journal medical research

THE

Journal of Medical Research

VOL. XLIV, No. i SEPTEMBER I923 No. I90

THE PATHOGENESIS OF EXPERIMENTAL PNEUMONIAIN THE RABBIT*

H. H. P3ERmAR(From the Pathological Laboratories, University of Pittsburgh, Pittsburgh, Pa.)

Introduction. This paper is the report of a study of experi-mental pneumonia in the rabbit, in which the interest is cen-tered particularly upon the earlier stages of the pathologicprocess. In carrying out the experiments the aim was to use aconstant small dose of pneumococci of practically uniformvirulence, and to vary only the time factor. The inevitablevariability of resistance on the part of the rabbits is apparentin the records of the experiments, but so far as could be de-termined they were all normal healthy animals.The work was stimulated by the studies of pneumonia in

monkeys reported in I920 by Blake and Cecil" 23. Theyoffered a new viewpoint on the question of pathogenesis, stat-ing that in lobar pneumonia the pneumococcus primarily in-vades the lung tissue at some point or points near the root ofthe lobe, whence the organisms spread throughout the lobe byway of the interstitial tissues and the lymphatic system. Thealveolar involvement they found to be primarily central, ex-tending radially to include more or less of the lobe or lobes in amassive lesion. They concluded, therefore, that lobar pneu-monia is primarily an interstitial infection of the lung. It waswith the idea of studying more closely this early process ofinvasion that the present work was undertaken.

* Received for publication May 7, I923.

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The technical procedure differed somewhat from that ofBlake and Cecil but the essential point was preserved, namely,to use not more than one cubic centimeter of fluid material asthe dose to be injected, thus avoiding the highly artificial con-ditions imposed by the presence in the lung of large quantitiesof fluid. Further, the respiratory tract was studied in detailfrom the epiglottis downward, this being accomplished byfollowing the same plan of blocktaking in every case.The results of this study, briefly, indicate that after one

cubic centimeter of an eighteen-hour culture of pneumococcustype I is instilled into the larynx or the upper end of thetrachea the fluid bearing the organisms is rapidly spread overthe surface of the trachea, down the bronchial tree and into theterminal bronchioles, ductuli alveolares, atria and alveoli.Position of the animal and gravity are important in determin-ing which lobe will receive most of the fluid culture. Aspira-tion during respiration is responsible for the presence of fluid inthe atria and alveoli. This can be very readily shown by in-troducing any colored fluid in I c.c. quantity into the trachea.The spread is by no means uniform throughout the lung. As

a rule, one or both lower lobes receive the greater part of theinjected culture. The result of bringing such a culture intocontact with the trachea and larger bronchi is the developmentof degrees of degeneration and desquamation of the cells of theepithelial lining with congestion of the subepithelial vessels,and after one or two hours the migration of leucocytes out ofthe vessels into the subepithelial stroma and into the epitheliallayer at the points of greatest damage. There is no lymphangi-tis developed at this stage, nor does the interstitial exudateextend from one focus to the next. The congestion and gather-ing of the cells of the inflammatory exudate is more striking inthe walls of the bronchioles and alveolar ducts. The walls ofthe atria and their subtending alveoli show intense congestionand leucocytes in increased numbers both within and withoutthe capillaries, while the alveoli themselves may or may notcontain a serous exudate. In these early lungs, the strikingreaction is in the walls of the bronchioles, alveolar ductules,atria and alveoli. It is a patchy lesion in the lungs but more of

2

PATHOGENESIS OF EXPERIMENTAL PNEUMONIA

the units of lung structure show involvement in one or twolobes than in the others.The process is then a progressive development of slight

laryngitis and tracheitis, more marked bronchitis and bron-chiolitis, and well marked exudate into alveolar ductules, atria,and alveoli. These localized patches of inflammation of thelung parenchyma tend to enlarge by direct extension. A moreor less perfectly developed border of alveolar inflammationfollows the fine bronchioles which bear small bud-like air-cellson their lateral aspects, and some extension takes place in theseregions.

After some hours, there is a new feature noted in the reaction.The interstitial tissues have become involved, due to the in-creased and infective lymphatic drainage from the areas ofacute parenchymal inflammation. Acute lymphangitis is nowa striking microscopic characteristic of the involved lung. Theperivascular, peribronchial and then peritracheal tissues areinvolved in a diffuse acute reaction and the lymph nodes at thehilus show acute lymphadenitis. Furthermore, after inter-stitial inflammation with acute lymphangitis has begun, theless involved portions of the pneumonic lobe and even otherlobes may show a predominating interstitial inflammation.This is due to the rapid interstitial spread peripherally alongthe lymphatics from the original foci of inflammation withinthe primarily involved lobe, or in the case of other lobes, to asimilar peripheral dissemination from the large lymphatictrunks at the hilus to the lymphatics of these lobes. The in-volvement of the pleura and pericardium takes place a littlelater than the other manifestations of interstitial involvement.On the other hand, the peribronchial lymph nodes show anacute inflammatory reaction very early, since the drainagefrom inflammatory foci in the lung is at first very rapid, and isswept along the main lymph channels to the nodes at the hilus.Septicemia never occurred in this group of animals withoutthe previous development of definite interstitial inflammationof the lungs.The pneumonic inflammatory process always tends to be

central and peribronchial in location and in the more advanced

3

stages the oldest lesion in a lobe is centrally placed. Thegradual development of a lobar distribution takes place by theenlargement and coalescence of the primary areas of broncho-pneumonia plus the rapidly extending acute interstitial in-flammation, once the latter has gotten under way.Thus this series of animals has shown that the pathogenesis

of experimental pneumonia in the rabbit consists in a rapid andalmost simultaneous development of inflammation of thetrachea and bronchial tree and of the terminal branches of thattree with the communicating atrial and alveolar structures.The extent of this involvement depends upon the distributionof the material introduced. The interstitial tissues, includingall the lymphatic networks and the lymph nodes, shortly be-come involved and after this has occurred the spread of theinflammation through the lung proceeds more rapidly thanbefore. The stage of interstitial involvement is important inhastening the development of lobar distribution of the lesion,as well as the induction of pleuritis, pericarditis, mediastinitis,and septicemia.

Literature. The literature on the subject of experimentalpneumonia is extensive, including as it does the greatest rangeof experimental procedure. The organisms known to causepneumonia have been studied in various grades of virulenceand in different types of animals. All the possible modes ofintroduction of organisms into the animal body have beenutilized. But as regards the genesis of pneumonic inflamma-tion, the investigations have generally corresponded to theview expressed by Ribbert 4' 5 that in primary pneumonias thebronchioles and immediately adjoining air cells are first in-volved in an acute exudative inflammation, which subse-quently extends in the lung parenchyma. Ribbert also notedthe occurrence of interstitial inflammation with acute lym-phangitis; and he described vascular thrombosis involving cap-imaries especially. Wadsworth 6 7, 8 stressed the importance ofresistance and virulence as factors in determining infection andthe type of lesion developed. He pointed out that if the bal-ance between these factors was properly adjusted, it was pos-

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sible to obtain typical lobar consolidation of the rabbit lung.He achieved this in a few instances by immunizing his animals,thus avoiding the rapidly fatal septicemia which had oftenkilled his rabbits before lung lesions developed. His firstreport contained an exhaustive review of the literature up tothat time. It is noteworthy that Wadsworth employed a smalldose of culture (i c.c.) in his intratracheal injections.

Meltzer and his co-workers 9,10* 11, 12, 13, 14 studied a variety oforganisms of different degrees of virulence and also certainchemicals and unorganized substances, using the dog as theexperimental animal and making intratracheal injections ofthese materials in large quantities, from 5 to 30 c.c. Theyobtained gross lobar consolidation with the more virulentstrains and could determine the lobe to be involved by con-trolling the direction of the catheter as it was introduced.Although this seems a highly artificial method, they were ablefor the first time to produce an experimental lobar pneumoniaat will. They believed that the large quantity of fluid usedobliterated many of the bronchial branches which enabled thepneumococci retained in the air passages to invade the lung.They suggested that in spontaneous pneumonia the mucoussecretion and fluid exudate in the air passages probably actedin the same manner.

Winternitz and his associates 15, 16, 17 published a series ofinteresting papers on various aspects of experimental pneu-monia in rabbits. Intratracheal doses of 5 c.c. were used intheir studies. The genesis of the infection was not directlydealt with, although phases of the problem of immunity werediscussed. They believed that forcible injection is necessary toproduce an experimental pneumonia and stated that if donegently, large doses of culture may be introduced into thelarynx and trachea without effect.

Sisson and Walker18 used B. Friedlander in 5 to 8 c.c. dosesin cats and produced a bronchopneumonia, which progressedto lobar consolidation. Rochaix and Durand 19 were able toproduce degenerative and inflammatory lesions of the bronchialtree and bronchopneumonia by the use of toxins of the Fried-lander bacillus intratracheally in the rabbit. Wadsworth 7had

5

not been conspicuously successful with this type of procedure,using a solution of washed pneumococci.

Armstrong,20 using a technique similar to that employed byWinternitz, reported findings which agreed essentially withthose of Wadsworth; namely, that the type of involvementdepended on the virulence of the organism. He also showedthat the degree of dissemination of the lesions depended uponthe amount of fluid used to suspend a given dose of organisms.Keegan 21 studied an animal-house epidemic of B. bronchi-

septicus infection among guinea pigs and concluded that thisinfection was a descending one which passed along the bron-chial tree without producing great change until the thin epi-thelium of the infundibulum was reached. Here, however, thestroma and capillary vessels were more easily affected by adiffusible toxin and it was here that he found the earliest well-developed acute exudate. The adjacent alveoli were almostequally involved, and the result after several days durationwas a more or less confluent lobular pneumonia. Certain pigsdied very early, and in these the outstanding feature was amarked interstitial inflammation, which Keegan suggested aspossibly being the result of unusually virulent organismswhich were able to penetrate the bronchial wall and involvethe lymphatics directly. Saelhof 22 described the same type ofpneumonia in the rabbit, both as a natural and an experi-mental infection. His findings presented a striking similarityto those reported by MacCallum,23 Klotz 24 and McMeans 25and others in human influenzal pneumonia. The lesion wasessentially a bronchopneumonia, more or less extensive andconfluent, according to the stage of development. The chiefdifferences were the more intense lymphangitis and the tend-ency to necrosis of the consolidated portions. This latter hefound to be especially distinctive of rabbit pneumonia due toB. bronchisepticus.The studies of Blake and Cecil 1, 2,3 on experimental and

spontaneous pneumonia in monkeys were unique because ofthe minute doses of pneumococci (i.o c.c. to o.oooooi c.c. of aneighteen-hour broth culture) required to produce the lesion, inthe absence of any procedures leading to artificial reduction of

6. PERMIAR


the resistance of the animal. In a word, they were able toobtain just the proper balance between the virulence of theorganism and the resistance of the animal to produce a typicalprimary pneumonia. The substance of their explanation ofthe pathogenesis of pneumonia has been given above. Theirviews were stated quite conservatively, however, and theywere not able to rule out primary inflammation of the bron-chioles, alveolar ducts, atria, and alveoli following direct en-trance of organisms into these structures from the larger airpassages. In only two instances were their experiments con-ducted for a period less than twenty-four hours. One of thesewas of twelve and one-half hours and the other of three hoursduration following intratracheal injection. It is in the latterthat interest centers from the standpoint of pathogenesis. Inthis experiment the virulent pneumococci from io c.c. of brothwere centrifuged down to a volume of I.5 c.c. and injected bytheir usual method. One is impressed by the fact that in thisexperiment a relatively enormous quantity of organisms wasemployed compared with the minute doses of equally virulentstrains used in their other experiments. The monkey, more-over, is evidently a very susceptible animal. It seems probablethat the inundation of all the lung tissues by pneumococci inthis case merely showed what would happen after three hoursunder given conditions; it does not follow, however, that thesame chain of events took place in the early stages of the re-maining experiments, in which the dose administered closelyapproached that producing spontaneous pneumonia. In thepresent experimental study of pneumonia in the rabbit it iswithin the first twelve hours that the primary bronchial dis-tribution with direct involvement of the air cells is to be found.Even before twelve hours have elapsed, early interstitial in-flammation, acute lymphangitis and pneumococcus septi-cemia have developed.

Miller 26 studied aspiration pneumonia following vagotomyin the rabbit and found that terminal bronchiolitis was theprimary lesion. He believed that the succeeding event was aninterstitial extension to the immediately adjacent alveolarwalls, rather than an involvement of the air cells at the termi-

7

nation of the air passages by direct extension over the surface orby aspiration. Following the appearance of the reaction in theperibronchial alveoli, an interstitial inflammation with lym-phangitis aided in producing extension of the lesion.The anatomical studies of W. S. Miller 27 are interesting in

this connection. He demonstrated bud-like air spaces branch-ing off directly from the terminal bronchioles, which explainsMuller's findings that these structures and the adjoining airspaces are involved simultaneously. The nature of the infec-tive agent in aspiration pneumonia can not be controlled. Thecommonest spontaneous lung infection in rabbits is that due toB. bronchisepticus. Muller described a severe interstitialreaction in aspiration pneumonia following vagotomy whichled to rapid extension of the inflammation in the lung. Therecent reports by Keegan 21 and Saelhof 22 on spontaneous andexperimental B. bronchisepticus infections of the rabbit lungnoted the great tendency in this infection toward interstitialinvolvement. It therefore appears possible that Miiller'srabbits suffered from spontaneous B. bronchisepticus pneu-monia.

It is not an easy matter to summarize the literature of ex-perimental pneumonia. But it may be worth while briefly torecapitulate the points brought out in the review presentedabove. It was shown that in producing experimental pneu-monia the size of the dose administered and the quantity offluid used to suspend it are less important than the virulence ofthe organism used, while other work indicated that the type oflesion obtained also depended in a large measure on the viru-lence and invasiveness of the organism. Opposite views havelong been and still are held on the precise mode of primaryinvolvement of the lung in acute pneumonia. The most recentevidence is that presented by Blake and Cecil in favor of alymphatic distribution of organisms in the lung. Others,among them Keegan and Saelhof, favored the bronchial dis-tribution of the infection, though Keegan stated that whenextremely virulent, the organisms appeared to invade directlythe lymphatics near the hilus. The infection was rapidly fatalin animals showing this type of lung involvement.

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Technique. The technical procedure followed may bebnrefly described as consisting of the instillation of i c.c. of aneighteen to twenty-four hour broth culture of virulent pneumo-cocci into the larynx or upper trachea of an etherized healthyrabbit. The animal was then allowed to live during intervalsvarying from one-half hour to seventy-two hours (as shown inthe tabulated protocols), when it was killed by ether andchloroform and autopsied at once. Cultures were taken as in-dicated. The lung was then gently injected through the pul-monary artery with Zenker's fluid, removed with the larynxand trachea and hardened in Zenker's fluid before sectioning.This naturally interfered with detailed gross study, but gavewell-fixed tissues for microscopic examination.

Several points require somewhat fuller description. Theorganism used was a laboratory strain of the Neufeld type Ipneumococcus. The virulence of this strain was enhanced bypassage through white rats, and the cultures used were madedirectly from the heart's blood of the rat on pH 7.5 infusionbroth (without dextrose or calcium carbonate). The virulencewas such that two or three drops of heart's blood from a ratdying as a result of intraperitoneal injection killed the succeed-ing rat in an average of twenty-one hours. This degree ofvirulence was selected arbitrarily and proved to be satisfac-tory. It is of the greatest importance to have a strain virulentenough to produce lung inflammation, and still not sufficientlyvirulent to cause septicemic death before the lungs are def-initely involved. After each interval in the work, duringwhich the culture was preserved on media, rat transmissionwas carried out until the above degree of virulence was ob-tained. The heart's blood used for inoculation from rat to rator from rat to broth for rabbit injection was always checkedby smear and culture on blood agar. Typing was done atintervals, and showed the reaction of the type I pneumococcus.The mode of inoculation of the animal was by a slender

rubber tube 2 mm. in diameter, which fitted snugly over ani8-gauge needle. This tube was re-enforced by a slendernichrome wire stilette, so that it could be bent to conformapproximately with the curve leading from the rabbit's teeth

9

IO PERM4AR

PATHOGENESIS OF EXPERIMENTAL PNEUMONIA II

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backward and downward into the larynx. The rabbit wasetherized and tied to a board which was then placed in an up-right position. The mouth was drawn open by means of loopshooked over the upper and lower teeth, and the tongue wasdrawn forward by padded forceps. It was then relatively easyto introduce the curved tube into the larynx, attach the tip tothe needle on a loaded syringe, and allow the dose of culture toenter the larynx and trachea. As the tube was rather long, itwas necessary to have a small amount of air behind the in-jected fluid to insure the delivery of the whole quantity. Theaim was always to introduce the fluid into the larynx or uppertrachea without force, and as far as possible to avoid trauma tothe larynx and upper trachea. This method is not as accurateas the direct tracheal injection used by Blake and Cecil, but itwas employed in the hope that the injections might be accom-plished without any trauma. However, as Winternitz, Smith,and Robinson28 have pointed out, traumatic injury to thelarynx is difficult to avoid. But it may be stated here that inour work such injury never gave rise to more than a localizedand usually quite superficial reaction, which caused no lym-phangitis and could therefore not be considered in discussingpathogenesis of pneumonia.The method adopted for cutting blocks for microscopic

study was designed to get a complete view of the larynx,trachea and lungs. The larynx was split longitudinally in theantero-posterior direction and both halves were sectioned. Thetrachea was cut crosswise, giving a succession of cylindricalblocks. At the bifurcation it was split lengthwise in the trans-verse diameter, so that the two main bronchi and the peri-bronchial lymph nodes were included in the section. Both ofthese blocks were cut. The lungs were divided into their lobes,and each lobe was cut so that the bronchus could be followedas far as possible, thus making a single block for each lobe.Duplicate sets of sections were made from all blocks andstained by hematoxylin and eosin, and by Haythorn's modifi-cation of the Gram-Weigert stain for bacteria in tissue.The materials thus obtained were given careful systematic

microscopic study. The results in each case, correlated with

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other pertinent data, are summarized in the accompanyingtable. The key to the arrangement of the protocols lies in theincreasing time interval of the various experiments.

Discussion. Before proceeding to an analysis of the resultstabulated above, it will be necessary to present in greater de-tail certain features of the above findings.The diagnosis of acute tracheitis, laryngitis, bronchitis and

bronchiolitis refers to swelling, degeneration, and desquama-tion of the epithelial lining accompanied by a more or lessmarked congestion and cellular infiltration below the tunicapropria and between the epithelial cells, with an exudate ofcells, mucus, serum, and fibrin on the free surface. Theselesions occasionally amounted to superficial ulcerations (al-though the ulcers recorded in the larynx were probably all dueto trauma). The type of cell concerned in the reaction was thepolymorphonuclear leucocyte and to a lesser extent the mono-nuclear phagocytic form, which will be termed in this work theendothelial cell, following the work of Haythorn,29 Foot,30Permar3' and others, as well as a recent and as yet unpublishedstudy of the mononuclear cells of the pneumonic exudatecarried out in connection with this work. The subepithelialtissues of the air passages normally contain a few scattered col-lections of lymphocytes and plasma cells, between the lym-phoid nodules of the submucosa. The cellular infiltration inquestion is an entirely distinct, essentially acute process. Inconsidering the microscopic features of the inflammatory reac-tion it must be kept in mind that the appearance of inflamma-tory cells in certain locations in the tissues is not an absoluteindication that the irritant calling out those cells is in the samelocation. The lung is peculiar in that it is made up of muchsurface as well as substance. With the injury present on thesurface it is clear that the cells of the developing exudate mustarise in the vascular structures and appear to infiltrate thesubstance of septa, alveolar walls, etc., before they can actuallyenter into the inflammatory reaction on the surface. Hence,it is necessary to demonstrate lymphangitis before one canindicate the presence of interstitial inflammation in the lung.

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The primary reaction was not associated with a lymphangi-tis. This point must be clearly established, since Winternitzand his associates 28 raised the point regarding lymphatic ex-tension to the lung from an acute infected injury of the larynxor upper trachea. There was no evidence of anything otherthan a localized inflammatory cell infiltration of the stromafound associated with these acute ulcerative lesions. Neitherthe deep nor the superficial lymphatic beds of the larynx,trachea or large bronchi showed inflammation until afterbronchiolitis and bronchopneumonia were established. Thenthe perivascular lymphatics were first involved, soon followedby generalized invasion of the lymphatics. At this time, theloose peritracheal tissues were included in the reaction de-scribed as a mediastinitis, and with this the superficial lym-phatics of the trachea showed an acute exudate as the result ofextension through the anastomosing channels from the deeplymphatic network; but this was essentially a part of anadvanced interstitial inflammation.The process by which the air spaces of the lung became in-

volved was similar to that giving rise to the inflammation ofthe air passages. In these experiments the infective materialgained entrance to the air spaces (atria and distal alveoli andthe bud-like spaces springing from the alveolar ducts) practi-caliy simultaneously with its distribution in the air passages.In spontaneous infection, an acute tracheo-bronchitis is prob-ably the primary occurrence, with subsequent aspiration intothe finer air passages and air cells of fluid exudate containingthe organisms. Where even i c.c. of fluid was introduced intothe trachea in the rabbit, a fairly wide immediate distributionin the lung took place. The lower lobes usually received thebulk of the dose. As previously stated, this may be easilyshown by the injection of i c.c. of a colored fluid, as methyleneblue solution. An animal so treated, and autopsied after abrief interval, shows a striking picture of patchy blue stainingof the lung, following the bronchial tree and most marked inthe lower lobes, one of which contains more of the injectedmaterial than the other.The inflammatory reaction in the atria and alveoli consisted

PATHOGENESIS OF EXPERIMrENTAL PNEUMONIA

primarily of engorgement and the rapid localization of poly-morphonuclear leucocytes in the walls of these structures.This was followed by a sero-fibrinous exudate into the airspaces, variable amounts of hemorrhage, and a cellular exudateconsisting of polymorphonuclear leucocytes and endothelialcells. In the adjoining, less severely infected alveoli the en-dothelial cell was frequently the first cell to appear. This wasseen at the margin of the progressing area of consolidation.Desquamation of epithelium undoubtedly took place, but itwas unimportant from the standpoint of the cells of theexudate. Definite damage to the respiratory epithelium wasdifficult to distinguish. It was best seen at the point where thealveolar ductule widens and the epithelium flattens to form theatrial lining. Here, loss of the epithelium could often bedemonstrated.The primary reaction in the atria and alveoli was then anal-

ogous to that in the air passages, with differences in degree andintensity conditioned by the differences in anatomic structure.The bronchioles and alveolar ducts showed a more marked sub-epithelial and intraepithelial leucocytic infiltration and de-veloped more acute exudate in their lumina than did the largerair passages. The atria and alveoli, being stirl thinner-walledand lined by a more specialized and less resistant epithelium,showed the most intense and most rapidly developed reaction.The other element in the inflammatory process, the true in-terstitial inflammation, was primarily a lymphangitis, withdiffuse involvement of the supportive tissues as the intensity ofthe reaction increased. The r6le of acute pneumococcic in-flammation rapidly extending throughout the lymphaticsystems of the lung in producing widespread parenchymal in-volvement, septicemia, pleurisy, pericarditis, mediastinitis,and lymphadenitis was unquestionably very important.The time at which interstitial pneumonia may be initiated

varies considerably. It may appear very early, as in RabbitII2 (killed two hours after injection), in which an early inter-stitial pneumonia was associated with the recovery of thepneumococcus from the heart's blood at autopsy. But thiswas an exceptional result, since the next appearance of inter-

I7

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stitial inflammation was recorded after a ten-hour interval,when it was again verified by a positive blood culture. How-ever, one of the ten-hour rabbits (No. 124) failed to show apositive blood culture, though there was an early acute inter-stitial inflammation in the sections examined.The rapidity of interstitial invasion must depend upon the

balance between virulence and resistance in the individualanimal. This was well brought out by certain of Blake andCecil's experiments, notably the three-hour experiment re-ferred to above. And the contrasting anatomical characteris-tics displayed by classical lobar pneumonia and the influenzalpneumonias Mustrate variations in resistance to infection asshown by a degree of localization in the former compared withwidespread and rapidly disseminated invasion of the lung inthe latter.No explanation is offered for the constant earlier involve-

ment of the perivascular lymphatics, although the corollaryfact that the peribronchial lymphatics were not the first to beinvolved was in line with the finding that the early acute in-flammatory process in the bronchial tree was superficial andwithout associated lymphangitis. To account for the primaryspread of the infection to the lung parenchyma by lymphaticextension was manifestly impossible from a study of thesematerials.The importance of the lymphatics in the later extension of

the inflammatory process cannot be too strongly emphasized.The spread of the interstitial inflammation from the site of thebronchopneumonic process in the more involved lobes towardthe hilus took place rapidly and was found fairly consistentlyin the experiments of ten hours duration or longer. It is note-worthy that the upper lobes, which presented as a rule little orno primary bronchopneumonia in the earlier stages, were fre-quently the seat of an extensive interstitial pneumonia afterthat process was well developed and had reached the hilus. Inthe lungs of animals showing well-established pneumonias,certain lobes or peripheral portions of lobes were found, whichshowed early bronchopneumonia but as yet no interstitialpneumonia.

PATHOGENESIS OF EXPERTMENTAL PNEUMONIA

As the experimental pneumonia was usually central, in-volvement of the pleura was delayed until the process reachedthose portions of the lung parenchyma in which the lymphat-ics drain toward the pleura. The earlier involvement of thelymph nodes at the hilus of the lung is to be explained on thebasis of the normal flow of lymph from the central portion ofthe lobe through the large lymphatic channels leading to thesestructures (Miller).27 Mediastinitis and pericarditis were stilllater events, not appearing in this series until well after posi-tive blood cultures had been consistently demonstrated. Theyappeared first in the successful experiment of twenty-fourhours duration (Rabbit I28).The variability of the development of septicemia has been

mentioned, as well as the fact that the perivascular lymphaticsare involved earlier than the peribronchial group. Inflamma-tion of the vascular wall was noted first in the rabbit killedafter an interval of twenty hours (Rabbit 129). The threecoats of the vessels were involved, but acute adventitial in-flammation arising by direct continuity from the infected peri-vascular lymphatics and acute endarteritis and endophlebitiswere the most outstanding lesions. Medial involvement wasrare. It could be demonstrated, however, and indicated theroute of invasion of the subintimal tissues. Vascular throm-bosis, usually of the septic type, was a finding constantly as-sociated with vascular inflammation and with the positiveblood cultures.Summarizing the data shown in the tabulated protocols it

will be seen that in twenty-seven attempts to induce experi-mental pneumococcus pneumonia in the rabbit, a degree ofsuccess was attained nineteen times. The majority of theseranged from early to well-marked bronchopneumonias whichtended to become confluent as the time interval of the experi-ment was increased and to be complicated by an acute inter-stitial inflammation of the lungs and by its sequelae. Of theeight excluded, six showed no lesions clearly attributable tothe action of the infecting agent. Of these, two were examinedafter an interval of only one-half hour; the second two weredear; the attempt to perform an intratracheal injection had

I9

possibly failed, but the element of uncertainty in the mode ofinfection as well as variations in the resistance of the animalsmust be taken into account here. The other two showed onlyrelatively mild inflammation of the large air passages. Thegroup of eight is completed by two instances in which the re-action appeared to be due to the pneumococcus, though truepneumonic consolidation did not develop and definite proof ofthe presence of the organism was lacking.

Positive blood cultures of pneumococcus were obtained fromthe heart's blood at autopsy in eleven of the nineteen animalsshowing pneumonia. All of these were animals in which inter-stitial pneumonia had developed. In two instances (RabbitsI25 and 129) the blood culture was negative, although inter-stitial pneumonia was present and fairly well developed. Thefault here was no doubt due to technical error in taking toolittle blood, as in one of them the organisms were demonstratedin the tissues.Pneumococci were demonstrated in the tissues in only nine

cases out of the nineteen. The irregularity of this result isdifficult of explanation. They were found in certain caseswhere no blood culture was taken, and not demonstrated incases where the blood culture was positive. More often theywere not found when the blood culture was positive; and inone instance (Rabbit 129) they were found in the tissuesthough the blood culture was negative. Repeated preparationswere made, without improving these results. Moreover, inthe majority of instances in which pneumococci were foundthey were by no means numerous. In the experiments ofshorter duration, the organisms found lay in surface exudateeither in air passages or alveoli. After interstitial inflammationhad developed they were found in both locations, sometimesmore readily in the exudate within and about the lymphatics.In two cases of intense pneumonia with interstitial involve-ment and septicemia they could be found in fair numberswithin the blood vessels. Granting that from some unex-plained cause the pneumococcus was not readily followed inthe tissues in all these cases, the factor of resistance is a possi-ble one to be considered. Certainly, the fact that an organism

PERM\AR20

PATHOGENESIS OF EXPERIMENTAL PNEUMONIA 2I

cannot be demonstrated to be actually present in tissue sec-tions is no indication that it has not been the exciting cause ofthe lesion.The results in the two seventy-two hour experiments were

unsatisfactory; in Rabbit IO9, the injection resulted in only amild reaction in the larynx and trachea and a slight bronchitis.In Rabbit I30, the pneumonic reaction was very slight, thoughacute interstitial pneumonia was present and the blood culturewas positive. This rabbit had shown unmistakable clinicalevidence of illness. This, therefore, indicated a very low resist-ance on the part of the animal, and the development of inter-stitial pneumonia and pneumococcus septicemia without theproduction of great local pneumonic inflammation.

It will be observed that the scope of this study does not in-clude the processes concerned in resolution or other modes ofhealing. The principles involved in the production of an acuteexperimental pneumonia and its development up to the stageof a lobar lesion constituted the present problem.Regarding the possibility of confusion of true experimentally

produced lesions with pre-existing lesions or with lesions arisingspontaneously as the result of anaesthesia, it is felt that thefindings as reported speak for themselves. The fairly consist-ent isolation of the pneumococcus type I, the less consistentidentification of the organism in the exudates and tissues andabove all the progressive intensity in the severity of the reac-tion are, it would seem, conclusive proofs of the specificity ofthe lesions produced. It is interesting, moreover, to note thatthe experimental work was carried out on two groups of ani-mals, one series running from October to January, the otherduring the following June; and that in each series the resultsare, with minor irregularities, quite comparable. The peculiarendothelial proliferative lesion described by Maitland, Cowan,and Detweiler 32 was present in a few of the lungs. It was notin the least confusing, due to its very mild chronic inflamma-tory character. So far as B. bronchisepticus infections areconcerned, the anatomical picture of these experimental pneu-monias lacked entirely the peculiar acute necrotic lesions sotypical of this spontaneous infection of the lung in rabbits.

It is quite a temptation, in concluding this discussion, todraw analogies between the experimental pneumonias de-scribed in the rabbit and the natural pneumonias of the hu-man, especially since the writer has long been engaged in thestudy of autopsy materials from pneumonias in the human.The primary importance of the aerogenic route of infection isno longer seriously questioned. We feel that the bronchial treeas the primary pathway of dissemination of the infection to thelung alveoli is also fairly well established. The importance ofthe interstitial inflammatory reaction, spreading widely by thelymph channels, is equally great, and it is evident that thisfeature of even the so-called classical lobar pneumonia hasbeen overlooked in the older study of the varieties of exudatein the various stereotyped "stages" of the lesion. One canalways, we believe, demonstrate interstitial inflammation inthe tissues from true lobar pneumonia, though it is not so out-standing a feature as in the atypical pneumonias such as thosefollowing influenza.The stages of lobar pneumonia as they are ordinarily taught

are gross rather than microscopic features. The study of agroup of sections from various parts of a consolidated lobe willshow great variety in the type of exudate predominating in thealveoli, indicating an origin in multiple foci which tend tocoalesce gradually and which do so more rapidly once the in-terstitial distribution of the pneumococcus has begun. Theextent and rapidity of the interstitial involvement is the factorof greatest importance in the production of lobar consolidation.These experimental animals illustrate how rapidly in certain

instances the barriers may be broken down, resulting in veryearly interstitial pneumonia and septicemia; or how againthe pneumonic reaction may be almost nil, with the early es-tablishment of a septicemia. And finally how, in the majorityof instances under the conditions of these experiments, a regu-lar, orderly sequence of events leading to a more or less perfectlobar consolidation of the lung takes place. Those animalswhich appear to have been unusually resistant to the infectionare of course open to the suspicion of never having properlyreceived the inoculation.

PERMIAR22


Since the experimental procedure is of necessity artificial toa certain degree, it is hard to apply its results in interpretinghuman pneumonia. The point previously made seems worthrestating in this connection, namely, that while an excessivedose of pneumococci has been introduced in an appreciablequantity of fluid into the rabbit lung, the lesion in the humanprobably originates as a localized tracheal and bronchial in-flammation with a variable amount of fluid exudate; and thatthis is aspirated, in the usual case, principally into one or twolobes, at which time the status of the two processes, humanand experimental, is essentially identical.

CONCLUSIONS

i. Experimental pneumonia in the rabbit originates as anacute inflammatory reaction of the trachea, bronchial tree,alveolar ducts, atria, and alveoli. The severity of the reactionincreases toward the terminal bronchioles and is greatest inthese structures, in the bud-like alveoli arising from them andin the alveolar ducts, atria, and alveoli. The process begins,therefore, as a bronchopneumonia, which tends to invade thetissues further by peripheral extension of the individual foci,leading to a coalescing consolidation.

2. Acute interstitial pneumonia develops early as the resultof an acute lymphangitis, which arises in the peripheral lym-phatics and extends both to the subpleural network of theperiphery of the lung and to the hilus. This augments theprimary bronchiogenic inflammatory process and tends tohasten the complete involvement of the affected lobe.

3. Acute lymphadenitis appears before acute lymphangitis,since toxins and bacteria (both free and phagocyted) are car-ried directly and rapidly to the lymph nodes at the hilus, wherethey are retained, ginvng rise to an acute local inflammatoryprocess. This reaction in the lymph nodes becomes very in-tense as acute interstitial inflammation with severe lymphangi-tis develops.

4. With extension of the interstitial inflammation, whichfirst involves particularly the perivascular lymphatic system,

23

24 PERMAR

arteritis and phlebitis with septic thrombosis, pleurisy, me-diastinitis, pericarditis, and septicemia are developed. Thislast may be the result of the direct involvement of vessel walls,or it may be due to the passage of organisms through the lymphnodes at the hilus and into the circulation by way of thethoracic duct.

5. The process is comparable in its essentials to that takingplace in the spontaneous pneumonias of man. The chief dif-ference is the more intense interstitial involvement seen in theexperimental pneumomnas.The writer desires to express his appreciation to Dr. S. R.

Haythorn for his interest and advice during the progress ofthis work and for his criticisms during the preparation of thereport. Thanks are also due Miss Grace Thomas for hercapable assistance in the experimental work and in the prep-aration of sections from the experimental material.

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