July 2007
Foods such as corn, lettuce and
potatoes have been found to accumulate antibiotics from soils spread with
animal manure that contains these drugs. The study results indicate that
organic foods are most likely to contain these drugs because manure is
often the main source of crop nutrients for organic food production.
Funded by the U.S. Department of Agriculture, USDA, scientists at the
University of Minnesota evaluated the impact of antibiotic feeding in
livestock production on the environment. Study results are published in
the July-August 2007 issue of the "Journal of Environmental Quality."
Plant uptake was evaluated in a greenhouse study involving three food
crops - corn, lettuce, and potatoes. The plants were grown on soil
modified with liquid hog manure containing Sulfamethazine, a commonly used
veterinary drug.
Sulfamethazine has been used to treat bacterial diseases in human and
veterinary medicine and to promote growth in cattle, sheep, pigs and
poultry.
In the University of Minnesota study, this antibiotic was taken up by all
three crops, with the highest plant tissue concentrations found in corn
and lettuce, followed by potato.
Concentrations in plant tissue also increased as the amount of antibiotics
present in the manure increased.
Concentrations of antibiotics were found in the plant leaves, and the
drugs also diffused into potato tubers, which suggests that root crops,
such as potatoes, carrots, and radishes, that directly come in contact
with soil may be particularly vulnerable to antibiotic contamination.
Holly Swanson Dolliver, the lead scientist in this study, notes that
antibiotics consumed by plants may be of particular concern to the organic
farming industry.
"Manure is often the main source of crop nutrients for organic food
production, since regulations prohibit the use of synthetic fertilizers,"
she says.
According to the USDA, producers must manage animal materials in a manner
that does not contribute to contamination of crops by residues of
prohibited substances, which includes antibiotics.
However, manure containing antibiotics are not formally banned or
prohibited.
The ability of plants to absorb antibiotics raises the potential for
contamination of human food supply, but group leader Satish Gupta says,
"The adverse impacts of consuming plants that contain small quantities of
antibiotics are largely unknown."
Consumption of antibiotics in plants may cause allergic reactions in
sensitive populations, such as young children. There is also concern that
consuming antibiotics may lead to the development of antimicrobial
resistance, which can render antibiotics ineffective.
Sulfamethazine is one of a class of drugs called sulfonamides used in
combination to treat a variety of infections in humans. At one time,
sulfonamides were widely used in the treatment and prevention of
infections such as urinary tract infections, chlamydia, rheumatic fever,
toxoplasmosis and malaria. The development of resistance in many of these
organisms has limited the clinical usefulness of these drugs, according to
the Access Project, a national program of the AIDS Treatment Data Network.
"There is sufficient evidence in experimental animals for the
carcinogenicity of sulfamethazine," according to the International Agency
for Research on Cancer, but this agency says there is "inadequate evidence
in humans for the carcinogenicity of sulfamethazine."
In a 1997 continuous breeding study in mice conducted by federal
government researchers with the National Institute of Environmental Health
Sciences, sulfamethazine reduced fertility in both males and females.
A 2002 study by the U.S. Geological Survey of concentrations of
antibiotics found in Midwestern streams during runoff events detected the
drugs in only 1 percent of water samples. Of the 37 antibiotics tested,
only sulfamethoxazole was detected in two samples.
On the issue of resistance to antibiotics that may develop as a result of
using antibiotic-laden manure, a study published in March in the journal
"Environmental Microbiology" suggests that "manuring of arable soils may
stimulate the spread of resistance genes by introduction of resistant
populations and antibiotics."
Dolliver and Gupta say further research is needed to investigate the
presence of antibiotics in edible parts of plants, especially vegetables
that are consumed raw, and how different plants absorb different
antibiotic compounds.
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