Antimicrobials in aquaculture | Wikipedia audio article

Antimicrobials in aquaculture | Wikipedia audio article


Antimicrobials destroy bacteria, viruses,
fungi, algae, and other microbes. The cells of bacteria (prokaryotes), such
as salmonella, differ from those of higher-level organisms (eukaryotes), such as fish. Antibiotics are chemicals designed to either
kill or inhibit the growth of pathogenic bacteria while exploiting the differences between prokaryotes
and eukaryotes in order to make them relatively harmless in higher-level organisms. Antibiotics are constructed to act in one
of three ways: by disrupting cell membranes of bacteria (rendering them unable to regulate
themselves), by impeding DNA or protein synthesis, or by hampering the activity of certain enzymes
unique to bacteria.Antibiotics are used in aquaculture to treat diseases caused by bacteria. Sometimes the antibiotics are used to treat
diseases, but more commonly antibiotics are used to prevent diseases by treating the water
or fish before disease occurs. While this prophylactic method of preventing
disease is profitable because it prevents loss and allows fish to grow more quickly,
there are several downsides. The overuse of antibiotics can create antibiotic-resistant
bacteria. Antibiotic-resistant bacteria can spontaneously
arise when selective pressure to survive results in changes to the DNA sequence of a bacterium
allowing that bacterium to survive antibiotic treatments. Because some of the same antibiotics are used
to treat fish that are used to treat human disease, pathogenic bacteria causing human
disease can also become resistant to antibiotics as a result of treatment of fish with antibiotics. For this reason, the overuse of antibiotics
in treatment of fish aquaculture (among other agricultural uses) could create public health
issues.==Overview==
The issue has two sides. In some foreign countries, clean water supplies
for aquaculture are extremely limited. Untreated animal manure and human waste are
used as feed in shrimp farms and tilapia farms in China and Thailand, in addition to the
collection of waste products accumulating from inadequate sewage treatment. In order to prevent the spread of bacteria
and disease in contaminated water, some foreign fish farms put U.S.-banned antibiotics into
their fishmeal. However, because the more stringent growing
regulations in the US increase the price of food, imports from nations without these regulations
are increasing based on price and profit.Between 1995 and 2005, the first ten years of the
NAFTA-WTO era in the US, seafood imports increased 65 percent and shrimp imports increased 95
percent. Today, 80 percent of American seafood is imported,
about half coming from aquaculture. China, Thailand and Vietnam together account
for 44 percent of seafood imports into the United States.The FDA has been testing for
chemicals in aquaculture products for over two decades. In November 2005, the testing program for
aquaculture drugs was revised to include antibiotics such as chloramphenicol, fluoroquinolones,
nitrofurans, and quinolones, as well as antimicrobial compounds like malachite green that are not
approved for use in aquaculture fish. From October 1, 2006, through May 31, 2007,
FDA tested samples of catfish, basa, shrimp, dace, and eel from China, finding twenty-five
percent of the samples to contain drug residues. FDA has approved five different drugs for
use in aquaculture as long as the seafood contains less than a mandated maximum residue
limit: florfenicol, sulfamerazine, chorionic gonadotropin, oxytetracycline dihydrate, oxytetracycline
hydrochloride, as well as a drug combination of sulfadimethoxine and ormetoprim. FDA has approved two drugs—formalin and
hydrogen peroxide—for which it has not set a tolerance.The FDA now enforces regulations
in the US requiring testing of certain imported products for antimicrobial agents under Import
Alert 16-131. The Import Alert provides that the use of
antimicrobials during the various stages of aquaculture, including malachite green, nitrofurans,
fluoroquinolones, and gentian violet, may contribute to an increase of antimicrobial
resistance in human pathogens and that prolonged exposure to nitrofurans, malachite green,
and gentian violet has been shown to have a carcinogenic affect. In a consumer brochure, the FDA describes
the reasoning for enforcement under the import alert: After FDA repeatedly found that farm-raised
seafood from China was contaminated, the agency announced on June 28, 2007, a broader import
control of all farm-raised catfish, basa, shrimp, dace(related to carp), and eel from
China. During targeted sampling, from October 2006
through May 2007, FDA repeatedly found that farm-raised seafood from China was contaminated
with antimicrobial agents that are not approved for use in the United States. More specifically, the antimicrobials nitrofuran,
malachite green, gentian violet, and fluoroquinolones, were detected. Due to limitations on funding and resources,
U.S. Government Accountability Office states that
only 1% of seafood, compared with 2% of all imports, is inspected and only 0.1% of all
seafood is tested for antibiotic residue.==Example antimicrobials=====
Copper alloys===Recently, copper alloys have become important
netting materials in aquaculture (the farming of aquatic organisms including fish farming). Various other materials including nylon, polyester,
polypropylene, polyethylene, plastic-coated welded wire, rubber, patented twine products
(Spectra, Dyneema), and galvanized steel are also used for netting in aquaculture fish
enclosures around the world. All of these materials are selected for a
variety of reasons, including design feasibility, material strength, cost, and corrosion resistance. What sets copper alloys apart from the other
materials used in fish farming is that copper alloys are antimicrobial (For information
about the antimicrobial properties of copper and its alloys, see Antimicrobial properties
of copper and Antimicrobial copper alloy touch surfaces). In the marine environment, the antimicrobial/algaecidal
properties of copper alloys prevent biofouling, which can briefly be described as the undesirable
accumulation, adhesion, and growth of microorganisms, plants, algae, tube worms, barnacles, mollusks,
and other organisms on man-made marine structures. By inhibiting microbial growth, copper alloy
aquaculture pens avoid the need for costly net changes that are necessary with other
materials. The resistance of organism growth on copper
alloy nets also provides a cleaner and healthier environment for farmed fish to grow and thrive. In addition to their antifouling benefits,
copper alloys have strong structural and corrosion-resistant properties in marine environments. Brass alloy netting cages are also currently
being deployed in commercial-scale aquaculture operations in Asia, South America, and the
USA. See antimicrobial properties of brass===
Methylene blue===Methylene blue is used in aquaculture and
by tropical fish hobbyists as a treatment for fungal infections. It can also be effective in treating fish
infected with ich, the parasitic protozoa Ichthyophthirius multifiliis. It is usually used to protect newly laid fish
eggs from being infected by fungus or bacteria. This is useful when the hobbyist wants to
artificially hatch the fish eggs. Methylene Blue is also very effective when
used as part of a “medicated fish bath” for treatment of ammonia, nitrite, and cyanide
poisoning as well as for topical and internal treatment of injured or sick fish as a “first
response”.===Ozone===
Ozone is added to seawater and used for the surface disinfection of haddock and Atlantic
halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted
virus which causes severe mortality in fish. Haddock eggs should not be treated with high
ozone level as eggs so treated did not hatch and died after 3–4 days.==Some problematic antimicrobials=====
Malachite green===In 1983, the FDA banned the use of malachite
green in aquaculture. Toxicity studies have shown that this chemical
can have serious toxic side effects. Malachite green is not actually an antibiotic,
but has antibiotic properties. Malachite green is somewhat stable within
the environment and, therefore, is detectable in fish that were treated with the chemical
at some point even after treatment has discontinued. After more stringent testing and inspection
by the governments of Western Countries, the use of malachite green began to wane and other
drugs began to become more prevalent.===Chloramphenicol===
While the U.S. has tested farm-raised shrimp for chloramphenicol since 1994, over the last
decade the FDA developed a more sensitive testing methodology and changed the levels
of detection for chloramphenicol in response to increasing discovery of traces of chloramphenicol
in imports. In response to the US discovery of chloramphenicol
in imported shrimp and subsequent increased testing sensitivity, the use of this compound
in aquaculture began to decrease.===Gentian violet===
Gentian violet, also known as crystal violet has antibacterial, antifungal, and antiparasitic
properties. This compound was used during the World War
I era as a topical antiseptic, but has been replaced in modern times with more modern
treatments. The FDA prohibits the use of gentian violet
in aquaculture because of numerous studies showing increased risk of certain cancers
related to the compound and a showing that the chemical is bioavailable in fish when
used in aquaculture.===Nitrofurans===
Nitrofurans are broad spectrum antibiotics, being effective against Gram-positive and
Gram-negative bacteria. In 1991, the FDA withdrew several approved
food animal nitrofuran products as a result of research showing nitrofurazone, one of
the nitrofurans, can produce mammary tumors in rats and ovarian tumors in mice. The FDA also concluded that some people may
be hypersensitive to this product. The FDA states, “Absolutely, no extra-label
use of the nitrofurans is permitted in any food animals, including seafood.” The FDA currently detains certain seafood
imports without physical examination due to nitrofuran use by the producer.===Floroquinolones===
Fluoroquinolones have been prohibited from extra-label use in the U.S. and many other
parts of the world in aquaculture because of public health concern about the development
of such antimicrobial resistance. Chinese authorities have acknowledged permitting
the use of fluoroquinolones in aquaculture, even though the use of fluoroquinolones in
food animals may increase antibiotic resistance in human pathogens compromising the effectiveness
of the use of this critically important class of antibiotics in human medicine. The Chinese government has established a higher
maximum residue limit than the US and research in China has shown that the Chinese are effectively
meeting the Chinese limits. Because of concerns about the presence of
fluoroquinolones in the food supply, not only in aquaculture, but also in foods like honey,
the U.S. is continuing to develop methods and strategies to detect illegal residues
and prevent their introduction into the U.S. food supply

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