Medical uses of silver
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The medical uses of silver include its incorporation into wound dressings to treat external infections, and its use as an antiseptic and disinfectant in medical appliances. Silver is also promoted within alternative medicine in the form of colloidal silver, although it has not been proven to be safe or effective.
The silver ion (Ag+) is bioactive and in sufficient concentration readily kills bacteria in vitro. Silver also kills bacteria in external wounds in living tissue, so physicians use wound dressings containing silver sulfadiazine (Ag-SD) or silver nanomaterials to treat external infections. Wound dressings containing silver are increasing in importance due to the recent increase of antibiotic-resistant bacteria, such as MRSA. The disinfectant properties of silver are used in medical applications, such as urinary catheters and endotracheal breathing tubes, where the silver content is effective in reducing incidences of catheter-related urinary tract infections and ventilator-associated pneumonia (VAP), respectively. Silver is also used in bone prostheses, reconstructive orthopaedic surgery and cardiac devices, as well as on surfaces and fabrics to reduce the spread of infection.
Since the 1990s, “colloidal silver”, a liquid suspension of microscopic silver particles, has been marketed as an alternative medicine, often claiming impressive “cure-all” qualities. The effectiveness of these products has never been scientifically proven, and in some jurisdictions, it is currently illegal to include such claims in product advertisements. Medical authorities and publications advise against the ingestion of colloidal silver preparations, because of their lack of proven effectiveness and because of the risk of adverse side effects, such as argyria. Historically, colloidal silver was also used as an internal medication to treat a variety of diseases. Their use was largely discontinued in the 1940s, due to the development of safe and effective modern antibiotics and concern about adverse side effects.
Silver has had some medicinal uses going back for centuries. The Phoenicians are said to have stored water, wine, and vinegar in silver bottles to prevent spoiling. In the early 1900s, people would put silver coins in milk bottles to prolong the milk’s freshness. Hippocrates, the “father of medicine”, wrote that silver had beneficial healing and antidisease properties. In the early 1900s, silver gained regulatory approval as an antimicrobial agent. Prior to the introduction of antibiotics, colloidal silver was used as a germicide and disinfectant. Physicians used it as an eyedrop for ophthalmic problems, for various infections, and sometimes internally for diseases such as tropical sprue, epilepsy, gonorrhea, and the common cold. Colloidal silver preparations (CSP) were used to treat or prevent gonorrhea and gonorrheal conjunctivitis. Although “silver products were infrequently promoted for oral use, benefits have been even more questionable.” With the introduction of antibiotics in the 1940s, the use of silver as an antimicrobial agent diminished. One well known, highly successful, brand name, silver colloid product in the period before 1940 was Argyrol.
Silver and most silver compounds have an oligodynamic effect and are toxic for bacteria, algae, and fungi in vitro. The oligodynamic effect is typical for heavy metals, such as lead and mercury, but, among the elements that have this effect, silver is the least toxic for humans. The antibacterial action of silver is dependent on the silver ion. The effectiveness of silver compounds as an antiseptic is based on the ability of the biologically active silver ion (Ag+) to irreversibly damage key enzyme systems in the cell membranes of pathogens. The antibacterial action of silver has long been known to be enhanced by the presence of an electric field. Applying a few volts of electricity across silver electrodes drastically enhances the rate bacteria in solution are killed. The antibacterial action of silver electrodes is greatly improved if the electrodes are covered with silver nanorods.
Electrolytically-dissolved silver has been used as a water disinfecting agent, for example, the drinking water supplies of the Russian Mir orbital station and theInternational Space Station. The World Health Organization includes silver in a colloidal state produced by electrolysis of silver electrodes in water, and colloidal silver in water filters as two of a number of water disinfection methods specified to provide safe drinking water in developing countries. Along these lines, a ceramic filtration system coated with silver particles has been created by Ron Rivera of Potters for Peace and used in developing countries for water disinfection.
In World War I, before the advent of antibiotics, silver compounds were used to prevent and treat infections. Silver compounds continue to be used in external preparations as antiseptics, including silver nitrate, which can be used in dilute solution as eyedrops to prevent conjunctivitis in newborn babies. Silver nitrate is also sometimes used in dermatology in solid stick form as a caustic (“lunar caustic”) to treat certain skin conditions, such as corns and warts.
According to Atiyeh et al. (2007), “The gold standard in topical burn treatment is silver sulfadiazine (Ag-SD), a useful antibacterial agent for burn wound treatment”. They do note, however, that silver-impregnated dressings do sometimes result in a slower healing process. Silver sulfadiazine cream (SSD Cream) replaced colloidal silver as the most common delivery system for using silver on the surface of burn wounds to control infection in the 1970s.
The US Food and Drug Administration has approved the use of a range of different silver-impregnated wound dressings.
Laboratory studies at the Biochemical Materials Research and Development Center of Jiaxing College, China, have shown silver-containing alginate fibres provide a sustained release of silver ions when in contact with wound exudates, and are “highly effective against bacteria”. A study administered by the Hull York Medical School found an antimicrobial barrier dressing containing silver provided “a highly effective and reliable barrier to the spread of MRSA into the wider hospital.”
More recently, dressings incorporating nanocrystalline silver or activated silver-impregnated substances have become available, which deliver higher concentrations of the active silver ion. As of 2006, more “than 10 dressings containing pure silver” were available. In particular, silver is being used withalginate, a naturally occurring biopolymer derived from seaweed, in a range of products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims.
Wound dressings containing silver are increasing in importance due to the increase of antibiotic-resistant bacteria, which has imposed clinical limits on the use of antibiotics. Chopra states topical silver is regaining popularity in the management of open wounds, “due largely to the spread of methicillin-resistant Staphylococcus aureus and the resultant reduction in first-line antibiotic prescribing”, and “[s]ome silver-based dressings appear to provide an effective alternative to antibiotics in the management of wound infection.” Silver has proven broad-spectrum antimicrobial activity that includes antibiotic-resistant bacteria, with minimal toxicity toward mammalian cells at low concentrations, and has a less likely tendency than antibiotics to induce resistance due to its activity at multiple bacterial target sites.
However, some sources still hold that the evidence for the effectiveness of silver-treated dressings is mixed, as the evidence is marred by the poor quality of the trials used to assess these products. Consequently, a systematic review by the Cochrane Collaboration found insufficient evidence to recommend the use of silver-treated dressings to treat infected wounds.
In medical appliances
The disinfectant properties of silver are used in some other medical applications, such as catheters and endotracheal breathing tubes. A study on the use of silver-alloy catheters by the University of Michigan School of Medicine concluded “The data supporting the use of silver alloy urinary catheters to reduce urinary catheter-related bacteriuria is reasonably strong.” The study also concluded silver alloy catheters are more effective than standard catheters for reducing bacteriuria in adults in hospital having short-term catheterization, and, although they cost about $6 more than standard urinary catheters, they may be worth the extra cost, since catheter-related infection is a common cause of nosocomial infection and bacteremia. Related meta-analysis also clarified discrepant results among earlier trials of silver-coated urinary catheters by revealing silver alloy catheters are significantly more effective in preventing urinary tract infections than are silver oxide catheters. These conclusions are supported by, among others, studies by the University Hospitals Leuven, Belgium and the University Hospital for Anesthesiology and Surgical Intensive Care, Halle, Germany.
In 2007, AGC Flat Glass Europe introduced the first antibacterial glass to fight hospital-acquired infection; it is covered with a thin layer of silver. Ionizable silver is also incorporated into fabrics to reduce the spread of bacteria.
Ventilator-associated pneumonia (VAP) causes substantial morbidity. A 2008 study by Kollef et al. concluded, “Patients receiving a silver-coated endotracheal tube had a statistically significant reduction in the incidence of VAP and delayed time to VAP occurrence compared with those receiving a similar, uncoated tube.” In addition, the U.S. Food and Drug Administration (FDA) has recently approved an endotracheal tube with a fine coat of silver for use in mechanical ventilation, after studies found it reduced the risk of ventilator-associated pneumonia.
The use of these devices is contraindicated for persons who are allergic to silver, and although they are widely used in hospitals, no thorough testing and standardization of these products has yet been undertaken.
A meta-analysis of 26 studies by the Cochrane Collaboration found that, while most were small and of poor quality, there was not enough evidence to support the use of silver-containing dressings or creams, as generally these treatments did not promote wound healing or prevent wound infections. Some evidence suggested that silver sulfadiazine had no effect on infection, and actually slowed healing.
Since about 1990, there has been a resurgence of the promotion of colloidal silver as an alternative medicine treatment, marketed with claims of it being an essential mineral supplement, or that it can prevent or treat numerous diseases, such as cancer, diabetes, HIV/AIDS, and herpes, as well as tuberculosis. Silver is not an essential mineral in humans; there is no dietary requirement for silver, and no such thing as a silver “deficiency”. No medical evidence supports colloidal silver as being effective for any of these claimed indications.
The commercial product referred to as “colloidal silver”, includes solutions that contain various concentrations of ionic silver compounds, silver colloids or silver compounds bound to proteins in water. Such products with concentrations of 30 parts per million (ppm) or less are typically manufactured using an electrolysisprocess, whereas those with higher concentrations of 50 ppm or more are usually silver compounds that have been bound with a protein. Colloidal silver preparations primarily deliver inactive metallic silver, rather than the active microbicidal silver ion.
No scientific evidence supports the effectiveness of colloidal silver in vivo. Some in vitro studies demonstrate an antibacterial effect of colloidal silver,although one study in 2004 of a colloidal silver solution marketed on the Internet showed no such antimicrobial activity. No clinical studies in humans demonstrate effectiveness, and a few report toxicity. The U.S. National Center for Complementary and Alternative Medicine has issued an advisory indicating the marketing claims made about colloidal silver are scientifically unsupported, the silver content of marketed supplements varies widely, and colloidal silver products can have serious side effects to the consumer, including “argyria,… neurologic problems (such as seizures), kidney damage, stomach distress, headaches, fatigue, and skin irritation. Colloidal silver may interfere with the body’s absorption of some drugs, such as penacillamine, quinolones, tetracyclines, and thyroxine.”
Although colloidal silver products are legally available at health food stores in the United States and Australia and are marketed over the Internet as a dietary supplement, it is illegal in the U.S. and Australia for marketers to make claims of medical effectiveness for colloidal silver. Ingestion of colloidal silver may result inargyria.
Adverse health effects
According to Lansdown, the risk expected due to clinical exposure to silver is “minimal”, as only chronic ingestion or inhalation of silver preparations leads to an accumulation of silver in the human body that can cause argyria, argyrosis (accumulation of silver in the eye), and other conditions. Silver-based products are contraindicated for people who are allergic to silver. The reference dose, published by the United States Environmental Protection Agency in 1991, which recommends the estimated daily exposure that is unlikely to incur an appreciable risk of deleterious effects during a lifetime, is 5 µg/kg/d; meaning 5 micrograms of silver per kilo of weight per person each day – about of 10 ppm colloidal silver per month for a person. An article from the National Center for Complementary and Alternative Medicine points out silver nitrate and silver sulfadiazine can have negative side effects, and they must be applied to the body externally and not taken internally.
The chronic intake of silver products and the silver buildup from colloidal silver can result in an accumulation of silver or silver sulfide particles in the hair, skin, kidneys, liver, heart and muscles due to high methionine-containing proteins, such as keratin, myosin, tropomyosin, troponin, and key dipeptide glutathione. Serious neurologic (such as seizures), renal, or hepatic complications, as well as headaches, stomach distress, fatigue, and skin irritation have been reported. if ingested, colloidal silver may react with certain drugs, such as Penicillamine, thyroxine, quinolones, and tetracyclines. One death has been reported in the medical literature which the authors felt was due to silver toxicity resulting from repeated oral ingestion of colloidal silver. Colloidal silver can reduce the absorption of some medications, including tetracycline and quinolone antibiotics and can bind to penicillamine, thereby reducing the effectiveness of those medications.
As in photography (where silver is used due to its reactivity with light), silver particles in the skin darken with exposure to sunlight, resulting in a blue or gray discoloration of the skin. This condition is known as argyria, which is a dermatological condition characterized by grayish-blue pigmentation of the skin, nails, gums, and deep tissues; and, in similar manner, it can lead to silver in the eye (argyrosis) and in other organs. Localized argyria can occur as a result of topical use of substances containing silver, while generalized argyria results from the chronic ingestion of such substances. Argyria was long believed to be irreversible, but recently, laser therapy has been used to treat it with satisfactory cosmetic results. The Agency for Toxic Substances and Disease Registry (ATSDR) describes argyria as a “cosmetic problem”, although some people consider it to be socially debilitating.
In August 1999, the FDA banned colloidal silver sellers from claiming any therapeutic or preventive value for the product, noting colloidal silver was being marketed for numerous diseases without evidence of safety or effectiveness. As a result, the product now has the status of a dietary supplement in the US; it can be promoted with general “structure-function” claims, but cannot be marketed as preventing or treating any illness. Following this ruling, the FDA has issued numerous Warning Letters to Internet sites that have continued to promote colloidal silver as an antibiotic or for other medical purposes.
In 2002, the Australian Therapeutic Goods Administration (TGA) found there were no legitimate medical uses for colloidal silver and no evidence to support its marketing claims. Given the associated safety risks, the TGA concluded “efforts should be made to curb the illegal availability of colloidal silver products, which is a significant public health issue.”