Many hospital infections are preventable

In 1850, Semmelweiss demonstrated that many hospital infections are preventable when he made the unpopular suggestion that puerperal fever (an infection in women who have just given birth, see Ch. 23) was carried on the hands of physicians who came directly from attending an autopsy to the delivery ward, without washing. A death rate of 8.3% was reduced to 2.3% by introducing the simple measure of handwashing before and after any clinical examination. Recent studies in the USA suggest that about one-third of all infections acquired in hospital can be prevented. Current US estimates place HAI costs associated with hospital infection at approximately 2 million infections leading to nearly 100 000 deaths at a cost of US$4–5 billion annually.

Urinary tract infections are the most common hospital infections in adults

The infections most commonly acquired in hospitals are:

The relative frequencies of these infections are illustrated in Figure 36.2. Each may be acquired from an exogenous or endogenous source, and even the ‘self-source’ may be derived from outside by the patient who becomes colonized with pathogens during his or her stay in hospital. Bacteraemia may arise from a variety of sources and may be:

Figure 36.2 The relative frequencies of different kinds of hospital infection vary in different patient groups, but urinary tract infections are the most common hospital-acquired infections.

Staphylococci and Escherichia coli are the most important Gram-positive and Gram-negative causes of infection, respectively, in hospitals

Almost any microbe can cause a hospital infection, though protozoal infections are rare. The pattern of hospital infection has changed over the years, reflecting advances in medicine and the development of antimicrobial agents. In the pre-antibiotic era, the majority of infections were caused by Gram-positive organisms, particularly Streptococcus pyogenes and Staphylococcus aureus. With the advent of penicillin and other antibiotics active against staphylococci, Gram-negative organisms such as Escherichia coli and Pseudomonas aeruginosa emerged as important pathogens. More recently, the development of more potent and broad-spectrum antimicrobials and the increase in invasive medical techniques has been accompanied by an increase in the incidence of:

Many of these organisms are considered as ‘opportunists’ – microbes that are unable to cause disease in healthy people with intact defence mechanisms, but that can cause infection in compromised patients or when introduced during the course of invasive procedures. Currently, coagulase-negative staphylococci, Staphylococcus aureus, and enterococci account overall for most healthcare-associated infections (Box 36.1).

Some infections historically associated with hospitals are now increasingly seen outside of the healthcare setting

Recent reports in numerous countries have documented the emergence of virulent MRSA strains causing infection in individuals outside of the healthcare system. These community-associated MRSA (CA-MRSA) can be transported into the healthcare environment, thus blurring the distinction between community-associated and healthcare-associated infection. This has prompted guidelines for differentiating the increasing number of CA-MRSA infections from those associated with healthcare, summarized in Box 36.2.

Viral infections probably account for more hospital infections than previously realized

These affect both patients and healthcare workers and include:

The risks of viral infections in hospital are summarized in Table 36.1.

Table 36.1 Examples of viruses causing hospital-acquired infection

Sources of hospital infection are people and contaminated objects

As stated above, the source of infection may be:

The source may become contaminated from an environmental reservoir of organisms, for example, contaminated antiseptic solution distributed for use into sterile containers (Fig. 36.3). Eradication of the source will also require eradication of the reservoir.

Figure 36.3 Hospital infections are spread by the same routes as infections spread in the community. The reservoir and the source of infection may be human or inanimate and may be one and the same (e.g. a nurse with an infected skin lesion). If the reservoir and source are distinct (e.g. contaminated distilled water supply used to prepare a variety of pharmaceuticals), both must be eliminated if the spread of infection is to be halted, otherwise the reservoir may continue to contaminate new sources. HBV, hepatitis B virus; IV, intravenous; RSV, respiratory syncytial virus.

Human sources may be:

The time period for which a human source is infectious varies with the disease (see Ch. 31). For example, some infections can be spread during their incubation period, others in the early stages of clinical disease, while others are characterized by a prolonged carrier state even after clinical cure (e.g. typhoid fever) (Fig. 36.4). Carriers of virulent strains of, e.g. Staph. aureus or Strep. pyogenes, may act as sources of hospital infection, although they themselves do not develop clinical disease. The carrier state may persist for a long time and go unnoticed unless there is an outbreak or, depending on the significance of the organism, a single case of infection that is traced to the carrier, e.g. a healthcare worker with chronic hepatitis B.

Figure 36.4 Pathogens differ in the time periods for which they can be disseminated from an infected person. For some, it is during the incubation period when infected people may not realize they are ill and infectious. Some people continue to carry organisms such as Salmonella typhi and hepatitis B virus long after they have recovered from the clinical disease. Opportunist pathogens are often members of the normal flora and may therefore be carried for long periods without the host experiencing any adverse effects.

Hospital infections are spread in the air and by contact and common vehicle

The important routes of spread of infection in hospitals are those common to all infections: airborne, contact and common vehicle. Examples of organisms spread by these routes in hospitals are illustrated in Figure 36.3. Although theoretically possible, vector-borne spread is very unusual in the hospital setting, as is sexually transmitted infection. It is important to remember that the same organism may be spread by more than one route. For example, Strep. pyogenes can be spread from patient to patient by the airborne route in droplets or dust, but is also transmitted by contact with infected lesions, for example on a nurse’s hand. In addition, a patient or healthcare worker with shingles can transmit VZV to a susceptible person having direct contact with rash blisters.

Underlying disease, certain treatments and invasive procedures reduce host defences

Host factors play a fundamental role in the infection equation, particularly in hospitals because of the high proportion of hospital patients with compromised natural defences against infection. The spread of an infectious agent to a new host can result in a spectrum of responses: from colonization, through subclinical infection, to clinically apparent disease, which may be fatal. The degree of host response differs in different people depending upon their degree of compromise. The very young are particularly susceptible because of the immaturity of their immune system. Likewise, the elderly suffer a greater risk of infection because of predisposing underlying disease, impaired blood supply and immobility, which contribute to stasis and therefore to infection in, for example, the lungs. In all age groups, underlying disease and the treatment of that disease (e.g. cytotoxic drugs, steroids) may predispose to infection (Fig. 36.5), while invasive procedures allow organisms easier access to previously protected tissues (Fig. 36.6). The important host factors to be considered in hospital infection are summarized in Table 36.2. Infections in the compromised host are discussed in more detail in Chapter 30.

Figure 36.5 Varicella in a patient with chronic myeloid leukaemia resulting in purpuric confluent lesions on the trunk.

(Courtesy of G.D.W. McKendrick.)

Figure 36.6 Child with infected Spitz–Holter valve used to relieve hydrocephalus.

(Courtesy of J.A. Innes.)

Table 36.2 Factors which predispose patients to hospital infection

Age	Patients at extremes of age are particularly susceptible
Specific immunity	Patient may lack protective antibodies to, e.g. measles, chickenpox, whooping cough
Underlying disease	Other (non-infectious) diseases tend to lead to enhanced susceptibility to infection, e.g. hepatic disease, diabetes, cancer, skin disorders, renal failure, neutropenia (either as a result of disease or of treatment)
Other infections	HIV and other immunosuppressing virus infections; patients with influenza prone to secondary bacterial pneumonia; herpes virus lesions may become secondarily infected with staphylococci
Specific medicaments	Cytotoxic drugs (including post-transplant immunosuppression) and steroids both lower host defences; antibiotics disturb normal flora and predispose to invasion by resistant hospital pathogens
Trauma     Accidental     Intentional	Burns, stab or gunshot wounds, road traffic accidents Surgery, intravenous and urinary catheters, peritoneal dialysis                 Disturb natural/host defence mechanisms

Hospital patients are not all at equal risk of infection. Some factors that predispose to infection can be influenced by, e.g. treating underlying disease, improving specific immunity and avoiding inappropriate use of antibiotics. Other factors such as age are unalterable.

A variety of factors predispose to wound infection

Wound infection or wound sepsis is characterized by the presence of inflammation, pus and discharge in addition to the isolation of organisms such as Staph. aureus. Extensive studies of postoperative wound infection have identified a number of predisposing factors:

Table 36.3 Risk factors for postoperative infections

Figure 36.7 Postoperative gangrenous cellulitis. There is a huge area of ulceration filled with gangrenous skin, with sloughing adjacent to the wound and surrounding cellulitis.

(Courtesy of M.J. Wood.)

Hospital infections affect both the patient and the community

Hospital infection may result in:

There are three main strategies for preventing hospital infection

For the reasons outlined above, the prevention of hospital infection deserves a very high priority, and the three main strategies are:

Exclusion of inanimate sources of infection is achievable, but it can be difficult to avoid contamination by humans

Exclusion of inanimate sources of infection is both desirable and, to a large extent, achievable. For example, the provision of sterile instruments and dressings, sterile medicaments and intravenous fluids, clean linen and uncontaminated food, and the use of blood and blood products screened for infectious agents. However, many of the sources of infection are human or are objects that become contaminated by humans, in which case exclusion is more difficult. Hospitals must attempt to prevent patient contact with staff who are carriers of pathogens. The problem is the identification of staff who are carriers of pathogens and their relocation to less hazardous positions.

Staff must undergo health screening before employment and should have regular health checks (Box 36.3). For example, in the UK all new healthcare workers (HCWs) are offered testing for HIV and hepatitis C. Hepatitis B immunization is offered and HCWs must know their post-immunization status (surface antigen negative or, if positive, e-antigen negative with a viral load of 103 genome equivalents/mL or less) before carrying out exposure-prone procedures (EPPs). It is critical that those carrying out EPPs who either do not know their post-immunization status or have not responded to the hepatitis B vaccine are checked to ensure that they do not have a current HBV infection or have a protective level of hepatitis B surface antibody. This is because HBV could be transmitted to the patients if the HCW carrying out EPPs is a hepatitis B carrier and also because the unprotected HCW is at risk of infection from a hepatitis B carrier patient. New HCWs carrying out EPPs must also be non-infectious for HIV (antibody negative) and hepatitis C (antibody negative or, if positive, negative for hepatitis C RNA).

Hospitals have blood-borne virus exposure policies for the management of healthcare workers and others who may have been exposed to viruses, including HBV, HCV, and HIV, having sustained a needlestick injury or mucous membrane splash from a potentially infected source. Prophylaxis includes active and/or passive immunization against hepatitis B and a short course of antiretroviral therapy for HIV exposure (see later). The risk of transmission is highest, ca. 33%, for HBV in unimmunized recipients, ca. 0.32% for HIV after a single needle stick injury, and HCV is thought to be between 1% and 3%. However, as reporting of exposure incidents and follow-up of the recipient improves, the better our understanding of the outcomes of the incident itself. Fortunately, most HCWs reported have recovered spontaneously or after having had ribavirin and pegylated interferon treatment.

In general, staff should be encouraged to report any incidences of infection (e.g. an infected cut or a bout of diarrhea). Appropriate immunizations should be offered and in some instances made mandatory. Work restrictions for personnel with selected infectious diseases are summarized in Box 36.3. However, healthy carriers of, for example, virulent staphylococci are difficult to identify unless bacteriologic screening is undertaken, which is not feasible on a routine basis. In addition, staff are sources of opportunist organisms such as coagulase-negative staphylococci or enterobacteria, which are part of their normal flora and cannot be excluded.

Ventilation systems and air flow can play an important role in the dissemination of organisms by the airborne route

Wards comprising separate rooms have been shown to afford some protection against airborne spread, and rooms with controlled ventilation are even better. However, neither prevents the carriage of organisms into the room on staff and their clothing, and some studies suggest that this is a more important route of infection than airborne spread. However, Legionella infection is acquired by the airborne route, and air-conditioning systems throughout the hospital should be maintained so as to prevent the multiplication of these organisms (see Ch. 19). Aspergillus infection in hospitals has been attributed to dissemination of the spores in hospital air, especially when building work is ongoing in the locality.

Ventilation systems in operating theatres must be properly installed and maintained to prevent the ingress of contaminated air and to minimize air currents carrying organisms from the staff in the operating room to the operation site. ‘Ultra-clean’ air is air passed through high-efficiency filters to remove bacteria and other particles and has been shown to contribute positively to a reduction in the number of postoperative wound infections developing after long orthopaedic operations.

Airborne transmission of infection can be reduced significantly by isolating patients

Patient isolation may be carried out:

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