Oxidizing Agents as Antiseptics and Disinfectants for Use With Animals

Chlorine exerts a potent germicidal effect against most bacteria, viruses, protozoa, and fungi by forming undissociated hypochlorous acid (HOCl) in water at acidic to neutral pH. It is effective against most organisms at a concentration of 0.1 ppm; however, much higher concentrations are required in the presence of organic matter. Alkaline pH ionizes chlorine and decreases its activity by decreasing its penetrability. Chlorine has a strong acidic smell. It irritates the skin and mucous membranes, including the respiratory tract, and can cause severe bronchospasms and acute lung injury. It is widely used to disinfect water supplies and objects (eg, utensils, bottles, pipelines) in dairies, creameries, and milk houses. Chlorine dioxide has replaced chlorine as a disinfectant for drinking water in some jurisdictions because it forms fewer by-products.

Electrochemically activated solution (ECAS) is a broad-spectrum bactericidal “metastable” solution containing free available chlorine species such as HOCl and hypochlorite generated from a dilute NaCl solution (~0.5%). It has disinfection activity equivalent to that of 80% ethanol and is superior to both 0.1% chlorhexidine and 0.02% povidone-iodine. ECAS has been suggested in place of formaldehyde fogging to disinfect factory farms; however, applications of ECAS have been limited because it corrodes processing equipment.

Inorganic chlorides include sodium hypochlorite (NaOCl) solutions (bleach). A 2%–5% NaOCl solution is a commonly used and effective disinfectant. NaOCl has been used as a disinfectant for more than a century. Contributing to its effectiveness as a disinfectant are the facts that it works against a broad spectrum of microbes; it works quickly against bacteria; it lasts a fairly long time in treated drinking water; it is easy to use, soluble in water, relatively stable, and relatively nontoxic at use concentrations; it has no poisonous residuals and no color; and it is inexpensive and easily available. Sodium hypochlorite–based disinfectants are widely used in veterinary surgery facilities, rescue shelters, farms, and homes. Because it is rapidly inactivated by contact with matter, sodium hypochlorite is an effective disinfectant only if the items are cleaned first.

The concentration of available chlorine and the pH of the solution determine the efficacy of sodium hypochlorite as a cleanser and disinfectant. The weak acid HOCl dissociates to the hypochlorite ion (ClO^–) and a proton (H⁺) in more alkaline solutions. HOCl is thought to give sodium hypochlorite its germicidal properties; the concentration of ClO ^– plays a large part in determining how efficiently sodium hypochlorite cleans. Therefore, the pH range that optimizes the germicidal activity of sodium hypochlorite likely differs from the pH range that optimizes its cleaning activity. Factors that decrease the activity of sodium hypochlorite include the presence of heavy metal ions, biofilms, organic material, low temperature, low pH, and UV radiation.

HOCl is not very stable in solution; it dissociates to Cl₂, H₂O, and other potentially hazardous components. At low pH, chlorine gas can be generated, and reaction with the ammonium in urine can cause chloramine to form. Oxidation of organic contaminants produces what are known as disinfection by-products (DBPs), which include chloroform and other trihalomethanes. However, DBP generation is more of a problem in large-scale disinfection—for example, when dealing with factory farm effluent—than when mopping the floor in a veterinary clinic, so it is not an issue for the general-practice veterinarian.

Hypochlorites kill most microbes; viruses and non-spore-forming bacteria, however, are more susceptible than spore-forming bacteria, fungi, and protozoa. Spores are controlled more effectively by bleach disinfection followed by steam cleaning than by bleach disinfection alone. Clinical uses of hypochlorites include chlorination of potable water to prevent Legionella spp colonization; chlorination of water distribution systems used in hemodialysis centers; cleaning of environmental surfaces; disinfection of laundry; decontamination of surfaces contaminated by blood, pus, feces, or urine; disinfection of equipment; decontamination of medical waste before disposal; and dental therapy. Even though other disinfectants are increasingly available, hypochlorite-based disinfectants are still widely used.

Organic chlorides contain chlorine weakly bonded to nitrogen, which is slowly released for germicidal activity. They are generally less irritant, more stable, and more convenient to use than hypochlorite solutions.

Sodium chloride (household salt) solution has long been used as a food preservative for its antimicrobial properties. Sodium chloride is a nutrient that affects different organisms in different ways. For example, some organisms are obligate halophiles: they require salt to survive and will lyse (break open) if their salt concentration drops too low. Other halophilic organisms are merely halotolerant, meaning that they do not need salt to survive but can tolerate moderately salty environments.

Salt (and sugar) inhibits microbial growth in several ways. The most notable is simple osmosis, or dehydration. Whether in solid or aqueous form, salt tries to reach equilibrium with the salt content of any food product it contacts. The effect is to draw available water from within the food to the outside and insert salt molecules into the food interior. As a result, the so-called product water activity (a_w)—a measure of unbound, free water molecules in the food that are necessary for microbial survival and growth—decreases. The a_w value of most fresh foods is 0.99; the a_w value necessary to inhibit the growth of most bacteria is roughly 0.91. Yeasts and molds, however, usually require even lower a_w values to prevent growth. When properly applied, these processes not only prevent spoilage of foods but, more important, inhibit or prevent the growth of foodborne pathogens such as Salmonella spp or Clostridium botulinum.

A simple homemade hypertonic saline solution (15 g household salt to 250 mL water, or 1 teaspoon salt to 0.5 pint water) can make an effective antimicrobial solution for bathing eyes affected by conjunctivitis until the results of laboratory tests arrive with the accurate diagnosis and indicate the specific treatment required.

The wide-ranging activity of iodine affects gram-positive and gram-negative bacteria, fungi, protozoa, and viruses. A solution containing 50 ppm iodine kills bacteria in 1 minute. Destruction of bacterial spores requires moist contact for at least 15 minutes. Iodine is poorly soluble in water but readily dissolves in ethanol, which enhances its antimicrobial activity. Iodine has low toxicity to tissues. Iodine surgical scrub is effective against methicillin-resistant Staphylococcus aureus(MRSA) and parvovirus.

Iodine can be prepared in many forms for antiseptic or disinfectant use. Iodine tincture contains 2% iodine and 2.4% sodium iodide (NaI) dissolved in 50% ethanol; it is used as an antiseptic. Strong iodine tincture contains 7% iodine and 5% potassium iodide (KI) dissolved in 95% ethanol; it is more potent but also more irritating than tincture of iodine when used as an antiseptic; it can be used as a teat or navel dip or as a sclerosing agent for draining tracts. Iodine solution contains 2% iodine and 2.4% NaI dissolved in aqueous solution; it is used as a nonirritating antiseptic on wounds and abrasions. Strong iodine solution (Lugol solution, also called aqueous iodine) contains 5% iodine and 10% KI in aqueous solution; it can be used as a water purifier or antiseptic.

Iodophors (eg, povidone-iodine and poloxamer-iodine) are combinations of iodine with a solubilizing agent or carrier; they are more stable and water-soluble than older formulations. Povidone-iodine consists of elementary iodine bound to the carrier polyvinylpyrrolidone. Solutions typically contain 5%–10% povidone-iodine and can be bought over the counter in most countries. Iodophors slowly release iodine as an antimicrobial agent. They are less irritating to skin than iodine compounds are, they do not sting or stain, and they are widely used as a preoperative scrub on surgeons' hands and patients' skin. Despite the absence of stinging or staining, repeated exposure to iodophors can result in dermatitis. They may also be corrosive to metals.

Iodophors are effective against bacteria, viruses, and fungi but less effective against spores; they remain active even in the presence of organic matter. Iodophor solutions retain good antimicrobial activity at pH <4; phosphoric acid is often mixed with iodophors to maintain an acidic medium. Iodophors have been used in teat dips to control mastitis, as dairy sanitizers, and as a general antiseptic or disinfectant for various dermal and mucosal infections. Iodophors should be used with caution around ears because iodine-based solutions are ototoxic (though less so than chlorhexidine), and high concentrations of alcohol-based solutions are the most harmful.

Povidone-iodine has been used for >60 years, has a good safety profile, and is included in WHO's list of essential medicines.

The main application is as fluoride for its anticaries effect; it is added to drinking water in many countries on the basis of this property. It is not widely used in animal health based on concerns for toxicosis.