Cleansers contain surfactants or detergents that remove dirt and contaminating organisms by solubilization and physical means. Cleansers are often a critical step to proper disinfection or antisepsis as removing gross contamination from an area prior to disinfection or antisepsis treatment maximizes their efficacy. Depending on the application and use, cleansing may be sufficient.

Cleansers can be classified into three types based on the presence and charge of the hydrophilic portion of the molecule: anionic, cationic, and nonionic. Soaps are anionic surfactants of the general structure R-COO−Na⁺. Dissociation in water to R-COO− liberates a molecule with both a hydrophilic and a hydrophobic portion, which can emulsify and solubilize hydrophobic dirt, fat, and protoplasmic membranes. Once solubilized, this contamination can be rinsed away with water. The ability to solubilize membranes renders soaps antibacterial against gram-positive and acid-fast bacteria. The anionic nature of soaps, however, causes them to be inactivated in the presence of certain positive ions such as free Ca⁺ in hard water and in the presence of cationic detergents. The mixture of soaps and quaternary ammonium compounds forms a precipitate, which terminates the activity of both compounds. Inclusion of antiseptic compounds in soap preparations has given them a wider antibacterial spectrum.

The quaternary ammonium compounds (QACs) are examples of cationic surfactants with germicidal activity. These compounds have been widely used as disinfectants (see Section Examples of disinfectant use in Veterinary Medicine). Cationic surfactants combine readily with proteins, fats, and phosphates and are thus of limited value in the presence of serum, blood, and other tissue debris (Huber, 1988). In addition, use with materials such as gauze pads and cotton balls makes them less germicidal owing to absorption of the active ingredients.

An antiseptic is a chemical agent that reduces the microbial population on skin and other living tissues. Because in most cases its mechanism of action involves nonspecific disruption of cellular membranes or enzymes, caution must be taken not to harm host tissue. An ideal antiseptic would have a broad spectrum of activity, low toxicity, high penetrability, would maintain activity in the presence of pus and necrotic tissue, and would cause little skin irritation or interference with the normal healing process.

The use of antiseptics has been suggested in situations which require maximal reduction of bacterial contamination (Larson, 1987) such as when defense mechanisms are compromised after surgery, during catheterization or insertion of other invasive implants, and in immunocompromised states due to immune defects, cytotoxic drug therapy, extreme old or young age, or extensive skin damage (burns and wounds).

Disinfection is a process that eliminates most, if not all, pathogenic organisms, excluding spore forms, from an inanimate object. Disinfection is sometimes incorrectly confused with sterilization, a process that completely eliminates all microbial forms by a physical or chemical means. True chemical sterilization necessitates the use of an EPA-registered agent capable of killing all infective organisms, including fungal and bacterial spores, usually within 10 hours. Sometimes, however, chemical sterilants can be considered disinfectants when shorter exposure periods are used. The treatment of objects that are too large to soak in disinfectant, such as cabinets, exam tables, chairs, lights, and cages, is considered surface disinfection. Immersion disinfection is the immersion of smaller objects in disinfectant for sufficient time to kill the majority of contaminating, pathogenic organisms.

The ideal characteristics of a disinfectant includes a broad spectrum, fast action, activity in the presence of organic material (including blood, sputum, and feces), compatibility with detergents, low toxicity, low cost, ease of use, and residual surface activity. They should not corrode instruments or metallic surfaces or disintegrate rubber, plastic, or other materials, and should be odorless and economical (Molinari et al., 1982).

The ability to kill different classes of microorganisms further categorizes disinfectants into high, intermediate and low levels. High-level disinfection destroys all microorganisms except high concentrations of bacterial spores. Intermediate-level disinfection inactivates acid-fast microorganisms, including Mycobacterium tuberculosis, most viruses and fungi, but not necessarily bacterial spores. Low-level disinfection kills most bacteria, some viruses, and some fungi, but not tubercle bacilli or bacterial spores. In addition, low-level disinfection usually occurs in less than 10 minutes.

A second classification system is intended to divide instruments and patient-care items into three categories based on risk of infection involved in their use (Spaulding, 1968). In this system, items are classified as: (i) critical – those that enter or penetrate skin or mucous membranes (e.g., needles, scalpels), usually at a sterile site; (ii) semicritical – those that touch intact mucous membranes (e.g., anesthesia equipment, endoscopes); and (iii) noncritical – those that do not touch mucous membranes but may contact intact skin (e.g., stethoscopes, cages, tables, food bowls). In general, items classified as critical should be sterilized, semicritical items require high-level disinfection, and noncritical items require low to intermediate-level disinfection.

Alcohols are one of the most popular antiseptic and disinfecting products, used every day in veterinary clinics and laboratories. Although many alcohols are germicidal, the two most commonly used as disinfecting agents are ethyl and isopropyl alcohol. These compounds are both lipid solvents and protein denaturants. They kill organisms by solubilizing the lipid cell membrane and by denaturing membrane cellular proteins. Alcohols are most effective when diluted with water to a final concentration of 70% ethyl or 50% isopropyl alcohol by weight. It is thought that at greater concentrations, initial dehydration of cellular proteins makes them resistant to the denaturing effect (Molinari and Runnel, 1991). Alcohols have excellent antibacterial activity against most vegetative gram-positive, gram-negative, and tubercle bacillus organisms but do not inactivate bacterial spores. They are active against many fungi and viruses, principally enveloped viruses due to alcohol’s lipid-solubilizing action.

The alcohols are not recommended for high-level disinfection or chemical sterilization due to their inactivity against bacterial spores and reduced efficacy in the presence of protein or other bioburden. Blood proteins are denatured by alcohol and will adhere to instruments being disinfected. Fatal Clostridium spp. infections have occurred postoperatively that were the result of contaminated surgical instruments that had been disinfected with alcohol containing bacterial spores (Nye and Mallory, 1923). After repeated and prolonged use, alcohols can damage the shellac mounting of lensed instruments, can swell or harden rubber and certain plastic tubing (Rutala, 1990), and can be corrosive to metal surfaces. Alcohols are flammable; thus caution must be taken in their storage and when used prior to electrocautery or laser surgery. In deciding between ethyl and isopropyl alcohol, it is important to consider isopropyl’s inactivity against hydrophilic viruses, its less corrosive nature, and the abuse potential for ethyl alcohol (grain alcohol).

Both isopropyl and ethyl alcohol are also commonly used, effective antiseptics, with only subtle differences in their action. Because their effectiveness is drastically reduced by organic matter such as feces, mucus, and blood, they are most effective on “clean” skin. They produce rapid reduction in bacterial counts (Lowbury et al., 1974), with contact times of 1–3 minutes, resulting in elimination of almost 80% of organisms. Rapid evaporation limits contact time; however, residual decreases in bacterial counts are seen to occur after the alcohol has evaporated from the skin. Although alcohols are among the safest antiseptics, toxic reactions have been reported in children. Alcohol can be drying to the skin and can cause local irritation. In efforts to minimize this drying effect, emollients such as glycerine have been added with good results (Larson et al., 1986).

Iodine and chlorine both demonstrate antimicrobial activity and are used as antiseptics or disinfectants. Elemental iodine has germicidal activity against gram-positive and gram-negative bacteria, bacterial spores, fungi, and most viruses. It exerts these lethal effects by diffusing into the cell and interfering with metabolic reactions and by disrupting protein and nucleic acid structure and synthesis. Iodine has a characteristic odor and is corrosive to metals. It is insoluble in water and thus is prepared in alcohol (tincture) or with solubilizing surfactants (“tamed” iodines). Tincture of iodine, used as early as 1839 in the French Civil War, is most effectively formulated as a 1–2% iodine solution in 70% ethyl alcohol. In this form, most (approximately 90%) bacteria are killed within 3 minutes of application. The antibacterial activity of this combination is greater than that of the alcohol alone. Tincture of iodine, however, is irritating and allergenic, corrodes metals, and stains skin and clothing. It is also painful when applied to open wounds and is harmful to host tissue; therefore, it can delay healing and increase the chance of infection. For these reasons, this preparation has fallen out of favor as an antiseptic or disinfectant. Strong tinctures of iodine have been used as blistering agents in the equine industry.

Efforts to reduce the undesirable aspects of tinctures while retaining the powerful killing action of iodine have led to the introduction of tamed iodines known as iodophors. In this preparation, iodine is solubilized by surfactants, which allow it to remain in a dissociable form. Application of this product allows for slow continual release of free iodine to exert its germicidal effects. The iodophors have a similar spectrum of activity to aqueous solution; are less irritating, allergenic, corrosive, and staining; and have prolonged activity after application (4–6 hours). Common solubilizing carriers include polyvinylpyrrolidone (called PVP-iodine or povidone-iodine, PI) as well as other nonionic surfactants, making iodophors excellent cleansing agents as well as antiseptics and disinfectants. Iodophor solutions retain their activity in the presence of organic matter at pH <4 (Huber, 1988). The water-soluble carriers have been postulated to interact with epithelial surfaces to increase tissue permeability, thereby enhancing iodine’s killing efficacy.

Proper dilution to 1% iodine is necessary for maximum killing effect and minimal toxicity. More-concentrated solutions are actually less efficacious, presumably due to stronger complexation preventing free iodine release. It takes approximately 2 minutes of contact time for release of free iodine (Lavelle et al., 1975). Literature reports indicate that iodophors are quickly bactericidal, virucidal, and mycobactericidal but may require prolonged contact times to kill certain fungi and bacterial spores. Iodophors formulated as antiseptics are not suitable as hard-surface disinfectants, due to insufficient concentrations of iodine.

Consideration must be taken of iodine’s ability to be systemically absorbed through the skin and mucous membranes. The extent of absorption is related to the concentration used, frequency of application, and status of renal function (the principal excretory route) (Swaim and Lee, 1987). Complications of iodophor absorption include increased serum enzyme levels, renal failure, metabolic acidosis (Pretsch and Meakins, 1976), and increased serum free iodide. If renal function is normal, serum iodine concentrations quickly return to normal. Clinical hyperthyroidism and thyroid hyperplasia have been reported after treatment with PI (Scheider et al., 1976; Altemeier, 1976).

Chlorine-containing solutions were first introduced by Dakin in the early 1900s in the chemical form of sodium hypochlorite. They are effective bactericidal, fungicidal, virucidal, and protozoacidal agents. The chemical forms most commonly used today include the hypochlorites (sodium and calcium) and organic chlorides (chloramine-T). In either form, the germicidal activity is due to release of free chlorine and formation of hypochlorous acid (HOCl) from water. The mechanisms of action of these compounds include inhibition of cellular enzymatic reactions, protein denaturation, and inactivation of nucleic acids (Dychdala, 1983). Dissociation of HOCl to the less microbicidal hypochlorite ion (OCl⁻) increases as pH increases, and thus the solution may be rendered ineffective above pH 8.0 (Weber, 1950). Mixing NaOCl with acid liberates toxic chlorine gas, and NaOCl decomposes when exposed to light.

Low concentrations of free chlorine are active against M. tuberculosis (50 ppm) and vegetative bacteria (<1 ppm) within seconds. Concentrations of 100 ppm destroy fungi in less than 1 hour, and many viruses are inactivated in 10 minutes at 200 ppm. Household bleach is 5.25% (52,500 ppm); thus dilutions of 1 : 100 to 1 : 250 should result in effective germicidal concentrations although more-concentrated solutions are often recommended (1 : 10 to 1 : 100).

The use of the hypochlorites as disinfectants are limited by several characteristics. Chlorine solutions are corrosive to metals and destroy many fabrics. Because chlorine solutions are unstable to light, they must be prepared fresh daily. Hypochlorites are inactivated by the presence of blood more so than are the organic chlorides (Bloomfield and Miller, 1989). They have a strong odor and are not suitable for enclosed spaces. In addition, hypochlorites may lead to irritation of mucous membranes and may form toxic bioproducts when interacting with other chemicals. Despite these shortcomings, chlorine solutions are commonly used as low-level disinfectants on dairy equipment, animal housing quarters, hospital floors, and other noncritical items. Of 12 disinfectant solutions evaluated for their ability to kill the dermatophyte Microsporum canis, those containing hypochlorite were most effective. Also found effective were benzalkonium chloride and glutaraldehyde-based products; phenolics and anionic detergents were considered inadequate (Rycroft and McLay, 1991). The hypochlorites are not recommended for routine use as antiseptics because they are very irritating to skin and other tissues and they delay healing. There is, however, research to suggest diluted household bleach can be applied to control superficial pyoderma in dogs.

Several compounds from a class called N-halamines (oxazolidinones or imidazolidinones) have been developed, which are water-soluble solids that have been shown to be bactericidal, fungicidal, virucidal, and protozoacidal in water disinfection at low total halogen concentrations (1–10 mg/l). They are noncorrosive and tasteless and odorless in water. They are extremely stable in water even in the presence of organic loads. Their potential use in poultry processing to control Salmonella has been evaluated (Smith et al., 1990).

Chlorhexidine (Chx) is popular synthetic cationic antiseptic compound (1-1′-hexamethylenebis[5-(p-chlorophenyl)biguanide]) with better activity against gram-positive than against gram-negative organisms. The compound lacks sporicidal activity. Chlorhexidine kills bacteria by disrupting the cell membrane and precipitating cell contents. It has also been suggested that membrane-bound adenosine triphosphatases, specifically inhibition of the F1 ATPase, may be a primary target for Chx (Gale et al., 1981). It is active against fungi, fairly active against M. tuberculosis, but poorly active against viruses. The antibacterial activity of Chx is not as rapid as that of the alcohols; however, as a 0.1% aqueous solution, significant killing action is evident after only 15 seconds. Additionally, Chx solutions have the longest residual activity, remaining chemically active for 5–6 hours and retaining their activity in the presence of blood and other organic material. Being cationic, it is inactivated by hard water, nonionic surfactants, inorganic anions, and soaps. Dilution with saline causes precipitation and its activity is pH dependent. It has extremely low toxicity even when used on intact skin of newborns (O’Neill et al., 1982). Chlorhexidine is available in a detergent base as a 4% solution or as a 2% liquid foam. Traditionally, it has widely been used as a presurgical antiseptic, wound flush, and teat dip. Formulations of chlorhexidine and alcohol have also been described and appear to improve efficacy. Its use as a disinfectant are not well described.

Polyhexamethylene biguanide (PHMB) is a polymeric biguanide with activity against gram-positive and gram-negative bacteria, including methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus equi. PHMB rapidly kills bacteria by disrupting the cytoplasmic membrane resulting in leakage and precipitation of cellular contents (Broxton et al., 1983). PHMB has been used to treat infections in the eye, mouth, and vagina and has been formulated in contact lens disinfectants and mouth rinses. It was shown to be nontoxic as a component of an ear flush for dogs (Mills et al., 2005) and when impregnated in a gauze wound dressing, reduced growth of underlying gram-positive and gram-negative bacteria in vitro (Lee et al., 2004).

Two related aldehyde disinfectants are formaldehyde and glutaraldehyde (GLT). Formaldehyde has antimicrobial activity both as a gas and in liquid form. Formalin, the aqueous form, is 37% formaldehyde by weight. It inactivates microorganisms by alkylating the amino and sulfhydryl groups of proteins and ring nitrogen atoms of purine bases (Favero, 1983). Formaldehyde is an effective but slow bactericide, virucide, and fungicide, requiring 6–12 hours contact time. It is effective against M. tuberculosis, bacterial spores, and most animal viruses, including foot-and-mouth disease virus. Its action is not affected by organic matter and it is relatively noncorrosive to metals, paint, and fabric. Formaldehyde alone is considered a high-level disinfectant and in combination with alcohol can be used as a chemical sterilant for surgical instruments. However, due to irritating fumes and pungent odor at low concentrations (approximately 1 ppm), and because the National Institute for Occupational Safety and Health requires it to be handled as a potential carcinogen, thereby limiting worker exposure time, formaldehyde’s use as a disinfectant has been limited.

Glutaraldehyde, a saturated dialdehyde, is similar to formaldehyde but without some of its shortcomings. It has better bactericidal, virucidal, and sporicidal activity than formaldehyde. Its biocidal activity is related to its ability to alkylate sulfhydryl, hydroxyl, carboxyl, and amino groups affecting RNA, DNA, and protein synthesis (Scott and Gorman, 1983). Acidic GLT solutions are not sporicidal; thus, they must be “activated” by alkalinizing agents to a pH between 7.5 and 8.5. Once activated, these solutions have a limited shelf life (14 days) due to polymerization of the GLT molecules (Rutala, 1990). Newer formulations (stabilized alkaline GLT, potentiated acid GLT, GLT-phenate) have increased shelf life (28–30 days) and excellent germicidal activity (Pepper, 1980). GLT has gained wide acceptance in high-level disinfection and chemical sterilization due to several favorable properties, including wide spectrum of activity. Low surface tension allows GLT to penetrate blood and exudate without coagulating proteins. It retains its biocidal activity in the presence of organic matter. It is noncorrosive to metal, rubber, and plastic and does not damage lensed instruments. GLT solutions must be used in well-ventilated areas, since air concentrations of 0.2 ppm are irritating to the eyes and nasal passages (CDC, 1987). Contact times of less than 2 minutes for vegetative bacteria, 10 minutes for fungi, and 3 hours for bacterial spores were necessary using a 2% aqueous alkaline GLT solution (Stonehill et al., 1963). Activity against the tubercle bacillus was found to be somewhat variable; at least 20 minutes at room temperature is needed to reliably kill these organisms with 2% GLT. When used as a high-level disinfectant, a minimum of 1% GLT should be used. GLT-phenate formulations should be used with caution since they were shown to be less effective than other aldehyde solutions in decreasing bacterial counts from some medical instruments (Ayliffe et al., 1986). GLT disinfectants were found to more effectively reduce duck hepatitis B virus infectivity when they contained additives such as alcohol, an ammonium chloride derivative, and a surfactant (Murray et al., 1991). The caustic nature of both formaldehyde and GLT makes them inappropriate as antiseptics, and in fact, protective gloves should be worn when using the aldehyde disinfectants.

Gluteraldehyde and QAC combinations have been formulated (e.g., Synergize™, Preserve International, Reno, NV) and largely marketed as a cleaner and disinfectant for use in animal (e.g., swine and poultry) production facilities.

Conflicting reports concerning hydrogen peroxide’s efficacy as a germicide make evaluating its utility in disinfection and antisepsis difficult. Although it has been reported to have bactericidal (Schaeffer et al., 1980), virucidal (Mentel and Schmidt, 1973), and fungicidal (Turner, 1983) activity, the activity of hydrogen peroxide is nonpenetrable and short lived. For this reason hydrogen peroxide antiseptic use is most valuable in the initial treatment of recently contaminated wounds. Because 3% hydrogen peroxide has been shown to be damaging to tissues, including fibroblasts (Lineweaver et al., 1982), it is not considered suitable for routine wound care. It is, however, considered a stable and effective disinfectant and is used in the disinfection of soft contact lenses. More recently, accelerated hydrogen peroxide products have been formulated to also contain a surfactant and stabilizer, which improve antimicrobial activity. These products are being implemented in many veterinary clinic settings for use as a disinfectant.

Other oxidizing agents include potassium peroxymonosulfate (PPMS), an oxidizing agent used in disinfection systems of pools and hot tubs. More recently, it has been formulated with potassium chloride and organic acids and salts (i.e., sulphamic acid, malic acid, sodium hexametaphosphate, and sodium dodecyl benzene sulphonate) resulting in a disinfectant effective against over 580 infectious agents including viruses, gram-positive and gram-negative bacteria, fungi (molds and yeasts), and mycoplasma (EPA Master Label). It is marketed as a powder because it is stable in solution for approximately 1 week. It is not inactivated by organic challenge and has been found to be user friendly to both humans and animals. It is widely used as a high-level disinfectant for surfaces in laboratories, dental care facilities, and hospitals; for decontaminating laundry; for air disinfection; and in food processing and transport. Use of peracetic acid, sodium perborate, benzyl peroxide, and potassium permanganate have also been reported in human and veterinary literature.

Carbolic acid, a phenol, is the oldest example of an antiseptic compound. However, due to severe toxicity, it is no longer appropriate for use as an antiseptic. These agents act as cytoplasmic poisons by penetrating and disrupting microbial cell walls. Most commercially available phenolic products contain two or more compounds that act synergistically, resulting in a wider spectrum of activity, including against M. tuberculosis. Sodium o-phenylphenol is effective against staphylococci, pseudomonads, mycobacteria, fungi, and lipophilic viruses, and against ascarids, strongyles, and tichurids. Cresols are substituted phenols and are more bactericidal and less toxic and caustic than phenols. Phenolics are not recommended for disinfection of anything other than noncritical items, because of residual disinfectant on porous materials causing tissue irritation even when the items have been thoroughly rinsed, because of strong odors, and because of absorption into feed.

Triclosan (Irgasan DP 300; 2,4,4′ trichloro-2′-hydroxydiphenyl ether) is a chlorinated diphenyl ether or bisphenol that possesses high antibacterial activity particularly against many gram-positive (e.g., Bacillus subtilis, Mycobacterium smegmatis, Staphylococcus aureus) and gram-negative bacteria (Escherichia coli, Salmonella enterica serotype Typhimurium, Shigella flexneri