Before the discovery of “viruses,” people believed that disease was caused by bacteria. There were rumblings and murmurs of invisible pathogens hidden within the fluids but most experiments were focused on cultivating bacteria from a diseased host and explaining pathogenesis based on the findings. This was no different for the flu. In 1892, German bacteriologist Richard Pfeiffer isolated a bacteria he believed was the causative agent of influenza. His discovery of this bacteria, later coined H. influenzae, and its association with the symptoms of disease known as the flu, became the prevailing causative theory for quite some time. However, as is the case with bacteria, numerous experiments showed Pfeiffers Bacillus could not always produce disease, could not be found in all cases of disease, and could not be transmitted from human-to-human. It bacame apparent that Pfeiffer’s bacillus was a dud.

https://viroliegy.com/2021/10/03/the-infectious-myth-busted-part-1-the-rosenau-spanish-flu-experiments-1918/

In 1931, a physician from Iowa named Richard Shope studied a condition in pigs said to be clinically and pathologically similar to the human flu. He had isolated a bacteria considered morphologically identical to Pfeiffer’s bacillus from the sickened pigs but he could not produce disease through experimentation with it by intranasal inoculation. This led him to attempt to filter a “virus” from the remaining fluids to see if it alone could cause disease. Below is a brief summary of the introductory history of influenza with Pfeiffer’s bacillus followed by Shope’s experiments:

Influenza: exposing the true killer

Abstract

“In the early 1930s, Richard Shope isolated influenza virus from infected pigs. Shope’s finding was quickly followed by the isolation of the influenza virus from humans, proving that a virus—not a bacterium, as was widely believed—caused influenza.

In 1892, German bacteriologist Richard Pfeiffer isolated what he thought was the causative agent of influenza. The culprit, according to Pfeiffer, was a small rod-shaped bacterium that he isolated from the noses of flu-infected patients (1). He dubbed it Bacillus influenzae (or Pfeiffer’s bacillus). Few doubted the validity of this discovery, in large part because bacteria had been shown to cause other human diseases, including anthrax, cholera, and plague.

The filtration question

When history’s deadliest influenza pandemic began in 1918, most scientists believed that Pfeiffer’s bacillus caused influenza. With the lethality of this outbreak (which killed an estimated 20 to 100 million worldwide) came urgency—researchers around the world began to search for Pfeiffer’s bacillus in patients, hoping to develop antisera and vaccines that would protect against infection. In many patients, but not all, the bacteria were found. Failures to isolate B. influenzae (now known as Haemophilus influenzae) were largely chalked up to inadequate technique, as the bacteria were notoriously difficult to culture (2).

The first potential blow to Pfeiffer’s theory came from Peter Olitsky and Frederick Gates at The Rockefeller Institute. Olitsky and Gates took nasal secretions from patients infected with the 1918 flu and passed them through Berkefeld filters, which exclude bacteria. The infectious agent—which caused lung disease in rabbits—passed through the filter, suggesting that it was not a bacterium (3, 4). Although the duo had perhaps isolated the influenza virus (which they nevertheless referred to as an atypical bacterium called Bacterium pneumosintes), other researchers could not reproduce their results. One of the doubters was Oswald Avery (Rockefeller Institute), who developed a culture media—chocolate agar—that optimized the growing conditions for B. influenzae and thus minimized false negative results from patient samples. Thus, the idea that flu was transmitted by a filterable agent (or virus) was dismissed.

Insights from pigs

Olitsky and Gates would not be vindicated until a decade later, when Shope—a young physician from Iowa then working on hog cholera at the Rockefeller Institute—turned his attention to swine influenza.

Pig farmers in Iowa had reported two outbreaks—one in 1918 and another in 1929—of a highly contagious, influenza-like disease among their animals. The disease bore such a remarkable resemblance to human flu that it was named swine influenza. Shope and his mentor Paul Lewis took mucus and lung samples from the infected pigs and attempted to isolate the disease-causing agent. They quickly isolated a bacterium that looked exactly like Pfeiffer’s human bacterium (and was thus called B. influenzae suis), but when they injected the bacteria into pigs, it caused no disease (5).

Shope then filtered the samples and, like Olitsky and Gates, found that the filtrate contained the infectious agent. Shope’s filtrate caused a highly contagious, influenza-like disease in pigs—albeit a more mild one than seen in naturally-infected pigs. Mixing the filtrate with the bacterium reproduced the severe disease. He concluded—correctly—that the filterable agent caused the infection, which then facilitated secondary infection with the bacterium (6). Shope published his results in a series of papers in The Journal of Experimental Medicine (5, 6).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2118275/

Shope’s experiments with influenza in pigs lent credence to the idea that an invisible filtrable “virus” was present in diseased fluids. However, he did not believe the “virus” nor the bacteria alone were capable of producing disease. He believed that they worked in concert together by some unknown mechanism. Presented below are highlights from his paper:

SWINE INFLUENZA IlI. FILTRATION EXPERIMENTS AND ETIOLOGY

McBryde, Niles, and Moskey (1) made five attempts to pass the etiological agent of swine influenza through small Berkefeld or Mandler filters but were unable to reproduce the disease by dropping such filtrates into the nostrils of normal hogs. In two of these experiments unfiltered material failed to produce the disease in their control animals. Although these experiments are too few in number to be conclusive, they indicate that the causative agent of swine influenza is not a filtrable virus.

EXPERIMENTAL

Since the studies in filtration reported by McBryde and his coworkers were not conclusive, the question of the filtrability of the etiological agent of swine influenza has been reconsidered. The results of ten filtration experiments with infectious material from the two strains of the disease obtained in 1928 and the two secured in 1930 (2) were inconstant and confusing. In these preliminary experiments the writer was not then cognizant of the possible etiological relationship existing between 1t. influenzae suis (3) and a filtrable agent to be described.

Material for filtration was prepared as follows.

Diseased lung and bronchial lymph nodes were minced with sterile scissors and added to bronchial exudate. This mixture was ground with sand in a mortar. When it had been reduced to a pasty and fairly homogeneous consistency, a 10 to 20 per cent suspension was made by gradually adding sterile distilled water or infusion broth (pH 7.3). It was then shaken with glass beads in a flask for l0 to 15 minutes and centrifuged. The supernatant fluid was removed by pipette and if more than moderately turbid it was centrifuged again. 24 hour bouillon cultures of B. prodigiosus were used to test the efficiency of all filters, and filtrates cultured in 1 cc. amounts on plain agar slants containing defibrillated blood were incubated at 37°C. for 48 to 72 hours before examination for growth.

Swine receiving filtered material were placed in carefully sterilized isolation
units where isolation precautions were taken. In certain experiments the control animal receiving unfiltered material and those receiving sterile filtrates were from the same source or even from the same litter.

Of the ten preliminary filtration experiments, three were at the time interpreted as negative, while in the remaining seven some evidence was obtained that the injected filtrate had contained an infectious agent. The disease induced by this filtrable infectious agent, however, was definitely not typical swine influenza and will be referred to hereafter as “filtrate disease.”

Clinically the filtrate disease was much milder than swine influenza. In most cases there was no elevation in temperature, while in a few a fever temperature for 1 day was observed. This was at marked variance with the 4 to 6 day fevers seen in typical swine influenza. The usual symptoms shown by filtrate-inoculated swine were a moderate and transient apathy, some diminution in appetite for a period not exceeding 3 days, occasionally a slight cough, and, as in typical swine influenza (2), a moderate or quite marked leucopenia. The extreme prostration so common in swine influenzal infections was not seen. In some instances the disease was so mild that it almost escaped recognition altogether. On this account and in the light of experiments to be outlined later in this paper, it seems possible that in the three preliminary experiments considered as negative, infections were actually produced but so mild in character that they escaped recognition.

The lesions exhibited at autopsy were similar in kind but different in extent, as
a rule, from those encountered in typical uncomplicated swine influenza (2).”

“The filtration experiments just outlined indicated that infectious material from experimental cases of swine influenza contains an agent capable of passage through Berkefeld filters V and N and possessing pathogenic properties when administered intranasally.

H. influemae suis, which was constantly encountered in culturing the respiratory tracts of animals with typical swine influenzal infections (3), was not found in similar cultures from animals with the
mild filtrate disease.

Anaerobic cultures of seven filtrates of swine influenza infectious material in blood broth and in 5 per cent serum bouillon over sterile rabbit kidney have failed to show growth. Four of the seven filtrates thus cultured were tested by intranasal inoculation into swine and all were found capable of inducing the mild filtrate disease.

Intranasal Inoculations with Mixtures of the Filtrable Agent and H. influemae suis

Since the only constant difference bacteriologically between the mild disease induced by the filtrable agent and typical spontaneous or experimental swine influenza lies in the absence of H. influenme suis in the filtrate-infected swine, the combination of the organism and the filtrable agent may be essential for the production of the natural disease. Experiments were conducted in which swine were inoculated intranasally with cultures of H. influenzae suis, which had been under cultivation for a long time (over 2 years in most instances), mixed with Berkefeld filtrates of infectious material from experimental cases of swine influenza.

In these experiments the isolation and filtration practice outlined above was followed. The cultures of H. influemae suis used were grown in defibrinated horse blood at the bases of plain agar slants in most instances for 24 hours. The undiluted blood culture was used in the inoculations and in all experiments the culture injected alone was identical with that mixed with filtrate before injection. The Berkefeld filtrate mixed with cultures of H. influemae suis was identical with that injected alone in individual experiments. With the exception of the first experiment, all animals used in individual experiments were from the same source and in most instances litter mates. The results of these experiments are recorded in Table I.

All eight of the swine infected by inoculation with Berkefeld filtered
infectious material or by contact with filtrate-infected swine developed only a mild disease. In some instances it was so slight as almost to escape recognition. None of the animals exhibited a febrile reaction. In 1 to 3 days after inoculation they appeared listless and apathetic for 2 or 3 days and there was some diminution in appetite. Those coming to autopsy showed enlarged and edematous cervical and bronchial lymph nodes, a small amount of tenacious mucous exudate in some of the smaller bronchi, and a scant scattered type of pulmonary atelectasis of one or more of the upper lobes of the lung.

The swine which were inoculated intranasally with pure cultures of H. influenzae suis were completely negative both clinically and at autopsy.

All the swine which received mixtures of the filtrable agent and H. influenzae suis developed a disease that was typical both clinically and at autopsy of swine influenza (4). Of the seven hogs infected either by direct inoculation with the filtrate-culture mixture or by contact with swine so infected, three developed typical swine influenza, and the disease was of about the same severity as that which developed in the control animals inoculated with unfiltered infectious material. Two others had a mild influenza, but in this instance the disease which developed in the control animal was also atypically mild. The remaining two swine developed exceptionally severe swine influenza and at autopsy both exhibited typical pneumonia. These two animals showed a more severe type of infection than did their controls infected by unfiltered material.”

Storage of Infectious Material

Experiments to test the keeping qualities of the agents of swine influenza have been complicated by differences in the period of survival of the two components. Pieces of atelectatic lung and bronchial lymph nodes from one experimentally infected swine were stored for 15 to 33 days and from another swine for 15 and 41 days in 50 per cent glycerol before testing them for infectivity. They have been found capable of inducing only the mild filtrate disease typical in its course and at autopsy. With one exception H. influenzae suis has not been demonstrable in cultures from the respiratory tract of swine infected with this material. With infectious material frozen and dried by Swift’s method (5) the disease induced by stored material was some- what different. Material that had been stored for 34 days proved capable of inducing only the filtrate type of disease when inoculated into two susceptible swine and H. influenzae suis was not found in the respiratory tracts of these two animals at autopsy. However, another tube of this same material tested after 54 days’ storage proved capable of inducing typical and rather severe swine influenza in which at autopsy H. influenzae suis was found in both the bronchial exudate and the atelectatic lung. It appears that the swine influenza virus is capable of storage in a dried state or in glycerol for at least 54 or 41 days, respectively, but that the bacterial component of the mixture is less resistant to such storage. The irregularity in the results obtained with dried infectious material may have been due to faulty freezing or drying of the particular tubes of dried material tested after 34 days’ storage, for it is difficult to understand why H. influenzae suis should not survive freezing and drying. If it were desirable to preserve both factors the virus could be maintained in a dried state or in
glycerol, while the organism could be kept under cultivation on artificial media and the two mixed before inoculation.

DISCUSSION

In a series of preliminary experiments to determine the pathogenic properties of bacteriologically sterile Berkefeld filtrates of infectious material from experimental cases of swine influenza, it has been possible in most cases to induce a definite but mild illness by the intranasal inoculation of swine with such filtrates. The disease thus induced has been transmissible by contact without altering its clinical or pathological characteristics. The possibility was thus eliminated that the filtrate disease is the result of inoculation with a toxic bacterial or tissue end-product or aggressin incapable of self-propagation in series. The observation that the illness which developed in animals exposed to cases of the filtrate disease does not differ in its clinical or pathological characteristics from that resulting from direct inoculation with the filtrable agent indicates that the mildness of the filtrate disease is not due to dilution of the inciting agent during filtration. Since the filtrate-induced disease has consistently been at variance with typical swine influenza, it was obvious that the disease induced by a filter-passing virus could not rightly be considered swine influenza. The impression gained after consideration of a series of these mild infections was that the disease both clinically and pathologically represented natural swine influenza in an incomplete form.

In the preceding paper (3) it was shown that a hemophilic bacillus, H. influenzae suis, was constantly demonstrable in the respiratory tracts of swine ill with influenza. It has been consistently absent from the respiratory tracts of swine ill with the filtrate disease. To test the possibility that swine influenza is the result of the two agents acting together, swine were inoculated intranasally with mixtures of the filtrable agent and H. influenzae suis. A disease typical of swine influenza in all clinical and pathological respects and indistinguishable from that induced by unfiltered infectious material resulted in all instances. Control animals receiving cultures of H. influenzae suis alone developed no evidence of illness and swine receiving the filtrable agent alone developed the mild filtrate disease. It seems permissible to interpret these experiments as indicating that swine influenza is due to a filtrable virus and H. influenzae suis acting together. Their mode of action is unknown although two possibilities are obvious:

The first possibility is that the pathological activities of the virus are such as to create a portal of entry for H influenzae suis and to furnish a favorable medium in which it can multiply. Under such an assumption the virus serves merely as an entering wedge for the organism and the latter determines the clinical picture and pathology. There can be little doubt from the data presented in this and the preceding paper (3) that, in the presence of the swine influenza virus, H. influenzae suis possesses invasive powers which it completely lacks when administered alone.

The second possibility is that the virus is the important component in contributing to the pathology and perhaps also to the symptoms characterizing the clinical picture, and that H. influenzae suis increases to a marked degree the pathogenic properties of the virus and hence the severity of the resulting disease. In this respect, the influence of H. influenzae suis on the pathogenic properties of the swine influenza virus suggests the effect of certain tissue extracts on various viruses pointed out first by Duran-Reynals (6) and later amplified by Hofcman (7).

Whatever the relation of the virus and the organism in respect to the disease, the data presented indicate that they act together.

The hypothesis is not new that a disease may be induced by a bacterium and an invisible agent, not readily demonstrable alone. It applies most directly to diseases in which the suspected bacterial agent, while readily and uniformly isolated from cases of the disease, either is completely incapable of reproducing the infection or very rapidly loses its ability to do so under conditions of artificial cultivation. It is possible that such organisms do not become non-pathogenic because of rapid loss of virulence but because of the absence of an invisible inciting agent. The proven association of organisms in swine influenza furnishes a tangible and experimentally reproducible justification of this hypothesis.”

The clinical picture of swine influenza, characterized by fever, anorexia, extreme prostration, leucopenia, and evidence of respiratory involvement and of muscular tenderness, is strikingly suggestive of human epidemic influenza. The onset is sudden, the course short, and convalescence usually uneventful. Death, when it occurs, is the result of an edematous type of pneumonia. The pathology of non-fatal swine influenza, characterized as it is by an exudative bronchitis and extensive pulmonary atelectasis, cannot be compared with the findings in non-fatal human influenzal infections because of our lack of knowledge of the latter. Probably the most significant similarity concerns the predominant bacterium encountered in the two conditions; H. influenzae suis is indistinguishable morphologically and culturally from H. influenzae. The frequency with which H. influenzae has been encountered in careful bacteriological studies of human influenza parallels the frequency of occurrence of H. influenzae suis in swine influenza, and, as in the case of the latter organism, has suggested an etiological significance. Without drawing analogy too far, the irregularity in the outcome of the filtration experiments reported, especially by French and English investigators, in attempting to determine whether a filtrable virus causes human influenza, is very similar to the experience of the writer in the early filtration experiments with swine influenza. The preliminary obstacles encountered in studying the nature of the etiological factors in swine influenza have had much in common with those met by investigators of human influenza. A careful investigation would seem warranted of the possibility that Pfeiffer’s bacillus and a filtrable agent act in concert to cause influenza in man.

SUMMARY AND CONCLUSIONS

1. It has been possible to demonstrate, in Berkefeld filtrates of infectious material from experimental cases of swine influenza, a virus which when administered intranasally to susceptible swine induced a mild, usually afebrile illness of short duration. The changes in the respiratory tract resembled those in swine influenza but were usually
   much less extensive. When the filtrable virus was mixed with pure cultures of H. influenzae suis and administered to swine a disease identical clinically and pathologically with swine influenza was induced. The data presented indicate that the filtrable virus of swine influenza and H influenzae suis act in concert to produce swine influenza and that neither alone is capable of inducing the disease.

doi: 10.1084/jem.54.3.373

In Summary:

• In the early 1930s, Richard Shope was said to have isolated influenza “virus” from infected pigs
• Shope’s finding was then said to be quickly followed by the isolation of the influenza “virus” from humans, proving that a “virus”—not a bacterium, as was widely believed—caused influenza
• Prior to this, in 1892, German bacteriologist Richard Pfeiffer isolated a small rod-shaped bacterium from the noses of flu-infected patients which he thought was the causative agent of influenza and dubbed it Bacillus influenzae
• Few doubted the validity of this discovery, in large part because bacteria had been shown to cause other human diseases, including anthrax, cholera, and plague
• Most scientists believed that Pfeiffer’s bacillus caused the 1918 Spanish flu
• The bacteria were found in many, but not all, of the sick patients
• Failures to isolate B. influenzae were largely chalked up to inadequate technique, as the bacteria were notoriously difficult to culture
• The first potential blow to Pfeiffer’s theory came from Peter Olitsky and Frederick Gates at The Rockefeller Institute
• Olitsky and Gates took nasal secretions from patients infected with the 1918 flu and passed them through Berkefeld filters, which exclude bacteria
• The infectious agent—which caused lung disease in rabbits—passed through the filter, suggesting that it was not a bacterium
• Although the duo had perhaps isolated the influenza “virus” (which they nevertheless referred to as an atypical bacterium called Bacterium pneumosintes), other researchers could not reproduce their results
• Oswald Avery (Rockefeller Institute), developed a culture media—chocolate agar—that optimized the growing conditions for B. influenzae which was said to minimize false negative results from patient samples
• Because of Avery’s method, the idea that flu was transmitted by a filterable agent (or “virus”) was dismissed
• Shope—a young physician from Iowa then working on hog cholera at the Rockefeller Institute—turned his attention to swine influenza
• In 1928 and 1929, pig farmers in Iowa noted a disease which resembled the human flu in pigs which was named swine influenza.
• Shope took mucus and lung samples from the infected pigs and attempted to isolate the disease-causing agent
• He isolated a bacterium that looked exactly like Pfeiffer’s human bacterium (and was thus called B. influenzae suis), but when it was injected into pigs, it caused no disease
• Shope then filtered the samples and, like Olitsky and Gates, found that the filtrate contained the infectious agent
• Shope’s filtrate caused an influenza-like disease in pigs—albeit a more mild one than seen in naturally-infected pigs
• Mixing the filtrate with the bacterium reproduced the severe disease
• He concluded that the filterable agent caused the infection, which then facilitated secondary infection with the bacterium

• McBryde, Niles, and Moskey made five attempts to pass the etiological agent of swine influenza through small Berkefeld or Mandler filters but were unable to reproduce the disease by dropping such filtrates into the nostrils of normal hogs
• Their experiments indicated that the causative agent of swine influenza was not a filtrable “virus”
• The results of ten filtration experiments with infectious material from the two strains of the disease obtained in 1928 and the two secured in 1930 were inconstant and confusing
• Diseased lung and bronchial lymph nodes were minced with sterile scissors and added to bronchial exudate
• This mixture was ground with sand in a mortar
• When it had been reduced to a pasty and fairly homogeneous consistency, a 10-20 percent suspension was made by gradually adding sterile distilled water or infusion broth (pH 7.3)
• It was then shaken with glass beads in a flask for l0 to 15 minutes and centrifuged
• In certain experiments the control animal receiving unfiltered material and those receiving sterile filtrates were from the same source or even from the same litter
• Of the ten preliminary filtration experiments, three were at the time interpreted as negative, while in the remaining seven some evidence was obtained that the injected filtrate had contained an infectious agent
• The disease induced by this filtrable infectious agent was definitely not typical swine influenza so Shope referred to it as “filtrate disease”
• Clinically the filtrate disease was much milder than swine influenza
• The usual symptoms shown by filtrate-inoculated swine were:
  1. No fever
  2. A moderate and transient apathy
  3. Some loss in appetite for a period not exceeding 3 days
  4. Occasionally a slight cough
  5. A moderate or quite marked leucopenia
• The extreme prostration so common in swine influenzal infections was not seen
• In some instances the disease was so mild that it almost escaped recognition altogether
• Shope then claims it was possible that in the three preliminary experiments considered as negative, infections were actually produced but so mild in character that they escaped recognition
• The lesions exhibited at autopsy were similar in kind but different in extent, from those encountered in typical uncomplicated swine influenza
• H. influemae suis, which was constantly encountered in culturing the respiratory tracts of animals with typical swine influenzal infections, was not found in similar cultures from animals with the mild filtrate disease
• Anaerobic cultures of seven filtrates of swine influenza infectious material in blood broth and in 5 percent serum bouillon over sterile rabbit kidney failed to show growth
• Shope believed the combination of the organism and the filtrable agent may be essential for the production of the natural disease
• Experiments were conducted in which swine were inoculated intranasally with cultures of H. influenzae suis, which had been under cultivation for a long time (over 2 years in most instances), mixed with Berkefeld filtrates of infectious material from experimental cases of swine influenza
• The cultures of H. influemae suis used were grown in defibrinated horse blood at the bases of plain agar slants in most instances for 24 hours
• The undiluted blood culture was used in the inoculations and in all experiments the culture injected alone was identical with that mixed with filtrate before injection
• All eight of the swine infected by inoculation with Berkefeld filtered infectious material or by contact with filtrate-infected swine developed only a mild disease
• In some instances it was so slight as almost to escape recognition
• The swine which were inoculated intranasally with pure cultures of H. influenzae suis were completely negative both clinically and at autopsy
• Of the seven hogs infected either by direct inoculation with the filtrate-culture mixture or by contact with swine so infected, three developed typical swine influenza, and the disease was of about the same severity as that which developed in the control animals inoculated with unfiltered infectious material
• Two others had a mild influenza, but in this instance the disease which developed in the control animal was also atypically mild
• Two animals showed a more severe type of infection than did their controls infected by unfiltered material
• In other words, 5 out of 7 pigs infected with the filtrate-culture showed the same disease symptoms as the “controls”
• Pieces of atelectatic lung and bronchial lymph nodes from one experimentally infected swine were stored for 15 to 33 days and from another swine for 15 and 41 days in 50 per cent glycerol before testing them for infectivity
• They had been found capable of inducing only the mild filtrate disease typical in its course and at autopsy
• With one exception H. influenzae suis was not demonstrable in cultures from the respiratory tract of swine infected with this material
• Material that had been stored for 34 days proved capable of inducing only the filtrate type of disease when inoculated into two susceptible swine yet H. influenzae suis was not found in the respiratory tracts of these two animals at autopsy
• Another tube of this same material tested after 54 days’ storage proved capable of inducing typical and rather severe swine influenza in which at autopsy H. influenzae suis was found in both the bronchial exudate and the atelectatic lung
• The irregularity in the results obtained with dried infectious material may have been due to faulty freezing or drying of the particular tubes of dried material tested after 34 days’ storage
• It was difficult for Shope to understand why H. influenzae suis should not survive freezing and drying
• In experiments with the bacteria filtrate, Shope states it has been possible in most cases to induce a definite but mild illness by the intranasal inoculation of swine with such filtrates
• Since the filtrate-induced disease had consistently been at variance with typical swine influenza, it was obvious that the disease induced by a filter-passing “virus” could not rightly be considered swine influenza
• The impression gained after consideration of a series of these mild infections was that the disease both clinically and pathologically represented natural swine influenza in an incomplete form
• Shope states that in a proceeding paper, it was shown that H. influenzae suis was constantly demonstrable in the respiratory tracts of swine ill with influenza yet it had been consistently absent from the respiratory tracts of swine ill with the filtrate disease
• Control animals receiving cultures of H. influenzae suis alone developed no evidence of illness and swine receiving the filtrable agent alone developed the mild filtrate disease
• It seemed permissible to interpret these experiments as indicating that swine influenza is due to a filtrable “virus” and H. influenzae suis acting together
• Their mode of action is unknown although two possibilities are obvious:
  1. The pathological activities of the “virus” creates a portal of entry for H influenzae suis and furnishes a favorable medium in which it can multiply
     • Under such an assumption the “virus” serves merely as an entering wedge for the organism and the bacteria determines the clinical picture and pathology
     • Shope stated that in the presence of the invisible swine influenza “virus,” H. influenzae suis possesses invasive powers which it completely lacks when administered alone
  2. The “virus” is the important component in contributing to the pathology and perhaps also to the symptoms characterizing the clinical picture, and H. influenzae suis increases the pathogenic properties of the “virus” and the severity of the resulting disease
• Shope stated whatever the relation of the “virus” and the organism in respect to the disease, his data indicated that they act together
• The hypothesis is not new that a disease may be induced by a bacterium and an invisible agent, not readily demonstrable alone
• It applies most directly to diseases in which the suspected bacterial agent, while readily and uniformly isolated from cases of the disease, either is completely incapable of reproducing the infection or very rapidly loses its ability to do so under conditions of artificial cultivation
• It is possible that such organisms do not become non-pathogenic because of rapid loss of virulence but because of the absence of an invisible inciting agent
• The clinical picture of swine influenza (fever, anorexia, extreme prostration, leucopenia, and evidence of respiratory involvement and of muscular tenderness) was strikingly suggestive of human epidemic influenza
• The pathology of non-fatal swine influenza could not be compared with the findings in non-fatal human influenzal infections because of a lack of knowledge of the latter
• H. influenzae suis in pigs is indistinguishable morphologically and culturally from H. influenzae in humans
• The frequency with which H. influenzae had been encountered in careful bacteriological studies of human influenza parallels the frequency of occurrence of H. influenzae suis in swine influenza which suggested an etiological significance (in other words, the bacteria found in humans caused the disease rather than the “virus”)
• The irregularity in the outcome of the filtration experiments reported by French and English investigators in attempting to determine whether a filtrable “virus” causes human influenza paralleled Shopes swine flu work
• He believed a careful investigation was warranted of the possibility that Pfeiffer’s bacillus and a filtrable agent act in concert to cause influenza in man
• Shope concluded that the data presented indicate that the filtrable “virus” of swine influenza and H influenzae suis act in concert to produce swine influenza and that neither alone is capable of inducing the disease

Shope could not produce the swine flu with the cultured bacteria he had originally believed caused the disease. He could not cause the swine flu with the invisible and assumed filterable “virus” either. It wasn’t until he combined the two toxic concoctions together that he sometimes produced a similar disease. In fact, he was certain neither the bacteria nor the “virus” alone could cause disease but only when they acted together.

Keep in mind that at this point in time there was no indirect Electron Microscope nor cell culture evidence attempting to “prove” a “virus” was present in the mixture. Much like today, it was assumed to be in there. No purification/isolation of any kind took place to separate a “virus” from everything else. Shope created toxic soups from pieces of diseased pig ateletatic lung and bronchial lymph nodes which he ground up and mixed with various chemicals/substances and then injected these mixtures up the noses of some pigs which sometimes caused them to get sick. It is on this basis that a swine flu “virus” was considered “isolated” which led to the “isolation” of Influenza A a few years later.