemic (1918) has swept. These reports by various authors are offered from different viewpoints, some investigators being impressed with certain features which they bring into marked prominence in their reports. It thus happens that up to the present there is a decided lack of uniformity in the opinions expressed upon different phases of the subject. The nature of the pathology of the past epidemic has given rise to many expressions of opinion as well as dogmatic statements, which are found to differ from those of others. It seems to us that this apparent confusion arises partly through the somewhat different characteristics of the disease as it has made its appearance in different centers. We hear it repeatedly stated that the types found in different military camps and urban communities were quite unlike those of other regions. It is evident that such differences in the clinical course actually did exist and that the epidemic though having a common foundation upon which the disease process was built differed in what might be looked upon as symbiotic complications during the early and acute stages. Differences in the nature of the findings in various communities also probably lay in the fact that the bacterial flora associated with the causative agent of influenza was quite different in different regions. We mention this here so that a full appreciation will be obtained for the differences in the pathological characters of the disease as they are found in one region or another. We appreciate, of course, that if the concomitant bacterial flora associated with the underlying cause of influenza, differs in different regions, so, too, will the bodily reactions differ within certain degrees. We are becoming more familiar with different types of bacteria, and the resulting inflammatory reaction which is often unique or at least particular, and that not uncommonly the nature of the inflammatory process suggests the type of bacterium involved. This argument, of course, must not be driven too far, for we well know that the same micro-organisms under different conditions can cause types of inflammatory reactions wholly divergent.
In as much as our observations are confined to a particular group of cases and the study of these was undertaken during the five weeks of the acute epidemic, these results are not to be compared with the collected statistics on influenza as they shall be made over a period beginning with the onset of the epidemic and ending with the last vestiges remaining after months or it may be years of time. Our observations are to be considered only in the light of the events taking place during the height of an epidemic wave. In as much as influenza presents itself during an epidemic in different forms, we shall again mainly limit the report upon our investigations of those cases having respiratory lesions. Our acute observations were made upon the tissues of those who had died of this disease. It is impossible, or nearly so, to fully study the tissues of those with lesser lesions and who recover. Hence, if we divide the influenza cases into those (1) without pulmonary lesions and (2) those with pulmonary lesions, we must state that all of our cases coming to autopsy fall in the second group. It is true that one of these having pulmonary lesions was not brought to his fatal termination by them but by a septicæmia arising in the middle ear. He had distinct lesions in his lungs. In other words, our autopsy material represents epidemic influenza in which the lung was definitely involved in an inflammatory state. In all but one of these the pulmonary lesion was the cause of death.
No doubt, if opportunity had presented itself to follow a large epidemic through months of its progress, during which late complications in various portions of the body would make their appearance, our analysis would give a different picture and the pulmonary factor for the fatal termination would not be in such prominence.