114 



THE AUSTRALIAN MUSEUM MAGAZINE. 



Rats and Fleas in Their Relation to Plague. 



By E. W. Ferguson, M.B., Ch.M. 

 Principal Microbiologist, Department of Public Health. 



It is a truism that has been demon- 

 strated on many occasions that we often 

 know little about the creatures that are 

 our closest associates, and it has needea 

 the discovery of their character as dis- 

 ease carriers to stimulate the study of 

 their life histories. The discovery ol 

 the role of the mosquito in malaria, of 

 the house fly as carrier of typhoid ana 

 dysentery, has resulted in a flood ot 

 light being thrown on these insects ana 

 their allies. In a similar way our in- 

 formation in regard to rats and fleas has 

 followed on the discovery of their con- 

 nection with human plague. 



Plague is a febrile disease caused by 

 a minute germ or l)acillus. In man the 

 disiease may occur in one of three form* 

 — Bubonic. Septicaemic or Pneumonic. 

 The bubonic fonn follows on the in- 

 oculation of the viinas and depends for 

 its character upon the arrest of the 

 plague baciUi in the nearest lymph 

 glands. The swollen and inflamea 

 glands constitute the "bubo." If the 

 bacilli succeed in passing this barrier 

 they may proceed to multiply in the 

 blood stream, this constituting septicae- 

 mic plague. This form is a terminal one 

 in fatal bubonic cases, but a case may 

 become septicaemic from the first. In 

 the pneumonic form the disease is lo- 

 cated in the lungs, and causes death in 

 hundred per cent, of cases. This form 

 of plague is highly infectious. 



The full knowledge of the manner in 

 which bubonic plague is transmitted and 

 the part played therein by the rat and 

 I'at flea is a quite recent acquisition. The 

 association of the rat with plague was 

 however at least suspected by the an- 

 cients, as is shown by the Biblical ac- 

 count of the outbreak of plag-ue amongst 

 the Philistines on the removal of the 

 ark of the covenant. It was for Dr. 

 Ashburton Thompson of Sydney to de- 

 monstrate that on epidemiological 

 grounds the disease could only spread 

 from rat to man through the agencv ot 



infected fleas, though previously it had 

 been shown that fleas were capable of 

 conveying the plague bacilli. The In- 

 dian Commissioners on Plague were 

 finally able to set the matter at rest by 

 direct experimental evidence. 



The main facts are simple and are 

 now a matter of common knowledge. In 

 a new locality plague is introduced by 

 infected rats who spread the disease 

 among the shore rats. In these animals 

 plagiie takes the form of a septicaemia, 

 that is to say the baeiUi or germs are 

 present and actively multiplying in the 

 blood; as many as 100,000,000 bacilli 

 may be present in 1 cubic centimetre ol 

 rat blood, and it is not a matter for an\ 

 wonder that bloodsucking insects like 

 fleas soon become themselves infected. In 

 turn the fleas infect other rats and, given 

 the chance, are capable of infecting hu- 

 man beings. 



In human bubonic plague there is- no 

 evidence that one person is infected 

 from another, in each case the source, 

 of infection is traceable back to the flea 

 and from the flea to the rat. 



RATS. 



There are many kinds of ra.ts in the 

 world, but practically only two species 

 are of importance in relation to plague. 

 These two, in common with the mouse, 

 are close associates of man, and with 

 him have travelled thi'oughout the world. 

 The two kinds are commonly known a* 

 the ])rown or sewer rat (Rattus norve- 

 picus) and the 1)lack rat (Battus rattus), 

 though the latter is more often grey ov 

 brown or even rufous in colour, ana 

 various subspecies have been described. 

 The two species differ in structure ana 

 in habits. The brown rat is the larger 

 and heavier species with coarser hair, 

 sliorier head, and smaller ears and shor- 

 ter tail, whilst the black rat is a, more 

 slenderly built animal with very long 

 tail and long ears. The brown rat, as 

 its alternative name implies, is a denizen 



