Perkinsosis in molluscs: A review

Abstract

-The genus Perkinsus includes protistan parasites infecting marine molluscs throughout the world, some of which are associated with mass mortalities. Life cycle involves vegetative proliferation within the host, by which a cell named trophozoite undergoes successive bipartitioning. Other stages have been observed in vitro or in vivo, depending on the species: hypnospore, zoosporangium and zoospore. Molecular taxonomy supports a close affinity between dinoflagellates and Perkinsus spp. Six species of Perkinsus are currently considered valid: P. marinus, P. olseni, P. qugwadi, P. chesapeaki, P. andrewsi and P. mediterraneus. Histology and, above all, incubation of host tissues in Ray's fluid thioglycollate medium (RFTM) are classic diagnostic methods. In addition, more sensitive and quicker molecular diagnostic techniques based on either immunoassays or PCR have been developed for Perkinsus spp. Epizootiological studies have shown a marked influence of water temperature and salinity on P. marinus infection in oysters Crassostrea virginica, thus determining parasite geographical range and temporal disease dynamics (seasonality). In vitro cultures have been established for four Perkinsus spp. Immune response to infection varies depending on host and involves phagocytosis or encapsulation of the parasite cells by host haemocytes. A polypeptide is secreted by clamTapes philippinarum haemocytes that could kill the parasite. In vitro cultured P. marinus cells secrete proteases that are likely involved in degradation of host tissues. P. marinus can suppress the toxic oxygen radicals produced by host haemocytes. In addition to host death, sublethal effects caused by Perkinsus spp. (reduction of fecundity, growth, and condition) may have significant ecological and economic implications. Various strategies have been assayed to mitigate the consequences of P. marinus epizootics on the oyster industry: modifications of management/culture procedures, selective breeding to obtain resistant oyster strains, and the use of triploid oysters and allochthonous oyster species. Some chemotherapeutants have been proved to inhibit or kill parasite cells in vitro.

Impact of Brown Ring Disease on the energy budget of the Manila clam Ruditapes philippinarum

“…The protozoan parasite Perkinsus marinus significantly reduces growth rate, condition index and gametogenesis of its host, Crassostrea virginica (Kennedy et al, 1995;Paytner, 1996;Dittman et al, 2001). A similar pattern has also been shown for Perkinsus olseni in the clam Tapes decussatus (see review in Villalba et al, 2004). The latter parasite also reduces the reproductive output of R. philippinarum (Ngo and Choi, 2004;Park et al, 2004).…”

“…P. olseni was also shown to reduce condition in the clams Tapes decussatus (see. review in Villalba et al, 2004) and Ruditapes philippinarum (Park et al, 1999). Our results show that BRD, as is the case with other pathological conditions, affects the energy budget of the Manila clam.…”

Freshwater Perkinsea and marine-freshwater colonizations revealed by pyrosequencing and phylogeny of environmental rDNA

“…The genera Perkinsus and Parvilucifera have until now been regarded as strictly marine parasites (Delgado, 1999;Noren et al, 1999;Erard-Le Denn et al, 2000;Villalba et al, 2004;Figueroa et al, 2008;Leander and Hoppenrath, 2008). However, in recent environmental surveys of freshwater lakes, the existence of taxa related to Perkinsea has been suggested (Richards et al, 2005;Lefèvre et al, 2008;Lepère et al, 2008).…”

“…There are also other reports of freshwater parasites with an outer morphology resembling Perkinsus (for example, a parasite of the freshwater cryptomonad Chilomonas paramecium), but there are no sequences available for these species (Brugerolle, 2002(Brugerolle, , 2003. Considering the position of the Rana parasite among the Perkinsea sequences in the tree (Figure 2), as well as the evidence from other studies indicating that putative members of Perkinsea are infecting a wide range of species, such as mollusks, frogs, dinoflagellates and cryptomonads (Brugerolle, 2002(Brugerolle, , 2003Green et al, 2003;Villalba et al, 2004;Davis et al, 2007), we suspect that the entire Perkinsea is parasitic. This is very important to clarify because it would alter the present view of the role of picoeukaryotes in 'microbial loops' and the carbon cycle in aquatic systems.…”

“…In addition, a highly abundant and widespread group uncovered from environmental 18S rDNA libraries, the so-called marine alveolates (MA), appear to be largely parasitic (see Guillou et al, 2008). To date, the Perkinsus and Parvilucifera (members of Perkinsea) and MA have only been identified in marine environments (Noren et al, 1999;Villalba et al, 2004;Groisillier et al, 2006;Guillou et al, 2008;Leander and Hoppenrath, 2008). However, microscopy observations, as well as reports of deeply diverging alveolate 18S rDNA sequences, point to the existence of freshwater Perkinsea species (Brugerolle, 2002(Brugerolle, , 2003Green et al, 2003;Richards et al, 2005;Davis et al, 2007;Lefèvre et al, 2008;Lepère et al, 2008).…”