PLAGUE— NOTES ON FLEAS \77 



Opsonic index rises markedly ten days later. 

 Guinea-pigs, mortality 66-6 per cent.; inoculated iG'G per cent. 

 Klein's protective inoculoiion prepared from the organs of dead 

 imimals of plague; he says : — 



(i) It requires ten to twelve days to prepare it; Haffkine's requires 

 four to six weeks. 



(2) A large amount can be prepared of uniform strength. 



(3) Its efficacy is easily standardized by injection into the rat. 



(4) Being dry and sterile it can be preserved without antiseptic and 



kept unaltered. 



(5) That its injection into the rat protects certainly many weeks. 



(6) Cost of its preparation is much less than Haffkine's. This 

 has not yet been tried on man. 



Masks of flannel cloth to cover the head tied about neck with 

 window of sheet celloidin are advisable. 



NOTES ON FLEAS. (Order Siphonaptera.) 



Fleas are active parasites of mammals and birds. They usually 

 have a preference for one host, and attack others with some reluctance. 

 They act as carriers or necessary intermediaries, e.g. — 

 (i) The common dog flea and the common European rat flea 



harbour the cysticercus of certain tapeworms of their respective 



hosts. 



(2) The trypanosome of the rat passes through certain stages of its 



developmental cycle in the European rat flea. 



(3) They are carriers of the plague bacillus among rats and other 



rodents, and from these to man. 



Pulex (Xenopsylla) cheopis most commonly infests house rats in the 

 tropics. 



The Pulex irritans (the human ilea) and others are capable of 

 carrying the plague bacillus. 



EXTERNAL APPEARANCE. 



Fleas are laterally compressed, wingless insects, with two piercing, 

 sucking mouth parts. The head is broadly articulated to the thorax; 

 all three segments of the thorax are distinctly independent. Eyes may 

 or may not be present ; when present they are simple and not com- 

 pound as in the house fly. A comb of teeth is found on the edge of 

 the cheek or on the lower edge of the head in most species. 



The antennas, two basal segments and a club with nine rings, lie 



behind and above the eyes. Bristles are seen irregularly placed about 



the head and thorax. They have a pair of longitudinally grooved, 



serrated, needle-like mandibles, which form an efferent tube for saliva, 



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