Ray's Fluid Thioglycollate Research Articles

Detecting Perkinsus-like organisms and Perkinsus marinus (Myzozoa: Perkinsidae) within new bivalve hosts in the southeastern Gulf of California.

Perkinsus marinus, a parasite myzozoa native from the Atlantic Ocean, has spread across various bivalve species in the Pacific Ocean. In the Gulf of California, clams like Larkinia grandis and Leukoma grata have become a secondary fishery, experiencing demand. During annual sampling (August 2017 - July 2018, n = 30/month), the Ray's fluid thioglycollate medium (RFTM) revealed presumptive hypnospores of Perkinsus sp. in L. grandis, with a low parasite prevalence (4.16%) and density (≤ 3.14 hypnospores g-1 tissue). From summer 2020 to summer 2021, P. marinus was seasonally identified in L. grata via staining and PCR tests, showing mean prevalence and density of 48.33% and 48 hypnospores g-1 tissue, respectively. The infection level was light in both clams. This parasite's presence in these new hosts within the southeastern Gulf of California confirms its ability to expand into non-native places, signaling a need for intervention strategies in managing and transporting mollusk species within the region.

Characterisation of common hypothetical surface peptides between protozoan parasites (Perkinsus olseni) originating from different geographical locations.

Perkinsus olseni and P. marinus are classified as notifiable pathogens by the World Organisation for Animal Health and are known to cause perkinsosis in a variety of molluscs globally. Mass mortalities due to these parasites in farms and in the wild have been a recurrent issue. Diagnosis for these protozoans is currently done using Ray's fluid thioglycollate medium method followed by optical microscopy or molecular assays. Both require a high level of skill and are time-consuming. An immunoassay method would make the diagnosis of perkinsosis quicker and cheaper. The present study used mass spectrometry-based proteomics to investigate common hypothetical surface peptides between different geographical isolates of P. olseni, which could be used to develop immunoassays in the future. Two peptides were identified: POLS_08089, which is a 42.7 kDa peptide corresponding to the 60S ribosomal subunit protein L4; and POLS_15916, which is a conserved hypothetical protein of 55.6 kDa. The identification of peptides may allow the development of immunoassays through a more targeted approach.

First record of Perkinsus marinus infecting Crassostrea sp. in Rio Grande do Norte, Brazil, using real-time PCR

A pathogen with high virulence potential in some host species, Perkinsus marinus remains a challenge for the ecological integrity of marine ecosystems and the health of bivalve molluscs. This study investigates the occurrence of P. marinus in Crassostrea sp. in estuaries of the Potengi River and the Guaraíras lagoon in Rio Grande do Norte, Brazil. A total of 203 oyster samples that tested positive for Perkinsus sp. in Ray's fluid thioglycollate medium (RFTM) were subjected to species-specific quantitiative PCR, where 61 animals (30.05 %) presented amplification graphs with a melting temperature of 80.1 ± 0.6 °C matching the positive control. This was the first record of P. marinus in oysters in these estuaries using qPCR as a diagnostic tool.

Molecular and histological identification of the protozoan parasite Perkinsus olseni in the blood cockle Anadara kagoshimensis (Tokunaga, 1906) occurring on the south coast of Korea

A recent report on co-infection by two Perkinsus species in mud arks Anadara trapezia (Bivalvia Arcidae) in Australia implies possible Perkinsus infection in other Arcidae species. Blood cockle Anadara kagoshimensis has been exploited since the 1960s on the south coast of Korea, and the landings have decreased due to mass mortality in recent years. During a histopathology survey of blood cockle on the south coast, we identified a Perkinsus-like pathogen in the visceral mass. Accordingly, we applied Perkinsus species-specific PCR and subsequent sequencing to identify the pathogen to species level using internal transcribed spacer (ITS) rDNA. Ray's fluid thioglycollate medium (RFTM) and immunofluorescence assays (IFA) were also used to quantify and visualize the infection in this novel host. Perkinsus-like organisms were mainly found in the hemal space of visceral mass connective tissue. The Bayesian inference tree based on ITS rDNA sequences is strongly affiliated with P. olseni, suggesting that the Perkinsus-like organism in A. kagoshimensis is P. olseni, commonly found in Manila clams in Korean waters. RFTM and subsequent 2 M NaOH digestion assay revealed that the infection intensity ranged from 8000 to 1,481,000 cells/g gill, with a mean of 1.2 × 105 ± 2.5 × 105, and the prevalence was higher in September (fall) with 13.5% (101/750) than in June (spring) (0.4%, 3/750). Histology revealed that heavily infected clams exhibited tissue inflammation suggesting that P. olseni infection could damage the structure and physiological function of A. kagoshimensis; although we could not find a significant difference in condition index between the infected and uninfected individuals.

Development of a simple host-free medium for efficient prezoosporulation of Perkinsus olseni trophozoites cultured in vitro.

The parasitizing stage (trophozoite) of the protozoan parasite Perkinsus olseni progresses to the dormant stage (prezoosporangium) immediately after the death of the host through physiologically and morphologically drastic changes. This development is reproducible in Ray's fluid thioglycollate medium (RFTM). In this study, supplementation with tissue extract from a host, the Manila clam, significantly improved the efficiency of development, as determined by the numbers and sizes of developed prezoosporangia. Similar results were seen following supplementation with boiled host tissue extract, which indicates that a thermally stable component of the host is required for the parasite's development. Subsequently, we found that a commercially available lipid concentrate significantly increased prezoosporulation without host tissue, suggesting that the lipids in host tissue enhance prezoosporangia development. Moreover, we determined that yeast extract, sodium thioglycollate, and sodium chloride were the only components of RFTM required for prezoosporulation. Based on these findings, we prepared a simple, host-free medium for P. olseni prezoosporulation-Lipid concentrate Yeast extract Medium (LpcYM)-consisting of yeast extract, lipid concentrate, sodium thioglycollate, and sodium chloride. We confirmed that the prezoosporangia developed in LpcYM produce zoospores that are infectious to Manila clams and that trophozoites of other Perkinsus species (P. marinus, P. honshuensis, and P. chesapeaki) also develop to prezoosporangia in this host-free medium. As LpcYM has the simplest composition of prezoosporulation media available thus far, it enables us to conduct molecular and biochemical studies examining the drastic transformation process of this parasite.

Perkinsus marinus in the pleasure oyster Crassostrea corteziensis cultivated on the southeast coast of the Gulf of California, Mexico

The protozoan Perkinsus marinus has been associated with high mortality episodes of the eastern oyster Crassostrea virginica in the USA. The presence of P. marinus on the pleasure oyster Crassostrea corteziensis cultivated in two estuaries on the southeast coast of the Gulf of California was evaluated. Oysters were collected monthly (September 2016 to September 2017) and analyzed using Ray's fluid thioglycollate medium (RFTM) and polymerase chain reaction (PCR). Water parameters and oyster biometrics were also recorded. Pathogen prevalence increased over time from 0 to 100% in oysters from La Pitahaya, and from 0 to 83.33% in those from Bacorehuis. At both oyster farms, infection intensity was light (<1×104 parasites g-1 wet tissue), pathogen prevalence and infection intensity were correlated with oyster size and weight, and there was a strong correlation between P. marinus prevalence and intensity (La Pitahaya r = 0.91; Bacorehuis r = 0.82). The oysters that resulted positive for P. marinus by RFTM also were assayed using PCR. P. marinus presence was confirmed in 98.27% (114/116) of the oysters from La Pitahaya, and 95.83% (46/48) of those from Bacorehuis. The detection of P. marinus confirms that this pathogen is well established in the area with high expression during the warmer season. Despite the light infection intensity of this parasite at both sites, health surveillance of this bivalve in the region is highly advisable.

The effects of environmental and nutritional conditions on the development of Perkinsus olseni prezoosporangia

Perkinsus olseni is a widely distributed protozoan pathogen that infects a wide range of marine mollusks. Prezoosporulation of P. olseni trophozoites is easily observed in Ray's fluid thioglycollate medium, but in nature, trophozoites within host tissue should be able to develop into prezoosporangia without any additional artificial medium after the host dies. How this process might work in field conditions remains poorly understood, however, partly because of the lack of appropriate in vitro assays. In this study, we observed that trophozoites of P. olseni successfully developed into prezoosporangia when mixed with minced tissue of the Manila clam Ruditapes philippinarum and placed in seawater. We were thus able to establish a new method to examine the development of P. olseni to prezoosporangia under artificially simulated natural environmental conditions. Using this method, we found that low temperatures (5 °C, 15 °C) significantly suppressed prezoosporangia development. In addition, we found that prezoosporangia were developed in a wide range of salinities (10–50 practical salinity unit) and that P. olseni requires some nutrition factors from host tissue for prezoosporulation to occur. Because the transmission of P. olseni among a host population highly depends on the developmental process of prezoosporangia, which leads to production of the infective zoospore stage, these results will help further our understanding of the parasite's infection dynamics in nature.

Development and validation of a specific real-time PCR assay for the detection of the parasite Perkinsus olseni

Perkinsus olseni is a protozoan parasite that infects a wide variety of molluscs worldwide, causing economic losses in the aquaculture sector. In the present study, a quantitative PCR (qPCR) assay was developed for the detection and quantification of P. olseni in clam gill tissue and hemolymph (Ruditapes philippinarum and R. decussatus), and the results were compared with those of the standard diagnostic methods recommended by the O.I.E. (World Organisation for Animal Health): Ray's fluid thioglycollate culture method (RFTM), a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay and histopathology. The efficiency, sensitivity and reproducibility of the newly described qPCR assay were also determined. The highest prevalence was detected using the qPCR assay, and the strongest linear correlation was obtained between the RFTM infection levels and the threshold cycle (Ct) number from the gill tissue. Although better results were obtained from gill than from the hemolymph in the qPCR assays, especially with lower infection levels of the parasite, a significant linear correlation was observed between Ct values from the gill and hemolymph. The qPCR assay that was developed in this study showed high sensitivity, specificity and reproducibility for the detection and quantification of P. olseni.

Two epizootic Perkinsus spp. events in commercial oyster farms at Santa Catarina, Brazil.

Perkinsus spp. have been detected in various bivalve species from north-east Brazil. Santa Catarina is a South Brasil state with the highest national oyster production. Considering the pathogenicity of some Perkinsus spp., a study was carried out to survey perkinsosis in two oyster species cultured in this State, the mangrove oyster Crassostrea gasar and the Pacific oyster Crassostrea gigas. Sampling involved eight sites along the state coast, and oyster sampling was collected during the period between January 2013 and December 2014. For the detection of Perkinsus, Ray's fluid thioglycollate medium (RFTM) and histology were used, and for the identification of the species, PCR and DNA sequencing were used. Perkinsus spp. was found by RFTM in C.gigas and C.gasar from São Francisco do Sul. This pathology was also detected in C.gasar from Balneário Barra do Sul both, by RFTM and histology. Perkinsus marinus was identified in C.gigas and C.gasar from São Francisco do Sul and Perkinsus beihaiensis in C.gasar from Balneário Barra do Sul. This is the first report of P.marinus in C.gigas from South America. Results of this preliminary study suggest that both oyster species tolerate the species of Perkinsus identified, without suffering heavy lesions.

EPIZOOTIOLOGY OF Perkinsus sp. INFESTING Crassostrea rhizophorae FROM THE SEMI-ARID REGION, BRAZIL

Abstract The coast of the semi-arid region of Brazil is known for the presence of large estuaries inhabited by numerous species of edible bivalve mollusks. The oyster C. rhizophorae is naturally found in estuarine environments along the Brazilian coast, fixed on substrates, mainly in the roots of mangroves. Protozoa of the genus Perkinsus are important parasites of marine bivalves, sometimes causing significant pathological damage throughout the world. This study reports the occurrence of parasites Perkinsus sp. in mangrove oysters from Porto de Mangue estuary (Rio Grande do Norte state) and Icapuí estuary (Ceará state), two estuaries of the semi-arid region of Brazil. In Porto do Mangue estuary, oysters were collected in December 2016, January and February 2017. In Icapuí estuary, oyster sampling occurred in September, October and November 2017. Two fragments of the gills were incubated in Ray's Fluid Thioglycollate Medium - RFTM. The ecological and parasitological parameters were calculated. Five of the ninety oysters collected in Porto do Mangue estuary were parasitized, which corresponded to a mean prevalence of 5.6%, and in Icapuí estuary 18 of the 120 oysters collected were parasitized, which corresponded to a mean prevalence of 15.0%. The intensity of infection by Perkinsus sp. ranged from very mild to mild.

Perkinsus olseni in the short neck yellow clam, Paphia malabarica (Chemnitz, 1782) from the southwest coast of India.

Parasites of the genus Perkinsus predominantly infect bivalves, and two species among them, P. olseni and P. marinus, are notifiable to OIE. P. olseni infections are known to cause extensive damage to wild as well as farmed bivalves globally with enormous implications to its fishery. Consequent to the initiation of a surveillance programme for aquatic animal diseases in India, Perkinsus infections were observed in many species of bivalves. The present paper describes P. olseni infections in the short neck yellow clam, Paphia malabarica from the southwest coast of India. Diagnosis of the parasite was carried out using Ray's Fluid Thioglycollate Medium culture, histology, in-situ hybridisation and molecular taxonomy. Pathology of infection and development of zoospores is also described. This forms the first report of a P. olseni infection in P. malabarica. High prevalence and intensity of infection of Perkinsus in clams raises concerns, as clam reserves in this geographical area sustain fisheries and the livelihoods of local fishing communities.

Parasites in two coexisting bivalves of the Patagonia coast, southwestern Atlantic Ocean: The Puelche oyster (Ostrea puelchana) and false oyster (Pododesmus rudis).

The purpose of this study was to compare the parasites of two coexisting bivalves, the edible Puelche oyster (Ostrea puelchana) and the false oyster (Pododesmus rudis) that lives attached to O. puelchana shells, and to investigate their host specificity. Samples from wild populations, 465 O. puelchana and 131 P. rudis, were collected seasonally during two years in the San José Gulf (northern Patagonia, Argentina) and were processed using standard histological techniques. To increase the natural low prevalences of Bonamia spp. and Perkinsus spp. that are present in wild populations, an in situ experiment was performed by maintaining captive sentinel bivalves at high densities inside a plastic mesh bag to enhance parasite transmission. Polymerase chain reaction (PCR) assays were used to test for apparent Bonamia sp. infections among captive sentinel O. puelchana specimens (n = 80), and Ray's fluid thioglycollate medium (RFTM) assays and histological immunoassays tested for apparent Perkinsus sp. infections among captive sentinel P. rudis specimens (n = 100). Despite histological observations that revealed the presence of microcells resembling Bonamia sp. infecting hemocytes of some Puelche oysters, PCR assays did not confirm that parasite identification. Among captive sentinel P. rudis that showed histological evidence of Perkinsus sp. infections, neither RFTM nor immunoassays confirmed such parasites. Ostrea puelchana from wild populations were occasional hosts for both Rickettsia-like organism (RLOs) and Urastoma-like turbellarians. In contrast, six parasite taxa infected P. rudis from coexisting populations, including RLOs, Urastoma-like turbellarians, an intracellular gregarine species, Nematopsis-like oocysts, an unidentified coccidian and a Perkinsus qugwadi-like protozoan. These results demonstrated specific infection patterns of the identified parasites in relation to their hosts.

Perkinsus beihaiensis (Perkinsozoa) in oysters of Bahia State, Brazil.

This study reports the pathogen Perkinsus beihaiensis in oysters of the genus Crassostrea on the coast of the State of Bahia (Brazil), its prevalence, infection intensity and correlation with salinity. Oysters (n = 240) were collected between October and December 2014 at eight sampling stations between latitudes 13°55'S and 15°42'S. The laboratory procedures included macroscopic analysis, histology, culture in Ray's fluid thioglycollate medium (RFTM), Polymerase Chain Reaction (PCR) and DNA sequencing. PCR and sequencing have been used for the genetic identification of oysters as well. Two species of oysters have been identified: Crassostrea rhizophorae and C. brasiliana. In both oyster species P. beihaiensis was the only Perkinsus species detected. In C. rhizophorae, the average prevalence was 82.8% by histology and 65.2% by RFTM. In C. brasiliana, the prevalences were 70.5% and 35.7%, respectively. The higher prevalence of P. beihaiensis in C. rhizophorae was probably influenced by salinity, with which was positively correlated (r> 0.8). In both oysters, P. beihaiensis was located mainly in the gastric epithelium. The infection was generally mild or moderate, without apparent harm to the hosts, but in cases of severe infection, there was hemocytical reaction and tissue disorganization. The generally high prevalence in the region suggests that oysters should be monitored with respect to this pathogen, especially in growing areas.

Primer registro de Perkinsus sp. (Protozoa, Apicomplexa) en el callo de hacha Atrina maura en Sinaloa, México

We evaluated the presence of the protozoan Perkinsus sp. by determining of its prevalence and level of infection in Atrina maura during 2012. Pen shells were collected per season beside an oyster farm located at Estero La Piedra, Sinaloa, Mexico, and analyzed by Ray's fluid thioglycollate medium and polymerase chain reaction. The higher prevalence (90%) and infection level (2.3) were obtained when temperature and salinity were higher. The Mackin scale showed a non-infected to moderate infection range. PCR was positive to Perkinsus sp. This is the first record of the presence of Perkinsus sp. in A. maura in Sinaloa, Mexico.

Perkinsus beihaiensis infectando a ostra Crassostrea rhizophorae em cultivo e em estoque natural na Baía de Camamu, Bahia, Brasil

Foi investigada a infecção de Perkinsus beihaiensis (Perkinsozoa) na ostra Crassostrea rhizophorae em um sistema de cultivo do tipo espinhel e em um estoque natural de ostras no manguezal adjacente ambos localizados no estado da Bahia, Nordeste do Brasil. As colheitas foram realizadas em outubro e novembro de 2012 e em janeiro de 2013. As ostras (n = 300) foram medidas, examinadas macroscopicamente quanto a sinais da infecção e submetidas às técnicas laboratoriais: histologia, ensaio em meio de cultivo de tioglicolato de Ray (RFTM), reação em cadeia da polimerase (PCR) e sequenciamento, que confirmou a identificação do patógeno. As análises histológicas e o RFTM mostraram, respectivamente, prevalência média de 93,3% e de 69%. A infecção foi geralmente leve ou muito leve. Não houve diferença significativa (p > 0,05) entre os ambientes em termos de prevalência ou severidade da infecção. Este é o primeiro registro de P. beihaiensis no estado da Bahia e o segundo em ostras do Brasil e América do Sul.

Epizootiology of Perkinsus sp. inCrassostrea gasar oysters in polyculture with shrimps in northeastern Brazil.

Bivalve culture is of considerable economic and social interest in northeastern (NE) Brazil. The polyculture is an alternative approach to traditional monoculture for reducing the environmental impact of shrimp farming and improving oyster culture. Perkinsus marinus and Perkinsus olseni were found infecting oysters in NE Brazil and can threaten oyster production. This study evaluated Perkinsus spp. occurrence in Crassostrea gasar during all production stages. Oyster spats were produced in a hatchery and grown in shrimp ponds in Rio Grande do Norte state. Perkinsus spp. were surveyed by Ray's fluid thioglycollate medium and confirmed by polymerase chain reaction. Prevalence and intensity of infection were determined in oysters until they reached 7 cm. Results showed that the broodstock was already infected by Perkinsus (60%), but the derived spats were Perkinsus-free. Oyster spats acquired Perkinsus infection when transferred to ponds. The prevalence gradually increased in the seven months following placement in ponds (73%), and then decreased to 17% by the tenth month. The infections were initially mild, but intensity increased at the final growth stage. In conclusion, it is possible to produce Perkinsus-free C. gasar oyster spats from infected broodstock, and their culture in shrimp ponds is feasible.

여름철 산란기에 있어 바지락 번식량의 공간적 변이와 기생 원생생물 Perkinsus olseni 감염이 바지락 번식에 미치는 영향

Spatial variation in the reproductive effort of Manila clam Ruditapes philippinarum is often closely associated with variation in the seawater temperature and food availability, which determines gonad maturity and the quantity of gamates produced during spawning. Previous studies also have reported that severe infection by the protozoan parasite Perkinsus olseni exerts a negative impact on clam reproduction, retarding gonad maturation or decreasing the reproductive effort. In the present study, we investigated impacts of P. olseni infection on the reproductive condition of Manila clam during a spawning season. Histology revealed that 54% of female clams in Wando off the south coast were in spawning, while only 10% of the female from Gomso and 0% of the female from Seonjaedo in Gyeonggi bay off the west coast were engaged in spawning at the end of May in 2004. Ray's fluid thioglycollate media (RFTM) assay was applied to assess P. olseni infection and indicated that the infection intensity in Wando (3,608,000 ± 258,000 cells/g wet tissue) was significantly higher than the levels in Gomso (1,305,000 ± 106,000 cells/g wet

Perkinsus sp. infections and in vitro isolates from Anadara trapezia (mud arks) of Queensland, Australia.

Perkinsus sp. protists were found infecting Anadara trapezia mud ark cockles at 6 sites in Moreton Bay, Queensland, Australia, at prevalences of 4 to 100% during 2011 as determined by surveys using Ray's fluid thioglycollate medium. Perkinsus sp. lesions were found among gill and visceral connective tissues in histological samples from several cockles, where basophilic, eccentrically vacuolated Perkinsus sp. signet ring trophozoites and proliferating, Perkinsus sp. schizont cells were documented. Two Perkinsus sp. isolates were propagated in vitro during August 2013 from gill tissues of a single infected A. trapezia cockle from Wynnum in Moreton Bay. DNA from those isolate cells amplified universally by a Perkinsus genus-specific PCR assay, and rDNA-internal transcribed spacer sequences respectively grouped them with P. olseni and P. chesapeaki in phylogenetic analyses. This is the first report of P. chesapeaki in Australia, and the first report of a P. chesapeaki in vitro isolate from an Australian mollusc host. Although P. olseni was originally described in 1981 as a pathogen of abalone in South Australia, and has subsequently been identified as a prevalent pathogen of numerous other molluscs worldwide, this is also the first report of a P. olseni-like in vitro isolate from an Australian mollusc host.

Development of a loop-mediated isothermal amplification method for detection of Perkinsus spp. in mollusks

Perkinsus is a genus of unicellular protozoan parasite responsible for mass mortality of several commercially valuable mollusks. Surveillance and inspection of its epidemiology in the field calls for convenient and rapid detection methods. Here, a loop-mediated isothermal amplification (LAMP) assay was developed to detect the presence of Perkinsus spp. in mollusks. Specific LAMP primers were designed targeting the conserved internal transcribed spacer 2 (ITS-2) region of the rRNA gene of Perkinsus spp. Using ITS-2 recombinant plasmid as a template, we optimized the LAMP reaction system and conditions and then evaluated the analytical sensitivity and specificity of the assay. The LAMP assay was validated using clam samples collected from coastal areas in eastern China and oysters imported to China and compared with the traditional Ray's fluid thioglycollate culture method (RFTM). Our results showed that the LAMP detection method for Perkinsus was successful. The detection limit was 10 copies of plasmid DNA. Compared to the RFTM assay, the LAMP detection method was more sensitive (56 versus 52 positive out of 60 samples). P. olseni and P. marinus from infected hosts were successfully detected by this method. The LAMP method is rapid, sensitive, and specific for Perkinsus spp. detection, and could be used to screen for perkinsosis both on farms and at ports.

Immunological responses of the mangrove oysters Crassostrea gasar naturally infected by Perkinsus sp. in the Mamanguape Estuary, Paraíba state (Northeastern, Brazil)

Perkinsus genus includes protozoan parasites of marine mollusks, especially bivalves. In the last four years, this parasite has been detected in mangrove oysters Crassostrea rhizophorae and Crassostrea gasar from the Northeastern region of Brazil. Hemocytes are the key cells of the oyster immune system, being responsible for a variety of cellular and humoral reactions, such as phagocytosis, encapsulation and the release of several effector molecules that control the invasion and proliferation of microorganisms. In Brazil, there is little information on perkinsosis and none on the immune responses of native oysters' species against Perkinsus spp. The objective of this study was to determine the effects of natural infection by Perkinsus sp. on the immunological parameters of mangrove oysters C. gasar cultured in the Mamanguape River Estuary (Paraíba, Brazil). Adults oysters (N = 40/month) were sampled in December 2011, March, May, August and October 2012. Gills were removed and used to determine the presence and intensity of the Perkinsus sp. infection, according to a scale of four levels (1–4), using the Ray's fluid thioglycollate medium assay. Immunological parameters were measured in hemolymph samples by flow cytometry, including: total hemocyte count (THC), differential hemocyte count (DHC), cell mortality, phagocytic capacity, and production of Reactive Oxygen Species (ROS). The plasma was used to determine the hemagglutination activity. The results showed the occurrence of Perkinsus sp. with the highest mean prevalence (93.3%) seen so far in oyster populations in Brazil. Despite that, no oyster mortality was associated. In contrast, we observed an increase in hemocyte mortality and a suppression of two of the main defense mechanisms, phagocytosis and ROS production in infected oysters. The increase in the percentage of blast-like cells on the hemolymph, and the increase in THC in oysters heavily infected (at the maximum intensity, 4) suggest an induction of hemocytes proliferation. The immunological parameters varied over the studied months, which may be attributed to the dynamics of infection by Perkinsus sp. The results of the present study demonstrate that Perkinsus sp. has a deleterious effect on C. gasar immune system, mainly in high intensities, which likely renders oysters more susceptible to other pathogens and diseases.