Journal of Clinical InvestigationVol. 43, No. 2, 1964

Relationship of Body Temperature to the Lethal Action ofBacterial Endotoxin *

ROGERP. ATWOODt ANDEDWARDH. KASS(From the Channing Laboratory, Mallory Institute of Pathology, Thorndike Memorial

Laboratory, Second and Fourth [Harvard] Medical Services, Boston City Hospital,and the Departments of Bacteriology and Immunology, and Medicine,

Harvard Medical School, Boston, Mass.)

Although an extensive descriptive literature onfever and the influence of temperature upon bio-logic systems exists, there has been relatively littledirect experimental evidence on whether fever isfavorable or unfavorable to survival in infectiousdiseases (1). Opinions on this question rangefrom the assumption that fever has evolved as amammalian response to infection that generallyhas survival value for the host to the attitude(reminiscent of that held by physicians of earliertimes) that fever is in itself deleterious and shouldbe prevented. Except in certain instances inwhich elevated body temperature directly affectssurvival of the infectious agent, little data areavailable with which to approach this question(1).

In 1909 Ruata observed that injections of vari-ous gram-negative bacilli were more lethal forguinea pigs kept at elevated than at ordinary en-vironmental temperatures (2). Two reports haveappeared indicating a potentiating effect of highenvironmental temperatures upon the mortality ofmice injected with endotoxin (3, 4), but in bothof these the effect was incompletely described andwas documented only as an incidental observation.

Recently, Connor and Kass (5) observed thatartificial fever, induced by means of elevated am-bient temperature, increased the susceptibility ofmice to the lethal action of endotoxin. A similareffect (though slightly less pronounced) has alsobeen observed in rats (6). It has long been

* Submitted for publication June 11, 1963; acceptedOctober 3, 1963.

Aided by grant no. AI 02582 from the National In-stitutes of Health, U. S. Public Health Service. Pre-sented in part at the Symposium on Bacterial Endotoxins,Institute of Microbiology, Rutgers, the State University,New Brunswick, N. J., September, 1963.

t Postdoctoral research fellow, U. S. Public HealthService.

known that mice, rats, and guinea pigs exhibithypothermia in response to endotoxin under usuallaboratory conditions in contrast to the hyper-thermia generally observed in man, rabbits, dogs,cats, and other species (7-16). In the experi-ments cited above (5, 6), when the mice and ratswere challenged with endotoxin at ordinary roomtemperature, they exhibited the usual hypothermicresponse, whereas at elevated ambient tempera-tures the animals exhibited a further rise in bodytemperature upon challenge with endotoxin. Theobserved increase in susceptibility to the lethalaction of endotoxin seen in the hyperthermic miceand rats might represent an artifact of the experi-mental situation resulting from the change fromthe usual hypothermic response to an artificiallyinduced hyperthermic response foreign to thesespecies.

The present studies were therefore undertakento determine whether induced fever alters thelethality of endotoxin in a species that regularlyexhibits fever in response to endotoxin. In ourexperiments, fever was produced in rabbits byelevated environmental temperatures or by phar-macological means. Animals with induced feverwere challenged with endotoxin, and their sus-ceptibility to the lethal action of endotoxin wascompared with that of appropriate controls.

It will be shown a) that fever induced either byphysical or pharmacological means increases thesusceptibility of rabbits to the lethal action of en-dotoxin and b) that prevention of fever by simplephysical means exerts a protective effect uponrabbits challenged with endotoxin.

Methods

Animals. A total of 120 female albino New Zealandrabbits, weighing 1.6 to 2.3 kg, were used. In the ani-mal colony they were caged singly and fed a standard

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commercial pellet diet. They had free access to water atall times. No animals were used whose base-line rectaltemperatures fell outside the range 38.0 to 39.80 C. Ani-mals were used only once. Shorn rabbits were preparedby restraining the animals without anesthesia andshearing the fur with fine-toothed electric clippers closeto the skin over the ears, face, head, neck, body, andlimbs, but sparing the paws.

Endotoxin. The endotoxin used was derived fromSalmonella typhosa (0901).1 Samples of the endotoxinwere suspended in sterile, pyrogen-free, physiologicalsaline at a concentration of 1,000 /Ag per ml. This opales-cent suspension was subdivided into sterile rubber-capped

.vials that were frozen and maintained at - 200 C. Foreach experiment a supply was thawed and used immedi-ately. Any endotoxin remaining in the thawed vial wasdiscarded. For lower dose ranges, dilutions with sterile,pyrogen-free saline were made from freshly thawedstock suspensions of endotoxin.

D-Lysergic acid diethylamide (LSD).2 LSD was sup-plied as a solution in sterile, pyrogen-free saline at aconcentration of 100 gg per ml and also contained 0.06,ug tartaric acid per ml added by the manufacturer tostabilize the LSD in solution.

Injections. Injections were made with sterile, pyro-gen-free disposable tuberculin syringes at volumes be-tween 0.2 and 1.0 ml. Larger doses of endotoxin wereadministered to shorn rabbits at the concentration of thestock suspension, thus requiring injections of 1 to 8 ml,which were given with larger disposable syringes. Allinjections of endotoxin and LSD were intravenous viathe marginal ear vein.

Environmental temperature. A warm room, lined withaluminum foil and measuring 6 X 7 X 4 feet was kept at370 ± 0.50 C by a thermostatically controlled electricheater. This provided dry heat (relative humidity 30 to40%o), the only moisture source being the animals' wa-ter bottles. The room was furnished with regular ani-mal cages and a work table. Floor to ceiling temperaturegradients were eliminated by a large electric fan.

Animals in the stock colony and laboratory were at 230± 30 C, hereinafter referred to as room temperature.

Body temperature recording. Rectal temperatureswere measured electronically with thermistor probesand a Telethermometer.3 The majority of rabbits ineach group were kept unrestrained in their cagesthroughout the experiments, but continuous recordingsof rectal temperatures were obtained from some animalsin each group with the Yellow Springs model 80 paperrecorder. During continuous recordings the animalswere restrained in the usual upright position, with freeaccess to water bottles, by a loosely fitting wooden neckclamp fastened to a flat board. Preliminary recordings of

1 Lot no. 452309. Purchased from the Difco Co., De-troit, Mich.

2 Obtained from the Sandoz Pharmaceutical Co., Han-over, N. J., through the courtesy of Dr. Craig Burrell.

3 Manufactured by the Yellow Springs Instrument Co.,Yellow Springs, Ohio.

temperature were made for 1 to 2 hours until a stablebase line was reached before injections were given.

Tabulation of data and statistical methods. In gen-eral, the lethal effect of endotoxin was manifested within24 hours, but in the shorn animals, it was necessary toextend the mortality tabulation to 48 and 72 hours be-cause of the long duration of endotoxin effects in theseanimals, whether leading to death or to recovery. Esti-mates of the LD.0 were made by the methods of Reedand Muench (17) and Berkson (18) and were in closeagreement. The reported LD,0 values were calculatedby the method of Reed and Muench. Ranges of bodytemperature measurements indicated as + refer to thestandard deviation of the mean.

Results

Comparative effects of endotoxin at 230 C and370 C. The lethality of the endotoxin was estab-lished in a group of 23 control rabbits given dosesranging from 50 pg per kg to 450 ug per kg andmaintained at room temperature (Table I). Datain this control group were collected concurrentlywith those from the other experiments. The na-ture of the response to lethal or sublethal dosesof endotoxin was not different from that de-scribed in the literature (19). No deaths oc-curred in less than 4 hours, the majority oc-curred between 4 and 8 hours, and all deathsin this group occurred within 16 hours after ad-ministration of endotoxin. With the large chal-lenge doses of endotoxin used in these experi-ments, there was no consistent relationship be-tween dose and febrile response (20) nor betweenthe height of the maximal febrile response andthe outcome, i.e., death or survival (Figure 1).Animals maintained at room temperature, andgiven endotoxin, generally survived beyond theearly maximal temperature elevation, whichranged from 1.80 to 3.10 C, and deaths occurredduring the defervescent phase. The estimatedLD50 at room temperature was 240 ug per kg(Table I), a figure in general agreement withthose reported by other workers using various en-dotoxins (16).

\Vhen uninjected rabbits were maintained at370 C, their rectal temperatures began to risewithin a few minutes and reached a mean level2.10 ± 0.9° C above the initial temperatureswithin 2 hours. After 24 hours at 370 C themean elevation of temperature was 1.80 ± 0.80 C(Figure 1). The animals exposed to the higher-temperature exhibited moderate panting, increased

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TABLE I

Influence of fever produced by elevated ambient temperature on the lethal action of endotoxin in rabbits

Ambient temp., 230 C Ambient temp., 370 C Ambient temp., 230 C Ambient temp., 370 Cbefore and after before and after before and 370 C after before and 230 C after

injection injection injection injection

Endotoxin Outcome Endotoxin Outcome Endotoxin Outcome Endotoxin Outcome

jg/kg ug/kg ;g/kkg jg/kg50 Survived 1.0 Survived 1.0 Survived 150 Survived87 Survived 1.0 Survived 2.0 Survived 150 Survived

130 Survived 2.0 Survived 2.0 Died 150 Survived200 Survived 4.0 Died 5.0 Died 150 Survived200 Survived 5.0 Died 5.0 Survived 150 Survived200 Died 5.0 Died 10 Survived 150 Survived200 Died 10 Survived 10 Survived 300 Died227 Survived 10 Died 10 Survived 300 Survived250 Survived 10 Died 10 Survived 300 Survived250 Survived 10 Died 10 Died 300 Died250 Died 10 Died 20 Died250 Died 25 Died 25 Died250 Died275 Died275 Survived300 Died300 Died300 Died400 Survived400 Died400 Died450 Died450 Died

Total no. ofrabbits: 23 12 12 10

LDio, ,g/kg: 240 4 13 300

warmth and vasodilatation in the ears and skin,and recumbent (but alert) posture. Also, someused drinking water or saliva to wet the fur ofthe head and fore-paws by "face-washing" be-

2 3 4 5TIME IN HOURS

FIG. 1. INFLUENCE OF AMBIENT TEMPERATUREUPON

FEVER AND SURVIVAL AFTER ENDOTOXIN CHALLENGE IN

RABBITS. Numbers appearing over symbols indicate num-

ber of animals represented by each point. Crosses indi-cate times of death of individual animals.

havior, presumably in attempts to regulate bodytemperature. In addition to the 22 animals heldat 370 C overnight in preparation for injection,12 rabbits of the same age and size range asthose used in this study were held at 370 C con-tinuously for 4 days with no mortality.

Animals injected with endotoxin and maintainedat 370 C were divided into two groups: thosemaintained and injected at room temperature andthen placed immediately at 370 C and those heldovernight at 370 C and then injected and contin-ued at that temperature after injection (Table I).The rectal temperatures of rabbits injected withendotoxin and then placed at 370 C rose morerapidly and reached consistently higher levelsthan those of animals held at room temperature(Figure 1). The elevated rectal temperatures ofanimals that had been held overnight at 370 be-fore the administration of endotoxin rose stillfurther after endotoxin and reached hyperthermiclevels similar to those of animals placed at 370 Conly after injection (Figure 1).

The LD60 of endotoxin was substantially smallerin animals exposed to 370 C before the injection

SurvIved o-OI Held at room temp.Died -I- J throughout

Suvived o-La Held at 37° CDied ..3. throughout

Survied V6 A-Injected at room te1 ,Died V1-Y J then placed at 370C

I I I I I

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ROGERP. ATWOODANDEDWARDH. KASS

of endotoxin than in those placed at 370 C only atthe time of challenge with endotoxin; this agreeswith observations in mice (5). Thus, the LD50of endotoxin was reduced from 240 pg per kg atroom temperature to 13 pg per kg in animals in-jected at room temperature and then placed at370 C, and to 4 ug per kg in animals maintainedat 37° C before and after injection (Table I).

In both groups of rabbits exposed to 37° C, in-tense vascular congestion became grossly visiblein the bulbar conjunctivae within 5 to 10 minutesafter injection with endotoxin. A similar ocularvascular reaction to endotoxin was observed inanimals kept at room temperature, but it did notdevelop until after fever had appeared and wasgenerally less intense than in animals at 37° C.This ocular hyperemia was not seen in uninjectedanimals at 370 C nor in those made febrile withLSD alone. Ophthalmoscopic inspection of thelarger vessels of the ocular fundi revealed nodistinctive vascular changes during fever andendotoxemia.

The lethal action of endotoxin was also ac-celerated at 37° C, with death occurring in mostanimals within 4 hours after injection. Survivalfor 9 hours after injection of endotoxin at thehigher temperature invariably indicated ultimatesurvival of the animal.

Temperature responses were similar in animalssurviving or succumbing to endotoxin at thehigher temperature, indicating that hyperpyrexiawas not the sole factor determining the lethaloutcome.

Effect of endotoxin during LSD-induced fever.LSD, an ergot alkaloid, was first shown to be py-rogenic for rabbits by Horita and Dille (21). Thefever in rabbits is produced by doses in the micro-gram range, has virtually no lag phase, is dose re-lated, and is predictable in its magnitude andduration (21, 22). Horita and Dille (21) alsoshowed that during LSD-induced fever in rabbitsthe surface temperature of the ears falls to hypo-thermic levels and the body surface temperatureremains unchanged as the deep body temperaturerises to hyperthermic levels, indicating that cu-taneous vasoconstriction and decreased heat lossare involved in LSD-induced pyrogenesis. Ourobservations confirm these findings and are ofspecial interest in contrast to the aforementioned

cutaneous vasodilatation observed in rabbits madefebrile by exposure to 370 C.

In the present studies, LSD, at a dose of 50 ,ugper kg at room temperature, produced fevers in rab-bits comparable to those others have observed(21, 22). In all experiments a standard intervalof 45 minutes elapsed between the administrationof LSD and the subsequent challenge with endo-toxin to allow time for LSD-induced fever to beat or near its maximal level. In 17 rabbits themean rectal temperature 45 minutes after in-jection of LSD was 40.6° ± 0.150 C, an elevationof temperature close to that obtained by placingthe animals at 370 C.

As shown in Figure 2, the administration ofendotoxin to rabbits pretreated with LSD resultedin further rises in rectal temperatures to levelssimilar to those observed in animals challengedwith endotoxin at 370 C. In Figure 2, as in Fig-ure 1, the temperature curves for those animalsthat survived and those that died have been plottedseparately and show no significant differences,suggesting that the lethal outcome was not deter-mined by the degree of hyperpyrexia alone.

Rabbits pretreated with LSD were indistinguish-able from those maintained at 370 C in their rapiddevelopment of tachypnea, dyspnea, diarrhea, andocular hyperemia in response to endotoxin. Pre-treatment with LSD accelerated the lethal out-come following challenge with endotoxin, andthe LD,0 of the LSD-treated group was 7 pg per

RECTALTEMP"C43p

39

38

0 1 2 3 4TIME IN HOURS

5 6 7

FIG. 2. INFLUENCE OF PHARMACOLOGICALLY (LSD)INDUCED FEVER UPON FEVER AND SURVIVAL AFTER ENDO-TOXIN CHALLENGE. Symbols as given for Figure 1.Ambient temperature, 23° C. LSD, D-lysergic acid diethyl-amide.

Ct= *-7*DE

t~~~~~~~~~~~~LSD~ ~ ~ LS -kwdbEdtx

154

42[

41

40

BODYTEMPERATUREAND THE LETHAL ACTION OF ENDOTOXIN

kg (Table II), a value comparable to that observedin animals made hyperthermic by the warmerenvironment.

Effect of LSD and endotoxin on shorn rabbits.The possibility was considered that pharmacologi-cal actions of LSD other than the pyretogenicproperty might influence the susceptibility of rab-bits to endotoxin by means unrelated to fever.To explore this possibility a way was sought toprevent the development of fever in rabbits givenLSD; rabbits shorn of their fur satisfied this re-quirement in that they did not develop fever whengiven LSD in the standard dose used. Shornanimals at room temperature have rectal temper-atures about 10 C lower than those with fur, asnoted by Grant (23) and Fekety (24). The ad-ministration of LSD in the standard dose to shornanimals was associated either with no change inrectal temperature or in a rise of rectal tem-perature of less than 10 C, resulting in tempera-tures no higher than the range for unshorn, unin-jected rabbits at room temperature.

When shorn animals at room temperature weregiven LSD, 50 lug per kg, and challenged withendotoxin 45 minutes later, hypothermia occurredin all animals 30 to 45 minutes after injection of

TABLE II

Influence of pharmacologically (LSD) induced fever onthe lethal action of endotoxin in rabbits*

Endotoxin Outcome

Ag/kg1.0 Survived1.0 Survived2.0 Survived2.0 Survived4.0 Survived4.0 Survived5.0 Survived5.0 Survived5.0 Died5.0 Died

10 Survived10 Died10 Died10 Died15 Survived15 Died25 Died30 Died30 Survived50 Died50 Died

Total no. of rabbits: 21 LDso, ug/kg: 7

* Ambient temperature, 230 C. LSD, D-lysergic aciddiethylamide.

2 3 4 5TIME IN HOURS

FIG. 3. INFLUENCE OF SHEARING (LOSS OF FUR) UPONBODY TEMPERATURERESPONSETO INJECTION OF LSD FOL-LOWEDBY ENDOTOXINANDTO ENDOTOXINALONE. Ambienttemperature, 230 C. LSD, D-lysergic acid diethylamide.

endotoxin (Figure 3). Rectal temperatures fell2.60 to 6.50 C below pretreatment levels, and thehypothermia persisted for 18 to 72 hours after asingle injection of endotoxin. No consistent re-latiQnship between the dose of endotoxin and thedegree of hypothermia was observed, nor wasthere any relationship between the degree of hy-pothermia and the outcome, i.e., death or survival,probably because all doses of endotoxin were rela-tively large and produced maximal physiologicaldisturbances that differed more because of indi-vidual animal variation than as a function ofdose. Using smaller doses of endotoxin, Granthas shown a relationship between dose of endo-toxin and degree of hypothermia in shorn animalsexposed to cold (23).

The hypothermic response was characterized bycrouching, immobility, coolness and cyanosis ofthe skin, diarrhea, failure to eat or drink, andmarked delay in either the lethal outcome or re-covery. No deaths occurred before 7 hours afterinjection of endotoxin, the majority of deaths oc-curred between 12 and 36, and several between36 and 48 hours. Surviving animals exhibitedhypothermia and the characteristic huddled ap-pearance for as long as 72 hours.

Comparison of the LD50 values for endotoxinin Tables II and III shows that the LD50 was60 times greater in shorn animals given LSD thanin comparable LSD-pretreated, intact animalsthat developed fever in response to LSD.

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ROGERP. ATWOODANDEDWARDH. KASS

TABLE III

Influence of prevention of fever by shearing on the lethalaction of endotoxin in rabbits*

Shorn rabbits given Shorn rabbits givenLSDt and endotoxin endotoxin alone

Endo- Endo-toxin Outcome toxin Outcome

pg/kg jig/kg100 Survived 150 Survived100 Survived 400 Survived150 Survived 400 Died200 Survived 700 Survived200 Died 1,000 Survived250 Survived 1,500 Died250 Died 2,000 Survived300 Survived 2,000 Survived300 Survived 2,500 Survived300 Survived 2,500 Died600 Survived 3,000 Died

1,000 Died 3,000 Survived1,000 Died 3,500 Survived1,500 Died 3,500 Died1,500 Died 3,500 Died2,000 Died 4,000 Died

4,000 DiedTotal no. of

rabbits: 16 17LD5o, ,ig/kg: 420 2,500

* Ambient temperature, 23° C.t D-Lysergic acid diethylamide,

jug per kg (see text).uniform in dose of 50

Shorn rabbits were given graded doses of en-

dotoxin at room temperature. As expected, theyresponded with hypothermia and with the same

delayed death or recovery, and the same generalappearance, as did shorn animals that had beenpretreated with LSD before endotoxin challenge.The hypothermia was somewhat deeper in shornanimals receiving endotoxin alone as comparedwith those pretreated with LSD (Figure 3), butthere was considerable overlap in the ranges ofhypothermia observed in the two groups.

As shown in Table III, the shorn animals sur-

vived remarkably large doses of endotoxin andhad an estimated LD5O of 2,500 ug per kg. Thisfigure, some six times higher than the LD50 forshorn rabbits pretreated with LSD, suggests thatLSD may have exerted some adverse effect upon

survival even in the absence of fever.A further demonstration of the effect of fever

upon the response to endotoxin was obtained byplacing shorn animals at 370 C overnight andthen challenging them with endotoxin. The in-crease in rectal temperature in 10 shorn rabbitsat 370 C was 1.5 + 0.6° C, a figure not far belowthat observed for furred rabbits at the same en-

vironmental temperature. Shorn animals chal-lenged with endotoxin at 370 C showed fevercurves similar to those of furred animals, and theestimated LD50 in this group was 16 utg per kg(Table IV). This figure is more than 150 timeslower than that for shorn animals at room tem-perature, but somewhat higher than that obtainedfor furred rabbits at the higher temperature.Presumably the latter difference reflects theslightly greater increase in body temperaturesthat occurred in furred animals kept at 370 C.

Finally, to rule out fixed injury resulting fromexposure to 370 C that might influence the re-sponse to endotoxin, 10 rabbits were held at 370 Covernight, then injected with endotoxin and placedat room temperature immediately after injection.The body temperatures of all of these animalsbegan to fall immediately after they were re-moved to the cooler environment, but their tem-peratures rose again in response to the challengewith endotoxin to levels comparable to those incontrol animals kept at room temperature through-out. The estimated LD50 was 300 ug per kg(Table I), a figure close to that obtained for con-trol animals maintained at room temperature be-fore and after endotoxin challenge. Eight ani-mals in this group survived endotoxin doses thatshould have been lethal had the animals continuedto react to endotoxin as they did at 370 C.

Autopsies were performed on animals dying ofendotoxin effects in every experimental group.Specimens for histologic examination were takenfrom kidney, liver, spleen, lung, myocardium, thy-mus, stomach, ileum, colon, and brain and were

TABLE IV

Influence of fever produced by elevated ambient temperatureon the lethal action of endotoxin in shorn rabbits*

Endotoxin Outcome

jAg/kg1.0 Survived5.0 Survived

10 Died10 Survived25 Died25 Survived50 Died

200 Died300 Died

Total no. of rabbits: 9 LD50, Ag/kg: 16

* 370 C.

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BODYTEMPERATUREAND THE LETHAL ACTION OF ENDOTOXIN1

fixed in formalin and stained with hematoxylinand eosin. Characteristic findings were common

to all groups. Scattered punctate hemorrhageswere found in the thymus, myocardium, and colon,and intense congestion was grossly and micro-scopically present in lung, liver, spleen, and kidneyof all animals examined. In all animals, we foundmargination of polymorphonuclear leukocytes inthe pulmonary vasculature and varying accumula-tion of these cells in the alveoli and bronchioles,generally more extensive in animals surviving forlonger periods before death. Otherwise, however,the morphologic findings in animals dying of en-

dotoxin effects were uniform for all of the ex-

perimental groups.

Discussion

The data presented here demonstrate that in-duction of fever increases the susceptibility ofrabbits to the lethal action of endotoxin, and con-

versely, that prevention of fever exerts a pro-

tective effect against such a lethal challenge. Theeffects of endotoxin were more rapid and more

severe in hyperthermic animals regardless ofwhether hyperthermia was produced by pharma-cological or physical means. Potentiation of thelethal action of endotoxin by artificial elevation ofbody temperature occurs in mice and rats (5, 6),two species that exhibit hypothermia in response

to endotoxin under usual laboratory conditions.The present data show that this effect is alsodemonstrable in rabbits, a species that normallyexhibits fever in response to endotoxin.

An effect of induced fever that may be perti-nent has been observed by Bannister (25). Vol-unteers, who at normal body temperatures ex-

hibited a certain febrile response to the adminis-tration of pyrogenic doses of endotoxin, were ob-served to have proportionately greater febrile re-

sponses to the same challenge when their restingbody temperatures had been elevated by physicalmeans. In the latter instances, as in the presentexperiments, fever produced by endotoxin seemedto be additive to that produced by physical means.

Bannister noted that at elevated environmentaltemperature minute doses of endotoxin producedin man dangerously high fever and discomfortof such severity (including transient hypotension)as to make further experimentation unwise. Hedemonstrated that subjects in a heating cabinet

with elevated body temperatures and profusesweating developed anhidrosis after administrationof endotoxin and that injection of pilocarpineinto the skin of the anhidrotic subjects led to lo-cal restoration of sweating in the injected area.This demonstration that the peripheral sweatingmechanism was intact led Bannister to concludethat inhibition of the sudomotor response resultedfrom the central action of endotoxin on the au-tonomic nervous system. The concept of a criticallocus of action of endotoxin in the autonomicnervous system has received earlier support fromthe work of Reilly and his colleagues (26-28),Penner and Klein (29), Zahl and Hutner (11),Bennett, Petersdorf, and Keene (30), and others.Recent observations of Porter and Kass (6) alsosupport the concept that the increased lethalityof endotoxin in the febrile animals is mediatedthrough a central mechanism. They observed thatendotoxin was lethal in smaller doses when in-jected directly into the carotid arteries of rats thanwhen administered via the aorta and that in ratsmade hyperthermic by elevation of the ambienttemperature, the LD,0 of endotoxin given via thecarotid route decreased in proportion to the de-crease in LDo by the systemic route. Stereotacticexperiments have also shown that ablation of aposterior hypothalamic area protects against anotherwise lethal dose of endotoxin (31).

Mediation of the lethal action of endotoxin byeffects on central thermoregulatory and vasomotorcenters is also suggested by the present observa-tions that a) elevated environmental temperatureand LSD increase susceptibility to endotoxin de-spite the opposite peripheral vasomotor responses(vasodilatation and vasoconstriction, respectively)that accompany their use to produce fever, b) thatperipheral vasomotor and thermal responses toendotoxin may be modulated by the resting bodytemperature during the early phases of the re-sponse to endotoxin (5, 6), and c) that LSD, anautonomic stimulatory agent (32, 33), appears toincrease significantly susceptibility to endotoxineven in animals that are prevented from becom-ing febrile by shearing their fur.'

4Gilbert's (34) experiments on the influence of pre-treatment with LSD upon the acute circulatory changesin cats after endotoxin challenge showed no adverseeffect of LSD, but the experimental circumstances wvereso different as to constitute no contradiction of the pres-

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ROGERP. ATWOODAND EDWARDH. KASS

Fever and endotoxin may interact at centralthermoregulatory and vasomotor control centersto produce the observed effects. Endotoxin, inaffecting such centers, would be especially damag-ing to centers that, as in the present experiments,are already undergoing increased metabolic ac-tivity resulting from the preparatory inducedfever.

There is evidence that induced fever in manpotentiates the effects of small doses of endotoxin.This is shown by the aforementioned observationsof Bannister (25) and also by studies, currentlyin progress, described by Wolff, Rubenstein, andMulholland (35). Fever was induced in volun-teers by a series of injections of the pyrogenicadrenal steroid etiocholanolone. Before and afterthe etiocholanolone, the subjects were challengedwith small doses of endotoxin. The responsesfollowing etiocholanolone were exaggerated ascompared to the response before the steroid-in-duced fevers. It seems likely, therefore, that theeffect of fever in increasing susceptibility to thelethal action of endotoxin may be applicable alsoto man. If this is true, it may carry importanttherapeutic implications for the clinical manage-ment of systemic endotoxin producing infections,particularly in the earlier stages when high feveris commonly present before shock and spontane-ous hypothermia supervene. Two reports haveappeared in which moderate induced hypothermiahas been used in the treatment of septic shock(36, 37). In both of these, the authors reportedthat hypothermia was of some benefit, but the caseseries were too small to draw valid quantitativeconclusions concerning differences in case salvagerate with hypothermia.

Whether prevention of fever in endotoxemia isprotective for man can only be settled by accumu-lation of a large and carefully matched case se-ries because of the tendency for the outcome to bedetermined inevitably (irrespective of therapeuticmeasures) by the underlying disorder that gaverise to endotoxemia and by other features of thecomplex host response to infection.

ent findings. The "initial, acute" circulatory responses toendotoxin challenge studied and illustrated by Gilbertwere those occurring within a few seconds after intra-venous administration of endotoxin to anesthetized catswith cannulae placed in their carotid and pulmonaryarteries.

Summary

1) The induction of fever in rabbits by physicalmeans (elevation of environmental temperature)or by pharmacological means (administration ofD-lysergic acid diethylamide, LSD) reduces theLD50 of bacterial endotoxin from approximately240 ug per kg to 4 to 13 jug per kg

2) Hyperthermic rabbits exhibit augmentedfebrile responses to endotoxin and acceleration ofendotoxic effects leading to an earlier lethal out-come.

3) When rabbits are shorn of their fur, theyinvariably respond to endotoxic challenge at roomtemperature with profound hypothermia and areresistant to the lethal action of endotoxin with anLD50 approximately 10 times higher than that ofunshorn animals challenged at room temperature.

4) When the body temperatures of shorn ani-mals are elevated by exposure to elevated environ-mental temperature, the febrile response and LD50of endotoxin are restored to levels characteristic ofunshorn animals.

5) LSD augments the lethal action of endotoxineven in shorn, hypothermic rabbits.

6) The findings are explained on the hypothesisthat increased metabolic activity of central vaso-motor and thermoregulatory loci accompanies py-rogenesis and that such loci are increasingly sus-ceptible to endotoxin as their metabolic activityincreases.

Acknowledgment

The authors gratefully acknowledge the technical as-sistance of Miss Mei Lie Uong.

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8. Sanarelli, J. Moyens de defense de l'organismecontre les microbes apres vaccination et dans laguerison. Ann. Inst. Pasteur 1893, 7, 225.

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