Researchers are trying to determine what antimicrobials are taken in by food plants through manure and what happens to those pharmaceuticals.
"We're still in the early stages of determining what the impact of this research will be," said Holly A.S. Dolliver, PhD, a professor at the University of Wisconsin-River Falls College of Agriculture, Food, and Environmental Sciences. "It's also important to note that we're trying to work on strategies to mitigate these risks as well."
Dr. Dolliver has been a researcher on projects including a 2006 study that demonstrated sulfamethazine uptake by corn, lettuce, and potatoes fertilized with swine manure spiked with the drug. The study was conducted with sandy soil and raw manure to simulate a worst-case scenario, and the antimicrobials were added to the manure directly, rather than fed to the swine that produced the manure.
"We chose a compound that had a slightly lower molecular weight than some of the other antibiotics," Dr. Dolliver said. "So we expected that, if antibiotics were being taken up, that this type of compound would be one that would be taken up."
Dr. Michael D. Apley, a professor of production animal medicine and clinical pharmacology at the Kansas State University College of Veterinary Medicine, points out studies have proved that some concentrations of antimicrobials can be taken up by plants, but says there is a need to determine what concentrations of these substances could actually result from current production practices. Other studies have indicated what concentrations of antimicrobials can survive in manure, and findings from those and the food crop-related studies could be used to design further research replicating field conditions.
Dr. Dolliver said more research is also needed to determine the behavior of various antimicrobial compounds in plants, and she noted that the type of antimicrobial, its mode of action, and its chemical properties will influence whether or how it will accumulate in plants.
"We need to have a better understanding of the levels of antibiotics in different types of manures, how these antibiotics degrade over time, whether they degrade in fruits and vegetables, and what impact cooking or handling of fruits or vegetables have on degradation of these antibiotic compounds," Dr. Dolliver said. "So there's still quite a bit of research that has yet to be done."
Dr. Apley said veterinarians should become well-versed on the research.
"The antibiotic debate has several key components, the biggest of which is zoonotic passage of organisms that have developed resistance while passing through animals, and another is what happens to antibiotics in the environment," Dr. Apley said.
Dr. Apley said it remains uncertain what impact small amounts of antimicrobials can have when they move through the food chain, and interpretation of a risk assessment would hinge on the risk people are willing to tolerate in keeping the benefits of pharmaceuticals for food animals.
Satish C. Gupta, PhD, a professor in the University of Minnesota Department of Soil, Water, and Climate, has collaborated with Dr. Dolliver on some studies. He said his research group is analyzing data from a summer 2009 field study in which hog and turkey manure mixed with antimicrobial feed or pure antimicrobials was used to fertilize greenhouse onions, carrots, potatoes, radishes, garlic, lettuce, cabbage, spinach, sweet corn, tomatoes, and peppers.
Dr. Gupta noted that his research group's previous studies have shown that plants can draw in small amounts of antimicrobials, but a person would need to eat large amounts of the vegetables grown in the studies to consume a dangerous amount of any pharmaceutical. The substances could pose a more substantial problem, however, for people allergic to those antimicrobials, he said.
One previous study involving piglet manure showed corn, green onion, and cabbage plants took in chlortetracycline but not tylosin, Dr. Gupta said. The researchers think tylosin was not found in the plant because it is a larger molecule.
Dr. Gupta noted that human-use pharmaceuticals may be more of a concern, as treated and recycled wastewater is used in producing fresh produce in some states.
Dr. Gupta believes that livestock and vegetable producers need to support further research.
"One of the things that I found in doing this research is that people are getting defensive about it, and I think they should be proactive and try to help answer the questions that the public is worried about," Dr. Gupta said. "I am in agriculture, and if we find there is something we are not doing right, then we'll find ways to address that question."
Source: American Veterinary Medical Association