Matilde Revelli, Contributing writer
Fish has recently seen skyrocketing numbers of consumption worldwide, with a 122% increase in the past 30 years. People seek it as a valid, more sustainable alternative to meat as it is rich in protein, Omega-3 and other essential nutrients, and it has a relatively low environmental impact. Raw fish consumption is a direct consequence of these consumption trends, together with the search for exotic foods or delicacies such as Latin American ceviche, Italian carpaccio, or Japanese sushi. Nevertheless, just like with any other food, “we are what we eat”, and what we might ingest through fish is not always healthy nutrients.
Fish (much like any other animal) can indeed be carriers of multiple elements that produce adverse health effects on our body, such as heavy metals (e.g. lead, copper, cadmium, mercury) and persistent organic pollutants (POPs), as well as pathogens, bacteria and parasites such as tapeworms. There is a wide range of risks for humans that derive from the ingestion of these particles: POPs, for instance, can disrupt the normal functioning of the endocrine system, or even cause cancer. The risk of exposure for humans, especially to bacteria and pathogens, is even higher if the fish is eaten raw or undercooked.
A category of pathogens has recently witnessed increased attention from scholars and medical practitioners: trematodes. Trematodiasis is a range of pathogen-induced infections caused by the ingestion of trematode parasites through undercooked or raw fish. Trematodes (also called ‘flukes’) are considered endemic in some areas of Southeast Asia, causing major public health problems in countries such as Cambodia, Laos, South Korea, Thailand, and Vietnam. Illnesses caused by trematodes include intestinal and lung infections, gallbladder pathologies and even bile duct cancer. In 2005, about 975 million people worldwide were at risk of infection by the three most common trematodes; furthermore, intestinal-fluke infections reportedly rose by 423% between 1995 and 2012.
Scary, right? But then why are these pathogens endemic of these areas, and how exactly do humans come to eat infected fish? The answer lies in (a) the role of fish in local cultures, and (b) the aquaculture system acting as a primary food source for rural populations. The way contamination happens is quite simple, and is pictured in the figure below. Parasite eggs are passed through infected animal or human feces that contaminate the water or soil. They seek snails as their first intermediate host, growing inside them until they become organisms called ‘cercariae’. The cercariae then swim independently until they find a suitable fish, their second intermediate host, and settle in its skin or flesh. Should this infected fish be ingested by humans, once it reaches the intestine and is broken down adults attach themselves to the intestine walls, producing new eggs to be released in the environment via their host’s feces.
One reason why trematodes are considered endemic to Southeast Asia is because, as mentioned in the first paragraph, fish is an important source of protein and other essential nutrients and thus often sought as a valid alternative to meat. Furthermore, the consumption of raw or undercooked fish is an integral part of the culture in many Asian countries: sushi and sashimi in Japan, saengseon hoe in South Korea, gỏi cá mai in Vietnam among others. Fish remains a basic source of food, especially for rural populations who often have access to their own aquatic ponds and therefore produce food for their own consumption. This brings us to the second point: the role of aquaculture.
Lack of a proper in-house bathroom or sewage system, especially in rural areas, can lead to contamination of the water and soil that feed into aquaculture ponds. A second aspect to consider is that these ponds are the prime environment for the type of snail that trematodes are likely to infect. A bigger snail population entails a bigger chance of being infected by trematodes, which then leads to increased risks of contamination for farmed fish and, ultimately, humans as well.
Aquaculture then plays a fundamental role in the potential development and distribution of trematodes. Fish farming has been on the rise in the past 30 years , with Asia being the major producer. However, this exponential increase requires attentive monitoring especially in areas where fish-borne trematodes are endemic. For instance, the grass carp ( Ctenopharyngodon idellus ) is a species whose cultivation has witnessed an important increase; it is also a major host for trematodes, and is traditionally eaten in China as raw fish (yusheng zhou).
Is raw fish safe to eat, then? Yes, but with due precautions. In order to limit the spread of trematodes and fish-borne infections, accurate monitoring is required at the very early stages of the food production chain, i.e. in aquaculture ponds where snails and fish dwell. A strategy may be to educate farmers about the importance of producing pathogen-free food, for instance through educational programmes on hygiene and food safety. Another method could be raising awareness among the population about sound eating habits or the risks that trematodiasis poses for humans. All in all, scholars agree on one thing: cooking or deep-freezing fish eliminates most dangers of becoming infected with trematode parasites. So, the next time you find yourself longing for sushi or other (raw) delicacies, remember to consider your health first!
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