THE SLEEPING SICKNESS 163 



great experience in tropical disease, had put forward 

 the suggestion that the sleeping sickness was due to the 

 infection of the patient by a minute thread worm (allied 

 to the ' vinegar-eel,' and one of a great class of parasites) 

 which he had discovered in the blood of negroes and had 

 named Filaria perstans. 



The occurrence of minute worms (true worms, neither 

 unicellular plants nor protozoa) in the blood of man was 

 first made known by Dr. Timothy Lewis, who described 

 the Filaria sanguinis hominis, as well as some other 

 most important blood-parasites, some years ago (1878), 

 when officially engaged in an enquiry into the cause of 

 cholera in Calcutta. Subsequently, in China, Manson 

 found that these little blood-worms were sucked up by 

 mosquitoes when gorging themselves on the blood of a 

 patient. It is, indeed, difficult to imagine how they 

 should escape passing into the mosquito with the blood. 

 Manson suggested that the minute worms (known to be 

 the embryos of a worm which, when adult, is about one 

 fifteenth of an inch long) are obliged to pass through a 

 mosquito in order to accomplish their development ; but 

 no proof of this suggestion has ever been made. We know 

 by abundant and repeated demonstration and experiment 

 that another blood-parasite the malaria parasite must 

 pass through a mosquito, in whose body it develops, and 

 by which it is carried to a new victim of infection. This 

 was suspected long ago by both peasants and doctors, and 

 experimentally proved by Ross ; but no such proof has 

 been given of the relation of Lewis's blood-worm to a 

 mosquito. The so-called Filaria perstans, discovered by 

 Manson in the blood of negroes, appears to be very dif- 

 ferent from the Filaria sanguinis hominis of Lewis. It is not 

 known how it gets into the blood ; and it is very 

 astonishing, and much to be regretted, that none of the 



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