"The results are reassuring," report the survey team, a European Union (EU)-funded collaboration between scientists from institutes in the UK, The Netherlands, Denmark and Spain. "To have an effect in the environment, antibiotic levels in soil and water would need to be much higher than those we have seen," they write in a report presented in June to the European Commission's Directorate General for Research in Göteborg, Sweden.

More than 3000 tons of veterinary antibiotics are used each year to treat farm animals in the EU, and estimates for usage in the US range from 8,500 to 11,200 tons. Some of the compounds are absorbed and degraded by the animals, but much is excreted and either released directly into the environment, or stored and applied to land as a fertilizing slurry.

When antibiotics began to be detected in ground water and streams, it became imperative to find out whether they were causing any damage. So three years ago, the ERAVMIS (Environmental Risk Assessment of Veterinary Medicines in Slurry) project was set up to find out how long the substances would persist in the environment, where they would end up - soil, ground water or surface water - and whether they harmed any of the organisms they came into contact with.

The team made slurry containing representative compounds from the major antibiotic classes. "Spiking 70,000 tons of slurry with antibiotics was, er, challenging," said Alistair Boxall of the Cranfield Centre for EcoChemistry, UK, who coordinated the project. They used concentrations of antibiotics typical of those expected with the treatment regimens found in the EU. In the US, antibiotics are used for growth promotion as well as veterinary treatment, and amounts released into the environment may be a little higher than in the EU, he says.

The slurry was spread on fields and the contamination of the soil, ground water and run-off water was monitored over time. Encouragingly, most of the antibiotics had degraded after two weeks, and even though some were still found in the soil eight months later, only very small amounts entered the ground water.

By investigating common aquatic plants, crops, animals and microbes, the team defined 'ecotoxicity' values for each antibiotic - the antibiotic concentration that was lethal and/or affected growth of those organisms. In general, the concentrations in the environment were more than an order of magnitude lower than the ecotoxicity values, and the researchers found no effects on crops, worms, fish or water fleas.

This is great news, says Dana Kolpin of the US Geological Survey, who has been monitoring water contamination by antibiotics. "When antibiotics first started turning up in waters, we thought, 'Oh Jeez, that can't be good,' but it looks like they're probably not having an immediate impact."

Kolpin has reservations about the effects on soil microbes, though. "When they first apply the slurry, is it affecting the microbial populations?" he asked. And, he notes, there are concerns that soil bacteria could develop mechanisms of antibiotic resistance.

Boxall says that the team are planning to study these issues, as the group did see some affects on bacteria, even at low levels of antibiotics. They'd also like to look at the effects of antibiotics in combination with pesticides and other common chemicals, to see whether there are any interactions.