THE PIROPLASM.4.SIDA 



307 



called agamonts) contain chromatin gran- 

 ules 0.4 to 2.0 jj. in diameter with a mean 

 of 1.2 li and produce macromerozoites 

 2.0to2.5jLt in diameter. Microschizonts 

 (sometimes called gamonts because they 

 are thought to produce sexual stages) con- 

 tain chromatin granules 0.3 to 0.8;^ in 

 diameter with a mean of 0. 5 pi and produce 

 micromerozoites 0.7 to 1.0 jj, in diameter. 



Life Cycle: The life cycle of this 

 species has been studied more than that of 

 other members of the family, but its details 

 in the tick are still uncertain. The most 

 important vector is Rhipicephalus appen- 

 diciilatus. Other vectors are R. ayrei, 

 R. capensis, R. evertsi, R. jeaiielli, R. 

 iieavei, R. slmus, Hyalonima excavatum, 

 H. dromedaril and H. truncatiim. Trans- 

 mission is stage -to -stage in all cases, 

 and not thru the egg. R. appendictdatiis, 

 for instance, acquires the infection as a 

 larva and transmits it as a nymph, or 

 acquires the infection as a nymph and 

 transmits it as an adult. The parasite 

 will not survive in the ticks thru more 

 than 1 molt. 



Reichenow (1940), who made a careful 

 study of the life cycle in cattle and in R. 

 appeiidicnlafHS, said that the great major- 

 ity of parasites die in the tick intestine. 

 A few succeed in passing thru the intes- 

 tinal wall into the body cavity and thence 

 to the salivary glands, where they invade 

 the secretory cells. Here they lie dor- 

 mant until after the tick has dropped off its 

 host, molted, attached itself to a new host 

 and started to suck blood. The parasites 

 then begin to multiply by repeated binary 

 fissions, filling the interstices between 

 the secretory droplets. They continue to 

 multiply, and finally the host cell is 

 greatly enlarged and filled with something 

 over 30, 000 tiny parasites. This requires 

 15 successive binary divisions. Very few 

 secretory droplets remain. The host cell 

 ruptures, and the parasites are released 

 into the lumen of the salivary ducts and 

 are injected into the host when the tick 

 sucks blood. It takes 3 days for the devel- 

 opmental process to be completed in 

 nymphs and 4, 5 days in adult ticks. 



The above process is completely asex- 

 ual. Gonder (1910, 1911), however, 



thought that there was a sexual stage in the 

 tick, and described a process of syngamy. 

 Cowdry and Ham (1932) also thought that 

 sex was involved, altho they admitted they 

 found no proof of it. According to their 

 account of the life cycle, two types of para- 

 site, large and small, emerge from the 

 erythrocytes in the tick's gut and become 

 applied to the surface of the gut epithelial 

 cells. Cowdry and Ham thought that fer- 

 tilization probably takes place here. 

 They said, "Very careful search was 

 made for fertilisation stages without con- 

 spicuous success. Large and small para- 

 sites were, however, occasionally ob- 

 served in contact, but it was difficult to 

 tell whether this was merely optical super- 

 position or whether actual union was taking 

 place„ Such appearances were detected in 

 0. 1 per cent or less of the parasites. " 



The parasites then enter the intes- 

 tinal cells, the small forms disappear, 

 and the large forms grow and give rise to 

 a stage without distinct nuclei which they 

 called a zygote. The zygote grows, a 

 nucleus reappears in it, and also a central 

 concentration of material. This central 

 concentration becomes more marked and 

 turns into a large, elongated, nucleated 

 organism which they called an ookinete. 

 The ookinete breaks out of the zygote into 

 the gut cell, enters the body cavity, makes 

 its way to the salivary glands, and enters 

 a salivary gland cell. Here it rounds up 

 and grows, surrounded by a colorless halo 

 of host cell cytoplasm, becoming so large 

 that it distends the host cell. Buds ap- 

 pear about its periphery which Cowdry 

 and Ham called sporoblasts; the parent 

 cell they called a sporont. The sporo- 

 blasts develop rapidly and produce sporo- 

 zoites about their periphery. These are 

 discharged into the lumen of the salivary 

 gland acinus and are introduced into the 

 animal when the tick feeds on it. 



Reichenow (1940) criticized the work 

 of Cowdry and Ham (1932) severely. He 

 said that the bodies in the intestinal cells 

 (the "zygotes"), could be found in both 

 infected and clean ticks and were there- 

 fore not a stage in the parasite's life 

 cycle. He found no structures which re- 

 sembled ookinetes. He considered the 

 "sporonts" to be degenerated tissue cells 



