Disinfection and Biosecurity in the Prevention and Control of Disease in Veterinary Medicine
Patrick J. Quinn
Bryan K. Markey
Finola C. Leonard
Eamonn S. FitzPatrick
Strategies for limiting the impact of infectious diseases on animal populations are often determined by the nature of the infectious agents responsible for disease production, the importance of the diseases produced on animal and human health, and the range of measures available to animal owners for the control and prevention of such diseases. Although there have been major advances in the development of chemotherapeutic drugs including antimicrobial compounds and anthelmintics as well as in the use of effective veterinary vaccines, infections caused by pathogenic microorganisms still constitute major obstacles to increased productivity, particularly in intensively reared animal populations. Among food-producing animals, infectious diseases may result in increased mortality, decreased production of meat, milk, or eggs and reproductive failure. The cost of chemotherapy is an additional expense for animal owners. In recent decades, increased reliance on intensive livestock production has resulted from increasing demands for food of animal origin and declining profit margins for producers. Although reliable data documenting losses attributable to disease in animals are difficult to determine, global estimates indicate average losses of more than 20%.1 The mortality rate associated with infectious diseases is strongly influenced by the species and ages of animals involved, by husbandry methods used, and by the measures employed to limit transmission of pathogenic microorganisms on the farm or in the production unit.
Characteristics of infectious agents, their modes of transmission, and both management and environmental factors influence the outcome of an encounter between a susceptible host and a virulent pathogenic microorganism. In some instances, a number of infectious agents either in combination or acting sequentially may cause well-recognized “complex” diseases, particularly affecting the respiratory tract. Such disease conditions may not be amenable to control through chemoprophylaxis, chemotherapy or vaccination, and other control measures relating to the animals’ immediate environment may be required to minimize adverse environmental conditions that predispose animals to opportunistic infections. For exotic infectious diseases of major economic importance in domestic animals, control measures include accurate identification of animals, control of their movement within a herd, flock or country, isolation followed by testing and where necessary, slaughter of infected animals. Contaminated buildings, equipment, and transport vehicles should be thoroughly disinfected to minimize transfer of pathogens to other locations. When dealing with endemic infectious diseases within a country, chemotherapy, chemoprophylaxis, and vaccination may be supplemented with appropriate disinfection and sterilization programs. Control measures applied to particular infectious diseases are determined by their status within a farm or country, their economic importance, and their public health significance.
TRANSMISSION OF INFECTIOUS AGENTSContamination of the environment, including buildings, equipment, transport vehicles, vegetation, water and soil, is an important consequence of outbreaks of many infectious diseases. Such contamination can occur on farms or in other locations where animals are reared or assembled for sales, shows, or sporting events. In association with outbreaks of many infectious diseases such as those that cause abortions in horses, cattle and sheep, extensive environmental contamination can occur as animals are aborting and subsequently from contamination of aborted fetuses and fetal fluids. Scavenger animals, transport vehicles, footwear and clothing of farm workers can amplify the extent of microbial contamination before effective control
measures are implemented. Salmonellosis, brucellosis, and leptospirosis as well as parvovirus, herpesvirus, and rotavirus infections are examples of diseases in which extensive environmental contamination occurs. Depending on their predilection sites in the host animal’s body, pathogenic microorganisms may be shed in secretions, fluids, excretions and animal products such as meat, eggs, and offal (Figure 52.1). For some infectious agents, aerosol transmission is an important mode of transfer among susceptible animals. Avian chlamydiosis, caused by Chlamydia psittaci, occurs in a wide range of both wild and domestic species. The organism, which is present in respiratory discharges and feces of infected birds, is usually acquired by inhalation or ingestion. In addition to its importance in avian species, C psittaci is recognized as a cause of sporadic disease in the human population, often with respiratory symptoms.
measures are implemented. Salmonellosis, brucellosis, and leptospirosis as well as parvovirus, herpesvirus, and rotavirus infections are examples of diseases in which extensive environmental contamination occurs. Depending on their predilection sites in the host animal’s body, pathogenic microorganisms may be shed in secretions, fluids, excretions and animal products such as meat, eggs, and offal (Figure 52.1). For some infectious agents, aerosol transmission is an important mode of transfer among susceptible animals. Avian chlamydiosis, caused by Chlamydia psittaci, occurs in a wide range of both wild and domestic species. The organism, which is present in respiratory discharges and feces of infected birds, is usually acquired by inhalation or ingestion. In addition to its importance in avian species, C psittaci is recognized as a cause of sporadic disease in the human population, often with respiratory symptoms.
Some infectious agents can survive for long periods outside the animal’s body, thereby facilitating prolonged exposure of healthy animals to sources of infection. Mycobacteria, coccidial oocysts, parvoviruses, and bacterial endospores are particularly able to survive for long periods in animal products, in the environment, in feces, soil and water, in animal buildings, and on pasture. Apart from clinically affected animals, carrier animals that appear clinically normal may shed microbial pathogens intermittently if stressed by transportation over long distances, by adverse housing conditions, or by severe climate changes.
The role of animal feeds in disease transmission became an issue of international importance following the unexpected appearance of bovine spongiform encephalopathy (BSE) in British cattle. The extreme resistance of the agent of BSE to thermal and chemical inactivation renders recycling food of animal origin, especially if derived from ruminants, an undesirable practice.
Insect vectors have an important role in the dissemination of infectious agents, especially viral pathogens. Wildlife reservoirs of infectious agents, both mammals and birds, often limit the efficacy of disease control measures and, in some instances, render them ineffective.
SURVIVAL OF INFECTIOUS AGENTS IN THE ENVIRONMENTMicrobial pathogens shed in the excretions, secretions, or body fluids of infected animals may contaminate farm buildings, transport vehicles, soil, water, pasture, food, and fomites. There is considerable variation in the survival times of animal pathogens in the environment, which, for Mycoplasma species and some viruses, may be days, unlike bacterial endospores, which may survive for decades in soil (Figure 52.2). For more labile pathogens, duration of survival is influenced by the number of infectious agents excreted by an infected animal, the availability of nutrients, competition from other microorganisms in the same environment, and other microenvironmental factors such as the type and amount of organic material present, temperature, pH, humidity, and exposure to ultraviolet (UV) light. Although accurate information relating to pathogen survival in the environment is available for some infectious agents,2,3,4 in many instances, the persistence of pathogens in soil, water, and animal waste is of uncertain duration. Infectious agents that are capable of prolonged survival in the environment include mycobacteria, coccidial oocysts, parvoviruses, bacterial endospores, and prions.
Lability is a feature of mycoplasmas, many enveloped viruses, and spirochetes, and these microbial pathogens usually have short survival times outside the animal body. In contrast, pathogenic mycobacteria, parvoviruses, and coccidial oocysts, because of their stability in the environment, remain viable in feces, soil, or contaminated buildings for many months and in favorable microenvironmental conditions for more than 1 year. Prions and bacterial endospores exhibit exceptional resistance to environmental factors and can survive for many years in soil.
Although the resistance of prions to physical and chemical inactivation has been well documented, the duration of their survival in the environment has been rarely reported. Scrapie-infected hamster brain homogenates mixed with soil and packed in perforated petri dishes retained infectivity for more than 3 years when buried in soil at ground level.5 This important report raises serious questions about the safety of burying the carcasses of sheep that had scrapie or bovine carcasses from animals with BSE. It also raises questions about the possibility of residual infectivity on pasture, which has been grazed by animals with transmissible spongiform encephalopathies (TSEs).
PRINCIPLES OF DISEASE CONTROLThe control measures applicable to a particular infectious disease are determined by its status in the country, its public health significance, and its importance internationally. Diseases that are endemic within a country may be confined to domestic animals or may be present in and transmitted from wildlife reservoirs. National governments implement control and eradication programs for infectious diseases in accordance with the importance of the diseases for the animal population, their public health importance, and their impact on national and international trade. Ultimately, the success of a disease control program is determined by the feasibility of the procedure, the reliability of the diagnostic tests, reservoirs of infection, and the national resources available for the implementation of the eradication program. Examples of the particular methods appropriate for the prevention, treatment, and control of particular infectious agents are presented in Table 52.1.
 FIGURE 52.1 Modes of transmission of infectious agents from infected to susceptible animals and relevant control measures. |
 FIGURE 52.2 Survival times of microbial pathogens in the environment at ambient temperature. The duration of survival is influenced by the number of infectious agents shed in the environment and by microenvironmental factors including the availability of nutrients, competition from other microorganisms in the same location, prevailing temperature, pH, humidity, and exposure to ultraviolet light. |
Biosecurity
The term biosecurity includes a range of measures designed to limit or prevent exposure of domestic animals on a farm or in a production unit to pathogenic microorganisms from sources outside the premises, referred to as bioexclusion. Appropriate measures for establishment of a closed herd or flock are illustrated in Figure 52.3. Biocontainment refers to procedures for preventing or limiting the spread of infectious diseases among animals on a farm, in kennels, in poultry production units, or wherever animals are housed or reared in close contact with each other. Infectious agents can be transmitted to healthy animals by purchase of infected animals, through contaminated feed,
by vectors, and from environmental sources. An effective biosecurity program has many components, all aimed at ensuring that the risks of healthy animals acquiring infection are minimized (Table 52.2). Design considerations for farms engaged in the implementation of biocontainment policies are illustrated in Figure 52.4.
by vectors, and from environmental sources. An effective biosecurity program has many components, all aimed at ensuring that the risks of healthy animals acquiring infection are minimized (Table 52.2). Design considerations for farms engaged in the implementation of biocontainment policies are illustrated in Figure 52.4.
TABLE 52.1 Methods for the prevention, treatment, and control of particular infectious agents | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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A number of well-defined measures can be used for the prevention and control of infectious diseases within a country or in a region of a country. These include exclusion of suspect animals, quarantine at point of entry, and isolation and slaughter of infected animals if an exotic disease is confirmed by clinical or laboratory tests. If infectious diseases are endemic in a country, control measures include vaccination, chemotherapy, and chemoprophylaxis. Effective control measures relating to the environment, animal waste, and animal products are central to the success of a disease eradication program. These include
Chemical disinfection of
buildings, bedding, and equipment
transport vehicles
footwear and clothing of workers
Chemical treatment of water supply, following disinfection of building
Chemical treatment of fluids, excretions, secretions
Heat treatment of milk and milk products; mandatory boiling of waste food if swill feeding to pigs is permitted
The forms and amount of animal waste generated on a farm are determined by the number and species of animals present, building design, and the type of material used as bedding for large animals or as poultry litter.
For buildings with slatted floors, animal waste is stored in slurry tanks. These tanks should be constructed to high specifications and have ample capacity to avoid overflowing of contents. Slurry spreading is usually restricted to defined times of the year when ground conditions are suitable for slurry tankers and when the risk of runoff is low. An interval of at least 2 months should elapse between the application of slurry to pasture and the commencement of grazing. A longer interval between the application of slurry and the commencement of grazing may be required if enteric pathogens such as Salmonella species or pathogenic acid-fast bacteria are likely to be present in the slurry.
For buildings with slatted floors, animal waste is stored in slurry tanks. These tanks should be constructed to high specifications and have ample capacity to avoid overflowing of contents. Slurry spreading is usually restricted to defined times of the year when ground conditions are suitable for slurry tankers and when the risk of runoff is low. An interval of at least 2 months should elapse between the application of slurry to pasture and the commencement of grazing. A longer interval between the application of slurry and the commencement of grazing may be required if enteric pathogens such as Salmonella species or pathogenic acid-fast bacteria are likely to be present in the slurry.
 FIGURE 52.3 Bioexclusion measures appropriate for establishment and maintenance of a closed herd or flock and farm policies aimed at minimizing the risk of introducing specific infectious agents to such animal populations. |
TABLE 52.2 Components of a biosecurity program for farm animals, including poultry | ||||||||||||||||||||||||||||||
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 FIGURE 52.4 Design considerations for farms engaged in the implementation of a biocontainment policy appropriate for the prevention or control of outbreaks of infectious disease in farm animals, including poultry. |
Straw used for bedding animals and litter from poultry houses should be composted for at least 2 months at a site remote from farm buildings before spreading on land used for tillage. Composting should take place on a site where runoff is minimized by location and by the design of the holding facility. Alternatively, the bedding material can be removed at regular intervals and dispersed on arable land. If an infectious disease has occurred on the farm from which the bedding material derived, composting should be carried out for at least 2 months before dispersal of the material on arable land.
SELECTION AND USE OF DISINFECTANTSControl of infectious diseases at farm level requires procedures that are practical, realistic, and cost-effective. Although a number of potentially useful physical methods for disinfection such as dry heat, moist heat, ionization radiation, UV light, and mechanical methods may be used for disinfection in many situations, chemical disinfection procedures usually find wider application than physical methods in clinical facilities and at farm level. By employing specific physical dispersal methods for chemical disinfectants, enhanced distribution and greater reliability in surface decontamination can be achieved.
A number of new chemical formulations, many based on combinations of antimicrobial compounds that have
been in continual use for decades, are gaining acceptance as effective disinfectants for infectious diseases of animals. Combinations of antimicrobial compounds that have received favorable evaluation in published reports include quaternary ammonium compounds (QACs) combined with glutaraldehyde (a proprietary disinfectant), accelerated hydrogen peroxide (containing a reduced concentration of hydrogen peroxide supplemented with acids, surfactants, wetting agents, and chelating agents), and hydrogen peroxide combined with silver nitrate. Characteristics of an ideal chemical disinfectant include
been in continual use for decades, are gaining acceptance as effective disinfectants for infectious diseases of animals. Combinations of antimicrobial compounds that have received favorable evaluation in published reports include quaternary ammonium compounds (QACs) combined with glutaraldehyde (a proprietary disinfectant), accelerated hydrogen peroxide (containing a reduced concentration of hydrogen peroxide supplemented with acids, surfactants, wetting agents, and chelating agents), and hydrogen peroxide combined with silver nitrate. Characteristics of an ideal chemical disinfectant include
Broad antimicrobial spectrum with effective activity against vegetative bacteria (including mycobacteria), bacterial endospores, fungal spores, enveloped and nonenveloped viruses, oocysts of pathogenic protozoa, and prions
Absence of irritancy, toxicity, teratogenicity, mutagenicity, and carcinogenicity for personnel implementing disinfection programs and also for animals occupying buildings where the chemical disinfectant will be used
Stability, with a long shelf life at ambient temperatures
Effectiveness against bacteria in biofilms or pathogenic microorganisms dried on surfaces
Compatibility with a wide range of chemicals including acids, alkalis, and anionic and cationic compounds
Retention of antimicrobial activity in the presence of organic matter
Solubility in water to the concentration required for effective antimicrobial activity
Absence of corrosiveness or chemical interactions with metallic fittings, plastic, rubber, or synthetic structural materials
Absence of tainting or toxicity following application to surfaces or equipment in dairies, meat, plants, or food processing areas
Moderately priced and readily available
Nonpolluting for groundwater or air, nontoxic for aquatic species and biodegradable
It is evident that no currently available antimicrobial compounds possess all of these attributes. With further refinements, chemical disinfectants with enhanced antimicrobial and fewer undesirable characteristics are emerging. Disinfection has become a central part of disease control strategies in the dairy industry, in the poultry industry, and in pig production. Teat dipping is an integral component of mastitis control programs for dairy cattle and routine cleaning, and terminal disinfection has become a standard procedure for the prevention of infectious diseases in pigs and poultry. Regardless of the species of animal involved or the intensity of production, disinfection should play a central role in the prevention and control of infectious diseases in farm animals or captive animals.
The relative susceptibility of pathogenic microorganisms to chemical disinfectants is illustrated in Figure 52.5. Marked differences in susceptibility occur among infectious agents, and consequently, selection of disinfectants for a disinfection program should be based on the particular infectious agents likely to be present and the spectrum of activity of the disinfectant selected. A clinical history of animals on the farm, laboratory reports, and other sources of relevant information can assist in the selection of a suitable chemical disinfectant for a particular farm. Ultimately, the choice of a disinfectant for a particular purpose should take into account its spectrum of activity, efficacy, susceptibility to inactivation by organic or inorganic materials, compatibility with surface materials or other chemicals such as detergents, toxicity for personnel and animals, contact time required, optimal temperatures, residual activity, corrosiveness and effect on the environment, and cost. Infectious agents also exhibit variation in their susceptibility to moist heat (Figure 52.6).
The Influence of Structural and Other Features of Microbial Pathogens on Disinfectant Efficacy
The great structural, biochemical, and metabolic diversity exhibited by bacterial, fungal, protozoal, and viral pathogens partially accounts for the wide variation in their patterns of resistance or susceptibility to chemical disinfectants. The presence of lipopolysaccharide (LPS) in the cell walls of gram-negative bacteria is associated with their intrinsic resistance to particular disinfectants. Mycobacterial resistance to disinfectants is linked to the hydrophobic nature of cell wall mycolic acids, which act as permeability barriers to hydrophilic disinfectants. The six-layered structure of bacterial endospores, the presence of dipicolinic acid in their cores, and their metabolically inactive state contribute to their exceptional resistance to chemical disinfectants and also to adverse environmental conditions. Fungal species etiologically implicated in human and animal infections share many common features in the structure of their cell walls, which are composed primarily of carbohydrates such as α-glucan polymers, chitin, mannans, and proteins. Composition, porosity, and thickness of fungal cell walls are features that may limit uptake of chemical disinfectants. As fungal cells age, their relative porosity decreases, probably due to increased cell wall thickness.
Viruses with lipid envelopes are susceptible to inactivation by lipophilic-type disinfectants, but nonenveloped viruses are resistant to the action of lipophilic disinfectants and demonstrate higher resistance to heat. Unlike conventional infectious agents, prions are resistant to many chemical compounds that readily inactivate pathogenic microorganisms. The basis of this exceptional resistance is not well understood but may
relate to the chemical composition of prions, which are described as being composed of altered or denatured protein.
relate to the chemical composition of prions, which are described as being composed of altered or denatured protein.
The oocysts of a number of protozoal parasites such as Cryptosporidium parvum are particularly resistant to most chemical disinfectants but are more sensitive to the effects of heat. The resistance of these oocysts to chemical inactivation is attributed to the inability of many chemical disinfectants to penetrate the rigid bilayer of acid-fast lipids in the oocyst wall and the inner layer of oocyst wall proteins. Structural and other features of particular infectious agents are presented in Figure 52.7. Distinguishing structural features of four pathogenic fungi are presented in Figure 52.8. Although a substantial amount of biochemical detail relating to fungal cell walls is available, much of this information cannot be readily illustrated in diagrammatic representations of these fungal agents.
 FIGURE 52.5 Microorganisms broadly ranked according to their relative susceptibility to chemical disinfectants. The effectiveness of disinfectants is influenced by many factors, including their composition and concentration, the presence of organic matter or other interfering substances, and the temperature at which they are used. Abbreviation: QACs, quaternary ammonium compounds. |
 FIGURE 52.6 Thermal inactivation of infectious agents by moist heat. The number of infectious agents initially present and sensitivity of the detection system used to determine survival or inactivation may alter the reliability of the results. |
 FIGURE 52.7 Structural, biochemical, and other features of bacteria, bacterial endospores, pathogenic fungi, pathogenic mycobacteria, protozoal oocysts, viruses, and prions, which may influence and, in some instances, determine their resistance to chemical disinfectants. Abbreviation: QACs, quaternary ammonium compounds. |
 FIGURE 52.7 Continued |
 FIGURE 52.7 Continued |
 FIGURE 52.7 Continued |
 FIGURE 52.7 Continued |
CHARACTERISTICS OF CHEMICAL DISINFECTANTS USED IN VETERINARY MEDICINETable 52.3 provides a summary of the different properties and spectra of action of the main disinfectant chemical groups.
Acids
Low concentrations of organic acids such as citric acid and acetic acid are nonirritating; mineral acids such as hydrochloric acid and sulfuric acid are corrosive and hazardous for workers. At pH values below 3, acids exert a bactericidal effect. Mineral acids are used as cleaning agents in food processing and for removing lime scale, milk stone, and other alkaline deposits in pipes, milking machines, and on surfaces. Organic acids are used as preservatives in food and in pharmaceutical products.
Both organic and mineral acids can inactivate the virus of foot-and-mouth disease. Hydrochloric acid at a 2.5% concentration, has been used for inactivation of the endospores of Bacillus anthracis on skins and hides.
Alcohols
Two forms of alcohol, ethyl alcohol and isopropyl alcohol, are widely used as disinfectants. They are relatively nontoxic, nontainting, colorless, and evaporate quickly without leaving a residue. Their solvent activity can damage rubber and some plastic material. Because they are flammable at concentrations recommended for disinfection, they should not be used close to naked flames.
When used at appropriate concentrations on intact skin, alcohols provide a rapid and effective reduction in the microbial skin population. This characteristic accounts for their extensive use in hand rinses and other preparations used for minimizing the transmission of flora acquired from infected patients in clinical environments.
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TABLE 52.3 General characteristics and antimicrobial spectrum of disinfectants commonly used in veterinary medicinea | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Alcohols denature protein and are lipid solvents. The most effective concentration of ethyl alcohol is approximately 70%. In the presence of organic matter, the antimicrobial activity of alcohols can be limited. The antimicrobial spectrum of alcohols includes gram-positive and gram-negative bacteria, acid-fast bacteria, Coxiella burnetii, some fungi, and many enveloped viruses. They are not sporicidal.
Aldehydes
As a group, aldehydes are highly reactive chemicals with a wide antimicrobial spectrum. Three aldehydes are used as disinfectants: formaldehyde, glutaraldehyde, and ortho-phthalaldehyde. A number of chemical disinfectants including aldehydes, ethylene oxide (EO), and β-propiolactone are alkylating agents: they inactivate enzymes and other proteins with labile hydrogen atoms such as sulfhydryl groups. Aldehydes react readily with amino, carboxyl, sulfhydryl, and hydroxyl groups on proteins, causing irreversible changes in protein structure. Some aldehydes react with amino groups on purine and pyrimidine bases in nucleic acids and with peptidoglycan.
Formaldehyde is a monoaldehyde that occurs as a gas, which is freely soluble in water. Formaldehyde solution, formalin, is an aqueous solution containing up to 38% formaldehyde with methyl alcohol added to prevent polymerization. Vapor-phase formaldehyde can be used for fumigation of sealed buildings. The vapor can be produced by evaporation of formalin, by the addition of formalin to potassium permanganate crystals, or by heating paraformaldehyde. The antimicrobial spectrum of formaldehyde is wide. It is effective against vegetative bacteria including mycobacteria, endospores, fungi, and viruses, but it acts more slowly than glutaraldehyde. In addition to its use as a disinfectant, formaldehyde is used in the preparation of veterinary vaccines and also in footbaths to prevent or treat foot lameness in cattle and sheep. Even at low concentrations, the irritating vapor and pungent odor of formaldehyde is evident. The use of formaldehyde as a broad-spectrum antimicrobial agent is declining due to its known toxicity and potential carcinogenicity. Chlorine-releasing agents should not be used in association with formaldehyde because bis(chloromethyl) ether, a potent carcinogen, is produced by the reaction of formaldehyde with hydrochloric acid, sodium hypochlorite, or other chlorine-releasing compounds.
Glutaraldehyde, a dialdehyde, is usually supplied commercially as a 2%, 25%, or 50% acidic solution. Although stable at acid pH, it is more active at values close to pH 8. It has high microbicidal activity against vegetative bacteria, bacterial endospores, fungal spores, and viruses. It is noncorrosive and usually does not damage rubber or plastic components. Glutaraldehyde is reported to have a marked inhibitory effect on bacteria within biofilms by penetrating and inhibiting bacterial activity and by contributing to bacterial detachment from the biofilm. This dialdehyde is widely used for high-level disinfection of thermosensitive medical equipment such as endoscopes and anesthetic equipment at various concentrations and exposure times depending on the formulation. Even at low levels, glutaraldehyde vapor is irritating for the eyes and mucous membranes. Concerns about the risks of exposure to glutaraldehyde have resulted in a marked decline in its use as a broad-spectrum disinfectant.
Ortho-phthalaldehyde, an aromatic aldehyde, has some characteristics in common with glutaraldehyde. It is bactericidal and virucidal, but its sporicidal activity is much less than that of glutaraldehyde. This chemical is stable over a wide pH range, has a mild odor, and is less irritating to mucous membranes than glutaraldehyde. Absence of corrosiveness and compatibility with most rubber and plastic components is another positive feature of this disinfectant.
Alkalis
The antimicrobial activity of alkalis is related to hydroxyl ion concentration. Alkalis are frequently used to increase the pH of many industrial sanitizers and cleaners and are sometimes used as cleaning and disinfecting solutions in their own right. Sodium hydroxide, potassium hydroxide, and sodium carbonate (washing soda) are the alkalis most often employed for cleaning the surfaces of buildings and transport vehicles. Calcium oxide (lime) has been used as a traditional disinfectant in agricultural locations for decades, and calcium hydroxide (slaked lime) is sometimes used for whitewashing building surfaces following an outbreak of an infectious disease. At high concentrations, these chemicals have effective microbicidal properties. Caustic alkaline solutions inactivate many viruses including foot-and-mouth disease virus, adenoviruses, and swine vesicular disease virus. A pH value above 12 may be required for inactivation of pathogens such as Mycobacterium bovis. Although sodium carbonate at a 4% concentration is used primarily as a cleaning agent, it is particularly effective against foot-and-mouth disease virus. At concentrations over 5%, sodium hydroxide has a wide antimicrobial spectrum including bacterial endospores. Prions, which are resistant to most decontaminating procedures, are inactivated by treatments including 2 M sodium hydroxide at 121°C for 30 minutes. Both sodium hydroxide and potassium hydroxide are corrosive for metals and hazardous for workers. All workers using strong alkaline solutions should be informed of their caustic nature and should wear eye protection, rubber gloves, and protective clothing.
Endospores of B anthracis have the ability to persist in the environment for decades, even under adverse environmental conditions. Outbreaks of anthrax are often associated with alkaline soil. Formerly, lime was used as a disinfectant when carcasses of animals, which had died
of anthrax, were being buried. Current scientific observations indicate that lime, rather than acting as a disinfectant for endospores of B anthracis, may actually prolong their survival.8
of anthrax, were being buried. Current scientific observations indicate that lime, rather than acting as a disinfectant for endospores of B anthracis, may actually prolong their survival.8
Ammonium hydroxide, described as a weak base, under some conditions, has been shown to inactivate coccidial oocysts that are resistant to the majority of standard chemical disinfectants. Strong solutions of ammonium hydroxide emit intense pungent fumes.
Biguanides
This group of cationic compounds includes chlorhexidine, alexidine, and some polymeric forms. Biguanides are widely used as aqueous solutions for hand washing and preoperative skin preparation. Because biguanides are cationic, their activity is greatly reduced by certain soaps and other anionic compounds. The most important member of this group, chlorhexidine, is available as dihydrochloride, diacetate, and gluconate. Chlorhexidine gluconate (CHG), which is water-soluble, is the form most commonly used. It binds to the negatively charged bacterial cell wall, altering the bacterial cell osmotic equilibrium.9 At low concentrations, this membrane-active agent inhibits membrane enzymes and promotes leakage of cellular constituents. When the concentration is increased, cytoplasmic constituents are coagulated and a bactericidal effect is observed. The optimal pH range for chlorhexidine is 5.5 to 8. Absence of toxicity is an important feature of biguanides.
Chlorhexidine has a wide antibacterial spectrum, which includes gram-positive and many gram-negative bacteria. It has limited antifungal activity. Some gramnegative bacteria such as Proteus species and Pseudomonas species may be highly resistant to this disinfectant, and, in addition, it is neither mycobactericidal nor sporicidal. Although it may be active against some enveloped viruses, the antiviral activity of chlorhexidine is variable, and it cannot be recommended as an effective antiviral disinfectant. Chlorhexidine-alcohol solutions are particularly effective as topical disinfectants: They combine the antibacterial rapidity of alcohol with the persistence of chlorhexidine at the site of application. Because it has longer residual activity on teat skin than many other disinfectants, chlorhexidine is often used in teat dips for mastitis control programs in dairy herds. The antimicrobial activity of chlorhexidine is reduced by the presence of organic matter. Because biguanides may be toxic for aquatic species, care should be taken when disposing of chlorhexidine solutions.
Gas Phase Sterilizing Agents
A number of chemical disinfectants with broad-spectrum antimicrobial activity are used for sterilizing medical devices that are heat sensitive. Ethylene oxide, formaldehyde and hydrogen peroxide (including some systems that use gas plasma as part of these processes) are among the vapor-phase disinfectant systems used for sterilizing heat-sensitive equipment. Some of these technologies are also used for area fumigation, such as hydrogen peroxide and formaldehyde.
At room temperature, EO is a colorless gas with a faint odor and an irritating effect on eyes and mucous membranes. It is soluble in water and a range of organic solvents. EO is toxic and can be flammable, and when present in the air at a concentration exceeding 3%, it forms an explosive mixture. This safety problem can be reduced by mixing EO with carbon dioxide or other suitable noninflammable gases. EO is generally not corrosive for metals and decomposes spontaneously into methane, ethane, and carbon dioxide. As an alkylating agent, EO reacts with amino, carboxyl, sulfhydryl, and hydroxyl groups, leading to denaturation of microbial proteins including enzymes and also nucleic acids. It is a highly effective antimicrobial agent with bactericidal, fungicidal, virucidal, and sporicidal activity. It does not inactivate prions. A desirable feature of the gas is its ability to penetrate a variety of materials including large packages, bundles of cloth, and certain plastics. Inactivation of microorganisms, however, takes place slowly. The antimicrobial activity of EO is influenced by relative humidity, temperature, gas concentration, contact time, and the presence of water vapor. Organic residues and salt crystals interfere with the activity of EO. Since the 1990s, EO has been listed as a mutagen and a human carcinogen, and consequently, there has been a decline in its use as a vapor-phase disinfectant. An additional concern relates to the potential risks arising from release of this alkylating agent into the environment in the immediate vicinity of establishments employing EO for low-temperature sterilization of equipment. Hydrogen peroxide gas (or vapor) processes are realistic alternatives to EO for sterilization of heat-labile equipment.
Halogens
Chlorine and iodine compounds are widely used in veterinary medicine for their antimicrobial activity. These compounds have wide antimicrobial spectra, are usually inexpensive, have low toxicity, and are convenient disinfectants in health care facilities, for agriculture buildings, food-processing units, and for veterinary facilities.
Chlorine Compounds
The antimicrobial activity of chlorine preparations is determined by the amount of available chlorine in solution. The stability of free available chlorine in solution is strongly influenced by chlorine concentration, pH, the presence of organic matter, and exposure to light. Chlorinereleasing compounds include sodium hypochlorite and N-chloro compounds, also referred to as organic chlorides. Chloramine-T, halazone, sodium dichloroisocyanurate,
and potassium dichloroisocyanurate are examples of organic chlorine compounds. The antimicrobial activity of these compounds is reported to be slower than that of hypochlorites, but some of these organic chloride compounds are less susceptible to inactivation by organic matter than sodium hypochlorite. Chlorine dioxide, which is a gas at room temperature, is being increasingly used for chlorination of potable water, also in swimming pools and for treatment of wastewater. This gas, which is soluble in water, forming a stable solution in the dark, decomposes slowly when exposed to light. At concentrations above 10% in air, chlorine dioxide is unstable and explosive. However, it is an effective antimicrobial compound and nonflammable and not explosive at concentrations used for sterilization. Chlorine dioxide has a broad antimicrobial spectrum. It is bactericidal, fungicidal, virucidal, and sporicidal and reported to be capable of inactivating prions. The sterilizing capability of chlorine dioxide is reported to be equivalent to EO. The antimicrobial activity of chlorine and its compounds is due to the formation of hypochlorous acid, which forms when free chlorine is added to water. Hypochlorites and chloramines undergo hydrolysis when added to water, leading to the formation of hypochlorous acid. This acid releases nascent oxygen, a powerful oxidizing agent. Chlorine compounds combine directly with bacterial cytoplasmic proteins and with viral capsid proteins. Sodium hypochlorite and other chlorine compounds are most effective at pH values below 7, and their antimicrobial activity is inversely proportional to the pH of the environment in which they are used. As diluted chlorine disinfectants lose their potency, fresh solutions should be prepared for use. Hypochlorites are among the most widely used disinfectants. Sodium hypochlorite is fast acting, nonstaining, and inexpensive. Its use in veterinary medicine, however, is limited by its corrosiveness, its inactivation by organic matter, and its relative instability.
and potassium dichloroisocyanurate are examples of organic chlorine compounds. The antimicrobial activity of these compounds is reported to be slower than that of hypochlorites, but some of these organic chloride compounds are less susceptible to inactivation by organic matter than sodium hypochlorite. Chlorine dioxide, which is a gas at room temperature, is being increasingly used for chlorination of potable water, also in swimming pools and for treatment of wastewater. This gas, which is soluble in water, forming a stable solution in the dark, decomposes slowly when exposed to light. At concentrations above 10% in air, chlorine dioxide is unstable and explosive. However, it is an effective antimicrobial compound and nonflammable and not explosive at concentrations used for sterilization. Chlorine dioxide has a broad antimicrobial spectrum. It is bactericidal, fungicidal, virucidal, and sporicidal and reported to be capable of inactivating prions. The sterilizing capability of chlorine dioxide is reported to be equivalent to EO. The antimicrobial activity of chlorine and its compounds is due to the formation of hypochlorous acid, which forms when free chlorine is added to water. Hypochlorites and chloramines undergo hydrolysis when added to water, leading to the formation of hypochlorous acid. This acid releases nascent oxygen, a powerful oxidizing agent. Chlorine compounds combine directly with bacterial cytoplasmic proteins and with viral capsid proteins. Sodium hypochlorite and other chlorine compounds are most effective at pH values below 7, and their antimicrobial activity is inversely proportional to the pH of the environment in which they are used. As diluted chlorine disinfectants lose their potency, fresh solutions should be prepared for use. Hypochlorites are among the most widely used disinfectants. Sodium hypochlorite is fast acting, nonstaining, and inexpensive. Its use in veterinary medicine, however, is limited by its corrosiveness, its inactivation by organic matter, and its relative instability.
Chlorine-releasing compounds are potent virucides. Chlorination, a standard treatment for preventing the spread of waterborne infectious diseases, is generally considered a safe procedure. Household bleach, which usually contains high concentrations of sodium hypochlorite, is used at suitable dilutions in dairies, food-processing plants, and for general disinfection of equipment and farm buildings. Health risks associated with the use of chlorine compounds appear to be limited. Detection of trihalomethanes in chlorinated water has raised concerns about the safety of this form of treatment of public water supplies because trihalomethanes are reported to be carcinogenic in laboratory animals. Currently, few alternative treatment methods for rendering public water supplies safe for human consumption are available. Advantages of chlorine compounds over other disinfectants include low toxicity at effective concentrations, wide antimicrobial spectra, ease of use, and relatively low cost.
Iodine Compounds
The antimicrobial activity of iodine has been recognized since the 1830s. As halogens, iodine and chlorine share some common characteristics: both are effective antimicrobial agents. Although less chemically reactive than chlorine compounds, iodine compounds are effective antimicrobials that are more active in the presence of organic matter than chlorine compounds. The antimicrobial activity of iodine is greater at acid pH than alkaline pH.
Elemental iodine, a bluish-black crystalline substance with a metallic luster, is only slightly soluble in water. Despite its low solubility, iodine was formerly used for its antimicrobial activity as an aqueous solution. Iodine is readily soluble in ethyl alcohol and in aqueous solutions of potassium iodide and sodium iodide. When dissolved in ethyl alcohol (tincture of iodine), high levels of free iodine are obtained. Disadvantages of using iodine solutions include instability, staining of skin and fabrics, toxicity, and skin irritation. Inorganic iodine has been largely replaced by iodophors in which iodine is complexed with surfaceactive compounds or polymers that allow both increased solubility and sustained release of free iodine. In iodophors, the iodine is bound to a carrier of high-molecularweight as micellar aggregates. When complexed, free iodine levels are limited and the disadvantages of using aqueous or alcoholic solutions are avoided. An iodophor in which iodine is complexed with polyvinylpyrrolidone, referred to as povidone-iodine, is a commonly used disinfectant. When an iodophor is diluted with water, dispersion of the micellar aggregates of iodine occurs leading to slow release of iodine. The amount of free iodine in iodophor solutions depends on the concentration used and, paradoxically, more concentrated solutions have less antimicrobial activity than diluted solutions. Within defined limits, as the degree of dilution increases, the bactericidal activity increases. The increased antimicrobial activity results from the higher level of free iodine in dilute solutions. For maximum antimicrobial effect, iodophor solutions should be diluted in accordance with the manufacturers’ instruction. Iodophors retain much of their activity in the presence of organic matter and are effective at both low and high temperatures.
When used at appropriate dilutions and at pH values below 5, iodophors have a broad range of antimicrobial activity. They are bactericidal, fungicidal, and virucidal. It has been suggested that the amino acids tyrosine and histidine in the viral capsid are specific targets of iodine. These disinfectants are reported to react with sulfhydryl groups. Some nonenveloped viruses are less sensitive than enveloped viruses to iodophors. It has been observed that when using iodophors, prolonged contact times are required to inactivate certain fungal species and bacterial endospores. With endospores, the spore coat and cortex are the sites affected. Reports of prolonged survival of Pseudomonas aeruginosa and Burkholderia cepacia in povidone-iodine solution have been attributed to the protection of these
bacteria by the presence of organic matter and inorganic material or by biofilm formation on the items being treated. Addition of alcohol improves the antimicrobial activity of iodophors. In some countries, alcoholic solutions of iodophors are widely used for disinfection of hands and sites prior to surgical procedures. Acidic iodophor solutions are used as sanitizers in the food industry. When employed as disinfectants in dairy plants, the pH of iodophors is kept acidic by the addition of phosphoric acid to ensure the removal of dried milk residues. Effective postmilking teat dipping is an important control measure for contagious mastitis in dairy cattle, and iodophors are among the common teat dips used for this purpose.
bacteria by the presence of organic matter and inorganic material or by biofilm formation on the items being treated. Addition of alcohol improves the antimicrobial activity of iodophors. In some countries, alcoholic solutions of iodophors are widely used for disinfection of hands and sites prior to surgical procedures. Acidic iodophor solutions are used as sanitizers in the food industry. When employed as disinfectants in dairy plants, the pH of iodophors is kept acidic by the addition of phosphoric acid to ensure the removal of dried milk residues. Effective postmilking teat dipping is an important control measure for contagious mastitis in dairy cattle, and iodophors are among the common teat dips used for this purpose.
Heavy Metals and Their Derivatives
The salts of mercury, lead, zinc, silver, and copper have been used for their antimicrobial activity in farming, horticulture, and in human and veterinary medicine for many years. The ability of extremely low levels of certain metals such as copper and silver to exert a marked inhibitory effect on bacteria, algae, and fungi has been observed for centuries. The antimicrobial and antifungal activity of heavy metal derivatives such as copper salts is attributed to their ability to inhibit enzymatic activity in microbial membranes and within the cytoplasm, by binding to sulfhydryl groups. The virucidal action of copper salts is attributed to their binding to thiol groups of protein molecules and also to their strong affinity for DNA.10 Three footbath treatment methods were evaluated for their ability to control digital dermatitis in dairy cows.11 Formalin (5% solution), a commercial footbath product (5% solution), and copper sulfate (10% solution) were used in the study. The results indicated that the commercial product performed better than formalin, and there was no therapeutic difference between copper sulfate and the commercial product. In a Norwegian study of infectious foot diseases of dairy cattle, a number of treatment methods were evaluated.12 Automatic stationary flushing with water alone had no beneficial effect on interdigital dermatitis or heel horn erosion. The claw horn of cows walking through a disinfectant containing 7% copper sulfate became harder, and the claw horn of cows that had their hind feet flushed with water became softer, compared with control animals. In the European Union (EU), the possible environmental impact of copper sulfate has been a cause of concern in recent years. Despite these concerns, few chemical compounds of comparable potency to copper sulfate for use in footbaths for cattle are currently available. To minimize the risks arising from the use of copper sulfate in footbaths, used solutions should be mixed with the contents of slurry tanks and subsequently dispersed widely on arable land. The algicidal activity of copper sulfate has been used to prevent algal growth on open bodies of water and pools. A number of copper compounds are used as preservatives in wood, paper, and paint industries.
Interference with sulfhydryl groups has been proposed as the likely method of interference by silver compounds with bacterial and fungal enzyme systems. Binding to nucleic acids has also been reported. Aqueous solutions of silver nitrate, which are bactericidal, have been used to prevent infection of burn wounds in human patients. The antibacterial effect of silver-releasing surgical dressings has been documented. Sand coated with silver has been used in filters for water purification, and silver-coated charcoal has been used for similar purposes. The virucidal activity of silver compounds against a range of enveloped and nonenveloped viruses has been described. Although the mode of action has not been explained, alteration of capsid proteins was proposed as the likely method of inactivation. Many published reports have confirmed the resistance of the fecal oocysts of C parvum to chemical disinfectants. Treatment of these oocysts with hydrogen peroxide combined with silver nitrate at a concentration of 3% completely eliminated oocyst infectivity for mice after exposure for 30 minutes.13
β-Propiolactone
This alkylating compound has a wide antimicrobial spectrum. It is bactericidal, fungicidal, sporicidal, and virucidal. The antimicrobial activity of β-propiolactone depends on its concentration and on relative humidity and temperature. It can be a highly effective gaseous sporicidal disinfectant. Demonstrable changes induced in viruses by β-propiolactone include structural alterations and modifications of viral capsids.14 This compound has been widely used for the production of inactivated viral vaccines. Health hazards associated with the use of β-propiolactone include skin lesions and eye irritation following direct exposure. The suspected carcinogenic activity of β-propiolactone has limited its use as a disinfectant in veterinary medicine.
Peroxygen Compounds
Broad antimicrobial spectra, safety for personnel engaged in disinfection programs, and absence of residual toxicity are dominant considerations in the selection and use of many chemical compounds for inactivation of infectious agents in human and veterinary medicine. Oxidizing agents, which include hydrogen peroxide, peracetic acid, ozone, and a number of peroxygen-based disinfectants, are antimicrobial compounds, which fulfill many of these requirements. Their wide antimicrobial spectra, low toxicity, and spontaneous breakdown into harmless products are attractive attributes of these strongly oxidizing agents. A number of published reports, however, indicate that some peroxygen compounds including peroxymonosulfates are ineffective as mycobactericidal agents. The characteristics of individual compounds determine their usefulness as disinfectants in veterinary medicine.
Hydrogen peroxide is a clear, colorless liquid, which is commercially available in concentrations ranging from 3% to 90%. It is a nonpolluting compound that rapidly degrades into water and oxygen. Because hydrogen peroxide solutions are unstable, benzoic acid or other suitable substances are added as stabilizers. This oxidizing agent is bactericidal, fungicidal, and virucidal depending on its concentration and how it is delivered (as a liquid, liquid formulation, or gas/vapor). There is a limited number of publications dealing with the virucidal activity of hydrogen peroxide. Under some conditions, the liquid disinfectant inactivates herpes viruses and rhinoviruses but not poliovirus.10 Virucidal and cysticidal activity have been shown with gas-based processes in particular.15 Hydrogen peroxide is an effective sporicidal agent, and its activity is enhanced by increased concentrations and elevated temperatures. In common with some other peroxygen compounds, the activity of hydrogen peroxide can vary against mycobacteria depending on the concentration, formulation, and delivery method. The presence of catalase and peroxidases in some bacteria can increase their tolerance to low levels of hydrogen peroxide. The antimicrobial activity of hydrogen peroxide is attributable to the formation of free hydroxyl radicals, which interact with lipids, proteins, and nucleic acids.16 Hydrogen peroxide vapor has been developed as an effective method for environmental decontamination. Two types are used: a noncondensing hydrogen peroxide vapor system and a condensing method. Both systems have proven effective against bacterial endospores and other infectious agents. The virucidal activity of a condensing hydrogen peroxide vapor system was evaluated against feline calicivirus, human adenovirus type 1, transmissible gastroenteritis, coronavirus of pigs, avian influenza virus, and swine influenza virus.17 The system proved virucidal for all the viruses included in the trial. Of note is that gas peroxide systems have been shown to neutralize the infectivity of prions.18 When an electromagnetic field is applied to a solution of hydrogen peroxide under appropriate conditions, the disinfectant becomes a gas plasma, a highly efficient disinfectant system with sporicidal activity. The term accelerated hydrogen peroxide is used to describe proprietary disinfectants containing reduced concentrations of hydrogen peroxide supplemented with compounds used in industrial cleaners such as surfactants, wetting agents, acids, and chelating agents. Disinfectants containing these compounds are reported to have a wide antimicrobial spectrum with short contact times. Undesirable features of these disinfectants include their reactions with brass, copper and aluminium, and some surgical instruments.
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