Fish and shellfish as sentinels of aquatic contamination: Global distribution and One Health implications of zoonotic Cryptosporidium and Giardia

Abstract

The zoonotic protozoans Cryptosporidium and Giardia are major drivers of the global gastrointestinal disease burden, circulating through complex human-animal-environment interfaces. This review synthesizes global epidemiological data to evaluate the role of fish and shellfish as reservoirs and potential conduits for zoonotic transmission. The pooled prevalence of Cryptosporidium spp. was 8.7% (858/9878) in fish and 16.5% (1459/8820) in shellfish. Likewise, the pooled prevalence of Giardia spp. in fish and shellfish were approximately 8.3% (30/361) and 6.2% (332/5392), respectively. Zoonotic Cryptosporidium species in fish included C. parvum, C. hominis, C. ubiquitum, C. scrofarum, and C. xiaoi. In shellfish, there were C. parvum, C. hominis, and C. meleagridis, with C. parvum dominant in both hosts. Zoonotic G. duodenalis assemblages A and B were detected in both fish and shellfish, with G. duodenalis assemblage A dominant in shellfish. These findings highlight that fish and shellfish, acting as either biological hosts or mechanical concentrators of environmental contamination, facilitate the dissemination of (oo)cysts through the aquatic food web. The presence of human-pathogenic genotypes in edible tissues poses significant food and water safety risks, exacerbated by anthropogenic pressures such as inadequate wastewater management and urbanization. This review emphasizes the necessity of a One Health framework to bridge the gap between environmental monitoring and public health surveillance. We advocate for integrated preventive measures, including enhanced molecular diagnostics and cross-sectoral management of aquatic ecosystems, to mitigate the transmission of foodborne and waterborne cryptosporidiosis and giardiasis.

Highlights

• •Global contamination by Cryptosporidium is 8.7% in fish and 16.5% in shellfish, by Giardia is 8.3% and 6.2%, respectively.
• •Zoonotic C. parvum and G. duodenalis assemblages A are dominant in both fish and shellfish.
• •Fish and shellfish accumulate zoonotic Cryptosporidium and Giardia, risking foodborneinfection when raw or undercooked.
• •Actionable One Health framework proposed to disrupt foodborne/waterborne spillover in aquaculture.

1. Introduction

Waterborne and foodborne parasitic infections represent a significant burden on global public health, with enteric protozoa like Cryptosporidium and Giardia are of significant importance to this challenge [1]. Cryptosporidium and Giardia pose unique risks due to their resilient life stages — Cryptosporidium oocysts and Giardia cysts can withstand harsh environmental conditions, including low temperatures, chemical disinfectants, and even some food processing treatments [2], [3]. This resilience allows them to persist in food systems long enough to reach human consumers, leading to infections that manifest as giardiasis and cryptosporidiosis. These diseases are characterized by gastrointestinal distress, including watery diarrhea, abdominal pain, bloating, and vomiting, which can escalate to life-threatening dehydration in vulnerable groups such as infants, the immunocompromised, and the elderly [4], [5], [6]. Beyond immediate illness, chronic infections may contribute to long-term nutritional deficiencies, especially in low-resource regions where access to healthcare and clean food and water is limited, highlighting the need to address all potential transmission routes of these pathogens [7], [8]. Currently, over 48 Cryptosporidium species are recognized [9]. Among these, C. hominis is the primary species pathogenic to humans, whereas C. parvum is the most frequently reported zoonotic species [9]. Giardia species include G. duodenalis, G. ardeae, G. agilis, G. microti, G. peramelis, G. psittaci, G. varani, G. cricetidarum, and G. muris. One of the most widely studied species is G. duodenalis, which is composed of eight assemblages (A–H), with assemblages A and B accounting for approximately 95% of human infections [9], [10].

While waterborne transmission of Cryptosporidium and Giardia has been extensively studied, with outbreaks linked to contaminated drinking water or recreational water being well- documented [11], [12], [13], [14]. A growing body of research has shifted attention to the food chain as a critical issue, yet not fully addressed in aquatic food safety monitoring and risk assessments [6], [15], [16]. Among food commodities, fish and shellfish emerge as particularly high-risk carriers due to their intimate connection to aquatic environments [17]. Unlike foods contaminated during post-harvest handling, fish and shellfish often become carriers in situ—in their natural habitats or aquaculture systems, creating a direct “environment- to- food” link that bypasses conventional food safety checks focused on processing or preparation. This distinction makes them unique carriers for Cryptosporidium and Giardia, as contamination can occur before the organisms are even harvested, leaving consumers vulnerable even if they follow proper cooking or handling guidelines [18], [19].

This review synthesizes current knowledge on the diversity, prevalence, and transmission of Cryptosporidium and Giardia species infecting fish and shellfish globally, with a critical focus on zoonotic species and their implications for food safety. We evaluate the potential public health hazards associated with consuming these aquatic hosts and emphasize the need for strategic preventive measures to mitigate consumer exposure and reduce the burden of foodborne cryptosporidiosis and giardiasis.

2. Global prevalence of Cryptosporidium and Giardia in fish and shellfish

2.1. Cryptosporidium spp. in fish

2.1.1. Species identified in fish

Cryptosporidium has been described in both fresh and marine fish, with the parasitic stages located on either the stomach or intestinal surface, or both. Cryptosporidium nasorum was the first identified Cryptosporidium species in Naso tang [20]. Nevertheless, this species was considered a nomen nudum due to the lack of viable oocyst measurements and taxonomically valid diagnostic features, as well as the fact that only developmental stages on the intestinal microvillous surface were described [21], [22], [23], [24]. In 2002, Álvarez-Pellitero and Sitjá-Bobadilla described C. molnari as a new species infecting the gills and stomach of gilthead seabream (Sparus aurata) and European sea bass (Dicentrarchus labrax) [25]. Since then, four other piscine-host-specific Cryptosporidium species have been recognized: C. huwi, C. bollandi, C. abrahamseni, and C. scophthalmi [25], [26], [27], [28]. In addition to fish-specific Cryptosporidium species and genotypes, several other vertebrate-associated Cryptosporidium species and genotypes have also been found in fish, including C. parvum, C. hominis, C. xiaoi, C. scrofarum, C. ubiquitum, and rat genotype III isolate. Furthermore, different genotypes have been identified in fish, including piscine Cryptosporidium genotypes, C. molnari-like genotypes, and several novel genotypes (Table S1, Fig. 1a).

Fig. 1.

Proportion of fish and shellfish samples positive for Cryptosporidium and Giardia.

2.1.2. Prevalence

The aggregated prevalence of Cryptosporidium spp. in fish is approximately 8.7% (858/9878). To date, Cryptosporidium presence in fish has been documented in eight countries with published epidemiological data, including Spain, Australia, France, Papua New Guinea, Vietnam, Azerbaijan, Iran, and the United Kingdom (UK). The reported prevalence of detection in these studies ranges from 1.1% (7/666) in Papua New Guinea to 20.0% (2/10) in Vietnam (Table 1; Fig. 2a). For the geographical distribution of existing evidence, the pooled prevalence across the sampled regions of the continents covered by included studies ranges from 5.1% to 15.1%. Specifically, the pooled prevalence in the sampled regions of four Asian countries (Vietnam, Azerbaijan, Iran) included in this review is 15.1% (21/139), which is higher than that of other continents with available data. The pooled prevalence in the sampled regions of European countries (Spain, France, UK) and Oceania (Australia, Papua New Guinea) is estimated to be 10.4% (666/6410) and 5.1% (171/3329), respectively (Table 1; Fig. 2a). However, according to current data, the major Cryptosporidium species vary among regions. However, according to current data, the major zoonotic Cryptosporidium species vary among regions. This is possibly related to the relative lack of research data, especially in Africa and South America.

Table 1.

Cryptosporidium and Giardia species and genotypes identified in fish in the world.

Location		Positive/total Samples (%)	Zoonotic Cryptosporidium spp. positive no.	Reference
Cryptosporidium
Asia	Iran	18.2% (4/22)		[29]
	Vietnam	20% (2/10)	C. parvum (2)	[30]
	Azerbaijan	14.0% (15/107)		[31]
	subtotal	15.1% (21/139)	C. parvum (2)
Europe	Spain	13.2% (640/4866)	C. parvum (54), . ubiquitum (2)	[[25], [32], [33], [34], [35], [36]]
	France	1.6% (24/1486)	C. parvum (21)	[[37], [38]]
	UK	3.5% (2/58)	C. hominis (2)	[39]
	subtotal	10.4% (666/6410)	C. parvum (75), C. ubiquitum (2), C. hominis (2)
Oceania	Papua New Guinea	1.1% (7/666)	C. parvum (5)	[40]
	Australia	6.2% (164/2663)	C. parvum (1), C. xiaoi (1), C. scrofarum (2)	[[26], [27], [28], [41], [42], [43], [44], [45], [46]]
	subtotal	5.1% (171/3329)	C. parvum (6), C. xiaoi (1)
Total		8.7% (858/9878)	C. parvum (83), C. hominis (4), C. scrofarum (2), C. xiaoi (1), C. ubiquitum (2)
Giardia
Asia	Iran	11.1% (1/9)	–	[29]
	subtotal	11.1% (1/9)	–
North America	USA	100% (1/1)	Assemblage B (1)	[47]
	subtotal	100% (1/1)	Assemblage B (1)
Africa	Egypt	3.3% (3/92)	Assemblage A (3)	[48]
	subtotal	3.3% (3/92)	Assemblage A (3)
Oceania	Australia	9.7% (25/259)	Assemblage A&B (26), Assemblage B (1)	[49]
	subtotal	9.7% (25/259)	Assemblage A&B (26), Assemblage B (1)
Total		8.3% (30/361)	Assemblage A (3), Assemblage B (2), Assemblage A&B (26)

Fig. 2.

Distribution of Cryptosporidium and Giardia spp. in fish and shellfish.

2.1.3. Occurrence by fish types

Cryptosporidium detection have been documented in 71 fish species, comprising 28 marine fish, 32 freshwater fish, and 11 ornamental fish species. The overall prevalence of Cryptosporidium presence in marine, freshwater, and ornamental fish are 11.9% (634/5319), 8.3% (172/2074), and 12.3% (41/332), respectively (Table S2). The zoonotic species C. parvum, C. scrofarum, C. xiaoi, and C. ubiquitum have been identified in marine fish [[37], [41]]. Additionally, two zoonotic species, C. parvum and C. hominis, have been detected in freshwater fish [[38], [39], [40]] (Table S2).

2.2. Cryptosporidium spp. in shellfish

2.2.1. Species distributions

To date, five known Cryptosporidium species (C. parvum, C. hominis, C. meleagridis, C. galli, C. baileyi) and one genotype (rat genotype IV) have been identified in shellfish (Table S3, Fig. 1b). The prevalence of Cryptosporidium in shellfish varies significantly across the sampled regions of continents covered by available studies. Based on cumulative data from included research, the pooled prevalence is highest in the sampled regions of South America at 20.6% (41/199), followed by Europe at 19.4% (843/4341), Asia at 18.3% (47/257), and North America at 11.8% (445/3782). With regard to species/genotype distribution, C. parvum, C. meleagridis, C. galli, and rat genotype IV have been detected in the sampled regions of Asia. In Europe, both C. parvum and C. hominis have been identified in shellfish. In North America, only C. parvum has been detected in shellfish, while in South America, C. parvum, C. baileyi, C. meleagridis, and C. hominis have been identified (Table 2). Data from Africa remains limited, with only scattered reports of Cryptosporidium presence in shellfish, highlighting a critical geographical gap in surveillance.

Table 2.

Cryptosporidium and Giardia species and genotypes identified in shellfish in the world.

Location		Positive/total Samples (%)	Zoonotic Cryptosporidium spp. positive no.	Reference
Cryptosporidium
Asia	Philippines	19.9% (40/201)	C. parvum (31), C. meleagridis (5)	[[50], [51]]
	Thailand	12.5% (7/56)	C. parvum (7)	[52]
	subtotal	18.3% (47/257)	C. parvum (38), C. meleagridis (5)
Europe	France	–	–	[53]
	Greece	2.5% (3/120)	–	[54]
	Italy	19.5% (721/3702)	C. parvum (403), C. hominis (60)	[[55], [56], [57], [58]]
	Netherlands	5.0% (9/179)	–	[59]
	Norway	42.9% (6/14)	–	[60]
	Portugal	32.7% (18/55)	–	[[58], [61]]
	Spain	35.2% (82/233)	C. parvum (82)	[[58], [62], [63], [64], [65]]
	UK	10.5% (4/38)	C. parvum (4)	[[58], [64]]
	Ireland	–	C. hominis	[[58], [66]]
	subtotal	19.4% (926/4582)	C. parvum (489), C. hominis (>60)
South America	Brazil	18.4% (33/179)	C. parvum	[[67], [68], [69]]
	Chile	40% (8/20)	C. parvum (8)	[70]
	subtotal	20.6% (41/199)	C. parvum (41)
North America	Canada	7.4% (101/1369)	C. parvum (66), C. meleagridis	[[71], [72], [73], [74]]
	USA	14.3% (344/2413)	C. parvum (335, include 311 mixed infection), C. hominis (9)	[[50], [75], [76], [77], [78], [79], [80], [81], [82]]
	subtotal	11.8% (445/3782)	C. parvum (368), C. meleagridis, C. hominis (9)
Total		16.5% (1459/8820)	C. parvum (936, include 311 mixed infection), C. meleagridis (>5), C. hominis (>69)
Giardia
Europe	Italy	2.5% (39/1542)	Assemblage A (89), Assemblage B (2), Assemblage A&B (3)	[[56], [57], [83]]
	Spain	37.2% (161/433)	Assemblage B (1)	[[64], [84], [85]]
	Netherlands	2.2% (4/179)	–	[59]
	Ireland	–	–	[66]
	Norway	50.0% (7/14)	–	[60]
	subtotal	9.7% (211/2168)	Assemblage A (89), Assemblage B (3), Assemblage A&B (3)
North America	Canada	2.1% (18/859)	Assemblage B (3)	[[72], [73], [74], [86]]
	USA	2.8% (31/1117)	Assemblage A (17), Assemblage C (9), Assemblage D (1), Assemblage B&C&D (4)	[[75], [87]]
	subtotal	2.5% (49/1976)	Assemblage A (17), Assemblage B (3)
Africa	Tunisia	1.6% (16/1020)	Assemblage A (16)	[88]
	subtotal	1.6% (16/1020)	Assemblage A (16)
Oceania	New Zealand	1.1% (1/90)	Assemblage B (1)	[89]
	subtotal	1.1% (1/90)	Assemblage B (1)
Total		6.2% (332/5392)	Assemblage A (122), Assemblage B (7), Assemblage A&B (3), Assemblage C (9), Assemblage D (1), Assemblage B&C&D (4)

2.2.2. Prevalence

Cryptosporidium spp. have been identified in shellfish in at least 15 countries based on published epidemiological studies, with the prevalence ranging from 2.5% (3/120) in Greece to 42.9% (6/14) in Norway (Table 2, Fig. 2b). Based on cumulative data from included studies, the pooled prevalence of Cryptosporidium spp. in shellfish is approximately 16.5% (1459/8820). The most prevalent Cryptosporidium species in shellfish samples is C. parvum (42.8%, 625/1459), followed by C. hominis (4.7%, 69/1459), C. meleagridis (0.3%, 5/1459), and rat genotype IV (0.2%, 3/1459). C. baileyi has only been found in oysters, in a mixed infection with C. parvum in the USA (the United States of America) (Table S3, Fig. 1b).

2.3. Giardia spp. in fish

2.3.1. Species identified

Much less is understood regarding Giardia in fish than has been understood about Cryptosporidium. Only G. duodenalis assemblages A, B, and E and Giardia microti have been reported in fish species (Fig. 1c). G. duodenalis assemblages A, E and C. microti have been reported in freshwater fish. G. duodenalis assemblages A and B have been reported in freshwater and marine fish, and G. duodenalis assemblage B has been reported in thresher sharks (Table S1). Notably, Giardia in fish often displays mixed infections among different assemblages, a pattern that could be attributed to variation in ecological roles and the pathways of environmental exposure [[48], [49]].

2.3.2. Prevalence by fish type

The aggregated prevalence of Giardia spp. in fish is approximately 8.3% (30/361) (Table S1). The coexistence of multiple Giardia assemblages in fish complicates the precise determination of infection rates for individual genotypes, as overlapping distributions of different assemblages have been recorded across various fish species and regions. Published studies indicate that freshwater fish are predominantly colonized or contaminated by G. duodenalis assemblages A, B, and E and G. microti, with an overall prevalence of 10.6% (23/217) based on cumulative data from the included studies (Table S2). In contrast, a lower infection rate is observed in marine fish: G. duodenalis assemblages A and B have been detected at a rate of 5.2% (7/135) (Table 1). Based on cumulative data from included studies, distinct distribution patterns of Giardia assemblages in fish are documented. G. duodenalis assemblages A, B, and E and G. microti have been reported in Australia; G. duodenalis assemblage A is prevalent in freshwater fish from Egypt; and G. duodenalis assemblage B has been identified in thresher sharks (Alopias vulpinus) from the USA. Additionally, Giardia spp. have been detected in freshwater fish from Iran (Table 1).

2.4. Giardia spp. in shellfish

2.4.1. Prevalence

To date, G. duodenalis assemblages A, B, C, and D have been found in shellfish distributed in nine countries The overall average detection rate is 6.2% (332/5392) in reported samples (Table 2, Fig. 2b). Regarding the geographical distribution, the prevalence across different regions ranges from 1.1% (1/90) to 9.7% (211/2168). Europe has a higher prevalence of 9.7% (211/2168) than other documented regions. The overall prevalence in North America, Africa, and Oceania are 2.5% (49/1976), 1.6% (16/1020), and 1.1% (1/90), respectively (Table 2).

2.4.2. Species distributions

Giardia species and their dominant distributions vary by region, based on current available epidemiological data (Table 2). In the sampled regions of Europe, G. duodenalis assemblages A and B have been identified, with assemblage A being the dominant genotype. For Africa, only G. duodenalis assemblage A has been detected in the limited published studies and sampled areas to date. In the sampled regions of North America and Oceania, G. duodenalis assemblages A, B, C, and D have been reported, with assemblage A as the dominant genotype (Table S3).

3. Zoonotic risk of Cryptosporidium and Giardia in fish and shellfish

3.1. Zoonotic risk of Cryptosporidium spp.in fish and shellfish

Several zoonotic Cryptosporidium species have been identified in fish, including C. parvum, C. hominis, C. ubiquitum, C. scrofarum, and C. xiaoi. Among these, C. parvum is the most frequently detected, with its presence documented in at least 19 piscine species. This species is known for its exceptionally broad host range, including bovids, primates, and marine mammals [90]. While humans serve as the primary susceptible hosts for the anthroponotic C. hominis, this species has recently been recorded in Salmon (Salmo salar) and Trout (Salmo trutta) [39]. Given that C. hominis is rarely found in non-human animals, its detection in fish likely reflects mechanical carriage following exposure to wastewater [91].

C. ubiquitum is recognized as an emerging zoonotic pathogen. Although it has been found in European sea bass in Spain [32], it is difficult to determine whether this represents a true biological infection or passive bioaccumulation in fish inhabiting human-altered environments or escaping from aquaculture facilities [[92], [93]]. The occurrence of C. ubiquitum in Spanish wastewater treatment plants further suggests that its presence in fish may stem from contaminated water sources rather than active colonization [94]. Likewise, the identification of the sheep-associated C. xiaoi in Australian school whiting is likely a result of environmental contamination from agricultural effluent [41]. It is critical to emphasize that while these pathogenic species have been detected in edible fish fillets, it remains unconfirmed whether they actively infect piscine hosts or are merely sequestered via bioaccumulation. Regardless of their biological status, the concentration of these parasites in fish tissues poses a significant potential public health risk [38].

In shellfish, identified zoonotic species include C. parvum, C. hominis, and C. meleagridis. Contamination with C. parvum has been reported globally, while C. hominis has been detected in oysters, clams, and zebra mussels across the USA, Italy, and Ireland (Table S4). The bioaccumulation of these oocysts in shellfish is primarily driven by the discharge of inadequately treated sewage into coastal waters [55]. C. meleagridis, the third most common cause of human cryptosporidiosis, has also been identified in various bivalves in Canada and the Philippines [[50], [51], [71], [95], [96], [97]]. A major source of this mechanical contamination appears to be avian excreta, as bivalve cultivation sites often provide perching spots for migratory and local birds [51]. The presence of C. meleagridis underscores the role of shellfish as environmental sentinels for fecal contamination and highlights the zoonotic risk associated with coastal ecosystems.

While true infection and direct transmission of C. molnari have been confirmed in gilthead seabream and sea bass [33], the role of fish and shellfish as mechanical carriers for human-pathogenic species is increasingly evident. To date, although biological transmission from these aquatic hosts to humans has not been directly observed, their capacity to concentrate and carry infectious oocysts renders them significant vectors in the waterborne and foodborne transmission chain.

3.2. Zoonotic risk of Giardia spp. in fish and shellfish

Zoonotic Giardia assemblage A and B have been detected in freshwater fish, marine fish, and cultured fingerlings (Table S2) [[48], [49]]. Given that these assemblages are prevalent in human populations, their presence in piscine hosts likely stems from environmental exposure to untreated human sewage and agricultural runoff.

In shellfish, the occurrence of these zoonotic assemblages is widespread. Giardia assemblage A has been detected in mussels, long-necked steamers, oysters, and clams across Italy, the USA, and Tunisia [[56], [57], [75], [83], [88]], while assemblage B has been reported in various bivalves in Italy, New Zealand, Spain, Canada, and the USA [[62], [72], [87], [89]] (Table S4). A distinct ecological pattern emerges, G. duodenalis assemblage A exhibits a higher prevalence of detection in fish and shellfish than assemblage B. This contrasts with global trends in human populations, where assemblage B is typically more prevalent [10]. This discrepancy suggests that assemblage A may possess a greater capacity for waterborne transmission and environmental persistence within aquatic habitats.

Fish and shellfish effectively serve as “litmus tests” for environmental contamination [88]. Although the majority of included studies originate from Europe, Australia, and North America, whereas data from Africa and South America are considerably scarcer. This disparity limits the global representativeness of our findings and may obscure region-specific transmission patterns or zoonotic risks. Extensive evidence confirms that humans and livestock shed Giardia cysts into aquatic ecosystems, where their occurrence is influenced by water turbidity and ambient temperature [[98], [99]]. However, a critical distinction must be made between mechanical carriage and active infection. While the presence of Giardia in shellfish is widely regarded as a result of passive bioaccumulation through filter-feeding, evidence in fish suggests a more complex relationship. Specifically, Yang et al. [49] observed large numbers of both Giardia trophozoites and cysts within the intestines of fish hosts. Since Giardia alternates its life cycle between the disease-causing trophozoite and the resistant cyst [100], the concurrent presence of both life stages suggests that zoonotic Giardia may successfully establish infections and proliferate within certain fish species, rather than merely being mechanically transported. Despite these localized findings of biological infection, the broader role of most aquatic species as mechanical carriers remains the primary concern for waterborne and foodborne transmission.

4. A one health perspective on prevalence heterogeneity

The marked variation in prevalence estimates across global studies is not merely a biological phenomenon but a reflection of the complex interplay within the human-animal-environmental interface. From a One Health perspective, the sensitivity of diagnostic tools represents a primary source of this data heterogeneity, as studies employing molecular techniques consistently yield higher detection rates and greater species resolution than those relying on conventional microscopy, which may suffer from false-negative results due to low (oo)cyst concentrations [101]. Host-specific ecological traits further complicate these comparisons. The inherently high filtration rates of bivalve mollusks facilitate a more pronounced bioaccumulation of pathogens compared to most pelagic fish species [51]. Furthermore, geographical disparities in prevalence often mirror regional anthropogenic pressures and the efficiency of local infrastructure. Regions characterized by intensive agricultural runoff or inadequate wastewater treatment facilities naturally exhibit elevated contamination levels in aquatic habitats [102]. Environmental conditions also play a critical role, as the lower prevalence observed in certain marine environments can be linked to high salinity and ambient temperatures that compromise the long-term viability and infectivity of Cryptosporidium and Giardia (oo)cysts [103]. These multifaceted drivers underscore that reported prevalence is not merely a reflection of parasite distribution, but is also significantly influenced by the diagnostic precision and the specific ecological context of each study [[104], [105]].

5. The critical role of fish and shellfish as food and water carriers

The primary routes for human exposure to Cryptosporidium and Giardia via fish and shellfish are directly linked to their consumption as food. While environmental water contamination serves as the initial source. The unique biological characteristics of aquatic animals, particularly their capacity for bioaccumulation which could transform them into potent carriers for foodborne transmission [106]. Fish and shellfish sequester Cryptosporidium oocysts and Giardia cysts from their aquatic habitats, a key step in the waterborne transmission pathway.

5.1. The waterborne transmission pathway

Fish and shellfish become contaminated with Cryptosporidium oocysts and Giardia cysts in their aquatic habitats, a key step in the waterborne transmission pathway. Cryptosporidiosis has been documented in a variety of livestock animals, including cattle, buffaloes, pigs, goats, sheep, horses, camels, donkeys, and poultry such as chickens and ducks [[107], [108]]. These protozoan oocysts/cysts are highly resistant to environmental stress and spread through the feces of infected humans, livestock, and wildlife [109]. Once Cryptosporidium and Giardia (oocysts/cysts) reach the water, they function as persistent waterborne pathogens. Cryptosporidium oocysts can survive for weeks to months in cool freshwater (4–20 °C) and even tolerate moderate salinity (up to 15 ppt), while Giardia cysts remain viable for up to several months in similar conditions [[110], [111]]. This persistence means that even transient fecal contamination can lead to long-term presence of pathogens in aquatic habitats, creating sustained risk. Aquatic organisms act as biological filters, accumulating these parasites in their tissues through filtration or ingestion. Among at least 416 outbreaks attributed to waterborne transmission of parasitic protozoa documented between 2017 and 2022, Cryptosporidium accounted for 77.4%, while Giardia was identified as the etiological agent in 17.1% [112].

Poultry, livestock, and rodent species may excrete feces into the environment, which subsequently enters water bodies. Fish and shellfish in the water accumulate these parasites in their bodies through filtration, thereby becoming carriers or transmitters. A pilot study of Cryptosporidium exposure risk among anglers in urban industrialized waterways estimated a 10–80% model-derived infection probability, based on fish and hand wash samples [113]. Additionally, engaging in recreational activities such as swimming in water can also lead to infections with Cryptosporidium and Giardia [97] (Fig. 3).

Fig. 3.

Schematic representation of possible zoonotic transmission routes.

5.2. The foodborne transmission pathway

Unsafe food accounts for nearly 600 million foodborne diseases and approximately 420,000 annual deaths [114]. Cryptosporidium and Giardia spp. are among the important causes in this regard. These parasites enter aquatic ecosystems through anthropogenic and zoonotic pathways, including sewage discharge, agricultural runoff, bird feces, and wildlife reservoirs [[51], [97]].

Bioaccumulation of Cryptosporidium and Giardia (oo)cysts occurs during filter-feeding by fish and shellfish. Even after undergoing depuration, a natural process in shellfish that reduces the microbial load in the body, a considerable number of (oo)cysts are capable of retaining their infectivity, leading to direct ingestion of infective stages of these parasites that possibly cause infection if shellfish are consumed raw or undercooked [[115], [116]]. Furthermore, the continued presence of Cryptosporidium and Giardia in the environment may increase the likelihood that fish and shellfish will filter out and concentrate oocysts, which could be harmful to seafood consumers [73].

On the other hand, many recent studies have revealed Cryptosporidium contamination in wastewater and recycled water used for agricultural irrigation [[117], [118]]. Humans use the waste water to irrigate crops and wash vegetables, thereby causing foodborne transmission. It is difficult to remove Cryptosporidium and Giardia oocysts from the surfaces of vegetables and fruits using common cleaning methods. Humans may ingest Cryptosporidium and Giardia under unknown circumstances. Given that a low infectious dose of less than 10 cysts or oocysts is sufficient to cause illness in humans, even a small amount of contamination can have serious consequences [118]. Vulnerable populations, such as young children, the elderly, and individuals with weakened immune systems, are at a particularly high risk of severe morbidity. This risk is more pronounced in areas with inadequate sanitation or lax food safety rules [114] (Fig. 3).

6. Integrating One Health to mitigate aquatic zoonoses

Integrated One Health strategies are essential to mitigate the complex public health risks associated with the transmission of Cryptosporidium and Giardia through the aquatic food chain. The One Health framework, which emphasizes the intrinsic interconnectedness of human, animal, and environmental health, is increasingly recognized as a critical paradigm for addressing fish-borne and shellfish-borne zoonotic diseases [[119], [120], [121]]. There is a need for multisectoral collaboration involving stakeholders ranging from seafood producers and veterinarians to public health authorities and clinicians to establish strong food safety protocols.

Despite its strategic importance, systemic challenges persist in the operationalization of this approach. The World Health Organization (WHO) has identified significant barriers, such as fragmented institutional structures, inadequate diagnostic capacities in developing regions, and the inherent complexity of interrupting multi-host transmission chains [122]. To overcome these limitations, enhanced coordination between human and veterinary health sectors is vital for real-time data sharing and early disease detection [121]. Furthermore, a persistent technical hurdle is the accurate molecular identification of parasitic assemblages, necessitating investment in advanced diagnostics to align mitigation strategies with the ecological and epidemiological drivers of zoonotic spillover.

Practical interventions must adopt a holistic perspective: (1) strengthening municipal and industrial wastewater treatment to minimize the environmental influx of resistant (oo)cysts; (2) implementing rigorous One Health-based monitoring in aquaculture facilities to ensure the biosecurity of farmed aquatic species; and (3) launching targeted public health campaigns to educate consumers on the risks of raw seafood consumption and proper handling techniques [123]. Ultimately, achieving global One Health integration remains a daunting yet imperative task. Moving away from fragmented, sector-specific responses toward a unified surveillance and management model is the only viable path to breaking the cycles of infection across human, animal, and environmental domains.

7. Conclusion

Cryptosporidium and Giardia represent significant zoonotic pathogens at the interface of aquatic ecosystems and human food safety. This review provides a comprehensive synthesis of Cryptosporidium and Giardia species and assemblages identified in fish and shellfish, consolidating evidence that positions these aquatic hosts as critical waterborne and foodborne vectors, particularly when consumed raw or minimally processed. The circulation of these parasites within interconnected aquatic environments reflects broader anthropogenic drivers, including rapid urbanization, intensified aquaculture, and climate-induced hydrological shifts. These factors collectively amplify the risk of environmental contamination and subsequent human exposure.

While molecular epidemiology has confirmed the presence of highly pathogenic genotypes (C. parvum, C. hominis, C. ubiquitum, C. meleagridis, C. xiaoi, and G. duodenalis assemblages A and B) in diverse aquatic species, significant knowledge gaps persist. Geographic disparities in surveillance infrastructure, variations in diagnostic sensitivity, and the ecological complexity of host-parasite interactions hinder the precise quantification of global zoonotic risks. Furthermore, the environmental resilience of these protozoans, exacerbated by the globalized seafood trade and cultural culinary preferences, underscores the systemic vulnerabilities in public health, especially in resource-limited settings with inadequate sanitation. Addressing these multifaceted challenges necessitates a One Health framework that integrates advanced molecular pathogen tracking with stricter multi-sectoral regulation of wastewater management near aquaculture and harvesting zones. Future research should prioritize the mechanisms of parasite persistence in edible tissues throughout the supply chain and evaluate the efficacy of cross-domain mitigation strategies. Ultimately, fostering community-driven education and interdisciplinary policy-making is essential to reduce the global burden of foodborne cryptosporidiosis and giardiasis linked to aquatic food systems.

The following are the supplementary data related to this article.

Table S1

Cryptosporidium and Giardia species and genotypes reported in fish globally.

Table S2

Prevalence of Cryptosporidium and Giardia in individual fish species

Table S3

Cryptosporidium and Giardia species and genotypes reported in shellfish globally.

Table S4

Prevalence of Cryptosporidium and Giardia in individual shellfish species

Ethical statement - studies in humans and animals

Not applicable.

Search strategy

This review was conducted using a structured search strategy to ensure a comprehensive narrative synthesis of the available literature. We performed a search for peer-reviewed articles published up to March 20, 2025, across four major databases: PubMed, Scopus, Science Direct, and Web of Science, using the search terms Cryptosporidium, Giardia, fish, and shellfish. The search was applied to specific fields, including “Title/Abstract” in PubMed, “Find articles with these terms” in Science Direct, “TITLE-ABS-KEY” in Scopus, and “Topic” in Web of Science, yielding a total of 137 relevant papers without language or publication date restrictions. Inclusion criteria were: (1) original research articles reporting the detection of Cryptosporidium and/or Giardia in fish or shellfish; (2) studies providing clear data on sample size and positive detections; and (3) articles published in peer-reviewed journals. Exclusion criteria were: (1) editorials, conference abstracts, commentaries, and opinion pieces; (2) studies focusing exclusively on environmental water samples without direct testing of fish or shellfish; (3) studies with insufficient data to extract prevalence information; and (4) duplicate publications. Data extraction was performed independently to capture key information, including geographical location, host species, and detection methods, which formed the basis for the subsequent data aggregation and narrative discussion of transmission dynamics.

CRediT authorship contribution statement

Na Li: Writing – original draft. Ziyang Qin: Investigation. Kaihui Zhang: Supervision. Pitambar Dhakal: Supervision. Junqiang Li: Supervision. Jinfeng Zhao: Supervision, Funding acquisition. Longxian Zhang: Supervision, Funding acquisition.

Funding

This work was supported by Henan Province Science and Technology Research Project (252102111025), the Key Research and Development Special Project of Henan Province of China (231111111500) and the National Key Research and Development Program of China (2022YFD1800200 and 2023YFD1801200).

Declaration of competing interest

The authors declare no conflict of interest.

Data availability

Data will be made available on request.