79       Disinfection

 

Infectious agents shed in the excretions or secretions of animals, or present in products of animal origin, may remain viable for long periods in the environment. Buildings, equipment, transport vehicles, soil, pasture, water and fomites may become contaminated by faeces or urine containing bacterial, viral or protozoal pathogens. Fungal pathogens such as dermatophytes may contaminate tables, building surfaces and grooming equipment. Respiratory secretions of sick animals may contain viral or bacterial pathogens and, following abortion caused by Brucella abortus, high numbers of brucellae may be present in foetal

fluids. Disinfection is an essential part of disease control programmes for both endemic and exotic diseases. It is also used to lessen the risk of disease transmission from animals to humans not only during the production stage but also at the processing stages in meat plants and dairies.

Disinfection implies the use of physical and chemical methods for the destruction of microorganisms, especially potential pathogens, on surfaces of inanimate objects or in the environment. There is wide variation in the susceptibility of infectious agents to thermal inactivation. Although both moist heat and dry heat can be used for the inactivation of microorganisms, moist heat is more effective and requires less time to achieve inactivation than dry heat. At temperatures above 80°C most vegetative bacteria are killed within seconds. Bacterial endospores are exceptionally thermoresistant and moist heat at 121°C for at least 15 minutes is required for their inactivation. Many enveloped viruses are labile at temperatures above 70°C. Non-enveloped viruses such as foot-and-mouth disease virus are thermostable and temperatures close to 100°C for more than 20 minutes may be required to inactivate such resistant viruses. The prions which cause transmissible spongiform encephalopathies are extremely resistant to thermal inactivation. Dry heat at 160°C does not inactivate these agents and traces of infectivity were detected following treatment in a muffle furnace at 600°C for 15 minutes. Autoclaving at 132°C for at least 4.5 hours is required for their inactivation.

Infectious agents vary widely in their susceptibility to chemical disinfectants. Most vegetative bacteria and enveloped viruses are readily inactivated by disinfectants. However, biofilm formation on equipment surfaces, especially in inaccessible locations, confers protection on microorganisms against chemical inactivation. Fungal spores and non-enveloped viruses are moderately resistant to chemical inactivation. Mycobacteria and bacterial endospores are resistant to many commonly used disinfectants. Prions are extremely resistant to chemical inactivation. High concentrations of sodium hypochlorite or autoclaving at 121°C for 30 minutes in 2 mol/L sodium hydroxide inactivated the prion agents tested.

Chemical compounds with antibacterial activity may react with the cell wall, cell membrane, nucleic acid or other cytoplasmic constituents. The targets of chemicals with sporicidal activity include the outer coat, the inner coat, the spore cortex and the small acid-soluble DNA-binding proteins in the endospore core. Virucidal disinfectants may react with nucleic acids, structural or functional proteins, glycoproteins and, in the case of enveloped viruses, with the lipid envelope.

In addition to the intrinsic resistance of bacterial endospores, mycobacteria and some Gram-negative bacteria to particular chemical compounds, the ability of a number of pathogenic bacteria to acquire resistance to chemical disinfectants has been observed in recent years. In a number of instances, there is evidence that resistance to disinfectants and antibiotics is genetically linked.

For the success of a disinfection programme, thorough cleaning should precede the application of disinfectant. The disinfectant selected should be active against the infectious agents present and it should be diluted to yield the correct concentration. Most disinfectants require several hours to inactivate infectious agents on surfaces. The presence of organic matter such as faeces, exudates, body fluids, bedding or food residues interferes with the antimicrobial activity of disinfectants and slows their action. Failure to inactivate infectious agents present in buildings, on equipment or in transport vehicles may be due to the selection of an inappropriate disinfectant, careless use of a potentially effective disinfectant or environmental factors. As no residual antimicrobial activity persists after disinfection, infectious agents may be reintroduced by infected domestic animals, fomites, food, on the footwear or clothing of personnel or by rodents.