Excystment

Those field studies that include water samples collected during winter months all show undetectable tamarensis concentrations (5, 18, 21). Given the relatively small volumes of water typically collected and counted, this does not preclude the presence of a few cells (the "hidden flora"), but it does indicate that motile populations are extremely small at best. Furthermore, since the growth rate of tamarensis is essentially zero at very low temperatures, the appearance of even a few hundred cells in early spring when waters are still very cold suggests that it is excystment and not division of surviving motile cells that initiates the bloom development. 

Nevertheless, the process of encystment dictates when a substantial fraction of a Gonyaulax population leaves the water column, regardless of the ability of motile cells to persist for extended periods. The encystment/excystment cycle can thus define the temporal limits of some blooms. More work is needed to evaluate the importance of this process relative to bloom termination from factors such as grazing or advection that decrease the numbers of motile cells. 

Andreassen, J., Hoole, D. and Befus, D. (1990) Complement-mediated leucocyte adherence of newly excysted and 4-day-old Hymenolepis diminuta. International Journal for Parasitology 20, 905-911. 

A number of reports have suggested a relationship between cysteine protease activity and excystment of metacercariae. Cysteine proteases may act alone or in concert with extrinsic host factors such as trypsin in the host gut (Intapan and Maleewong, 2001 Li et al., 2004). In vitro, Paragonimus ohirai metacercarial cysteine protease activity is induced by addition of the bile salt, sodium cholate and parasite... 

Ikeda, T. (2003) Involvement of cysteine proteinases in excystment of Paragonimus ohirai metacercariae induced by sodium cholate and A23187. Journal of Helminthology 77, 21-26. 

Intapan, P. and Maleewong, W. (2001) In vitro excystation of Paragonimus heterotremus metacercariae. Journal of Parasitology 87, 1184-1186. 

C02 is frequently the predominant duodenal gas and plays an important role in the physiology of cestodes in two ways (a) as a source of carbon atoms for metabolic functions via C02 fixation (Chapter 5) (b) as a trigger in the hatching of cyclophyllidean eggs (p. 192) or the excystment of larval cysts. Production of C02 results from the interaction of HC03 and H+, the latter being derived from secretions from the gut, biliary and pancreatic... 

IMPORTANCE TO INTESTINAL PARASITES Bile is of particular importance to intestinal parasites, for in many cases it is involved in the trigger mechanisms concerned in the hatching of helminth eggs, excystation of protozoan cysts, evagination of cestode scoleces and excystment of trematode metacercaria. In relation to cestodes, its role in... 

Although excystment and evagination are theoretically separate processes, in many cases the second follows so closely after the first that they cannot readily be separated. In some species, a particular enzyme may be required to digest the cystic membranes completely before the larva escapes in others, the same enzyme may so activate the larva that it breaks out of its enveloping membranes before they are fully digested. Requirements for... 

Table 9.1. Requirements for excystment and/or rapida evagination of some cyclophyllidean larvae for these results to be of value the duration of treatment and the physico-chemical conditions of the medium must be...

Another and more convincing explanation for this discrepancy of previous and recent studies related to the inactivation of Cryptosporidium refers to the methods of viability testing. So, it seems that in vitro viability assays (chemical excystation and vital stains) that have been used previously may have significantly underestimated the inactivation efficacy of UV-C irradiation of the parasites compared with in vivo infectivity assays applied in recent studies using neonatal mouse models (Craik et al, 2001). This was also demonstrated by UV inactivation of Giardia muris cysts using MP Hg lamps (Craik et al., 2000). 

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