Meat is a highly consumed American commodity. The average US citizen consumes 67 pounds of beef per year  and 92 pounds of chicken. Coccidiosis is an intestinal disease caused by bacteria. Coccidia is common in livestock and causes loss of appetite, fatigue, dehydration and diarrhea. It is considered a significant disease calves and chickens that can result in fatality. Due to increasing demand for poultry and red meat, ranchers looked to prevent meat production loss from coccidiosis. The first coccidiostat, Sulfaquinoxaline, was produced in 1965. Coccidiostats are used to treat coccidiosis once symptoms are already showing. Anticoccidants, on the other hand, stop the bacteria responsible for coccidiosis before the disease has infected the livestock. Ionophores were the first anticoccidants. They were produced in the early 1970s then approved by the FDA 1975. The European Union banned antibiotics for the use of growth promotion in 2006. While ionophores are technically a class of antibiotics, they were not included in the ban due to their effectiveness as an anticoccidant. Ionophores are still used as anticoccidants. Concerns are raised as levels of popular commercial ionophores have been found in water and soil surrounding farmlands.
Figure 1: Data obtained from United States Department of Agriculture
The most common ionophores are monensin, salinomycin, lasalocid, tetronasin, lysocellin, narasin, and laidlomycin. They can be produced biologically by microorganisms for intracellular ion transport. Ionophores can be produced synthetically, however, the biological ionophores are used as anticoccidants. Ionophores are considered nontherapeutic antibiotics. They disrupt the ion concentration gradient causing ruminant bacteria to enter a futile ion cycle making them unable to maintain an effective metabolism . The gram negative bacteria that the ionophores eliminate are responsible for inefficient feed digestion causing body weight gain to remain steady while food intake decreases . These bacteria are also response for coccidiosis making the ionophores useful anticoccidants.
Ionophores disrupt the ion concentration gradient in two modes of action. The carrier ionophore mode of transportation is shown in figure 2 (a). The carrier ion shields the charge of that particular ion making it easier to pass through the hydrophobic membrane. Channel forming ionophores, shown in figure 2 (b), create a hydrophilic pore within the membrane. This allows ions to pass through without interacting with the membrane.
Ionophores are deemed to be emerging environmental contaminants due to their heavy use in poultry and livestock production for the last 55 years. Ionophores have relatively low amounts of studies on occurrence, fate and toxicological effects. The purpose of this paper is to outline the potential hazard, to characterize that hazard and identify possible courses of remediation for the past and future use of ionophores.
Ionophores are fed to the livestock and then can either remain in the animals’ tissues or are defecated in the form of the parent compound and its metabolites. Studies to chicks have shown that even when treated with a large amount of ionophore the tissue concentration is low. At zero time withdrawal from poultry, monensin has been reported at levels of 4.6 ug/kg in the liver and 2.0 ug/kg in muscle. After one day of freezer storage, levels of monensin in kidney, liver, skeletal muscle and cardiac muscle were found to be 7.0, 7.5, 2.25 and 3.95 mg/kg . The level of monensin was recorded at 1.46 mg/kg after 80 days of being frozen in the liver. Several studies have found that there is passage of monensin from the chicken to the egg–. The monensin in the egg has been found to decrease rapidly and considered negligible when the egg reaches the consumer. Salinomycin was found to degradate within a number of days in chicken tissue. There is an absence of studies of the pervasiveness of ionophores in cow tissue and if the acute amount in chicken tissue effects the other animals eating the meat. Ionophores have been in heavy usage since their onset so it is unlikely that the amounts remaining in chicken and cows are toxic. That does not rule out that there could be acute effects associated with these ionophores to the people who eat them.
Ionophores in livestock diet are not completely metabolized before leaving the body through defecation. Figure 3 shows the amount of ionophores detected through the manure of various livestock animals’ feces. Cows and sheep excrete 10% of Monensin as the parent compound. Breakdown products are metabolized through demethylation, decarboxylation and hydroxylation reactions. There are over 50 metabolites with no single metabolite prevailing. The biological activity of the metabolites is considered less than the parent compound. The ionophores that are not metabolized or remain in the tissue are defecated.
|Animal||Mass of Ionophore Defecated
(mgkg-1 dry soil)
|Type of ionophore|
Once the ionophore is excreted it can be used as broiler litter or composted to be used as fertilizer. Broiler litter is a combination of spilled feathers, excretion and other materials that are used as bedding in poultry farming. The broiler litter is treated with moisture, litter amendments, acidifiers and litter re-utilization. Litter can be reused for a number of years , so it is of interest to see if ionophore levels persist in the reused broiler litter. Composting manure is a process that minimizes the volume, kills pathogens and improves soil quality.Composting takes place in four main stages as shown in figure 4. The change between the stages is represented by a temperature change brought on by different microbial and bacterial degradation. The total time to compost a pile can take 3-6 months depending on the season and the size of the pile. Understanding how long ionophores persist and what step they breakdown at during composting is of importance to recognize the amount that can escape into the environment.
Degradation of monensin and salinomycin was not observed in broiler litter stacking over a 2 month time period. Studies have shown after 3 months of composting the amount of monensin degrades by 50%. This monensin is further diluted by spreading the compost and broiler litter throughout the fields. Monensin and salinomycin spread through a dry field was shown not to degrade. Rainfall brought on a 1.5 week half life to monensin while salinomycin was stable in the top soil for over three weeks. Biodegradation of monensin in was found to be 13, 15 and 18 days for silty loam, clay loam and sandy loam soils respectively where they reported several unidentified compounds were formed. Monensin was shown to be stable in water for over 30 days at pH 5, 7, and 9. Salinomycin was stable in water above pH 7 but below had half lives of less than 5 days. The abiotic, acid catalyzed degradation appears to be effective on the majority of ionophores. Monensin, lasalcoid and salinomycin have respectively logKow of 3,2 and 8 , . This gives the potential for bioaccumulation. Figure 5 shows the potential environmental fate for ionophores. The likelihood of binding to soil will have a great effect on whether it ends up in water. Ionophores persistence in water indicates they could be a hazard if there is bioaccumulation in the water ways.
Seasonal change and local climate will have a definite effect on the spreading of ionophores.
Hazard Characterization and Dose Response Assessment
The hazard characterization for livestock has been well defined and studied over the past 55 years. There is a narrow range from preferred concentration of ionophore feed and toxic dose of ionophore feed. Chickens tend to be the least sensitive to ionophores followed by cows then horses. Headlines from ionophore toxicity include “accidental poisoning of 17 dogs with lasalocoid”, “High mortality due to accidental salinomycin intoxication in sheep” and “ionophore poisoning in Angora goats”. The common theme with these incidents and articles like these is that there was improper mixing of the ionophore into the feed of the animal or improper use of the ionophores. The most interesting of these stories involves the poisoned goats. They developed ionophore poisoning after being fed chicken manure which still had a toxic level of the ionophore present. The toxicosis of livestock is, overall, tangential to the purpose of this study. The toxic dose relationship between ionophores and earthworms, plants and fish will be investigated for the hazard characterization.
Earthworms are presented with ionophores due to their exposure with livestock feces through compost spreading. Reproduction, growth, avoidance and mortality were selected at the endpoints due to them having a prevalent ecological effect and adversely effecting higher trophic levels.
Plants were chosen due to coming contact with ionophore rich soil. Radish were particularly sensitive to monensin. Emergence and growth were selected as endpoints because of ecological relevance. Table 4 shows emergence effect endpoints while table 5 shows growth endpoints both for monensin. Monensin influences the Golgi apparatus of plants. After minutes of contact with monensin, Golgi apparatus of a variety of plants being to swell due to the influx of ions.
The effect on fish is deemed important due to the potential for runoff in a rainfall event and ionophores longevity in water. A variety of fish will be represented due to the lack of data for just one species of fish. The only effects that are well documented in fish is mortality data. There is lack of data available on the mode of action through fish.
There is a lack of data on ionophores toxic effects that does not pertain to livestock. The information provided in this section was largely the work of the European Food Safety Authority. This data was more concerned with the toxic end points and ensuring the environmental concentration would stay below that. More data is required to make a statement on the mode of action, toxicokinetics and toxicodynamics for the species examined in this section.
Field studies that measure the level of ionophores around farm have shown that ionophores do accumulate in the surrounding area. Kim et al. found in northern Colorado monensin, salinomycin and naracin ranging in quantities of .01-2.0ug/L in surrounding watershed. They found concentrations as high as 90ug/kg in the surrounding sediment. Wang et al. found values of monensin ranging between 1-8 ug/L in particular lakes, streams and ponds in china with median values being approximately 2.6ug/L. Kaczala et al. reported monensin, salinomycin and narasin concentrations in river water at .036, .007 and.038 ug/L respectively. They reported values of 31.5, 30.1 and 16.3 ug/kg in river sediments of monensin, salinomycin and narasin respectively. The studies report consistently that in surrounding waters of farms that use veterinary antibiotics there is an expected level of ionophore antibiotics in those waters.
The value of ionophores found in the environment surrounding farms is enough to cause damage to the ecosystem. The occurrence of ionophores reported is enough to cause mortality in the species examined in the hazard assessment section.
An exposure assessment of the surround area would be beneficial. To perform this exposure assessment, I would first find several farms who use ionophores, preferably a few different ionophores and allow for samples to be taken of their farmland. Data would be taken from broiler litter, compost piles and fertilizer. It would be important to take samples from the area surrounding these zones to see if the xenobiotics are spreading through rainfall events. Next any standing water near the farms and rivers would be examined. It would be beneficial to measure concentrations downstream of the farms and upstream to get a comparison to see the persistence of the ionophores. It would be interesting to take samples before and after a rainfall event to see the effect it has on spreading the ionophores. It would be of interest find earthworms and rainbow trout in the field to see the possibility of biomagnification. To analyze the concentrations LC/MS analysis would be employed.
Ionophores persistence could cause a large drop in plant life and sediment dwelling creatures. The ionophores could cause a reduction of the growth in not only plants in the ecosystem but in the farmland itself. Avoidance or mortality caused to earthworm could cause further ecological harm to the developing plants in the area as worms would not be there to aerate the soil and break down nutrients for plant use. This decrease in plant life would have effects that could range through the trophic levels. This would occur through absence of plants, so the lower trophic levels would have nothing to eat. This would in turn cause the higher trophic levels with nothing to eat.
The effect on the water organisms could be significant. The bio accumulation in sediment could be harmful to the animal life. Mcgregor et al. ensures that monensin is not toxic to macrophytes at environmentally relevant concentrations. In certain locations, the concentration of monensin in the water is high enough to be fatal to fish. The presence of ionophores in fish tissue could be hazardous to bird in contact with the water and eating the fish. The toxic concentration for birds other than turkeys and chickens are not well documented so it would be largely unknown what would happen to them. There was a case where ostriches were accidentally fed monensin. Symptoms included ataxia, dyspnea and death. The accidental poisoning happened over 14 days. Ostriches, in general, would not be the main species of concern. It is showing that ionophores damage most type of fowl, so the possibility for harm is present.
Ionophores are continuously fed to farm animals to quickly fatten them to make more prophet. The hazards of ionophores to the farm animals are attributed mostly to human error. This human error results in miscalculating the amount of ionophores are needed in the food or not thoroughly mixing the ionophores in the cattle feed. The possibility that ionophores could make it into the edible meat of livestock is present. It does not appear that it arrives in toxic doses but it would be interesting to see what concentrations are present in the meat if any as it has shown that monensin can remain in other organs. The ionophores exit livestock through defecation with a partial amount of the original monensin intact. The metabolization of ionophores leaves many metabolites with decreased efficiency of transmembrane ion displacement. It would be interesting to see the characterizations of these metabolites because while sources claim that they are less harmful there does not seem to be any study cementing that knowledge.
Ionophores are excreted through the livestock with a portion of them intact. There is a lot of data available for this portion of ionophore toxicity. Researchers have found the half lives of ionophores in sediment and composting. Roughly half of the initial ionophores that make it to the compost pile persist to being spread through the growing area. The remaining ionophores can prevent full growth or emergence of plants while present in the soil. Ionophores are toxic to earthworms and can inhibit reproduction to earthworms in the soil that are beneficial to farms and surrounding plant life. Rainfall events can spread the ionophores to surround bodies of water where they are much more persistent and can bioaccumulate causing issues. It would be interesting to see how much ionophores escape the compost piles after a large rainfall event. The compost piles pose the problem of what animals could get into the compost piles and the toxins seeping into the ground from the pile itself. Data is abundant and rightfully so because this is the portion where the ionophore makes it into the environment.
The spread compost threatens earthworms and agricultural plant life before spreading through natural events. The earth worms can cause behavioral and mortal effects at environmentally relevant concentrations to both earthworms and a variety of plant life. Earthworms could exhibit reproduction inhibition, avoidance or fatal effects. Plants could see a reduction of growing or not sprouting. The ionophores do break down rapidly in wet soil while staying persistent in dry soil. In the farm area itself, the plants would regularly require water, so the soil would stay wet. Research has found that there are levels of ionophores present in the soil surround farms
Ionophores that make it into the water are not hazardous to macrophytes while are near hazardous concentrations for fish. The highest concentrations of ionophores are surrounding farms where they are used. Most fish data is for mortality effects involving ionophores. It would be interesting to see more research on the behavioral effects of fish responding to ionophores performed. It would be interesting to see more aquatic data occurrence data in various parts of rivers, upstream and downstream from farms using ionophores. The ionophores exist in both the sediment and the water itself with the ionophores being persistent in waters with pH above 6.
The composted pile
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