Human health and diseases

Human exposure to marine-borne diseases can be costly, not only to infected individuals but to entire communities. Direct costs can include: death, medical diagnosis, treatment, investigation of outbreaks, and subsequent monitoring. Indirect costs can include: lost wages and productivity, migration costs, chronic pain and suffering, cost of psychological follow-up, mental health costs, and loss of recreational activities.

Globally, marine pathogens cause

approximately 250 million cases of illness a year, with 5 million annual cases of gastrointestinal illness due to bathing in contaminated water in the USA alone (Ralston et al., 2011). Currently, there are no estimates for the global economic costs associated with this. There are, however, estimates available from the USA, which place figures at approximately US$ 1 billion per year (Ralston et al., 2011). A breakdown of the costs show that US$ 350 million is spent on known marine pathogens and toxins that cause

food-borne diseases; US$ 300 million is spent on seafood-borne diseases with known etiology (i.e.

cases with unknown pathogenic etiology are excluded from this estimation); US$30 million of damage is caused by the Vibrio bacteria; and US$ 300 million is spent on gastrointestinal illnesses arising from bathing in contaminated water (Ralston et al., 2011).

The two primary routes of transmission for marine borne disease are the consumption of contaminated seafood and direct exposure to marine pathogens in the marine environment.

However, a third route of impact on human health - which deserves further attention - is the effect of anthropogenic pollution on the spread, transfer and potential exacerbation of marine-borne illnesses. The following outlines a few examples of the frequently reported illnesses associated with these transmission and impact routes, and the economic data currently available regarding the costs of these illnesses.

Shellfish and other food contamination

Contaminated seafood causes more illnesses per weight consumed than any other food category (CSPI, 2015; Elbashir et al., 2018).

Caused by a variety of bacteria, viruses, parasites and toxins, illnesses can range from mild gastroenteritis to life-threatening symptoms (Iwamoto et al., 2010). Coastal populations are particularly vulnerable to seafood-borne illness, with an average consumption per capita that is 15 times higher than non-indigenous country populations (Cisneros-Montemayor et al., 2016).

6 http://www.fao.org/food/food-safety-quality/scientific-advice/jemra/risk-assessments/vibrio0/en/

While viruses are one of the most common cause of seafood-related infections, bacterial agents have been associated with most hospitalizations and deaths (Butt et al., 2004).

Parasitic infections from seafood consumption seem more uncommon (and less costly) than bacterial or viral infections. However, the actual public health impact posed by parasites via shellfish remains unclear, largely due to severe under-reporting and the lack of data and minimal evidence of infection transmission, globally and locally (Robertson, 2007). Better monitoring is urgently required to understand the risks posed by these food-borne parasites (Singh et al., 2014).

Costs of seafood-borne illness

According to US studies, the acute health care costs resulting from marine-borne pathogens in the USA are estimated at approximately US$1 billion annually (Ralston et al., 2011). This includes US$350 million due to known marine pathogens and toxins that cause food-borne diseases; and US$300 million due to seafood-borne disease with known etiology (cases with unknown pathogenic etiology are excluded from this estimation). Most of the costs of seafood-borne illness are associated with premature deaths (US$300 million), followed by medical care (US$25 million) and lost productivity (US$15 million) (Ralston et al, 2011⁶).

Of all the marine-borne pathogens, Vibrio and Norovirus (Norwalk virus) are among the costliest. Vibrio spp. is the main species involved in seafood-and seawater-borne illness worldwide (with Vibrio vulnificus being the costliest). Enteric viruses, such as human noroviruses are among the most infectious, and one of the main causes of gastroenteritis worldwide (Rincé et al., 2018). Ciguatera Fish-Poisoning (discussed in section 3.3) is the most frequently reported seafood-toxin illness in the world (Friedman et al., 2017), and remains in the top five of costliest illnesses in the USA.

Figure 7: Overall cost of pathogen-specific, marine-borne illness in the USA

(Source: Ralston et al, 2011) 

Costs of Vibrio infections

Facts about the Vibrio bacteria

Vibrio bacterial infections are among the most costly seafood-related illnesses in the USA, and post a significant burden on the healthcare sector (Iwamoto et al., 2010). For example, in the USA alone, the annual health costs of seafood related illnesses associated with Vibrio vulnificus, Vibrio parahaemolyticus, Vibrio alginolyticus are estimated to be US$ 350 million (Heng et al., 2017). Studies demonstrate that premature deaths (i.e. death before the age of 75) account for a large proportion of the total treatment costs associated with seafood-related illnesses (US$ 306 million), followed by medical care (US$ 25 million), hospitalization (US$ 6 million), and loss of productivity (US$ 15 million) (Ralston et al., 2011 in Heng et al., 2017).

Despite causing a lower incidence of illness than other bacteria or pathogens, V. vulnificus is a leading cause of seafood-associated fatality, with approximately 95 cases reported with 85 hospitalizations and 35 deaths per year globally (Heng et al., 2017). Just in the USA, V. vulnificus infections account for 45 hospitalizations and 16 deaths annually (Centre for Disease Control, 2013, cited in Ralston et al, 2011). Accordingly, V.vulnificus is recognized as one of the costliest of all marine-borne pathogens, accounting for 66% of seafood-related illness health costs and 26% of the total health costs, one third of the total seafood-related illness and more than 85%

of the costs of direct exposure to the Vibrio pathogen (Heng et al., 2017).

Vibrio cholerae is often transmitted by contaminated drinking water, but the consumption of fish or seafood that has been in contacted with contaminated water can also serve as a frequent transmission route. There are an estimated 1.3 to 4.0 million cases of cholera per year, with 21,000 to 143,000 deaths worldwide (Ali et al, 2015 cited in WHO, 2018e). However, the global health burden and costs of cholera remain largely unknown and possibly underestimated given that most cases are not reported and, in many cases, the specific pathway of exposure to Vibrio cholerae in patients remains unknown (Chowdhury et al., 2016).

Vibrios are common in marine environmental infections especially in warmer regions, where the bacteria thrive (e.g. US southeast coast, South America, Asia), whereas there are fewer cases reported in Europe⁷. Anthropogenic warming of ocean water, increased coastal flooding and encroachment of saltwater further inland, could further increase the risk of human interaction with pathogenic Vibrio species, such as V. vulnificus, V. cholerae, and V.

parahaemolyticus (Froelich and Daines, 2020).

7 https://earthobservatory.nasa.gov/images/91591/bacteria-thrive-as-ocean-warms

Costs of norovirus virus (NoV) 

While rarely causing deaths, some of the milder illnesses from seafood consumption or direct recreational exposure, such as Norovirus(Norwalk virus or NoV), can also be costly.

The third most commonly reported pathogen associated with seafood in the USA (mostly from contaminated shellfish, such as molluscs and raw oysters), NoV causes 29% of all seafood-associated illnesses and 10% of hospitalisations (Iwamoto et al, 2010). In the USA alone, costs are estimated at US$18 million annually to manage an estimated 184,000 cases, making it one of the top five most costly pathogens causing marine-borne illness in the USA (Ralston et al, 2011).

Most incidence and regional distribution data are available for developed countries (including USA, Europe, Japan), with a lack of incidence data from high mortality settings and low-income countries, including for example, significant data gaps in Africa (Kreidieh et al., 2017).

In addition, while considerable global cost and loss data are available regarding NoV associated foodborne diseases, fewer studies quantify the global health and economic burden specifically associated with marine-borne sources (including cases with mild symptoms).  Given the significant costs and losses associated with food-borne NoV cases, and the fact that 50% of NoV outbreaks are due to the consumption of contaminated shellfish (Alfano-Sobsey et al., 2012), we conclude that this deserves more detailed monitoring and surveillance. Further studies are recommended to understand the incidence and role exerted by NoV on the burden of diarrheal illnesses resulting from marine-borne sources. 

Illnesses from direct marine exposure

With over a million bacteria characterized as being present per mL of seawater, and 10 to 100 times as many viral particles, the risks of developing an infectious disease - from recreational or occupational uses of marine water, or exposure to beach and sand sediment, zooplankton and animals - is very high, but one that remains poorly understood (Young, 2016).

Some of the human pathogens identified in bathing waters include for example: Adenovirus,

Noroviruses, Enteroviruses, Hepatitis A, Escherichia coli, Staphylococcus aureus, Bacteroides spp., Clostridia, Pseudomonas spp., Salmonella spp., Giardia spp and Vibrios spp.

(Young, 2016). There is also growing evidence that beach sand can also harbour harmful microbes such as pathogenic bacteria, viruses and fungi in much larger concentrations that beach water itself; however, little research has been conducted regarding the health outcomes of direct and indirect exposure to differing sand qualities (Sabino et al., 2014, Solo-Gabriele et al., 2016, Abreu et al., 2016).

Spread of marine-borne illness via ballast water

The treatment and discharge of ballast water by ocean-going vessels has the potential to significantly impact human health through the worldwide spread of aquatic invasive species, pathogens (such as V.cholerae) and toxic organisms, such as harmful algae. The significant transfer of ballast water (approximately 2 to 3 billion tonnes per year) from one continent to another and between oceans (Werschkun et al., 2014) poses a significant potential bacteriological risk for all international water routes. In addition, human health can be impacted through occupational and non-occupational exposure (oral, dermal and inhalation) to the disinfection-by-products (DBPs) used for ballast water treatment and disinfection, which can include chlorine (Werschkun et al., 2014). Detailed risk assessments are required to identify the incidence of transmission of pathogens and toxins causing human illness via the various exposure pathways described above; and the economic and health burden that this represents.

It is unclear from current research whether it is possible to contract the COVID-19 virus from exposure to faeces in recreational waters; there are still many unknowns regarding potential transmission of this virus via sewage and wastewater (Carducci et al., 2020).

Anthropogenic pollution - exacerbating human illnesses

While there is significant evidence regarding the human health risks caused by bacterial and viral pathogens and marine toxins, there are much fewer studies to-date regarding how the

consumption and/or exposure to anthropogenic pollutants in the marine environment affect the incidence and human health costs of marine-borne diseases. There is evidence of human infectious diseases being transferred from anthropogenic sources and spreading infections to new populations and geographic locations (Young, 2016). However, there are no data available regarding the economic and health burden that these illnesses represent.

This section outlines a few examples of anthropogenic pollution (e.g. transfer of pathogens via ballast water, synthetic organic chemicals, metals and microplastics), which have the potential to adversely impact human health, cause or exacerbate human illnesses – and which deserve further study in order to understand their scale and magnitude in light of a changing ocean climate, and the health and economic burden that they represent.

Costs of illnesses from direct marine exposure

Given that the symptoms of marine infectious diseases from direct marine exposure (e.g.

gastrointestinal, respiratory, dermatologic, and ear, nose and throat infections) are mild and often self-reported, it is a challenge to understand the true incidence of infection and associated health burden (Sridhar and Deo, 2017). Some studies estimate that approximately 250 million cases of gastroenteritis occur worldwide each year (5 million in the US) as a result of bathing in contaminated water (Ralston et al, 2011), and 50 million or more cases associated with severe respiratory disease (Shuval, 2003).

While rarely causing death, there is evidence that even the milder illnesses caused by direct marine exposure can also be costly. For example, gastrointestinal illness from beach recreation in the USA is estimated to cost US$300 million, with US$30 million from direct marine exposure to the Vibrio species (Ralston et al., 2011).

Synthetic organic chemicals

Derived from industrial activities, synthetic organic chemicals include a wide range of chemicals, including petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), chlorobiphenyls, chlorinated dioxins, industrial solvents, pesticides and herbicides.

Approximately 9% of dietary exposure to synthetic organic chemicals is from fish and shellfish caught in freshwaters, estuaries and in coastal zones (near shore versus in the open ocean) (Hellberg et al., 2012).

Classified for their persistence, bioavailability, tendency to bioaccumulate and toxicity, the consumption of synthetic organic chemicals has been linked to cause several health issues, including immune modulation (Hernrotha et al., 2018). For example, human ingestion of herbicides and pesticides has been associated with endocrine-disruption, immunosuppression and developmental problems (Fleming et al., 2006). Consumption of PAH-contaminated bivalves has also been associated with lung cancer, low birth rates, and decreased fecundity (Fleming et al., 2006). Persistent Organic Pollutants (POPs), such as PCBs and dioxins found in seafood can be particular hazardous when fish is consumed in large amounts by subsistence anglers, pregnant women and young children (Hellberg et al., 2012).

While some studies report an association with prenatal exposure to PCBs and dioxins with childhood neurodevelopmental problems, other studies report no adverse effects (Nakajima et al., 2006, Hellberg et al., 2012).

Known to be immune modulators, further research is required to determine the long-term impacts of consuming seafood contaminated with synthetic organic chemicals, and the health costs that these impacts represent.

Mercury and other metals

While there are many sources of metals in the marine environment (some naturally existing, and others introduced by anthropogenic sources), the consumption of seafood is recognized as one of the major sources of non-occupational, organic mercury exposure to humans (Hellberg et al., 2012, Sheehan et al, 2014). In addition to neurodevelopmental impacts for pregnant women and foetus, the consumption of mercury can also have cardiovascular impacts on human health (Hellberg et al., 2012).

The highest human health risks of mercury are found in seafood consuming populations in coastal regions and tropical riverine populations, including south-eastern Asia, western Pacific and the Mediterranean (Fleming et al., 2016, Sheehan et al., 2014). Environmental chronic exposures have been reported in populations that depend on fishing, including in Amazonia, Coastal Peru, Seychelles, Faroe Islands, the Arctic and New Zealand (Fleming et al., 2006, Sheehan et al., 2014). At particular risk are women of reproductive age who live in low to middle-income countries (where access to information on methyl mercury content in seafood is not widely available) - and rely on seafood for at least 20% of their protein intake (estimated at approximately 400 million women worldwide (Budnik and Casteleyn, 2019). Higher risk of mercury exposure is also identified in subjects who consume self-caught fish versus store-bought fish, given their tendency to target predatory species, which contain the highest mercury concentrations (Eagles-Smith et al., 2018, Budnik and Casteleyn, 2019). It is expected that the relative exposure risk in distinctive populations is likely to change in response to globalization, population growth, resource availability (changed fisheries and “fishing down”

of marine food webs), and climate change.

Other metals found in seafood include cadmium, lead and arsenic. For example, cadmium (a known human carcinogen) can bioaccumulate in the marine environment (found to bioaccumulate in shellfish, and particularly mussels, more than in fish), posing several human health risks, among the most important being proteinuria and renal failure (Fleming et al., 2006).

Overall, there is a need for more accurate information about heavy metal contaminant exposures (particularly low-level exposure) from human fish consumption, thus informing policy makers regarding the balance between fish consumption benefits and the potential adverse effects resulting from heavy metal exposure.

Microplastics

Given their high persistence in the environment, and the fact that they are accumulating in different marine ecosystems at increasing rates, microplastics (MPs) and nanoplastics (NPs) are considered an emerging issue of great concern as they may compromise human food security, food safety, and consequently human health (Barboza et al., 2018).

In addition to the absorption and transfer of chemicals and hydrophobic pollutants (such as metals and heavy metals) - which are released into the gastro-intestinal tracts of organisms when the MPs are ingested (Godoy et al., 2020) - MPs can also spread exotic invasive species and dangerous pathogens (Barboza et al., 2018). This suggests that exposure to marine plastic debris (either from marine bathing or ingestion) could increase the risk of human disease via new contamination and infection routes. For example, plastic beads (or ‘nurdles’) can act as rafts for harmful bacteria (including E.coli and Vibrio spp.), raising the potential for “pathogens to be transported over large distances and survive for much longer than normal” (McVeigh, 2019, Rodrigues et la., 2019). There are higher risks of human infection, particularly for children, resulting from marine bathing or exposure (e.g. ingestion) to plastic nurdles on beaches (Rodrigues et al., 2019).

To-date, the human health impacts associated with the consumption of MPs and NPs through the ingestion of shellfish, fish, salt, seaweed and other marine organisms, air or dermal

exposure, remain largely unknown (Barboza et al., 2020, CIEL, 2019, Mercogliano et al., 2020).

Our collective understanding of the sources, fate, exposure, bioavailability and toxicity of MPs and NPs and their associated impacts in the marine environment and for humans is limited. It is unknown to what extent pathogenic organisms can survive on plastic debris and transmit infectious diseases in humans through the consumption of seafood, and the physical and chemical toxicity thereof – which is likely dependent on size, associated chemicals and dose (Smith et al., 2018, Revel et al., 2018).

Considering the ubiquitous nature of microplastic pollution in the marine environment, the toxic effects that have been found in animals (including lipid oxidative damage in the brain), and the potential risks posed to humans through the ingestion of microplastic contaminated seafood - more research on human exposure to microplastics through the marine food web and the toxicity of the consumption of these particles to humans is of urgent priority (Barboza et al., 2020, CIEL, 2019).

The risks of plastic pollution go beyond microplastics, and these risks to human health, ecosystems, and national economies are grave and interlinked. The degradation and modification of marine and coastal systems causes negative socio-economic effects on tourism, fisheries, shipping, and human health (Raes et al., 2021). The build-up of plastic litter on beaches can have a large impact on a country’s economy, wildlife, and the physical and psychological wellbeing of individuals (Jain et al., 2021). The major economic cost of this plastic debris is the reduced aesthetic appeal of coastal areas. This adversely affects the tourism industry, leading to a loss of output, revenue, and employment (Jain et al., 2021). The risks of plastic pollution on fisheries – including the broader economic dimensions of export revenue, livelihoods and employment, food security, marine ecosystems, and biodiversity – are well documented (Raes et al., 2021).

4. Evidence of impacts