586 



NA rURE 



[April 23, 1908 



Liston's ingenious method of experimentation with 

 guinea-pigs. It has been shown that these animals 

 are very rarely infected from soil or clothes grossly 

 contaminated with cultures of the Bacillus pestis, or 

 from other plague-infected guinea-pigs, so long as 

 lleas are excluded. On the other hand, they readily 

 contract plague when exposed to the attacks of fleas 

 which have fed on plague-infected animals. Follow- 

 ing up this method, the commission placed healthy 

 guinea-pigs in buildings where it appeared that 

 plague was contracted, and found that the test 

 animals died of plague, and, further, that the fleas 

 they picked up in these surroundings could be trans- 

 ferred to fresh aniinals in the laboratory and infect 

 them also with plague. An exceedingly interesting- 

 experiment on these lines was carried out in Sion 

 ^'illage. One part of this village was evacuated by 

 the inhabitants on the discovery of a dead rat. The 

 commission at once took advantage of the " guinea- 

 pig method," and substituted a population of healthy 

 guinea-pigs for the decamped inhabitants. These 

 guinea-pigs were confined to their several houses, 

 and all chance of direct spread of infection among 

 them was excluded. In these circumstances an 

 epizootic appeared among the new population, which 

 progressed through the village, following the dis- 

 tribution of a simultaneous epizootic among the rats 

 (A/, ratftis). The guinea-pig plague was so severe 

 that thirtj'-six out of fifty-one animals put in the 

 houses died. 



The evidence brought against the direct spread of 

 plague from man to man rests on the immunity of 

 hospital staffs, and of friends of patients in hospital, 

 who very frequently lived at the bedside throughout 

 the course of the disease. Again, the village of 

 Worli offered an example of what often occurs. 

 Three cases of plague were imported into the village, 

 but in the absence of an epizootic the infection did 

 not spread. 



With regard to the beginning of an epizootic in a 

 fresh locality, it seems that infection may be carried 

 bv a healthy man and spread from him to the rats 

 of his dwelling. Such infection, the commission 

 supposes, is flea-borne. The yearly recrudescence ol 

 plague may be due to such fresh importation, or to 

 the persistence of acute plague throughout the year, 

 aft'ecting only a few rats. The commission found in 

 two Punjab villages that cases of chronic plague 

 abscess in rats occurred at intervals all the year 

 round. Whether such cases might be able to re- 

 kindle an epizootic of acute plague is not clear, and 

 seems to demand further experiment. 



With regard to the spread of infection during an 

 epidemic, the Queensland report gives us valuable 

 data. Mr. Burnett Ham, dealing with small 

 epidemics, and a population more amenable than 

 that of Bombay to sanitary regulation, was able to 

 trace definitely the source of infection in a large 

 majority of cases to houses, stores, or ships where 

 the presence of infected rats was proved. An excep- 

 tional occurrence was the outbreak of pneumonic 

 plague in Maryborough in 1905. The infection in 

 this instance spread directly from patient to patient ; 

 nine cases occurred. 



The seasonal exacerbation of plague remains un- 

 accounted for. In Queensland and in Bombay, in 

 opposite hemispheres, the plague-seasons still broadly 

 coincide, though the plague-maxima in Queensland 

 varied considerably — from February to August — in 

 different years. A study of the figures given for the 

 variations in the flea-infestation of rats does not 

 reveal a complete explanation of the phenomenon. For 

 instance, the Punjab experiments show that fleas 



NO, 2008, VOL. 77] 



become frequent months before the plague season 

 begins. Fleas were actually on the decrease when 

 rat-plague was rising to its maximum in Kascl hnd 

 Dhand. Nor can we suppose that the temperature 

 in Bombay in the cold season is low enough to keep 

 plague in abeyance until the spring. We learn from 

 the reports that the Tiiean temperature of Bombay 

 for the cold season is about 75° F., and a glance at 

 the chart, which gives meteorological data in con- 

 nection with the Queensland epidemics, shows that 

 plague may appear, spread, and maintain itself 

 under a mean temperature varying between 50° and 

 70° F. The Queensland repbrt, in addition to 

 epidemiological data, gives a large amount of clinical 

 information, and experiments on the prophylactic 

 and curative value of anti-pest sera. Experiments 

 with rat-viruses carefully maintained at high viru- 

 lence gave results more encouraging in the labor- 

 atory than in the field. 



Major G. Lamb is responsible for the third volume 

 under notice, and he has used the opportunity to 

 give us a very clear outline of the present state of 

 knowledge on the subject of plague. He also indi- 

 cates the lines along which sanitary measures may 

 be taken with advantage. While stating that this 

 summary represents his personal opinions, he shows 

 how these opinions arise logically out of the ex- 

 periments carried out by the commission. 



The lacunae in our knowledge are brought into a 

 healthy prominence by a summary of this nature. 

 We do not yet possess trustworthy information as to 

 rat population (part ii. B.), while our only means 

 of taking a census is that of recording the percentage 

 number of rats caught per traps set. One thing is 

 pretty clear-:— that the most efficient trapping will 

 never of itself reduce the rats of any considerable 

 area below the minimum necessary for the spread of 

 an epizootic. 



Part iii. disposes of transmission of infection by 

 direct contact, by the air, by food, or through the 

 soil, and gives the proofs for transmission by the 

 rat-flea. Direct contagion certainly occurs in 

 pneumonic plague, but this is so rare as to fall out- 

 side the main problem. 



Though man plays a minor rdle in the spread of 

 an epidemic (part iv.), yet he is directly concerned in 

 the importation of the disease into healthy areas 

 (part vii.). It has been shown that hungry rat-fleas 

 will cling to man and yet refrain from feeding for a 

 considerable time. The man in the meantime may 

 have travelled some distance and have arrived at a 

 house where rats are abundant. His guests will 

 then leave him for their natural hosts, and thus the 

 rats may become infected while the man remains 

 healthy. 



At the end of the plague season in any locality 

 plague may die out completely, or the epizootic may 

 continue at a low level through the off-season. In 

 the latter case the locality becomes a focus from 

 which infection will be spread, with the arrival of 

 the next plague season, over the surrounding 

 country. Thus rats with acute plague are caught in 

 Bombay all the year round. 



The suin of these investigations may perhaps be 

 expressed as follows. Extinction of rats, extinction 

 of fleas, or exclusion of rats from dwellings might 

 anv one of them put an end to human plague, but 

 alf are exceedingly difficult to realise. On the other 

 hand, the conditions which determine the off-plague 

 season are not fully known, and may still give an 

 indication of some practicable method of fighting 

 the disease. Further investigation is urgently 

 demanded. L- N. 



