Environmental dispersal of Cryptosporidium parvum oocysts and cross transmission in cultured bivalve molluscs

Investigations from Northern Greece on mussels cultivated in areas proximal to wastewaters discharges, as a potential source for human infection with Giardia and Cryptosporidium

Marine bivalves are usually cultivated in shallow, estuarine waters where there is a high concentration of nutrients. Many micro-pollutants, including the protozoan parasites Giardia duodenalis and Cryptosporidium spp., which also occur in such environments, may be concentrated in shellfish tissues during their feeding process. Shellfish can thus be considered as vehicles for foodborne infections, as they are usually consumed lightly cooked or raw. Therefore, the main objective of this study was to investigate the presence of both parasites in Mediterranean mussels, Mytilus galloprovincialis that are cultivated in Thermaikos Gulf, North Greece, which is fed by four rivers that are contaminated with both protozoa. Moreover, the occurrence of these protozoa was monitored in treated wastewaters from 3 treatment plants that discharge into the gulf. In order to identify potential sources of contamination and to estimate the risk for human infection, an attempt was made to genotype Giardia and Cryptosporidium in positive samples. Immunofluorescence was used for detection and molecular techniques were used for both detection and genotyping of the parasites. In total, 120 mussel samples, coming from 10 farms, were examined for the presence of both protozoa over the 6-month farming period. None of them were found positive by immunofluorescence microscopy for the presence of parasites. Only in 3 mussel samples, PCR targeting the GP60 gene detected Cryptosporidium spp. DNA, but sequencing was not successful. Thirteen out of 18 monthly samples collected from the 3 wastewater treatment plants, revealed the presence of Giardia duodenalis cysts belonging to sub-assemblage AII, at relatively low counts (up to 11.2 cysts/L). Cryptosporidium oocysts (up to 0.9 oocysts/L) were also detected in 4 out of 8 samples, although sequencing was not successful at any of the target genes. At the studied location and under the sampling conditions described, mussels tested were not found to be harboring Giardia cysts and the presence of Cryptosporidium was found only in few cases (by PCR detection only). Our results suggest that the likelihood that mussels from these locations act as vehicles of human infection for Giardia and Cryptosporidium seems low.

Protocol standardization for the detection of Giardia cysts and Cryptosporidium oocysts in Mediterranean mussels (Mytilus galloprovincialis)

Marine bivalve shellfish are of public health interest because they can accumulate pollutants in their tissues. As they are usually consumed raw or lightly cooked, they are considered to be a possible source of foodborne infections, including giardiosis and cryptosporidiosis. Although data indicating contamination of shellfish with Giardia cysts and Cryptosporidium oocysts have been published, comparing results from different studies is difficult, as there is no standardized protocol for the detection and quantification of these parasites in mussels, and different researchers have used different analytical approaches. The aim of this study was to identify and characterize the most sensitive protocol for the detection of Giardia cysts and Cryptosporidium oocysts in shellfish. In an effort to test the sensitivity and the detection limits of the protocol, every step of the process was investigated, from initial preparation of the mussel matrix through detection of the parasites. Comparative studies were conducted, including several methods previously applied by other researchers, on commercial mussels Mytilus galloprovincialis spiked with a known number of (oo)cysts of both parasites. As preparation of the mussel matrix plays an important role in the sensitivity of the method, different techniques were tested. These included: (ia) removal of the coarse particles from the matrix with sieving, (ib) extraction of the lipids with diethyl ether, and (ic) artificial digestion of the matrix with pepsin digestion solution, and (ii) the use or not of immunomagnetic separation (IMS) for the concentration of the (oo)cysts. Pre-treatment of the mussel homogenate with pepsin digestion solution, followed by IMS, then detection with a direct immunofluorescence assay, achieved the highest sensitivity: 32.1% (SD: 21.1) of Giardia cysts and 61.4% (SD: 26.2) Cryptosporidium oocysts were recovered, with a detection limit of 10 (oo)cysts per g of mussel homogenate. The outcome of the current study was the standardization of a protocol, with defined detection limits, for the detection of these two protozoan transmission stages in mussels, in order to be used as a reference technique in future studies. Further advantages of this protocol are that it uses the whole mussel as a starting material and does not require difficult handling procedures. The method has potential to be applied in larger surveys and, potentially, to other species of shellfish for the detection of these parasites. However, the composition (lipid to protein ratio) may be of relevance for detection efficiency for some other species of shellfish.

Public health risks associated with food-borne parasites

Parasites are important food‐borne pathogens. Their complex lifecycles, varied transmission routes, and prolonged periods between infection and symptoms mean that the public health burden and relative importance of different transmission routes are often difficult to assess. Furthermore, there are challenges in detection and diagnostics, and variations in reporting. A Europe‐focused ranking exercise, using multicriteria decision analysis, identified potentially food‐borne parasites of importance, and that are currently not routinely controlled in food. These are Cryptosporidium spp., Toxoplasma gondii and Echinococcus spp. Infection with these parasites in humans and animals, or their occurrence in food, is not notifiable in all Member States. This Opinion reviews current methods for detection, identification and tracing of these parasites in relevant foods, reviews literature on food‐borne pathways, examines information on their occurrence and persistence in foods, and investigates possible control measures along the food chain. The differences between these three parasites are substantial, but for all there is a paucity of well‐established, standardised, validated methods that can be applied across the range of relevant foods. Furthermore, the prolonged period between infection and clinical symptoms (from several days for Cryptosporidium to years for Echinococcus spp.) means that source attribution studies are very difficult. Nevertheless, our knowledge of the domestic animal lifecycle (involving dogs and livestock) for Echinoccocus granulosus means that this parasite is controllable. For Echinococcus multilocularis, for which the lifecycle involves wildlife (foxes and rodents), control would be expensive and complicated, but could be achieved in targeted areas with sufficient commitment and resources. Quantitative risk assessments have been described for Toxoplasma in meat. However, for T. gondii and Cryptosporidium as faecal contaminants, development of validated detection methods, including survival/infectivity assays and consensus molecular typing protocols, are required for the development of quantitative risk assessments and efficient control measures.

Alteration of host-pathogen interactions in the wake of climate change - Increasing risk for shellfish associated infections?

The potential for climate-related spread of infectious diseases through marine systems has been highlighted in several reports. With this review we want to draw attention to less recognized mechanisms behind vector-borne transmission pathways to humans. We have focused on how the immune systems of edible marine shellfish, the blue mussels and Norway lobsters, are affected by climate related environmental stressors. Future ocean acidification (OA) and warming due to climate change constitute a gradually increasing persistent stress with negative trade-off for many organisms. In addition, the stress of recurrent hypoxia, inducing high levels of bioavailable manganese (Mn) is likely to increase in line with climate change. We summarized that OA, hypoxia and elevated levels of Mn did have an overall negative effect on immunity, in some cases also with synergistic effects. On the other hand, moderate increase in temperature seems to have a stimulating effect on antimicrobial activity and may in a future warming scenario counteract the negative effects. However, rising sea surface temperature and climate events causing high land run-off promote the abundance of naturally occurring pathogenic Vibrio and will in addition, bring enteric pathogens which are circulating in society into coastal waters. Moreover, the observed impairments of the immune defense enhance the persistence and occurrence of pathogens in shellfish. This may increase the risk for direct transmission of pathogens to consumers. It is thus essential that in the wake of climate change, sanitary control of coastal waters and seafood must recognize and adapt to the expected alteration of host-pathogen interactions.

Bioaccumulation of human waterborne protozoa by zebra mussel (Dreissena polymorpha): interest for water biomonitoring

Cryptosporidium parvum, Giardia duodenalis and Toxoplasma gondii are ubiquitous pathogens, which waterborne transmission has been largely demonstrated. Since they can be found in various watercourses, interactions with aquatic organisms are possible. Protozoan detection for watercourses biomonitoring is currently based on large water filtration. The zebra mussel, Dreissena polymorpha, is a choice biological model in ecotoxicological studies which are already in use to detect chemical contaminations in watercourses. In the present study, the zebra mussel was tested as a new tool for detecting water contamination by protozoa. In vivo exposures were conducted in laboratory experiments. Zebra mussel was exposed to various protozoan concentrations for one week. Detection of protozoa was realized by Taqman real time qPCR. Our experiments evidenced C. parvum, G. duodenalis and T. gondii oocyst bioaccumulation by mussels proportionally to ambient contamination, and significant T. gondii prevalence was observed in muscle tissue. To our knowledge, this is the first study that demonstrates T. gondii oocyst accumulation by zebra mussel. The results from this study highlight the capacity of zebra mussels to reveal ambient biological contamination, and thus to be used as a new effective tool in sanitary biomonitoring of water bodies.

The potential for marine bivalve shellfish to act as transmission vehicles for outbreaks of protozoan infections in humans: a review

Most marine molluscan bivalve shellfish feed on suspended phytoplankton which are trapped from water pumped across the gills by ciliary action. Pathogenic microorganisms in the water may be filtered by the gills during feeding, and become concentrated in the digestive glands/tract. If these pathogens are not excreted or inactivated by the shellfish, or in subsequent preparatory processes, they may be ingested by consumers, the shellfish thereby acting as vehicles of infection. The protozoan parasites Cryptosporidium spp., Giardia duodenalis and Toxoplasma gondii have the potential to be transmitted in this way, and here we review the accumulating knowledge on the occurrence and survival of the transmission stages of these parasites in shellfish, whilst also emphasising the considerable gaps in our knowledge. Relevant information is particularly lacking for T. gondii, which, in comparison with Cryptosporidium spp. and G. duodenalis, has been relatively under-researched in this context. Although it seems evident that these shellfish can accumulate and concentrate all three of these parasites from the surrounding water, whether Giardia cysts remain viable and infectious is unknown, and some evidence suggests that they may be inactivated by the shellfish. Although both Toxoplasma and Cryptosporidium apparently retain their infectivity for prolonged periods in shellfish, the actual public health threat posed by these parasites via these shellfish is unclear, largely because there is minimal evidence of infection transmission. Reasons for this apparent lack of infection transmission are discussed and it is recommended that the potential for transmission via shellfish consumption is recognised by those concerned with investigating transmission of these infections.

Effectiveness of standard UV depuration at inactivating Cryptosporidium parvum recovered from spiked Pacific oysters (Crassostrea gigas)

When filter-feeding shellfish are consumed raw, because of their ability to concentrate and store waterborne pathogens, they are being increasingly associated with human gastroenteritis and have become recognized as important pathogen vectors. In the shellfish industry, UV depuration procedures are mandatory to reduce pathogen levels prior to human consumption. However, these guidelines are based around more susceptible fecal coliforms and Salmonella spp. and do not consider Cryptosporidium spp., which have significant resistance to environmental stresses. Thus, there is an urgent need to evaluate the efficiency of standard UV depuration against the survival of Cryptosporidium recovered from shellfish. Our study found that in industrial-scale shellfish depuration treatment tanks, standard UV treatment resulted in a 13-fold inactivation of recovered, viable C. parvum oocysts from spiked (1 x 10(6) oocysts liter (-1)) Pacific oysters. Depuration at half power also significantly reduced (P < 0.05; ninefold) the number of viable oocysts recovered from oysters. While UV treatment resulted in significant reductions of recovered viable oocysts, low numbers of viable oocysts were still recovered from oysters after depuration, making their consumption when raw a public health risk. Our study highlights the need for increased periodic monitoring programs for shellfish harvesting sites, improved depuration procedures, and revised microbial quality control parameters, including Cryptosporidium assessment, to minimize the risk of cryptosporidiosis.

A histological study of the transit of Cryptosporidium parvum oocysts through clams (Tapes decussatus)

A histological study was carried out to investigate the transit of Cryptosporidium parvum oocysts through the clam Tapes decussatus. Spat of approximately 5-7 mm shell length were maintained in a tank of natural sea water contaminated with purified C. parvum oocysts. The experiment lasted 240 h and, every 24 h, five specimens were killed, placed in Bouin's fixative, and processed routinely for histological examination. Sections (3 mum) cut from the all body tissues were stained with modified Gomori's trichrome for their accurate identification; the oocysts were detected by a direct immunofluorescence procedure. Oocysts were detected in siphons, gills, stomach, digestive diverticula, and intestine. The oocysts present in the intestine were free or mixed with the intestinal contents; therefore release of these oocysts with the feces should favour dissemination of contamination. Oocysts were found in branchial mucus and within the interfilamentary spaces, which suggests the occurrence of repeated filtrations and the possibility that the retained oocysts maintain their infective capacity.

Epidemiology of Cryptosporidium molnari in Spanish gilthead sea bream (Sparus aurata L.) and European sea bass (Dicentrarchus labrax L.) cultures: from hatchery to market size

A long-term epidemiological study of Cryptosporidium molnari in aquacultured European sea bass (ESB) and gilthead sea bream (GSB) was performed in different types of facilities on the Atlantic, Cantabric, and Mediterranean coasts. Four types of studies were carried out. In study A, fish raised from juveniles to marketable size (ongrowing stage) were periodically sampled in three different types of cultures. Studies B and C focused on hatchery and nursery facilities. In study D, occasional samplings were performed during mortality or morbidity outbreaks. As a general trend, C. molnari was more prevalent in GSB than in ESB. Data on the distribution pattern of C. molnari in total sampled GSB (studies A, B, and D) had a variance higher than the mean (overdispersion). In GSB (study A), the type of ongrowing system (sea cages, earth ponds, or indoor tanks) was found to have no significant effect. There was a significant relationship between the presence of the parasite and both fish weight and season. The highest infection values were recorded in spring. Prevalence and intensity had convex weight profiles, with a peak in 30- to 100-g fish. In study D, the prevalence of infection was higher in fish recently introduced in sea cages and in preongrowing systems. In studies B and C, fish were almost never infected before entering the postlarval and nursery facilities. The parasite seems to enter the host mainly through the water in production steps with less stringent water treatment. Recirculation systems and fish cannibalism could contribute to oocyst concentration and dispersion in aquaculture facilities.