There has never been in the history of medicine so many experiments on human beings as have been carried out in the attempts to discover the etiological factor in the recent pandemic of influenza. Davis has called attention to a successful human inoculation with pure cultures of B. influenzæ which he performed in 1906. During the present investigation at least 200 men have volunteered as experimental subjects, and the results of many different methods of attempting to transmit the disease, have been disappointing and inconclusive. I will not attempt to review the reports at present available, as a great deal of the work done has not yet appeared in print. The important point is that the results do not affect the various views held as to the causative agent in pandemic influenza nor the massive evidence for transmission of the disease under natural epidemic conditions.
It is my opinion, as expressed above, that practically all of the population are rapidly infected during such a pandemic as we have had. The resistant have escaped, and it would appear to be very difficult to break down this resistance. The human experiment carried out by Pettenkofer on himself and his assistantwith vibrion choleræ is an example, but we have numerous others demonstrating the same kind of phenomena in most of our diseases of established bacterial origin. In diphtheria we have an explanation in the varying antitoxic content of the sera, but we really know very little of what are the actual factors in preventing or determining infection among exposed individuals in the natural history of most diseases. The reports of Leonard Hill and Gregor are well worth reading in this connection, as well as the editorial in the same number of the British Medical Journal. We are not in a position to be very dogmatic on the causes of epidemics. The mere presence of the bacteria or any other living virus is not in itself sufficient to explain the phenomenon, and one of the chief objects of this paper is to indicate from the collected facts, that in the words of Flexner, “the case against the influenza bacillus is not proved.”
1. B. influenzæ is one of a group of hemophilic bacteria and there are probably strains of this organism which may be differentiated which will lead to further subdivisions of the group.
2. B. influenzæ as we understand it today, is distinguished by its morphological and staining characters; its requiring hemoglobin in some form for its development; its showing symbiotic reactions with other bacteria which stimulate its growth; the production of a toxine and its usual low pathogenicity for animals.
3. The media found most favorable for its growth are those containing blood with the hemoglobin content altered in certain ways, (1) by heating, (2) the addition of various chemicals, (3) by the action of other bacteria or their products. The heated blood agar I have found to be a most efficient and readily prepared medium.
4. Since B. influenzæ is so difficult to isolate, it is necessary to be very cautious in interpreting results unless the greatest effort has been made to demonstrate the presence of this organism.
5. B. influenzæ should be considered, from the evidence at hand, as the bacterial causative agent in epidemic influenza, and it should be recognized that secondary infections following theprimary attack by this organism are both frequent and important. This view I believe the logical one, unless much more convincing evidence than we have today may demonstrate another more probable living virus as the cause.
6. B. influenzæ is a frequent etiological factor in purulent and chronic bronchitis, broncho-pneumonia and other acute and chronic respiratory infections, in meningitis, endocarditis, sinusitis, conjunctivitis and other conditions, as well as in complications of many other diseases.
7. There are many carriers of the bacillus among our population, both in apparently normal individuals and in those suffering from chronic infections of bronchi, sinuses or other parts.
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.