WESTERN PRODUCER — We’ve come a long way from the days of “mad hatters.”
Historically, hat makers, who used mercury in the process of turning the pelts of beaver and other fur-bearing animals (many of which originated in Canada) into luxury top hats, suffered from severe, debilitating neurological disease.
Nowadays, the most common ways that people encounter mercury are through eating seafood, dental amalgam fillings and through workplace exposures. But mercury is widely found in nature, resulting in accumulation and poisoning in certain groups of animals.
Unlike many modern chemicals, mercury is a naturally occurring element that is present in the soil and rock that makes up the outer layer of the Earth. It enters the atmosphere through volcanic activity, normal rock off-gassing, forest fires and human activities such as burning coal and garbage, smelting, and mining. Once in the air, it can settle into waterways through rain and other precipitation.
This is where the link between mercury and fish begins. Microbes convert the mercury in water into an organic form and when microbes are eaten by small aquatic animals, this organic form is accumulated in tissues. Fish eat these small organisms like shellfish.
As with many toxins, higher levels of the food chain generally lead to larger and larger animals and with this, accumulation of mercury in the muscle and other tissues. Apex sea-dwelling predators such as tuna, shark, orca and whales have some of the highest levels of mercury through this bioaccumulation process.
Fresh water fish also experience this. In Canada, fish with high mercury levels include walleye, bass and pike.
Mercury is an insidious poison. It accumulates in nearly every tissue in the body and can attach to immune cells. At a molecular level, mercury binds to and alters the structure of proteins, which are essential to normal bodily functions.
It also crosses the placenta to enter fetuses during gestation, as well as concentrates in milk. In this way, it can interfere with brain development in young mammals. Fish with high levels of mercury also suffer negative effects, including developmental abnormalities in fry fish, reduced reproduction and neurological abnormalities.
Clinical signs associated with mercury depend on the type of mercury and exposure route. For instance, elemental mercury, such as is used in thermometers and light bulbs, readily vaporizes if the container is broken. Animals that inhale these vapours can experience severe lung damage resulting in trouble breathing that may be fatal. Mercury fumes are heavier than air, so will settle near floors where pets and children tend to hang out.
If eaten, inorganic mercury is corrosive to the mouth and digestive system, resulting in vomiting, diarrhea, dehydration and possibly death.
The nervous system is particularly affected by mercury poisoning, resulting in a range of signs. Animals may be blind, unco-ordinated and experience muscle tremors or convulsions. In severe cases, affected individuals may be depressed, stop eating, become paralyzed or die. Similar effects are seen in people with mercury poisoning.
According to Health Canada, Canadians are generally at low risk of mercury poisoning. The same is likely true for our domesticated animals, which would have similar exposures as people.
The difference lies with wildlife and populations that rely on harvested marine mammals and fish for their main food source. People in Arctic communities in particular, who eat whales, seals and fish as a substantial proportion of their diets, may increase the risk of mercury toxicity in pregnant women and their children.
For people and animals, limiting exposure to mercury is a good idea but must be balanced with the benefits of eating fish.
In addition to an important sustainable protein source for many Canadians, fish contain high levels of omega-3 fatty acids, which studies have linked to a variety of improved health parameters. Examples of fish that contain low mercury include anchovy, herring, mussels, rainbow trout, salmon and shrimp.
Fish are also fed to pets in the form of commercial and prescription diets, particularly those with allergy and skin conditions. Fish-based diets are high in omega-3 fatty acids that have anti-inflammatory effects. Pets can eat bone-free fish but tuna, which can be high in mercury, should be avoided.
At a broader scale, we should support efforts to limit mercury generated from human activities. Reducing mercury contamination in waterways is important to protect fish and the animals and people that depend on them.
Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.
