Poxvirus infection of the baboon (Papio cynocephalus)*

Poxvirus infection of the baboon (Papio cynocephalus)* R. L. HEBERLING,1 S. S. KALTER,2 & A. R. RODRIGUEZ 3 Ten serial passages of monkeypox (MPV), vaccinia, variola, and chimpanzeepox (chimp-9) viruses were performed in baboons (Papio cynocephalus) via skin scarification. Comparisons of clinical and virological results indicate that MPV and vaccinia are very closely related and that variola and chimp-9 viruses are identical. These findings suggest that infections of simians with chimp-9 virus resulted from contact with variola virus, the source of which is still unknown. On the other hand, MPV in monkeys may have resulted from contact with recently vaccinated humans, serial passage through simian hosts resulting in the biological alterations that produced MP V. Poxviruses of different types exist in many species of mammals and birds. This widespread distribution of virus presents a potential threat in any effort made to eradicate human poxvirus disease. Thus, as all indications are that smallpox is fast disappearing from geographical areas known to harbour this virus, the recent and unexpected occurrence in West Africa of a disease clinically indistinguishable from variola was viewed with some alarm. The etiological agent isolated and identified from these human cases was shown to be similar to other viruses in the variola- vaccinia group in its biological characteristics and to have the properties of classical monkeypox virus (MPV). The existence of a poxvirus disease of monkeys was first described by von Magnus and his co-work- ers in 1958 (15). Since then, a number of monkeypox outbreaks have been reported in simian colonies. However, human disease due to close contact with MPV-infected animals or with naturally occurring disease in wild monkey populations has not been reported (1). The close similarity between human and simian poxviruses has provoked inquiries regarding the clinical and epidemiological relationships of these * From the Southwest Foundation for Research and Education, San Antonio, TX, USA. 1 Foundation Scientist, Microbiology and Infectious Diseases. 2 Director, Microbiology and Infectious Diseases. ' Associate Foundation Scientist, Microbiology and Infectious Diseases. diseases. Of immediate concern is the possibility that monkeys may serve as a reservoir for smallpox. In general, serological surveys (2, 13, 14) failed to sub- stantiate this consideration; nevertheless, sufficient numbers of seropositive animals have been reported by several laboratories to keep this possibility open (2, 8, 9, 12, 13, 14). Another consideration that lent itself to investi- gation was that a poxvirus variant with altered patho- genicity could develop as a result of passage through monkeys or interchange between different species of primates (or possibly other animals). Therefore, a study was initiated involving the serial passage of monkeypox, variola, vaccinia, and chimpanzee pox- viruses in the baboon (Papio cynocephalus). Chim- panzee poxvirus was selected because it is a non- human primate isolate with properties similar to those of variola virus (11). This paper reports com- parative observations of the disease and virus stud- ies resulting from 10 passages of each virus in baboons. MATERIALS AND METHODS Animals The baboons used in this study were immature animals of both sexes, ranging in age from 4 to 22 months. These animals were born in the colony maintained at the Southwest Foundation for Re- search and Education (SFRE), and their mainten- ance and husbandry have been described in detail elsewhere (10). 3520 - 285 BULL. WORLD HEALTH ORGAN., Vol. 54,1976 R. L. HEBERLING ET AL. Viruses The Copenhagen strain of monkeypox virus (15), the chimpanzee isolate designated as chimp-9 (11), the Lilly vaccine strain of vaccinia, and the Harvey strain of variola major (ATCC) were used as inocula. Stock cultures of these viruses were prepared by cultivation on chorioallantoic membrane (MPV) or Vero cells (chimp-9, variola). Vaccinia virus was obtained commercially and was used as glycerolated calf lymph. The original titres for these viruses per 0.1 ml were as follows: monkeypox virus 107-° TCID50 chimp-9 virus 104.7 TCID50 vaccinia 108-0 TCID50 variola 106 3 TCID50 Titrations were performed on primary baboon kid- ney cells. Inoculaition of animals Each animal was sedated with phencyclidine or ketamine. The back was then clipped, cleansed with 70% ethanol, and air dried, and an area approxi- mately 10 by 10 cm was scarified with the point of a hypodermic needle. Virus was inoculated by rub- bing the scarified area with cotton-tipped swabs saturated with virus suspension. Serial passage was made in a similar manner following harvest of the virus on day 7 or 8 by scraping the pustular lesions with a sterile scissors blade. The collected material was suspended in 5.0 ml of Eagle's minimal essential medium containing 10% fetal calf serum and antibiotics. Passage from animal to animal was carried out immediately on collection of the virus without prior freezing and thawing. T'he remaining virus suspension was stored at -90°C for further study. Infected animals were observed at frequent inter- vals for signs of clinical illness. Throat and rectal swabs, as well as blood, were periodically collected for virus isolation, serology, and haematology. In certain instances animals were sacrificed for virus isolation attempts from various tissues. Virus recovery and serology The procedures used for the isolation of virus from various specimens as well as the technique for per- forming the haemagglutination inhibition (HI) test have been described in detail elsewhere (6, 7). Vac- cinia and MPV antigens were used to test the sera of all animals for HI antibody. Virus isolations from tissue were generally performed by inoculation of primary baboon kidney cells with small pieces of fresh tissue mince. RESULTS Initial evidence of infection was similar for all four viruses inoculated-namely, induration along the scarifications and the development of erythema. Following the appearance of clinical illness, the animals varied in the severity of the disease observed. Monkeypox virus The first-inoculated baboon showed erythematous and papular lesions along the lines of scarification and an associated rise in temperature (rectal) 3-4 days after infection. This condition progressed in severity, maximum fever being noted on the 6th day along with enlarged and inflamed axillary and inguinal lymph nodes. Over the next few days, numerous secondary lesions appeared on the trunk, extremities, and face, including the mouth. These lesions became vesicular, pustular, and haemorrhagic, at which time they were scraped for passage material. Even- tually the animal recovered, the lesions developing scabs and healing. Infection of the second baboon followed a some- what similar course, but with a marked decrease in the number of lesions secondary to the site of inocu- lation. Baboons inoculated subsequently in this series manifested lesions only at the inoculation site. These lesions, which developed between days 3 and 7, were initially papular, changing to haemorrhagic pustules. Fever and adenopathy developed between the 4th and 6th days post inoculation. A second series of baboons, younger than the first group (Table 1), was inoculated in an attempt to repeat the interesting observations noted in the first group. In general, the first two inoculated ani- mals clinically resembled those observed in the first experiment except for the extent of secondary lesions, which were markedly reduced in number. These lesions, restricted to the face, neck, and groin, were small, dry, and persisted for only a short period of time. Pustular lesions at the site of inoculation con- tained increasing amounts of fluid with each pass- age, tending to resemble vaccinia lesions. Healing progressed rapidly in the first three animals. The scarified sites in passages 4-8, however, became increasingly haemorrhagic and subcutaneously oede- matous, extending to the hips and base of the tail on the dorsal side, to the abdomen and chest on the ventral side, and producing a " puffy " appear- ance. Baboons of passages 4, 5, 6, and 8 died in a similar fashion on either day 9 or 10 post inocu- lation, whereas the baboon of passage 7, which ap- 286 POXVIRUS INFECTION OF THE BABOON Table 1. Serial passage of monkeypox virus in baboons Virus titre Appearance Passage Animal Age (days) in harvest Outcome Aeoneper 0.1 ml Oucoeefsecondsege(-logio TCID50)lein Experiment 1 1 1 - B844 female 273 6.5 . . . 2 1-B847 female 281 7.0 ++ 3 1 -B833 male 336 6.5 4 1-9991 female >365 7.0 5 1 -B836 male 340 7.5 Sacrificed at >365 days a - 6 1-9911 female >365 6.0 7 1-9921 female >390 6.5 8 1-9981 female >365 6.5 9 1-8852 female 306 6.0 10 1-9971 female >365 7.0 Sacrificed at 7 days - Experiment 2 1 1 X243 male 106 6.5 Sacrificed at 35 days + 2 1 X262 female 96 6.5 Sacrificed at 116 days + 3 1 X265 female 93 7.0 Sacrificed at 180 days 4 1 X316 female 84 6.5 Died at 9 days 5 1X317 female 88 6.0 Died at 10 days + (edema) 6 1 X31 9 female 89 6.5 Died at 9 days + 7 1X326 male 80 7.0 Died at 17 days + (edema) 8 1 X328 female 86 5.5 Died at 9 days + (edema) 9 1 X330 male 84 5.5 Sacrificed at 132 days 10 1 X339 male 80 6.5 Sacrificed at 7 days a Animals sacrificed after 7 days were in good health and considered to be survivors. peared to be recovering, died on day 17 with macro- scopic evidence of a pneumonia (virus was not isolated from lung tissue). The results of these two series of inoculations are presented in Table 1. The amount of virus found in the lesions of infected baboons was fairly uniform in both experi- ments (Table 1). The results of virus isolations from throat, rectal swabs, and blood in the second experi- ment are presented in Table 2. Virus was constantly isolated from throat, stool, and blood samples. Persistence of virus in these sites was generally restricted to the first 7 days post inoculation, but virus was also found 10, 14, and 28 days post infection in throat or rectal swabs. HI antibody varied in individual animals but was generally de- tected by the 7th day, the highest titre being attained about the 14th day. Titres started falling after 42 days, but were still evident after 70 days. Haema- tological studies showed a leucocytosis approxi- mately 7 days post infection when skin lesions and fever were maximum and virus was readily demon- strated in throat and rectal swabs, as well as in the blood. The predominant leucocytes observed were polymorphonuclear neutrophils, which showed a toxic reaction, and immature forms (stab cells). HI antibodies usually were detected when the leuco- cyte count returned to normal between day 8 and day 14 post infection. Bacteriological examination of all specimens failed to demonstrate a specific etiological organism. In the first experiment the tenth animal (1-9971) in the series was sacrified 7 days after infection 287 R. L. HEBERLING ET AL. Table 2. Isolation of monkeypox virus and Hi titre of inoculated baboons Virus isolation a Maximum Organs Passage Animal no. Throat Rtl vaccination positive swab swab Blood b HI titre for virus c,d Experiment 1 1 B844 - + (7) ND >320 - (>365) 2 B847 - - ND >320 - (>365) 3 B833 - - ND >320 - (>365) 4 9991 + (7) - ND >320 - (>365) 5 B836 - - ND 160 - (>365) 6 9911 + (10) - ND 160 - (>365) 7 9921 + (7) + (7) ND 20 - (>365) 8 9981 + (8) - ND 160 - (>365) 9 B852 + (7) - ND 160 - (>365) 10 9971 - - ND - Spl.; Ax. Ly.N. (7) Experiment 2 1 1X243 + (14) + (7) + (7) >320 - (35) 2 1X262 - + (7) + (7) 160 - (116) 3 1X265 + (7) + (28) + (7) >320 - (180) 4 1X316 + (7) + (7) + (7) < 10 Notdone (9) 5 1X317 + (7) - + (7) 40 Lu.; Ly.N. (10) 6 1X319 + (7) + (3) + (7) < 10 He.; Lu.;Thy.; Li.; Kid.; Mes.; Ax., Ing. Ly.N. (9) 7 1X326 - + (7) + (3) 160 Ly.N. (17) 8 1X328 + (7) + (7) + (7) < 10 Ad.; Kid.; Lu.; Thy.; Ly.N. (9) 9 1X330 - - - >320 - (132) 10 1X339 + (7) + (7) - < 10 Ax., Ing. Ly.N. (7) a Isolation on day shown in parentheses. b ND, not done. c Ad, adrenal; Ax, axillary; He, heart; Ing, inguinal; Kid, kidney; Li, liver; Lu, lung; Ly.N., lymph node; Mes, mesenteric; SpI, spleen; Thy, thymus. d Numbers in parentheses indicate day of death or sacrifice. (Table 1). Virus was readily isolated from the spleen and axillary lymph nodes of this baboon (Table 2). In the second experiment, baboon tissues from passages 5-10 (the ninth passage animal sur- vived and was sacrificed at day 132) obtained upon death or sacrifice from day 7 to day 17 were con- sistently positive, poxvirus being obtained from heart, lung, thymus, liver, kidney, adrenal, lymph nodes (axillary, inguinal, and mesenteric), and sub- cutaneous edematous areas (Table 2). None of the tissues from survivors contained virus. Vaccinia virus Inoculation of baboons with vaccinia virus resulted in a clinical picture similar to, but less severe than, that following inoculation of the Copenhagen strain of MPV. Papules developed on the third day in all 10 baboons and rapidly progressed to large haemor- rhagic pustules by day 7 or 8, when material was collected for passage. Vaccinia lesions were gen- erally limited to the site of inoculation with only 1-2 secondary pustules developing in several of the animals. Vaccinia lesions were larger and more pus- 288 POXVIRUS INFECTION OF THE BABOON Table 3. Isolation of vaccinia virus and Hi titre of inoculated baboons Lesion Virus isolation a_Maximum Organs Animal no. titre Throat Rectal Blood b vaccination positive(-logia ) swab swab Blo' HI titre for virus c, d B914 7.0 + (6) _ ND - Li.; Thy.; He. (6) a B784 5.5 + (4) + (4) - 160 - (85) B916 5.7 + (4) + (7) - 40 - (1) B788 6.0 + (4) + (7) - 40 - (113) B919 7.0 + (4) + (4) + (4) 320 - (15) B874 6.0 + (8) + (8) - 320 - (42) B921 6.3 + (6) - - 160 - (17) B887 6.5 + (7) - - <640 - (60) B926 6.5 + (4) + (7) - ND - (50) B897 5.5 + (7) + (7) - - Ax. Ly.N. (7) a Isolation on day shown in parentheses. b ND, not done. c Ax, axillary; He, heart; Li, liver; Ly.N., lymph node; Thy, thymus. d Numbers in parentheses indicate day of death or sacrifice. tular than the MPV lesions, eventually becoming haemorrhagic and necrotic prior to healing. As seen in Table 3, virus was always detected in the lesion and could be isolated with ease from throat and stool specimens from day 4 to day 8 following infection. Viraemia was not determined except in passage 5. Positive serology was indicative of infection, but again there was variation between individual animals. As seen with the other poxvirus infections, HI antibody titres rapidly decreased after reaching a peak. The two animals that were sacrificed 6 and 7 days after infection were found to have virus in the liver, thymus, heart, and axillary lymph nodes. A leucocytosis occurred during the first week of infection; however, the polymorphonuclear neutrophils did not show as marked a toxic reaction as that seen in MPV-infected animals, nor were there as many stab cells. Chimp-9 virus Inoculation of baboons with this virus resulted in a somewhat different clinical picture from that observed following monkeypox and vaccinia virus inoculations. Erythematous and papular lesions were noted at the site of inoculation 3-4 days after inocu- lation. These lesions changed over the next 3-4 days to vesicles and pustules, at which time the virus was generally harvested and the next passage made. Secondary lesions were either totally absent or limited to one or two near the site of inoculation. Very little fluid was found in the vesicles, which healed rapidly without complications. The dryness of the lesion was very similar to that observed in the animals inoculated with variola virus. Poxvirus was isolated from scarified lesions and from throat and rectal swabs and blood as early as 3 days (blood and throat) and as late as 21 days (stool) after infection. As virus isolation attempts were not continued past 21 days it is not known whether virus was excreted after this time. Animals sacrificed 7 days after infection yielded poxvirus from lungs, spleen, adrenals, pancreas, and mesenteric lymph nodes, and from the site of inoculation (Table 4). The HI titre reached a maximum of 1: 80 at days 13-24 then fell to 1: 10 or less, but antibody was still evident in some animals as late as 83 days after inoculation. Titres were generally the same for both antigens (vaccinia and MPV), but were not detected until the leucocyte count returned to normal about day 9 post infection. Haematological studies showed that a leucocytosis was evident by days 7-10, when the skin lesions were at their maximum and virus was appearing in the throat and intestinal tract. Stab cells were gen- erally absent or few in number and polymorpho- nuclear neutrophils did not appear to be toxic. 289 R. L. HEBERLING ET AL. Table 4. Isolation of chimp-9 virus and HI titre of inoculated baboons Lesion Virus isolation a Maximum Organs Animal no. titre vaccination positive (-logI) Tshwoabt Rectal Blood HI titre for virus b, c B988 4.5 + (8) + (8) - _ Spl. (8) B992 5.5 + (9) - + (5) 80 - (83) B993 5.0 - + (21) _ 80 - (81) B994 5.7 + (7) + (7) - 80 - (78) B995 5.0 + (7) - + (3) - Lu.; Ad.; Pa.; Mes. Ly.N.; SpI. (7) B997 4.7 + (7) + (7) + (7) <10 - (22) B999 4.5 - - - 40 - (59) B972 5.0 - - + (7) 20 - (48) B973 5.0 - - - 40 - (43) B970 5.3 + (3) -- - (7) a Isolation on day shown in parentheses. b Ad, adrenal; Lu, Lung; Ly.N., lymph node; Mes, mesenteric; Pa, pancreas; SpI, spleen. c Numbers in parentheses indicate day of death or sacrifice. Variola virus Variola virus inoculations produced lesions that progressed from papules to vesicles and pustules during the first week of infection. The most notable feature of the variola lesions, in which they some- what resembled those produced by chimp-9 virus, was their relative dryness. The dryness made for difficulties in collecting passage material at the time skin scrapings were taken on days 7-8 after infection. Progress of these lesions appeared to be Table 5. Isolation of variola virus and HI titre of inoculated baboons Lesion Virus isolation a Maximum Organs Animal no. itre vaccination positive(-9ogia) swab Rectal Blood HI titre b for virus c, d B915 6.7 + (4) + (4) - - Ax. Ly.N. (7) B787 5.2 + (7) - + (4) 80 - (71) B918 6.5 + (4) + (7) + (7) > 20 - (11) B871 5.7 - - - 160 - (86) B920 5.5 - - - > 40 -(14) B886 5.7 - - - 80 - (84) B922 6.0 - - - 80 - (18) B888 6.3 - - - ND - (84) B925 7.5 + (7) - + (3) > 20 - (15) B904 5.3 - - - > 20 - (8) a Isolation on day shown in parentheses. b Not done. c Ax. Ly.N., axillary lymph node. d Numbers in parentheses indicate day of death or sacrifice. 290 POXIVIRUS INFECTION OF THE BABOON slower than that seen in lesions due to MPV or vaccinia virus, but virus was isolated from the site of inoculation in all passages (Table 5). Secondary lesions were not seen in any of the passages. Variola virus also was isolated from throat, stool, and blood during the first week of infection (days 3-7), as well as from an axillary lymph node of the first animal sacrificed on day 7 post infection. No virus was recovered from tissues on the indicated days (Table 5) when the animals were sacrificed. Antibody (HI) was determined in all animals after day 8 post infection (Table 5). The leucocytosis observed in all the poxvirus baboons was not seen until the end of the first week or later. In general, the haematological picture was quite similar to that seen in chimp-9 infected animals. DISCUSSION The overall pattern of infectivity appeared similar in monkeypox (Copenhagen), vaccinia, variola, and chimp-9 poxviruses passaged through baboons. Some notable differences were observed, however, suggest- ing that these four viruses may perhaps be separated into two groups: (I) monkeypox and vaccinia, and (2) chimp-9 and variola. These differences, which are summarized in Table 6, related to the appear- ance of lesions, the development of haematological changes, and the HI antibody response. Additional studies (unpublished observations) also lend support to the suggested separation of the viruses into two groups. Suspensions of vaccinia, chimp-9, and variola viruses obtained from the pass- age lesion showed no alteration in plaque morphol- ogy when tested on Vero, LLCMK4, and primary baboon kidney cell cultures. Monkeypox virus pla- ques, however, became larger and more like those of vaccinia with passage, especially when observed in baboon kidney cell cultures. Gispen & Brand- Staathof (4) previously showed that the Copenhagen strain of MPV produced small haemorrhagie pocks and a smaller number of larger white pocks on chicken embryo chorioallantoic membrane. Passage in baboons may have selected for this white pock variant. The course of the disease in baboons infected with MPV was more vaccinia-like than variola-like, and this similarity tended to increase with passage. In the first experiment with MPV no deaths were observed and the early passage demonstrated more extensive secondary lesions spreading over the entire body. The number and severity of these lesions diminished with each subsquent passage. In the second experi- ment more than 50% of the animals succumbed after the fourth passage. While these experiments may appear to be somewhat contradictory, it must be emphasized that the greatest number of fatalities occurred in the second experiment, in which animals less than 3 months of age were used. This contrasts with the first experiment, in which the animals were generally over I year of age. Increased resistance to MPV with age was observed by us in an earlier experiment when newborn baboons died as a result of MPV infection, whereas 1-month-old animals Table 6. Results of serial passage of poxviruses in baboons Virus Appearance Secondary Alterations Leucocy- HI antibody of lesions lesions during passage tosis Appearance Peak Decline Monkeypox Extensive with Generalized in first Secondary lesions 4-7 10-12 12-15 27-30(Copenhagen) less fluid than two passages, spreading diminished in number vaccinia; 3-7 to head, face, and with passage; plaque days extremities morphology altered in tissue culture Vaccinia Purulent and 1 or 2 localized None observed 4-7 11-15 18-21 28-34(IHD) localized, 3-7 days Chimp-9 Dry and localized; Totally absent or 1-2 None observed 7-10 9-14 13-23 17-34 3-7 days localized Variola Dry and localized; Totally absent None observed 6-11 8-15 11-14 21-29(Harvey) 3-7 days a Generally accompanied by neutrophil increase and fever. 291 R. L. HEBERLING ET AL. survived (5). More animals of similar ages are ob- viously necessary for each passage so that more sig- nificant interpretation of the data is possible. As stated above, chimp-9 virus lesions consistently more resembled those caused by variola virus rather than those produced by vaccinia virus or MPV. The major characteristics common to chimp-9 and variola virus infections were the dryness of the lesions and the failure to develop secondary lesions. How do these data help in relating human disease to viruses derived from nonhuman primates? Virus with the properties of monkeypox virus has been isolated on a number of occasions during outbreaks of disease in captive monkeys. Chimp-9 virus, as well as several similar viruses previously recovered from symptomless cynomolgus monkeys, were iso- lated from kidney cell cultures derived from " nor- mal" animals. Biological characterizations of these virus isolates indicate they are very close to variola virus. Passage of chimp-9 and variola viruses through baboons without any evidence of alterations con- tinues to support the view that these two viruses are related. Recent comparative studies by Gispen (3) have also demonstrated close similarity between chimp-9 and variola viruses. These findings may be interpreted as indicating that chimp-9 virus (and those viruses related to it) resulted from an accidental infection of the particular animal (or of its tissues). The original source of infection, however, remains obscure, although there were human cases of variola in the geographical areas where these monkeys were obtained. The data reported here also demonstrate the simi- larity between chimp-9 virus and variola in their pathological properties. MPV differs significantly from these two viruses but does resemble vaccinia. Passage of MPV through baboons increases this resemblance, leading to a vaccinia-like plaque variant as evidenced by growth in cell culture. This alteration in pathogenicity suggests that vaccinia virus and the Copenhagen strain of MPV may have common biological components. It is conceivable that the original infection of monkeys resulted from contact with recently vaccinated humans. The MPV infection first seen in macaques (15) and in subsequent occur- rences may have led to alteration of the vaccinia virus, giving rise to a virus population with mixed properties. Under the conditions of our first attempt at serial passage of MPV in baboons, the wild type vaccinia virus was selected. The relationship of MPV to vaccinia requires further study of the properties of the various primate poxvirus isolates and infection of nonhuman pri- mates using different routes of inoculation and ani- mals of different species and ages for elucidation. Gispen (3) has demonstrated that MPV differs from vaccinia virus by virtue of certain antigens: 4'vc") and " va " in vaccinia and " mo " in MPV. As these antigens apparently are surface antigens, their pres- ence may reflect passage through different hosts. Further, various monkey sera when absorbed with vaccinia or monkeypox viruses lost their titres for both of those viruses (some titre remained for MPV when absorbed with vaccinia), again suggesting the very close relationship between the two viruses. In attempting to understand the epidemiology of human infection with monkeypox virus the question of latency has some relevance. Inasmuch as several poxviruses have been isolated from presumably nor- mal animals, we examined various tissues, especially kidneys, whenever one of the animals in this study was sacrificed as long as 1-2 years post infection. In no instance was a poxvirus recovered later than 3 weeks post inoculation. Here, too, additional stud- ies are necessary to confirm or refute this possibility. Serological testing of simian sera may also lend some support to the suggestion of simian infection. While initial serum testing was equivocal or nega- tive (2, 13, 14), more recent studies in several labora- tories (8, 12) have demonstrated seropositive mon- keys. The serological data reported here also demon- strate that titres, at least as measured by the HI test, decrease rapidly. However, limited studies by us indicate that detectable HI antibody may last as long as I year. After this, serum neutralizing anti- body is still present and it is, therefore, to be pre- ferred for serum surveys. All these studies emphasize the need for in-depth investigations along the lines described in this report. Differences in pathogenicity have been noted. It is now necessary to ascertain whether or not passage of a poxvirus through a simian host will result in changes making it indistinguishable from other recog- nized poxviruses. 292 POXVIRUS INFECTION OF THE BABOON 293 RESUME INFECTION 'A POXVIRUS DU SINGE PAPIO CYNOCEPHALUS On a passe en serie, par scarification cutanee, sur des singes Papio cynocephalus divers poxvirus (monkeypox ou MPV, vaccine, variole, et le virus isole du chimpanze ou echimp-9))) en vue de comparer 1'evolution de la maladie produite par ces virus et de determiner si les proprietes de ces derniers etaient modifiees au cours des passages. Dans tous les cas, il y avait une induration et un erytheme le long des scarifications 3 ou 4 jours apres l'inoculation. Ensuite, les lesions passaient par les stades papules, vesicules, pustules, crouites, puis finale- ment elles guerissaient. Les lesions observees sur les singes infectes de MPV devenaient de plus en plus pustu- leuses avec les passages, ressemblant aux lesions dues au virus de la vaccine. Les lesions pustuleuses suscitees par virus de la variole et chimp-9 etaient semblables, et beaucoup plus seches que les lesions dues A la vaccine. Des lesions secondaires, en dehors de celle du point d'ino- culation, s'observaient surtout avec le MPV, mais seule- ment apres le premier et le deuxieme passage au cours de la premiere experience. Chez les jeunes animaux, les lesions secondaires apparaissaient plus regulierement. Elles etaient rares avec les trois autres virus. On observait une leucocytose et de la fievre 4 A 7 jours apres l'inocu- lation de MPV et de la vaccine, et 6 A 11 jours apres l'inoculation des virus de la variole et chimp-9. Peu apres, apparaissaient des anticorps dont le titre atteignait son maximum 2 A 3 semaines plus tard. On a isole du virus de la gorge et d'ecouvillonnages rectaux, du sang et d'echantillons d'organes au cours de la premiere semaine qui suivait l'inoculation, et cela pour les quatre virus; cependant, l'isolement etait moins frequent dans le cas des virus chimp-9 et de la variole que dans ceux du MPV et de la vaccine. L'infection a e mortelle seule- ment chez de jeunes singes inocules de MPV; tous les autres animaux ont survecu et ont ete sacrifies. II a gene- ralement e impossible d'isoler le virus A partir des survivants deux semaines apres l'inoculation. Le MPV isole A partir du siege de l'inoculation apres 10 passages sur Papio cynocephalus produisait en culture de cellules des plages qui ressemblaient A celles que donne le virus de la vaccine; elles etaient plus grandes que les plages du MPV d'origine. Pour les autres virus, aucune altera- tion des caracteres n'a etd notee en culture de cellules A la suite des passages. 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