Sources of Infectious Agents

Humans (patients, personnel, or visitors) are the primary source for infectious agents in the health care setting, but inanimate objects (e.g., contaminated medical equipment, linen, medications) have also been implicated in transmission. A person may have an acute infection with symptoms, may be in the asymptomatic or incubation period of the disease, or may be colonized by pathogens without symptoms. People may also serve as their own source of infection, via endogenous flora. This latter process is called autogenous infection.

Susceptible Hosts

Susceptibility and resistance to infection vary greatly. Some individuals may be immune to infection or able to resist colonization. Others exposed to the same organism may carry it but show no symptoms. Other individuals may develop clinical disease. Host factors, such as poorly controlled diabetes mellitus, extremes of age, and underlying acquired (HIV infection) or iatrogenic (through chemotherapy or anti–tumor necrosis factor inhibitors) immunodeficiency, can enhance susceptibility to infection. Surgical incisions and radiation therapy impair defenses of the skin and organ space. Medical devices, such as urinary tract catheters, central venous catheters, and endotracheal tubes, allow pathogens to increase the risk of infection by impeding local host defenses and providing biofilms that may facilitate adherence of pathogens.

Hospital-acquired or nosocomial infections are infections that are acquired in the hospital. The high incidence of nosocomial gram-negative bacterial pneumonia is associated with factors that promote colonization of the pharynx with these organisms. Gram-negative colonization dramatically increases in critically ill patients, which increases the likelihood of the development of these pneumonias.⁴ Most nosocomial pneumonias occur in surgical patients, especially patients who have had chest or abdominal procedures. In these patients, normal swallowing and clearance mechanisms are impaired, allowing bacteria to enter and remain in the lower respiratory tract. Intubations, anesthesia, surgical pain, and use of narcotics and sedatives impair host defenses further. The risk of pneumonia is not the same for all surgical patients. Patients at the highest risk include elderly patients, severely obese patients, patients with chronic obstructive pulmonary disease (COPD) or a history of smoking, and patients with an artificial airway in place for long periods.⁵

Patients with an artificial tracheal airway are at high risk for nosocomial pneumonia for several reasons. Typically, patients requiring prolonged intubation already have one or more factors predisposing to infection, such as severe COPD. Another risk factor may be increased upper airway colonization with gram-negative bacteria. Because the tube bypasses the normal protective mechanisms of the upper airway, bacteria have easy access to the lower respiratory tract. Finally, handling of these tubes increases the likelihood of cross contamination, particularly during suctioning.

Some pneumonias occur primarily in immunocompromised hosts. Physicians may purposefully suppress a patient’s immune response with drugs, as in organ transplant cases. Alternatively, immunosuppression may be a result of underlying disease, as with AIDS. Immunocompromised hosts, regardless of cause, are highly susceptible to infections, especially infections caused by opportunistic bacteria, fungi, or viruses.

Modes of Transmission

The three major routes for transmission of human sources of pathogens in the health care environment are contact (direct and indirect), respiratory droplets, and airborne droplet nuclei (respirable particles <5 µm). Table 4-1 provides examples of the common transmission routes for selected microorganisms.⁶

Decreasing Host Susceptibility

Decreasing inherent host susceptibility to infection is the most difficult and least feasible approach to infection control. Hospital efforts at this level focus mainly on employee immunization and chemoprophylaxis. Certain immunizations are recommended for susceptible health care personnel to decrease the risk of infection and the potential for transmission to patients and coworkers within the health care facility. The Occupational Safety and Health Administration (OSHA) mandates that employers offer hepatitis B vaccination. Vaccinations of health care workers in the absence of evidence of immunity against varicella, rubella, and measles should be encouraged.⁸ In addition, health care personnel in facilities that care for young infants and children should receive the adult acellular pertussis vaccine. Health care personnel without medical contraindications should also receive an annual influenza vaccination.⁹

Antimicrobial agents and topical antiseptics may be used to prevent outbreaks of selected pathogens. Postexposure chemoprophylaxis is recommended under defined circumstances for Bordetella pertussis (whooping cough), N. meningitidis (meningococcal meningitis), Bacillus anthracis (anthrax), influenza virus, HIV, and group A streptococci.⁶

A large percentage of HAIs are due to device-related infections, including ventilator-associated pneumonia (VAP), catheter-related bloodstream infection, and catheter-associated urinary tract infection. The best way to decrease host susceptibility to a device-related infection is first to limit device use and second to ensure that devices are placed and maintained appropriately. Prevention bundles—defined as the use of multiple different evidence-based best practices to prevent device-related infection—have been shown to decrease the incidence of HAIs significantly.10,11 Boxes 4-1 and 4-2 list the components of the central line bundle for vascular catheter placement and the VAP bundle. Institutions should be committed to these processes of care, and individual health care workers should be familiar with these practices and execute them on a routine basis.^12,13

Eliminating the Source of Pathogens

It is impossible to eliminate all pathogens from any working environment. Nonetheless, standard infection control procedures always include efforts to eliminate pathogens, and recommended practices for cleaning and disinfecting noncritical surfaces in patient care areas should be followed. Infection control procedures designed to remove environmental pathogens fall into two major categories: general sanitation measures and specialized equipment processing.

If the environment is dirty, all other infection control efforts are futile. General sanitation measures help to keep the overall environment clean. General sanitation aims to reduce the number of pathogens to a safe level. This reduction is achieved through sanitary laundry management, food preparation, and housekeeping. Environmental control of the air (using specialized ventilation systems) and water complements these efforts.

The goal of specialized equipment processing is to decontaminate equipment capable of spreading infection. Equipment processing involves cleaning, disinfection, and sterilization (when necessary). Methods that kill bacteria are bactericidal, whereas methods and techniques that inhibit the growth of bacteria are bacteriostatic. Methods that destroy spores are sporicidal, and methods that destroy viruses are virucidal.

Interrupting Transmission

General sanitation measures and equipment processing have limits. To prevent the spread of infections between patients and to keep themselves healthy, health care personnel also must take measures to stop infection. There are two tiers of HICPAC and Centers for Disease Control and Prevention (CDC) transmission precautions: standard precautions and transmission-based precautions.⁶

Standard Precautions

Standard precautions are intended to be applied to the care of all patients in all health care settings all the time. This is the primary strategy for the prevention of health care–associated transmission of infections among patients and health care personnel. From a health care worker protection perspective, application of standard precautions on a routine basis is recognition that all blood, body fluids, secretions, and excretions with the exception of sweat and urine may contain transmissible infectious agents. To mitigate against this risk, a health care worker should employ personal protective equipment (PPE). PPE refers to various barriers and respirators used alone or in combination to protect mucous membranes, skin, and clothing from contact with infectious agents. Gloves, gowns, masks, eye protection, and face shields should be employed depending on the anticipated exposure.

The application of standard precautions by health care personnel during patient care depends on the nature of the interaction and the extent of anticipated blood, body fluid, or pathogen contact. For some patient care situations, only gloves are required. In other cases, gloves, gowns, and face shield may be required. Box 4-3 describes standard precautions, including hand hygiene; use of gloves, masks, and eye protection; equipment handling; and patient placement.

Hand Hygiene

The importance of hand hygiene to reduce the transmission of infectious agents cannot be overemphasized and is an essential element of standard precautions.¹⁴ Hand hygiene includes handwashing with either plain or antiseptic-containing soap and water for at least 15 seconds and the use of alcohol-based products (gels, rinses, and foams) containing an emollient that does not require the use of water. In the absence of visible soiling of hands, approved alcohol-based products are preferred over antimicrobial or plain soap and water because of their superior microbicidal activity, reduced drying of skin, and convenience. The quality of performing hand hygiene can be affected by the type and length of fingernails and by wearing jewelry. Artificial fingernails and extenders are discouraged because of their association with infections.¹⁴ Figure 4-2 illustrates the proper technique for handwashing.

Gloves

Gloves protect both patients and health care workers from exposure to pathogens that may be carried on the hands of health care workers. Gloves protect caregivers from contamination when contacting blood, body fluids, secretions, excretions, mucous membranes, and nonintact skin of patients and when handling or touching visibly or potentially contaminated patient care equipment and environmental surfaces.¹⁴

Caregivers should wear sterile gloves whenever performing invasive procedures. A single pair of nonsterile disposable gloves (e.g., latex, vinyl, nitrile) may be used for routine patient care. Gloves should be changed, regardless of use, between each patient contact and after any direct contact with infectious material, even if in the middle of a procedure. After removing the gloves, caregivers must always wash their hands. Gloves may have small, invisible defects or may be torn during use. The hands can be contaminated during removal of the gloves. For these reasons, the wearing of gloves should never be used as a substitute for handwashing.

Mouth, Nose, Eye, and Face Protection

Face protection is an important component of standard precautions because the mucous membranes of the eyes, nose, and mouth are particularly vulnerable to some types of pathogens. Masks protect mucosal surfaces against splashes or sprays but should not be confused with particulate respirators that are recommended for protection from small particles (as described subsequently for airborne isolation [AI]). The wearing of masks, eye protection, and face shields in specified circumstances when exposures are likely to occur (e.g., bronchoscopy suite) is mandated by the OSHA Blood-borne Pathogen Standard.

Respiratory Protection

Respiratory protection (use of NIOSH-approved N-95 or higher level respirator) is intended for diseases (e.g., M. tuberculosis, SARS, smallpox) that could be transmitted through the airborne route.⁶ The term respiratory protection has a regulatory context that includes components of a program required by OSHA to protect workers: (1) medical clearance to wear a respirator, (2) provision and use of appropriate NIOSH-approved fit-tested respirators, and (3) education in respirator use. Information on types of respirators can be found at www.cdc.gov/niosh/npptl/respirators/respsars.html.

Gowns, Aprons, and Protective Apparel

Isolation gowns and other apparel (aprons, leg coverings, boots, or shoe covers) also provide barrier protection and can prevent the contamination of clothing and exposed body areas from blood and body fluid contact and transmissible pathogens (e.g., respiratory syncytial virus and Clostridium difficile). Selection of protective apparel is dictated by the nature of the interaction of the health care worker with the patient, including anticipated degree of body contact with infectious material.⁶ In most instances, gowns are worn only if contact with blood and body fluid is likely. Clinical coats and jackets worn over clothing are not considered protective apparel. Isolation gowns should always be donned with gloves and other protective equipment as indicated. As with gloves and masks, a gown should be worn only once and then discarded. In most situations, aseptically clean, freshly laundered, or disposable gowns are satisfactory.

The emergence of SARS and the ongoing concerns for pandemic infection have led to a strategy of preventing transmission of respiratory infections at the first point of contact within a health care setting (e.g., physician’s office) termed respiratory hygiene/cough etiquette that is intended to be incorporated into infection control practices as one component of standard precautions.⁶