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Organochlorine compounds in pet food

As pet owners, we are always following the newest recall or examining the ingredient lists with a fine tooth comb. But what about the contaminants that aren’t the retailer or distributors fault? What about cancer causing compounds that we don’t consider because they are already in everything? 

Carcinogens like organochlorine compounds (OCCs) are often overlooked when it comes to pet food and this is because they are likely to be found in trace amounts in all food (including human food). This article will examine what organochlorine compounds are, the risks, and which foods may contain the highest concentrations of these toxic compounds.

What are OCCs?

Let’s start off by asking what is an organochlorine compound and where do they come from? OCCs are persistent environmental pollutants that can potentially result in devastating health effects. There are many different types and groups of OCCs. The most common types found in food are dioxins, PCBs and organochlorine pesticides. These compounds originate from combustion and burning of materials. This can include the burning of industrial materials, as well as plants and trees (through forest fires mostly).

Harmful emissions into the environment, smoke and smog causing pollution in the atmosphere

All groups of OCCs are classified as persistent organic pollutants due to their longevity in the environment and their resilience to biodegrade. Most organochlorines have a half-life of 10 to 20 years once deposited in the environment. This poses a problem when it comes to food source contamination, as these compounds are easily transferred through aquatic systems and in the air after combustion. 

OCCs are lipophilic in nature, meaning that they tend to bioaccumulate in fatty tissue. This poses a problem overtime as these compounds can increase in concentration since the body does not readily metabolize and excrete them. As a result, these compounds can remain in the body for long periods of time. 

Biomagnification is also a risk when considering OCCs. It is a term that refers to the accumulation of toxins through different species in the food chain. For example, a worm is contaminated with OCCs, then a salmon eats 10 worms and finally a grizzly bear eats 5 salmon. The OCC concentration has significantly increased in concentration and biomagnified in the grizzly bear, putting that animal more at risk for toxicity. This is why biomagnification is a notable risk factor for dogs, cats and humans, since we are carnivorous/omnivorous animals at the top of the food chain.

OCCs in pet food

You may be asking yourself, how does this apply to me and my pets? As stated above, dogs and cats are at the top of the food chain, with the amount of meat that is in their diet. This puts them at greater risk for bioaccumulation of OCCs. Let’s take a moment to examine the different components of pet food and where the greatest risk for OCC exposure is.

Fish and fish oil are common ingredients in pet food and unfortunately they are probably the ingredients with the highest concentration of OCCs. Fish are often viewed as a healthy ingredient due to the high amounts of omega 3 fatty acids. Omega 3s are great for helping to reduce inflammation and improve cognitive function. Unfortunately, since fish have such fatty tissue, this means that they easily bioaccumulate OCCs. There is also a difference between farmed and wild fish. A study conducted by Hites et al. (2004), indicated that the accumulation of OCs is greater in farmed fish than in wild fish. The researchers determined that the difference is greatly due to the difference in diet. All farmed fish samples were fed high amounts of fishmeal and oil. Due to market volatility, farmed fish are much less expensive than wild fish, so unfortunately it is rare to see wild fish used in pet food.

Cartoon illustration of fish underwater avoiding bait on fish hook

Plants are added to pet food as a source of fibre, vitamins and minerals. Plant ingredients are the least likely source of OCCs out of most ingredients. When compared to animals, plants are generally very low in fat. Therefore, there is a lower risk of bioaccumulation. There are still trace amounts of OCCs found in and on plants however. This is usually a result of atmospheric deposition and absorption through the soil. Ingredients such as grains and legumes have the least exposure to environmental toxins, as they are enveloped in a pod or shell and are not exposed to aerosol or pesticide deposition. Broad leaf plants like spinach, kale, grasses and forages are plant sources with the highest concentration of OCCs. This is because they contain more surface area for aerosolized OCC deposition. While not commonly used in dog and cat foods, grasses and forages are fed to livestock, whose protein and fat is used in companion animal diets. Therefore, pets are exposed to forage-based OCCs indirectly as a result of bioaccumulation in livestock. 

Aerial Shot Of Organic Vegetables On Farm Plot An aerial shot of various organic vegetables on a small urban farm plot

Terrestrial animal protein is probably the second most concentrated source of OCCs in pet food. This includes protein sources like beef, lamb, chicken, tukey, pork, etc. While terrestrial livestock species can also ingest OCCs from water sources, their main mode of OC accumulation is from feed. As previously stated, since grains, legumes and by-products are relatively low in organochlorine compounds, forages are considered the most important route of exposure for livestock species. 

Of the different terrestrial animals used for protein in pet food, ruminant protein, such as beef and lamb, tend to contain the greatest concentration of PCBs and dioxins. Researchers Kim et al. (2004) compared the concentrations of seven different PCB congeners in beef, pork and chicken fat. It was noted that beef fat contained the overall greatest accumulation of PCBs. The researchers attributed this to the fact that out of the three species used in the study, cattle have the longest lifespan before slaughter, therefore allowing them to accumulate more OCCs in their bodies. Another theory that the study proposed is that ruminant species can accumulate more OCCs due to their diets. Ruminant animals are fed a higher percentage of forage substrates compared to other livestock species. Higher forage consumption lends to a higher possibility for bioaccumulation of organochlorine pesticide ingestion.

Panoramic shot of cows on pasture at sunrise, back light.

Pork is one of the lowest OCC containing protein sources, compared to poultry, beef and lamb. Researchers Molcan et al. (2017) theorized that the low OCC accumulation could be due to a high expression of a detoxifying enzyme, CYP1A1, in porcine tissues. This enzyme allows for pigs to more easily detoxify and excrete OCCs, rather than the compounds just bioaccumulating in their tissues.

Drinking water is a near negligible source of OCCs in pets. While OCCs can be transported through water, they cannot bioaccumulate or dissolve in it. A study conducted by researchers Wei et al. (2015) conducted a risk assessment on the composition and distribution of organochlorine pesticides in South China. Samples for the study were collected from nine different sources across Southern China in order to determine OCC concentration and whether there were potential human health concerns. The average of total OCC concentration was determined to be 25.7 ng/L. After a health risk assessment, it was decided that the water from the nine different sources was safe for drinking. This complies with the Canadian Drinking Quality Guidelines, which sets the safe level of dioxins in drinking water to be below 5000 ng/L.

Thirsty yellow labrador retriever drinking water from the plastic bottle his owner in an open field

Most pet food is processed to some degree. Whether that’s steaming fresh cooked diets or extruding kibble diets, some form of heat is usually added to the processing step of pet food. Unfortunately, processing generally does not decrease the concentration of OCCs and in some cases may actually increase the toxicity. With heating methods, such as extruding, flaking, flame peeling and heat sterilization, more dioxins may be produced and deposited onto the food.

Pet food is most commonly fed as either a kibble, fresh-cooked, raw or in wet-canned form. Dry kibbles contain the lowest amount of protein and contain the highest percentage of vegetables/grains. This is why kibble has the lowest risk of OCC contamination when compared to other forms of feed. Conversely, canned food and raw diets contain a higher ratio of both protein and fat, therefore containing a higher concentration of OCCs. While canned food is processed and heated to 120°C to achieve sterility, it is not high enough to destroy OCCs.

Toxic effects of OCCs

Organochlorine compounds pose a risk to all animal species and can be toxic at both acute (immediate) and chronic (prolonged) doses. In the case of pets, OCCs usually cause adverse effects due to chronic toxicity. 

OCCs cause toxicity through a specific mechanism in the cells of the body. OCCs enter the cell membrane and are transported to the nucleus by bonding to a specific cellular receptor called the aryl hydrocarbon receptor. Once in the nucleus, the aryl hydrocarbon receptor binds to the DNA and turns on transcription mechanisms. Transcription mechanisms cause the cell to produce an enzyme called CYP P450, that can bioactivate other toxins in the body. The aryl hydrocarbon receptor can also alter gene expression, which can consequently lead to cell death, carcinogenesis (cancer formation), and impaired systemic function. 

3d rendering of DNA Strands

Due to the genotoxic and bioactivating effect of OCCs, bioaccumulation of these compounds can lead to a multitude of systemic issues. Chronic exposure to animals has been shown to be the cause of various types of cancer and acute exposure being contributed to a reduction in immune, reproductive, endocrine and nervous function. 

Unfortunately, there is a problem with narrowing down the specific effects that OCCs cause. This is because while there may be many toxicological effects seen with OCC contamination, some symptoms may not have been caused by OCCs directly, and are instead the result of a toxic metabolite generated by the increase of CYP P450 enzymes. Overall, the extent of the specific effects of OCCs depend on the cumulative influence of different factors such as age, species, bioaccumulation and diet.

Prevention and Veterinary Treatment for Bioaccumulation and Toxic Effects

Prevention is the best and most important method in reducing the bioaccumulation and toxic effects of OCCs. There are multiple approaches that can be taken to prevent OCC contamination. An extremely critical method of OCC prevention is reduction in the ingestion of contaminated food. Since meat and other high fat sources are the greatest source of OCCs in the diet, it would be beneficial to remove or reduce these sources in the diet. Furthermore, selection of protein sources with low contamination would be optimal regarding prevention. Choosing sources of protein such as lentils or pork would contain lower concentrations of OCCs than sources such as fish or beef. Trimming the fat off of meat sources would also help to reduce OCC ingestion.

A purebred border collie is indoors eating food from his dog bowl. In this frame he is looking down at his meal while eating his dinner pieces of dog food spill over the edge onto the floor.

Exercise also helps to reduce OCC bioaccumulation. Due to the lipophilic nature of OCCs, they will naturally deposit in fat. If an animal has a lower fat index, OCCs will be less likely to bioaccumulate and are instead excreted from the body. 

There is limited veterinary treatment for the bioaccumulation of OCCs. The most common practices are just to treat symptoms associated with the toxic effects of OCCs such as chemotherapy or administration of pharmaceuticals. If symptoms and conditions are extremely progressive, animals are commonly euthanized as veterinary treatment is not always effective and is expensive. 

Take home message

Organochlorine compounds are not contaminants to immediately panic about. There is no way to completely eliminate them from pet food. The best thing to do is consider diets that are lower in major OCC sources. For example if you want to feed your dog a diet with fish in it, consider a diet with dual protein. For example a turkey and salmon diet may be a better option than just salmon. Also consider opting for diets that contain a healthy amount of plant products. For instance, feeding a kibble or fresh food diet over a raw or canned diet due to the lower OCC containing plant ingredients. Implementing a good exercise routine and keeping your dog's BMI low is another way to avoid the bioaccumulation of OCCs. Overall, OCCs are inevitably found in trace amounts in pet food but there are steps you can take to reduce bioaccumulation in your pet.

Boston Terrier refusing food being fed to him on a spoon by his owner
Boston Terrier refusing food being fed to him on a spoon by his owner

View Sources

Health Canada. 2017. Guidelines for Canadian drinking water quality. Published on February 21, 2017. (2017 February 26)

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