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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 clam Tapes 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.
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