Toxoplasma gondii Infection in Marine Animal Species, as a Potential Source of Food Contamination: A Systematic Review and Meta-Analysis.

PURPOSE: Many marine animals are infected and susceptible to toxoplasmosis, which is considered as a potential transmission source of Toxoplasma gondii to other hosts, especially humans. The current systematic review and meta-analysis aimed to determine the prevalence of T. gondii infection among sea animal species worldwide and highlight the existing gaps.
METHODS: Data collection was systematically done through searching databases, including PubMed, Science Direct, Google Scholar, Scopus, and Web of Science from 1997 to July 2020.
RESULTS: Our search strategy resulted in the retrieval of 55 eligible studies reporting the prevalence of marine T. gondii infection. The highest prevalence belonged to mustelids (sea otter) with 54.8% (95% CI 34.21-74.57) and cetaceans (whale, dolphin, and porpoise) with 30.92% (95% CI 17.85-45.76). The microscopic agglutination test (MAT) with 41 records and indirect immunofluorescence assay (IFA) with 30 records were the most applied diagnostic techniques for T. gondii detection in marine species.
CONCLUSIONS: Our results indicated the geographic distribution and spectrum of infected marine species with T. gondii in different parts of the world. The spread of T. gondii among marine animals can affect the health of humans and other animals; in addition, it is possible that marine mammals act as sentinels of environmental contamination, especially the parasites by consuming water or prey species.

Introduction

Marine species constitute a very diverse group of animals with global distribution, mostly along coastal regions or habitat [1]. The human population density in coastal areas greatly increased during the recent decades and zoonotic pathogens can be transmitted to humans directly or indirectly from marine animals [2]. Thus, the health of marine mammals can substantially influence human’s well-being. Toxoplasmosis, caused by the intracellular protozoan Toxoplasma gondii, is a zoonotic infection with felids as definitive hosts, and a wide range of homoeothermic vertebrates as intermediate hosts [3, 4]. Pregnant women and immunocompromised patients are at a higher risk for developing the clinical disease with harsh outcomes, including congenital toxoplasmosis (hydrocephalus, chorioretinitis, and cerebral calcifications) and life-threatening encephalitis [5-7]. Understanding T. gondii transmission routes in wild, free-ranging marine mammals is problematic. There are three possible routes by which marine animals could become infected with T. gondii, including: ingestion of oocysts, ingestion of bradyzoites in tissue cysts of other intermediate hosts or vertically. Oocysts are shed via cat feces into the environment, which can readily infect several animal species [8, 9]. Small T. gondii oocysts show remarkable resistance to common disinfectants and remain alive in moist surroundings, even when exposed to a vast range of salinity and temperature conditions. This environmental tolerance leads to in fast and extensive dispersal of infection, particularly following heavy rain falls. The runoff originated from rainfalls alongside wastewater outfalls being likely contaminated with stray/feral cat fecal material make a huge depot of infective oocysts, which are usually discharged into a water body, i.e., sea and ocean, posing potential risk of T. gondii infection in those species dwelling in marine habitats [10]. In another way, marine animals acquired infection through ingestion of T. gondii protozoal cyst containing numerous bradyzoites. In areas where definitive hosts are rare and the viability of oocysts are likely limited due to freezing conditions, such as the Canadian Arctic, this could explain how animals are exposed to T. gondii. A number of investigators have pointed out that oocysts and bradyzoites of T. gondii are concentrated by oysters, clams and mussels during filter-feeding activity. It is noteworthy that the role of vertical transmission of toxoplasmosis in marine animals is unknown [9]. These are highly promising findings, but the precise mode of transmission is still open to question. Experimentally, oocyst sporulation occurs in seawater, remaining infective for animals for 6–24 months, depending on the temperature [11, 12].

During the last decades, a number of studies have reported T. gondii infection in marine animals, such as cetaceans, pinnipeds, sirenians, and sea otters (Enhydra lutris) [13-16]. Disseminated clinical disease has also been documented in adult or sometimes neonate marine mammals from Europe, USA, and Australia [17-19], with some degree of morbidity observed, for example, in the sea otters [13, 20, 21] and in the Pacific harbor seal (Phoca vitulina richardsi) [22, 23]. Furthermore, it seems that some species have been threatened and endangered in part due to toxoplasmosis [3, 24].

The increasing amount of anthropogenic toxicants discharged into the marine environment, as well as morbillivirus infection, can suppress the immunity of marine mammals and give rise to clinical toxoplasmosis susceptibility, yet in others cases, no links to concurrent disease have been identified [25, 26]. Since T. gondii is a pronounced hallmark of aquatic pollution and marine species are superb sentinel animals in marine life [27-29], it would be beneficial to assess the status of T. gondii infection in these animals. Thus, the current systematic review and meta-analysis aimed to investigate the prevalence of T. gondii infection among marine animal species worldwide and highlight the existing gaps.

Materials and Methods

Search Strategy

This study was prepared and performed in accordance with the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) statement [30]. Data were systematically searched and collected from English language databases including PubMed, Science Direct, Google Scholar, Scopus, ISI Web of Science, published from inception to 1 January, 2020 by two investigators (FR and ASP).

The search process was performed using the following keywords and medical subject headings (MeSH) terms: “Toxoplasma gondii”, “Toxoplasmosis”, “T. gondii” in combination with “fishes”, “marine mammals”; “oyster”, “Shellfish”, “mussels”, “dolphin”, “shark”, “crab”, “seal”, “sea lion”, “whale”, “sea otter”, “porpoise”, “shrimp”, “Manatees”, “Walruses”, “Eel”, “crayfish”, and “turtle”. To avoid missing of any paper, the reference list of relevant papers was screened manually.

Study Selection

For the first screening, the two independent authors (ASP and FR) surveyed the title and the abstract of all papers returned from the search process. To ensure the eligibility for inclusion to the systematic review, full texts of papers were also reviewed by investigators (ASP and FR), and any disagreement on articles selected was resolved.

Quality Evaluation

Selected articles were assessed according to a checklist used in previous studies [31]. This checklist was based on contents of the strengthening the reporting of observational studies in epidemiology (STROBE) checklist containing questions about various methodological aspects such as type of study, sample size, study population, data collection approaches and tools, sampling methods, variables estimation status, methodology, research objectives and demonstration of results according to the objectives [32]. For each question, a score was attributed and articles with a score of at least seven were selected articles. In addition, any disagreements with selected papers were reviewed by another author.

Selection Criteria and Data Extraction

Papers were included in the meta-analysis with the following criteria: (1) original articles; (2) studies in English language; (2) articles available in full-text; (3) studies that evaluated the prevalence of T. gondii infection in marine animals. On the other hand, the exclusion criteria entailed: case reports, review articles, letter to the editor, unclear or not technically acceptable diagnostic criteria, insufficient information, congress articles, as well as those with unavailable full-text. After reviewing all articles, papers without sufficient information and that did not obtain the minimum quality score were excluded.

Meta-Analysis

In this study, a forest plot was used to visualize the summarized results and heterogeneity among the included studies. The size of every square indicated the weight of every study as well as crossed lines presented confidence intervals, CI. To assess heterogeneity index, Cochran’s Q test and I2 statistics were applied. Additionally, a funnel plot was designed to determine the small study effects and their publication bias, based on Egger's regression test. The meta-analysis was conducted using Stats Direct statistical software (http://www.statsdirect.com). A P value less than 0.05 was considered statistically significant. Additional meta-analysis was performed based on the type of host, location and diagnostic method.

Results

A total of 5175 papers were analyzed by exploration of PubMed, Science Direct, Scopus, Google Scholar, and ISI Web of Science databases, and finally 55 records were found to be eligible for the current systematic review and meta-analysis. The searching and study selection procedures are illustrated in Fig. 1. Based on Continent, the highest number of investigations was from Europe (30 studies) with a total prevalence of 12.99%, and marine mustelids were the most infected group with 53.12%. It is also worth noting that 24 studies from North America were included in this systematic review, indicating a total prevalence of 21.15%, and an exceptionally high infection rate among cetaceans was observed in this continent (80.85%). In Asian countries, a low prevalence rate of 1.78% was reported and the pinnipeds were the most infected group with 29.2%. In South America, a pooled prevalence of 8.03% was reported with the highest infection in cetaceans (30.35%). In Oceania, the pooled prevalence was 17.73% and cetaceans were the most infected species (26.12%). In addition, the pooled prevalence rate in Antarctica was 39.21% in pinnipeds. On the other hand, no reports were found for the North Pole and the African continent (Fig. 2).

Fig. 1

Flowchart describing the study design process

Fig. 2

Pooled prevalence of T. gondii in marine animal species in different continents

According to Table 1, T. gondii infection was detected in dolphins (45 entries), whales (29 entries), seals (31 entries), sea lions (5 entries), sea otters (10 entries), porpoise (3 entries), oysters/mussels/shellfish (11 entries), fishes (4 entries), shrimp (2 entries), manatees (2 entries), walruses, eel and crayfish (single record for each) using serological and/or molecular techniques. Most reports were from the USA and Brazil with 24 records for each country, followed by Scotland (15 records), Italy (13 records), China (10 records), Spain (9 records), Canada and United Kingdom (8 records for each), Mexico (5 records), Norway and Russia (4 records for each), New Zealand (3 records), Japan (2 records) as well as single records from Iran, Turkey, Portugal, Netherlands, Peru, Australia and Solomon Islands. Altogether, eight serological methods were employed to determine T. gondii infection among marine animals. These include the modified agglutination test (MAT) as the most used technique (41 records), followed by immunofluorescence antibody test (IFA) (30 records) and immunohistochemistry (IHC) (21 records). Moreover, 17 entries used conventional polymerase chain reaction (PCR), being this the most used molecular technique, followed by nested-PCR (7 records) and quantitative PCR (qPCR) (4 records). Subgroup analysis (Table 2) showed that most studies were focused on cetaceans (whale, dolphin and porpoise) (36 studies), whereas the highest prevalence rate of T. gondii infection belonged to marine mustelids (sea otter, 10 studies) with 54.8% (95% CI 34.21–74.57%). Pooled proportion of T. gondii infection in dolphin species was of 51.07%. According to Egger’s test, the prevalence rates in cetaceans (P value = 0.0489) and pinnipeds (P value = 0.0004) were statistically significant.

Table 1

Detection of Toxoplasma gondii in marine animals (sorted by scientific name and publication date)

Species	Location	Continent	Test	Sample size	Positive (%)	References
Dolphin
Tursiops truncatus	USA	North America	MAT	141	138 (97.9)	Dubey et al. [17]
Sousa chinensis	Australia	Australia	IHC	4	4 (100)	Bowater et al. [47]
Stenella coeruleoalba	Spain	Europe	MAT	36	4 (11.1)	Cabezón et al [48]
Delphinus delphis	Spain	Europe	MAT	4	2 (50)	Cabezón et al. [48]
Tursiops truncatus	Spain	Europe	MAT	7	4 (57.1)	Cabezón et al. [48]
Phocoena phocoena	Spain	Europe	MAT	1	1 (100)	Cabezón et al. [48]
Grampus griseus	Spain	Europe	MAT	9	0	Cabezón et al. [48]
Tursiops aduncus	Solomon Islands	Oceania	Immunoblotting	58	8 (13.8)	Omata et al. [49]
Tursiops truncatus ponticus	Russia	Europe	ELISA	59	27 (45.7)	Alekseev et al. [50]
Tursiops truncatus	USA	North America	MAT	52	27 (51.9)	Dubey et al. [44]
Tursiops truncatus ponticus	Russia	Europe	ELISA	74	39 (52.7)	Alekseev et al. [51]
Tursiops truncatus	USA	North America	MAT	7	7 (100)	Dubey et al. [18]
Delphinus delphis	United Kingdom	Europe	Sabin Feldman	21	6 (28.5)	Forman et al. [52]
Grampus griseus	United Kingdom	Europe	Sabin Feldman	1	0	Forman et al. [52]
Lagenorhynchus acutus	United Kingdom	Europe	Sabin Feldman	1	0	Forman et al. [52]
Tursiops truncatus	United Kingdom	Europe	Sabin Feldman	1	0	Forman et al. [52]
Stenella coeruleoalba	United Kingdom	Europe	Sabin Feldman	5	0	Forman et al. [52]
Stenella coeruleoalba	Italy	Europe	IFA	8	4 (50)	Di Guardo et al. [53]
Tursiops truncates	Italy	Europe	Nested-PCR and MAT	8	7 (87.5)	Pretti et al. [54]
Stenella coeruleoalba	Italy	Europe	Nested-PCR and MAT	6	6 (100)	Pretti et al. [54]
Inia geoffrensis	Brazil	South America	MAT	95	82 (86.3)	Santos et al. [55]
Tursiops truncatus truncatus	Mexico	North America	MAT	63	55 (87.3)	Alvarado-Esquivel et al. [56]
Tursiops truncatus gillii	Mexico	North America	MAT	3	3 (100)	Alvarado-Esquivel et al. [56]
Cephalorhynchys hectori	New Zealand	Oceania	PCR	49	17 (34.7)	Roe et al. [57]
Tursiops truncatus	Spain	Europe	IFA	24	2 (8.3)	Bernal-Guadarrama et al. [58]
Stenella coeruleoalba	Italy	Europe	IFA	18	8 (44.4)	Profeta et al. [59]
Tursiops truncatus	Italy	Europe	IFA	3	2 (66.6)	Profeta et al. [59]
Grampus griseus	Scotland	Europe	IFA	7	2 (28.5)	et al. [26]
Delphinus delphis	Scotland	Europe	IFA	13	2 (15.4)	van de Velde et al. [26]
Stenella coeruleoalba	Scotland	Europe	IFA	9	0	van de Velde et al. [26]
Lagenorhynchus albirostris	Scotland	Europe	IFA	6	1 (16.6)	van de Velde et al. [26]
Stenella coeruleoalba	Italy	Europe	PCR	10	6 (60)	Pintore et al. [60]
Tursiops truncatus	Italy	Europe	PCR	1	1 (100)	Pintore et al. [60]
Steno bredanensis	Brazil	South America	IHC	3	0	Costa-Silva et al. [61]
Lagenodelphis hosei	Brazil	South America	IHC	2	0	Costa-Silva et al. [61]
Sotalia guianensis	Brazil	South America	IHC	27	1 (3.7)	Costa-Silva et al. [61]
Tursiops truncatus	Brazil	South America	IHC	4	1 (25)	Costa-Silva et al. [61]
Pontoporia blainvillei	Brazil	South America	IHC	102	0	Costa-Silva et al. [61]
Stenella frontalis	Brazil	South America	IHC	6	0	Costa-Silva et al. [61]
Stenella longirostris	Brazil	South America	IHC	5	0	Costa-Silva et al. [61]
Stenella clymene	Brazil	South America	IHC	6	0	Costa-Silva et al. [61]
Stenella coeruleoalba	Brazil	South America	IHC	2	0	Costa-Silva et al. [61]
Delphinus delphis	Brazil	South America	IHC	1	0	Costa-Silva et al. [61]
Delphinus delphis	Brazil	South America	IHC	1	0	Costa-Silva et al. [61]
Inia geoffrensis	Brazil	South America	IHC	1	0	Costa-Silva et al. [61]
Whale
Balaenoptera acutorostrata	Norway	Europe	MAT	202	0	Oksanen et al. [62]
Delphinapterus leucas	USA	North America	MAT	3	0	Dubey et al. [17]
Globicephala melas	Spain	Europe	MAT	1	0	Cabezón et al. [48]
Orcinus orca	Japan	Asia	PCR	8	1 (12.5)	Omata et al. [49]
Delphinapterus leucas	Russia	Europe	ELISA	147	7 (4.7)	Alekseev et al. [51]
Megaptera novaeangliae	United Kingdom	Europe	Sabin Feldman	1	1 (100)	Forman et al. [52]
Ziphius cavirostris	United Kingdom	Europe	Sabin Feldman	1	0	Forman et al. [52]
Physeter macrocephalus	Portugal	Europe	qPCR	5	0	Hermosilla et al. [63]
Balaenoptera physalus	Italy	Europe	IFA	1	0	van de Velde et al. [26]
Globicephala melas	Italy	Europe	IFA	1	0	van de Velde et al. [26]
Balaenoptera physalus	Scotland	Europe	IFA	1	0	van de Velde et al. [26]
Orcinus orca	Scotland	Europe	IFA	3	0	van de Velde et al. [26]
Globicephala melas	Scotland	Europe	IFA	10	4 (40)	van de Velde et al. [26]
Balaenoptera acutorostrata	Scotland	Europe	IFA	5	0	van de Velde et al. [26]
Mesoplodon bidens	Scotland	Europe	IFA	4	0	van de Velde et al. [26]
Physeter macrocephalus	Scotland	Europe	IFA	2	0	Alekseev et al. 2017 [64]
Balaenoptera borealis	Scotland	Europe	IFA	1	0	Iqbal et al. [65]
Delphinapterus leucas	Russia	Europe	ELISA	87	10 (11.5)	Profeta et al. [59]
Delphinapterus leucas	Canada	North America	PCR	34	15 (44.1)	Profeta et al. [59]
Globicephala melas	Italy	Europe	PCR	1	0	Pintore et al. [60]
Kogia sima	Brazil	South America	IHC	7	0	Costa-Silva et al. [61]
Peponocephala electra	Brazil	South America	IHC	5	0	Costa-Silva et al. [61]
Globicephala macrorhynchus	Brazil	South America	IHC	3	0	Costa-Silva et al. [61]
Physeter macrocephalus	Brazil	South America	IHC	3	0	Costa-Silva et al. [61]
Kogia breviceps	Brazil	South America	IHC	2	0	Costa-Silva et al. [61]
Megaptera novaeangliae	Brazil	South America	IHC	2	0	Costa-Silva et al. [61]
Orcinus orca	Brazil	South America	IHC	2	1 (50)	Costa-Silva et al. [61]
Mesoplodon europaeus	Brazil	South America	IHC	1	0	Costa-Silva et al. [61]
Balaenoptera physalus	Italy	Europe	PCR	7	1 (14.2)	Marcer et al. [66]
Seals
Phoca groenlandica	Norway	Europe	MAT	316	0	Oksanen et al. [62]
Phoca hispida	Norway	Europe	MAT	48	0	Oksanen et al. [62]
Cystophora cristata	Norway	Europe	MAT	78	0	Oksanen et al. [62]
Phoca vitulina	USA	North America	MAT	380	29 (7.6)	Lambourn et al. [67]
Phoca vitulina	USA	North America	MAT	311	51 (16.4)	Dubey et al. [17]
Phoca hispida	USA	North America	MAT	32	5 (15.6)	Dubey et al. [17]
Erignathus barbatus	USA	North America	MAT	8	4 (50)	Dubey et al. [17]
Phoca largha	USA	North America	MAT	9	1 (11.1)	Dubey et al. [17]
Phoca fasciata	USA	North America	MAT	14	0	Dubey et al. [17]
Phoca groenlandica	Canada	North America	MAT	112	0	Measures et al. [68]
Cystophora cristata	Canada	North America	MAT	60	1 (1.6)	Measures et al. [68]
Halichoerus grypus	Canada	North America	MAT	122	11 (9)	Measures et al. [68]
Phoca vitulina	Canada	North America	MAT	34	3 (8.8)	Measures et al. [68]
Phoca vitulina stejnegeri	Japan	Asia	ELISA	77	3 (3.9)	Fujii et al. [9]
Phoca vitulina vitulina	Spain	Europe	MAT	56	3 (5.3)	Cabezón et al. [48]
Halichoerus grypus	Spain	Europe	MAT	47	11 (23.4)	Cabezón et al. [48]
Pusa hispida	Canada	North America	DAT	788	80 (10.1)	Simon et al. [69]
Erignathus barbatus	Canada	North America	DAT	20	2 (10)	Simon et al. [69]
Phoca vitulina	Canada	North America	DAT	9	2 (22.2)	Simon et al. [69]
Leptonychotes weddellii	Antarctic Peninsula	South America	DAT	31	13 (41.9)	Rengifo-Herrera et al. [70]
Mirounga leonina	Antarctic Peninsula	South America	DAT	13	10 (76.9)	Rengifo-Herrera et al. [70]
Lobodon carcinophaga	Antarctic Peninsula	South America	DAT	2	1 (50)	Rengifo-Herrera et al. [70]
Arctocephalus gazella	Antarctic Peninsula	South America	DAT	165	4 (2.4)	Rengifo-Herrera et al. [70]
Arctocephalus gazella	Antarctica	Antarctica	DAT	21	12 (57.1)	Jensen et al. [71]
Leptonychotes weddellii	Antarctica	Antarctica	DAT	33	17 (51.5)	Jensen et al. [71]
Mirounga leonina	Antarctica	Antarctica	DAT	48	11 (22.9)	Jensen et al. [71]
Arctocephalus australis	Peru	South America	IFA	27	0	Jankowski et al. [72]
Halichoerus grypus	Scotland	Europe	IFA	13	0	van de Velde et al. [26]
Phoca vitulina	Scotland	Europe	IFA	17	2 (11.7)	van de Velde et al. [26]
Phoca vitulina richardsi	Alaska	North America	IFA	34	0	Bauer et al. [73]
Pusa caspica	Iran	Asia	MAT	36	30 (83.3)	Namroodi et al. [74]
Sea lions
Zalophus californianus	USA	North America	MAT	45	19 (42.2)	Dubey et al. [17]
Otaria flavescens	Mexico	North America	MAT	2	0	Alvarado-Esquivel et al.[56]
Zalophus californianus	Mexico	North America	MAT	4	2 (50)	Alvarado-Esquivel et al. [56]
Zalophus californianus	USA	North America	IFA	1630	46 (2.8)	Carlson-Bremer et al. [75]
Phocarctos hookeri	New Zealand	Oceania	ELISA	50	5 (10)	Michael et al. [76]
Sea otters
Lontra canadensis	USA	North America	LAT	103	46 (44.6)	Tocidlowski et al. [77]
Enhydra lutris nereis	USA	North America	IFA	223	115 (51.5)	Miller et al. [78]
Enhydra lutris nereis	USA	North America	IFA	80	29 (36.2)	Miller et al. [78]
Enhydra lutris kenyoni	USA	North America	IFA	21	8 (38.1)	Miller et al. [78]
Enhydra lutris kenyoni	USA	North America	IFA	65	0	Miller et al. [78]
Enhydra lutris nereis	USA	North America	Microscopic test	35	15 (42.8)	Miller et al. [79]
Enhydra lutris	USA	North America	MAT	145	107 (73.7)	Dubey et al. [17]
Lontra canadensis	USA	North America	IFA	40	7 (17.5)	Gaydos et al. [80]
Lutra lutra	Scotland	Europe	IFA	32	17 (53.1)	van de Velde et al. [26]
Enhydra lutris kenyoni	USA	North America	MAT	70	65 (92.8)	Verma et al. [81]
Porpoise
Phocoena phocoena	United Kingdom	Europe	Sabin Feldman	70	1 (1.4)	Forman et al. [52]
Phocoena phocoena	Netherlands	Europe	MAT	31	4 (12.9)	van de Velde et al. [26]
Phocoena phocoena	Scotland	Europe	IFA	98	2 (2)	van de Velde et al. [26]
Oysters/mussels/shellfish
Mytella guyanensis	Brazil	South America	Nested PCR	300	0	Esmerini et al. [82]
Crassostrea rhizophorae	Brazil	South America	Nested PCR	300	10 (3.3)	Esmerini et al. [82]
Mytilus galloprovincialis	Turkey	Europe	HRM	53	21 (39.6)	Aksoy et al. [37]
Ostreae concha	China	Asia	PCR	398	0	Zhang et al. [83]
Mytilus galloprovincialis	Italy	Europe	qPCR	53	7 (13.2)	Marangi et al. [84]
Crassostrea virginica	USA	North America	PCR	230	4 (1.7)	Marquis et al. [85]
Crassostrea rhizophorae	Brazil	South America	PCR	624	17 (2.7)	Ribeiro et al. [86]
Oysters	China	Asia	Nested PCR	998	26 (2.6)	Cong et al. [87]
Perna canaliculus	New Zealand	Oceania	Nested PCR	104	13 (12.5)	Coupe et al. [88]
Mytilus edulis	China	Asia	Nested PCR	2215	55 (2.4)	Cong et al. [89]
Crassostrea virginica	USA	North America	qPCR	1440	446 (30.9)	Marquis et al. [90]
Fishes
Carassius auratus	China	Asia	PCR	309	0	Zhang et al. [83]
Cyprinus carpio	China	Asia	PCR	309	0	Zhang et al. [83]
Hypophthalmichthys molitrix	China	Asia	PCR	456	1 (0.2)	Zhang et al. [83]
Fishes	Italy	Europe	qPCR	147	32 (21.7)	Marino et al. [91]
Shrimp
Penaeus monodon Fabricius	China	Asia	PCR	426	0	Zhang et al. [83]
Macrobrachium nipponense	China	Asia	PCR	813	1 (0.1)	Zhang et al. [83]
Manatees
Trichechus manatus	Mexico	North America	MAT	3	0	Alvarado-Esquivel et al. [56]
Trichechus inunguis			MAT	74	29 (39.1)	Mathews et al. [15]
Walruses
Odobenus rosmarus	USA	North America	MAT	53	3 (5.6)	Dubey et al. [17]
Eel
Monopterus albus	China	Asia	PCR	98	0	Zhang et al. [83]
Crayfish
Procambarus clarkii	China	Asia	PCR	618	4 (0.64)	Zhang et al. [83]

IHC immunohistochemistry, IFA immunofluorescence antibody test, DAT direct agglutination test, LAT latex agglutination test, HRM real time PCR/high-resolution melting analysis, IHAT indirect hemagglutination test

Table 2

Pooled prevalence of Toxoplasma infection in marine animals and subgroup analyses

Types of animals (species)	No. of studies	Prevalence (95% CI)	Heterogeneity			Egger’s test
			I2	Q	P value	T	P value
Cetaceans (whale, dolphin, porpoise)	36	30.92 (17.85–45.76)	97.5	1377.98	< 0.0001	4.87	0.0489
Pinniped (seals, sea lions, walruses)	18	12.16 (7.28–18.09)	96.3	460.63	< 0.0001	4.59	0.0004
Sirenians (manatees)	2	26.51 (2.46–63.69)	–	2.62	0.1049	–	–
Marine fissipeds (sea otter)	6	54.8 (34.21–74.57)	96.6	147.12	< 0.0001	−0.42	0.9593
Fishes (fish, eel)	5	1.64 (0.02–7.22)	96.2	105.71	< 0.0001	4.34	0.1065
Decapoda (crayfish, shrimp)	3	0.26 (0.03–0.73)	57.1	4.35	0.1132	–	–
Mollusca (oysters, mussels, shellfish)	10	7.45 (2.06–15.81)	99.1	962.83	< 0.0001	7.56	0.067

Discussion

The present systematic review and meta-analysis aimed to determine the prevalence rate of T. gondii infection worldwide. The obtained data were categorized based on the species of marine animals, continents, and diagnostic techniques. Among marine animals, the prevalence of T. gondii infection was higher in the population of sea otters (54.8%). In a study, Miller et al. [33] suggested that coastal freshwater runoff is a risk factor for toxoplasmosis in southern sea otters (Enhydra lutris nereis) in southern California. Furthermore, it has been shown that exposure to T. gondii among sea otters was highly influenced by individual animal prey choice and habitat use [34]. Toxoplasmosis had considerable morbidity and mortality rates in the sea otter [35]. T. gondii encephalitis in sea otters causes high mortality rate and is responsible for slow population recovery, particularly for the endangered Southern sea otter [27]. In addition, cetaceans were the most infected animals in North America, South America, and Oceania.

Modified agglutination test (MAT) was the most applied diagnostic assay for T. gondii detection in marine animals. This technique is widely employed in research of toxoplasmosis in humans and in all species of animals because it is considered as a rapid and simple approach without the requirement for special facilities [36]. Molecular methods, particularly polymerase chain reaction (PCR) and nested PCR, were used in marine animals usually as a food source for humans like fishes, shrimp, oysters, and crayfish, amongst others. Some studies indicate that consumption of contaminated raw shellfish and mussels can be considered a significant health danger due to their ability to infect a wide variety of hosts such as other marine animals and humans. However, they are particularly at risk for T. gondii infection, and therefore, they can be considered a bioindicator for monitoring waterborne pathogens [37, 38]. The high prevalence rate of T. gondii in the examined marine species may indicate that the nearby terrestrial environment in the studied area was heavily contaminated by T. gondii, and consequently, contamination was transferred to the aquatic environment. Furthermore, marine hosts may associate with T. gondii infection as paratenic hosts in some area [39]. Hence, contamination of marine animal species is an important bioindicator for contamination of aquatic environments.

Each cat, as final host for T. gondii, shed over 3–810 million oocysts. The sporulation of the oocysts takes 1–5 days, and they can remain infective in the soil for up to 18 months [40]. Furthermore, experiments showed that oocysts of T. gondii can sporulate in sea water and survive at 4 °C for 24 months and then infect mice [12]. One important factor in infected hosts is the strain of the parasite, which plays a major role in the toxoplasmosis prognosis. So far, the genotypes T. gondii were classified as classical types I, II, III, mix/recombinant atypical, and African lineages [41]. Comparison between T. gondii genotypes from the marine and terrestrial environments would help clarify routs and mechanisms of land-sea transmission. Type I strains, which are highly virulent and pathogenic, can lead to acquired ocular toxoplasmosis in individuals with disseminated congenital form of T. gondii [42, 43]. Aksoy et al. [37] reported T. gondii type 1 infection in Mytilus galloprovincialis (Mediterranean mussel), one of the most consumed shellfish in Turkey. The authors suggested that these types of contaminated seafood may be involved in the transmission of the parasite to humans and other hosts. Type II T. gondii strains are the vast majority of human infections and have a worldwide distribution. Type II strains are causative agents for numerous asymptomatic toxoplasmosis cases in Europe, it can be pathogenic for two important categories of subjects, namely immature fetuses and immunocompromised individuals [43]. On the basis of a previous study, Dubey et al. [44] showed Type II T. gondii from a striped dolphin (Stenella coeruleoalba) in Costa Rica. It is noteworthy that Type III T. gondii in mice are classified as avirulent strain. Study carried out by Hancock et al. [45] showed the first report of type III T. gondii in a Hawaiian monk seal. This genotype was determined to be restriction fragment length polymorphisms (RFLP) of the SAG2 gene. On the other hand, it has previously been shown that Type X strains of T. gondii are virulent for southern sea otters from coastal California [27]. Additionally, one interesting study has demonstrated Type X strains of T. gondii in canids, coastal-dwelling felids, nearshore-dwelling sea otters, and marine bivalve. It is assumed that contaminated runoff to feline faecal rapidly reaches sea from lands, and otters could be infected with T. gondii via the consumption of filter-feeding marine invertebrates [46].

The prevalence rate of marine T. gondii infection in various regions of the world was very different, and ranged from 0 to 100%. These differences may originate from different types of marine animals, sample sizes, and diagnostic approaches in the reviewed studies. Regarding continents, North America showed the highest T. gondii infection in marine animals that may suggest the level of fecal contamination of the soil and water reservoirs. Our analysis also showed that there is either no available data (Africa) or very limited literature (Antarctica, Oceania, and South America) on the prevalence of T. gondii infection in significant parts of the globe. Therefore, it is essential to conduct more studies to determine the putative role of T. gondii on marine species. The main limitation expressed in the included studies regarding prevalence of T. gondii infection in marine animal species was related to the use of different diagnostic methods with varying sensitivity and specificity due to their great impact on the results. The use of an accurate and reliable technique can help to correctly interpret the results of T. gondii prevalence in marine species in different parts of the world.

Conclusion

The results of current study indicated that the global prevalence rate of T. gondii infection was high in marine animals. It is well demonstrated that T. gondii parasite has a very successful adaptation in aquatic environments. Despite the worldwide range and broad marine animals host record of T. gondii infection, there was no evidence regarding toxoplasmosis in these animals in most parts of the world. Therefore, it is necessary to develop surveillance for detection of T. gondii in aquatic animals in different regions with appropriate molecular and serological techniques. It is also important to know the ecology of this parasite in aquatic environment to design appropriate strategies for monitoring, controlling, and prevention of the transmission of toxoplasmosis to humans or other hosts.

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