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In Leishmania transmissions, the presence of parasites in the 

 mouthparts of the infecting sand fly is probably not as critical as 

 massive infections in the region of the stomodeal valve and cardia 

 which give rise to the small "infective" forms. Some Lu . diabol ica 

 and Lu. shannoni involved in successful transmissions manifested no 

 parasites in the mouthparts upon dissection but had massive 

 infections in the cardia, with great numbers of short-slender, highly 

 active forms present. 



Bray (1974b) stated that "factors governing the numbers of 

 promastigotes in the sand fly capable of being delivered to the skin 

 of the vertebrate, and the mechanism or mechanisms which cause them to 

 be deposited in the skin, remain completely unknown and . . . 

 constitute the most important gap in our knowledge of the disease" 

 (Bray, 1974a, p. 98). Several researchers have suggested that, in the 

 act of biting, parasites from the stomodeal valve, or apparently 

 blocking the pharynx or esophagus, may be forced forward and deposited 

 in the skin (Bray, 1974b; Molyneux, 1977; Lainson et aj_., 1977). 

 Adler and Theodor (1935) opposed this blockage theory on the grounds 

 that the strong dilator muscles of the ci barium and pharynx would 

 widen the lumen sufficiently for blood to pass without difficulty. 

 Molyneux (1977) accepted Adler and Theodor's explanation but believed 

 the blockage could occur in nonpumping organs such as the esophagus 

 and stomodeal valve. This appears to be true in well developed 

 infections of L mexicana in Lu, diabol ica and Lu. shannoni . In three 

 and four-day-old infections, numbers of attached parasites in the 

 region of and immediately posterior to the stomodeal valve were 

 so great, that the gut became distended, as if under positive 



