In recent years, environmental toxicologists and chemists have discovered that drugs and their breakdown products, once assumed to be broken down in sewage treatment plants, are showing up in streams, lakes and coastal waters. Although much of the focus has been on drugs used in human medicine, there is also awareness of the possibility that animal drugs may also end up in these surface waters. Drugs (and their breakdown products) eliminated by animals usually do not go through the same kinds of treatment processes typical for human waste before reaching streams or groundwater. Unlike pesticides, which undergo extensive testing to determine potential environmental hazards before they can be put on the market, pharmaceuticals may not receive as much scrutiny.

Work in my laboratory is looking at the toxicity to aquatic animals of some of the drugs commonly used in food animals. We hope that our results will provide a better basis for sound judgments about environmental risks that may be associated with the use of these drugs. This is an important part of the equation when it comes to making decisions about managing animal wastes to reduce potential environmental problems that may become the focus of regulatory scrutiny.

A fundamental principle that governs the toxic effects of any chemical on a living organism is that the likelihood or extent of the effect is governed by the amount of the chemical absorbed by the organism. Toxicity is therefore related to the amount of the chemical in the environment, how much of it is actually absorbed and how potent it is in causing harmful effects in the particular species exposed.

For aquatic animals, exposure to a chemical can result from contact with it dissolved in water, bound to sediment or present in the animal’s food. Some chemicals dissolve well in water; others stick to sediment at the bottom of a stream or suspended in the water. It is important to understand how a chemical behaves in the aquatic environment so that we have an idea of how animals might be exposed to it. This information allows us to identify the most appropriate approaches for designing laboratory studies that accurately reflect how aquatic organisms will be exposed in the environment.

In most of our studies, we start by using an aquatic flatworm (Planaria) to screen chemicals for toxicity. This species is inexpensive and easy to work with and can be exposed to chemicals dissolved in water or mixed with sediment. Further testing is conducted on chemicals of interest using other organisms, such as a tiny crustacean (Hyalella) or larval fish (fathead minnows) that are more labor-intensive (and expensive) to work with. This allows us to observe the effects of a chemical in a range of species that may differ in their sensitivity. Larval fathead minnows and Hyalella are also widely used for toxicity testing for regulatory purposes.

Our initial studies have shown that many of the drugs used in animals have low toxicity to aquatic life. This includes chlortetracycline, tylosin, sulfamethazine and metronidazine, drugs representing a variety of types and uses.

The ionophore antibiotics, monensin and lasalocid, appear to be substantially more toxic to the aquatic organisms that we have tested so far. Because of its widespread use in cattle feed, we have focused our current efforts on monensin. It was found to be relatively toxic to fathead minnow larvae when present in water and to invertebrates (both flatworms and Hyalella) when added to sediment. For fathead minnows, the median lethal concentration was approximately 5 parts per million in water. Sediment concentrations of 20 ppm were toxic to Planaria, but 1 ppm was lethal to Hyalella.

So far, we have mainly looked at the lethal effects of monensin. Some preliminary work with Planaria suggests that monensin may affect behavior related to survival at sublethal concentrations. If monensin causes similar effects on swimming speed and responses to environmental stimuli in fish, lower environmental levels of this drug may be of concern because of effects on survival.

Ken Carlson’s studies on the Cache la Poudre River showed that monensin concentrations were well below those shown to be toxic in our studies. However, a recent report from Canada suggested that sediment levels of monensin could approach those causing toxicity in our experiments. We are currently trying to determine how much monensin is present in water and sediment in different streams in northern Colorado. We are also seeking funding to do similar work in a Nebraska watershed with a high density of cattle feeding facilities.

Our studies suggest that there may be reason for concern that ionophores may affect aquatic life. We are continuing our work in this area to see if we can better define the concentrations of monensin in environmental systems that will adequately protect aquatic life, through a combination of laboratory and field research.

Howard Ramsdell, Associate Professor
Department of Environmental and Radiological Health Sciences