Fleas and Lice as Carriers of Trypanosoma lewisi 213 
a specific poison. The majority of them are tropical in distri¬ 
bution. 
Though we are concerned especially with the species which infest 
man, we shall first consider two of the trypanosomes of lower animals, 
known long before any of those of man had been found. 
Fleas and Lice as Carriers of Trypanosoma lewisi .—Trypanosoma 
lewisi, the first mammalian trypanosome known, is to be found in the 
blood of wild rats. Like its host, it appears to be cosmopolitan in 
distribution, having been reported from several localities in the 
United States, Brazil, Argentine, England, Germany, France, Italy, 
Russia, Asia and Africa. 
This species is usually regarded as non-pathogenic, but in experi¬ 
mental work, especially with white rats, heavy infestations often 
result fatally, and naturally infested specimens sometimes show 
evidence of injury. Rats which have been infested exhibit at least 
temporary immunity against new infection. 
Trypanosoma lewisi is transmitted from rat to rat by fleas and 
by lice. Rabinowitsch and Kempner (1899) first found that healthy 
rats which were kept with infested rats, showed trypanosomes in 
their blood after about two weeks. They found the trypanosomes 
in the alimentary canal of fleas which had fed on the diseased rats. 
On teasing such fleas in physiological salt solution and inoculating 
them into fresh rats they were able to produce the infection. Finally, 
they showed that the fleas which had fed upon infested rats were 
able to carry the parasites to healthy rats. Corresponding experi¬ 
ments with lice were not successful. Prowazek G9°5) found in the 
rat louse ( Hceniatopinus spinulosus ) organisms which he regarded 
as developmental stages of the Trypanosoma lewisi. He believed 
that the sexual cycle was undergone in this insect. 
Nuttall (1908) readily transmitted the trypanosomes through the 
agency of fleas, ( Ceratophyllus fasciatus and Ctenopthalmus agyrtes). 
He believes that these insects are probably the chief transmitters 
of the parasite. He was also able to transmit it from diseased to 
healthy rats through the agency of the rat louse. He was unable 
to trace any developmental stages in the louse and inclined to the 
opinion that Prowazek was deceived by the presence of extraneous 
flagellates such as are known to exist in a number of blood-sucking 
arthropods. 
Nuttall concludes that since three distinct kinds of blood-sucking 
insects are capable of transmitting Trypanosoma lewisi it appears 
