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Bivalve molluscs are filter feeders and as a consequence they may
bioaccumulate in their tissues viruses that infect humans and higher
vertebrates. However, there have also been described mortalities of bivalve
molluscs associated with viruses belonging to different families. Mass
mortalities of adult Portuguese oysters, Crassostrea angulata, among French livestocks (between
1967 and 1973) were associated with irido-like virus infections.
Herpesviruses were reported in the eastern oyster, Pacific oyster and
European flat oyster and lately in scallops and clams. Disseminated
neoplasia, a proliferative cell disorder of the circulatory system of
bivalves, although of uncertain aetiology, has been suggested to be caused
by retroviral infections. Other viruses described in bivalves are
interpreted as members of the Papovaviridae, Togaviridae, Reoviridae,
Birnaviridae and Picornaviridae. However, the lack of bivalve cell lines
renders difficult virus isolation from molluscs although some viruses can be
isolated in fish cell lines.
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.
Among mollusc pathogens, paramyxeans are an important group of protistan
parasites belonging to the genera Marteilia and Marteilioides. Marteilia refringens and M. sydneyi are of particular concern given
their potential impact on mollusc aquaculture world-wide. Aber disease and
QX disease are currently listed by the OIE, the World Organisation for
Animal Health. After more than thirty years of existence in the scientific
literature, these organisms still pose questions and research challenges to
investigators. This paper reviews current knowledge of the group and key
references. The review was focused on taxonomy, epidemiology, pathology and
potential control methods for these organisms in order to enhance
understanding of paramyxean infection issues in mollusc aquaculture.
This review summarizes the current state of knowledge regarding the two
primary proliferative diseases of commercially important marine bivalves.
Disseminated neoplasia is characterized by the presence of large (2−4 times
the diameter of normal hemocytes), anaplastic, circulating cells that have a
hyperchromatic and often pleomorphic nucleus containing one or more
prominent nucleoli. Prevalence exceeding 90% has been reported; the
disease is progressive and can result in significant mortality of affected
populations. Softshell clams, Mya arenaria, and mussels, Mytilus trossulus,
from the east and west coasts of North America, respectively, and cockles,
Cerastoderma edule, from Ireland, appear to be
especially susceptible. Disseminated neoplasia can be transmitted to
uninfected individuals, indicating an infectious (perhaps viral) etiology,
the expression of which may be aggravated by environmental degradation.
Gonadal neoplasia consists of small, basophilic, undifferentiated cells that
originate as small foci in gonadal follicles where they proliferate and
eventually invade surrounding tissues. This disease primarily affects
M. arenaria in
Maine (USA) and Mercenaria spp. in Florida (USA) at prevalences up to 50%.
Most affected individuals are female. Limited field studies to date indicate that
the disease progresses slowly and mortality rates are low. The major impact
is most likely a reduction in reproductive effort. The finding that
prevalence of gonadal neoplasia is higher in hybrid Mercenaria spp.
suggests a genetic etiology. Precise determination of the etiology and other aspects
of both diseases will benefit greatly from future advances in cellular and molecular
Brown ring disease (BRD) in Ruditapes philippinarum and Ruditapes decussatus is a shell disease caused by Vibrio tapetis. This
disease has begun in 1987 in clams beds in Brittany (France) and then has
spread along the European Atlantic coast. Since about fifteen years,
research on BRD has progress a lot and the purpose of this review is to give
a short description of BRD in clams in retrospect and in addition of recent
and pertinent results. Diagnosis including isolation, biochemical,
serological and molecular characterization has been developed to identify
and detect the pathogen. Therefore, Vibrio tapetis-like strains has been detected in other host
specie, in cultured fishes during mortalities, such as the wrasse,
Symphodus melops in Norway and the halibut, Hippoglossus hippoglossus in Scotland. Mechanisms of host-pathogen
interactions modulated by environmental factors were studied using in vivo
bacterial challenge and in vitro bio-assays. According to these studies, adherence
and cytotoxic factors have been yet identified as virulence factors;
identification and characterization of virulence genes are in progress.
Field and experimental studies confirmed the significant effect of
temperature on the development of BRD and on clam defense related
activities. A significant increase in temperature over 21 °C may have a
preventive effect on the development of the disease.
The main microbial diseases affecting marine cultured bivalves have been
revised on the basis of the etiologic agents, pathogenesis and
pathogenicity. Several recent bivalve-interaction models have been studied,
including Pecten larvae-Vibrio pectinicida, brown ring disease,
juvenile oyster disease, Pacific
oyster nocardiosis and summer mortalities of oysters. In addition, the
taxonomy and phylogeny of new potential bivalve pathogens and their
virulence factors have been established.
Facing the difficulty of identifying bacterial strains associated with
molluscan diseases (mainly vibriosis), polyphasic approaches have been
developed to correlate the phenotype and genotype of potential pathogens. By
evaluating likely virulence mechanisms, developing biotests to screen
virulent strains and characterising the genes implicated in pathogenesis, a
new generation of diagnostic tools, based on potential virulence, will be
developed. Acquisition of pertinent diagnostic tools will be of major
benefit in disease management, health surveillance and monitoring will
contribute to maintaining sustainable aquaculture industries.
The current status of the Haplosporidia is reviewed as well as recent
information on Haplosporidium nelsoni, the causative agent of MSX disease in oysters. Recent
molecular phylogenetic analyses with greatly increased taxon sampling
support monophyly of the Haplosporidia and hypothesize placement of the
group as sister taxon to the phylum Cercozoa. Oyster pathogens in the genus
Bonamia should be considered haplosporidians based on molecular sequence data.
Thus, the group contains 4 genera: Uropsoridium, Haplosporidium, Bonamia and Minchinia. Molecular phylogenetic analyses
support monophyly of Urosporidium, Bonamia and Minchinia, but Haplosporidium forms a paraphyletic clade. Reports of
haplosporidia worldwide are reviewed. Molecular detection assays have
greatly increased our ability to rapidly and specifically diagnose important
pathogens in the phylum and have also improved our understanding of the
distribution and biology of H. nelsoni and H. costale. Much of the data available for H. nelsoni has been
integrated into a mathematical model of host/parasite/environment
interactions. Model simulations support hypotheses that recent H. nelsoni outbreaks in
the NE United States are related to increased winter temperatures, and that
a host other than oysters is involved in the life cycle. Evidence is
presented that natural resistance to H. nelsoni has developed in oysters in Delaware
Bay, USA. However, in Chesapeake Bay, USA H. nelsoni has intensified in historically
low salinity areas where salinities have increased because of recent drought
conditions. Efforts to mitigate the impact of H. nelsoni involve selective breeding
programs for disease resistance and the evaluation of disease resistant
Our understanding of the microcell oyster parasites of the genera Bonamia
and Mikrocytos has expanded in recent years with the application of
ultrastructural and especially molecular biological research approaches.
Molecular phylogenetic analyses of SSU rRNA genes have united three species,
Bonamiaostreae, Bonamiaexitiosa, and
Mikrocytos (now Bonamia) roughleyi,
in a microcell clade within the Haplosporidia, supporting both early and
recent ultrastructural observations. Ultrastructural and molecular
phylogenetic evidence has emerged that Mikrocytos mackini, on the other
hand, is a unique protist with unusual adaptations for a parasitic
existence. DNA probes and polymerase chain reaction (PCR) assays promise
new insights into the life cycles, transmission, and diversity of these
organisms. The development of Ostrea edulis lines selected for B. ostreae
resistance will increase the viability of aquaculture industries for this
species and, combined with rapidly developing biotechnological approaches
for studying host defenses and host-parasite interactions, will allow
greater insight into the nature of phenomena such as resistance and
tolerance to disease in oysters.