Parasites used as heavy metal bioindicators

Did you know that more than half of the world’s animal species are parasitic? A parasite is an organism that lives in or on another organism (commonly referred to as the host) and benefits by deriving nutrients from the host species. Parasites can be found almost everywhere in the world, where they latch on to a wide variety of hosts such as ants, fish and humans. One example of a truly gruesome parasite is the tongue-eating louse. This parasite enters through the gills of fish, digging its claws into the fish’s tongue, causing the tongue to fall off! It then replaces the fish’s tongue, feeding on the host’s blood or mucus.

tongue parasite

The tongue eating louse parasite in its fish host (larger louse is the female, smaller louse is the male)


Other examples of parasites include tapeworm, leeches and head lice.

leechesrainbow trout

Examples of parasites: to the left: leeches parasitizing a fish ; to the right: adult tapeworm found in rainbow trout

It is hard to imagine that parasites could be of use to any animal species, especially to humans. However, certain parasites are able to accumulate heavy metals at much higher concentrations than their fish host’s tissues or the environment. Because of this ability, they can be sensitive bioindicators of environmental pollutants as compared to other more commonly used bioindicators (such as fish muscle and various mussel species).


Rainbow trout: Example of a commonly used bioindicator used for freshwater systems.

Note: Bioindicators are used to assess the concentrations of various pollutants in the environment

For example, Brazova and his colleagues (2012, Sensors, 12: 1424-8220) compared heavy metal concentrations in perch, a freshwater fish, with two of its main parasite species, in order to determine which species is a better bioindicator of heavy metal pollution. The study was undertaken at a water reservoir in a region of Slovakia, known for its intensive mining and ore processing activities.

They found that both parasites had significantly higher metal concentrations than the host’s tissues and that the two parasites differed in their propensity to accumulate different pollutants (i.e. there is a species-specific preference from some metals, where one parasite species accumulated certain metals more intensively than the other parasite species and visa-versa). Both parasite species also showed to have a relatively high accumulation of the heavy metal cadmium, whereas there was shown to be a low concentration of cadmium concentrated in the water and bottom sediments of the reservoir that was under study. This implies that even very low concentrations of heavy metals in the environment can be detected using parasites.


An example of perch and one of the main parasites found within the intestines of perch ( used in the study done by Brazova et al. (2012)

Why should we care?

Heavy metals occur naturally in the environment, but anthropogenic activities (such as mining, burning fossil fuels, dumping industrial waste and other industrial activities) have increased concentrations to toxic levels in some habitats. This is frightening as heavy metals cannot be broken down, once released, they accumulate in the environment, especially in lake, marine and estuarine sediments.


mercury sign

The most dangerous heavy metals are cadmium, lead and mercury, which are highly toxic metals that cause major harm to human heath at even very low concentrations

Heavy metals are taken up by organisms that feed in these habitats, becoming increasingly concentrated as one moves up the food chain due to a process termed biomagnification. Humans, as consumers of fish, sit atop the food chain and can therefore be exposed to potentially lethal doses of heavy metals. For example, fish in more than 50 percent of freshwater systems in Sweden are polluted with mercury above World Health Organisation standards; therefore the people inhabiting these areas are advised to refrain from eating these toxic fish. The long-term consumption of fish meat can cause a variety of disorders such as alterations to the nervous system, brain damage, reproductive diseases. Eating contaminated fish meat can also inhibit growth, affect your respiratory system, shorten your lifespan and cause various cancers.

Heavy metal pollution is not just a localised problem, only affecting industrial areas or areas situated near coal-powered stations. There have been reports that high mercury levels exist in relatively remote areas of the world such as in Greenland and the Arctic, showing us that tiny particles of mercury are able to drift over relatively long distances.   There are hence serious environmental, economic and social impacts associated with heavy metal pollution.



Diagram showing how heavy metal concentrations increase in organisms as you move up the food chain

Because parasites are sensitive indicators of environmental pollution and are able to detect even very low levels of pollution, they can be used as early warning indicators of a variety of pollutants as they are more sensitive than other bioindicators and can therefore warn us before pollution levels (especially heavy metal concentrations) reach frighteningly high concentrations in the environment. Parasites can also be used as a monitoring tool in areas where industrial activities are operating nearby or in the case of industries that have stopped operating; parasites can be used to determine whether the area that was affected by the industry is safe to utilize.

Parasites are incredible organisms that play a major role in the functioning of all major ecosystems and we are only starting to understand the many uses that parasites may offer.


Brázová, T., Torres, J., Eira, C., Hanzelova, V., Miklisova, D., Salamun, P. 2012. Perch and its parasites as heavy metal biomonitors in a freshwater environment: the case study of the Ružín Water Reservoir, Slovakia. Sensors 12: 1424-8220.


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