RICKETTSIA-LIKE ORGANISMS 185 



of phlebotomus fever in the earthquake districts is due to the 

 abundance of ideal breeding places for the gnats furnished by 

 the ruined walls. It is possible that other species of Phlebotomus 

 may also transmit the disease. 



The gnats become infective about a week after feeding on an 

 infected person. The incubation period of the disease in man is 

 about four or five days. Natural immunity is extremely rare, 

 but in most cases an immunity of variable duration results from 

 an attack of the disease. 



No specific cure has yet been discovered. Prevention lies 

 in avoiding the bites of Phlebotomus papatasii and in reducing 

 their numbers as far as possible by methods described on p. 473. 

 In case of prolonged residence in an endemic region, there is 

 little hope of escaping infection, and willful exposure to it at a 

 time when the disease will be least inconvenient is usually ad- 

 visable, in view of the usually persistent immunity which results. 



Rickettsia-Like Organisms 



Within the past few years there have been described as the 

 probable cause of a number of human diseases, organisms of very 

 minute size, frequently less than half a micron in diameter, which 

 may show a number of morphological types. Round or diplococcal 

 forms are of most frequent occurrence, but there also occur minute 

 rod-shaped bodies and occasionally threadlike forms. The re- 

 lationships of these organisms is very obscure. They are looked 

 upon by some investigators as bacteria, and by others as protozoa, 

 while still others consider them merely by-products of the infection. 

 In their staining reactions they resemble spirochaetes rather than 

 bacteria, and the possibility of their representing modified granule 

 stages of spirochaetes has been suggested. They are non-motile, 

 and very difficult to cultivate. They occur in a number of kinds 

 of arthropods, usually multiplying on or in the epithelial cells of 

 the alimentary canal, but some species, at least, occur in other 

 organs also, including the reproductive glands, and are hereditarily 

 transmitted. The resemblance to spirochsetes is further borne 

 out by the way in which reproduction takes place by granule- 

 shedding from elongated threadlike forms, and by their hereditary 

 transmission among arthropods. Species of Rickettsia or closely 

 allied forms have been found in human lice (Pediculus), Mal- 

 lophaga, fleas, bugs (Cimex), sheep ticks (Melophagus), and true 



