-220- 



avirulent in Balb/c mice. In contrast, midgut promastigotes obtained 

 seven to ten days after fly infection were highly virulent (Sacks and 

 Perkins, 1984). These authors did not describe the morphologies of the 

 three and ten-day promastigotes obtained from the sand fly midguts. 

 Their findings are consistent with the observations of L mexicana 

 development in Lu . diabol ica in that transmissions to uninfected 

 hamsters occurred only after at least four days of parasite 

 development, corresponding to the point in the life cycle at which 

 division ceased and the short-slender, highly active flagellates first 

 appeared. It is probable that the ten-day inoculum of L. tropica 

 promastigotes from the midgut of Lu. anthophora , used by these workers 

 to infect Balb/c mice, contained short-slender, highly active 

 flagellates, and that the high virulence may have been due to their 

 presence. Because of their rapid, random movement, these parasites, 

 as well as being present in the pharynx and proboscis, are also found 

 throughout the alimentary tract in six to ten-day infections. This is 

 evidence of morphological change to an infective stage which Sacks and 

 Perkins (1984) may have overlooked. 



Gross differences in behavior and pathogeneicity between in vi vo 

 and culture forms were pointed out by Adler and Theodor (1927). 

 Differences in ultrastructure of Leishmania in the sand fly and in 

 culture were also pointed out by Molyneux et aj_. (1975), who said that 

 although the ultrastructures of j_n vivo and j_n vitro parasites are 

 similar, the changes in the organization or organelles and the 

 configuration of the parasite seen in the different parts of the gut 

 of the vector are not clearly revealed in culture. They concluded 

 that cultural forms may be misleading and should not be considered 

 identical to those in the vector (Molyneux et _§_!_., 1975). 



