Essential Microbiology for Dentistry. 5th ed.

Chapter 36. Principles of infection control

Co-editor; Dr Caroline Pankhurst formerly Senior Lecturer in Oral Microbiology, Kings College London, UK

Cross infection

Cross infection may be defined as the transmission of infectious agents between patients and patients, and patients and staff, within a clinical environment. Transmission may result from person-to-person contact or via contaminated objects (fomites), through the airborne route via splatter, aerosols or via a common source such as the dental unit waterlines (Fig. 36.1). Organisms capable of causing cross infection in humans are derived from:

 other human sources (the most important)

 animal sources (less important)

 inanimate sources (of least importance).

Principles of infection transmission

Transmission of infection from one person to another requires:

1. a source of infection: the person with the infection is called the index case

2. a mode or vehicle by which the infective agent is transmitted, for example, blood, droplets of saliva, instruments contaminated with blood, saliva and tissue debris. (animals, birds or insects may act as vehicles or vectors of transmission, e.g., in malaria, dengue, Zika but are not described here)

3. a route of transmission, for example, inhalation, ingestion.

It can be helpful to think of these steps in the transmission of infection as a 'chain of infection' that can lead to disease unless it is interrupted by infection prevention and control measures. Links in the chain can be broken, halting further transmission and preventing disease. Vaccination and drug therapy are used to protect health care workers in the practice from the infected 'source patient'; drug therapy with antimicrobials is used to treat the disease, if acquired. Infection control and prevention

measures such as personal protective equipment (PPE) can block modes and vehicles of transmission and prevent access of microbes to the mucosa of the respiratory tract or mouth or gut by inhalation or ingestion.

Source of infection

The sources of infection in clinical dentistry are mainly human; they include:

1. People with overt infections who liberate large numbers of organisms into the environment (e.g., droplets and discharges from the mouth or other portals; wounds, ulcers and sores on the skin). Fortunately, in routine clinical dentistry, few patients with acute life-threatening diseases are seen. Common infections circulating in the community can be transmitted in the dental environment from patients or staff with overt infections, such as herpes viruses, influenza viruses, Staphylococcus aureus.

2. People in the prodromal stage of certain infections. During the prodrome or the incubating period, the organisms multiply without evidence of infection; although patients are healthy at this stage, they are highly infectious. Viral infections, such as measles, mumps and chickenpox (varicella), are easily spread in this manner. Person incubating mumps or measles are infectious for 5-6 days before symptoms appear and they become aware of the illness. Hence, for instance, the requirement for occupational vaccination of dental health care workers and students with the mumps, measles and rubella (MMR) vaccine and varicella vaccine.

3. People who are healthy carriers of pathogens and can be classified as:

 convalescent carriers

 asymptomatic carriers.

Convalescent carriers are those who suffer an illness and apparently recover, although blood and secretions of the individual act as persistent reservoirs of infective organisms. For example, following diphtheria or streptococcal sore throat, the organisms may persist in the throat for some time and infect others or, in the case of hepatitis B, patients may recover fully, although they may carry the infectious agent in the blood for a considerable period. The latter are called chronic carriers.

Fig. 36.1 Routes and modes by which infection may spread in the dental clinic.

Asymptomatic carriers give no history of infection as they may have unknowingly had a non-apparent or subclinical infection (recognized merely because of the presence of specific antibodies in the person's blood). Nevertheless, these individuals may carry infective microbes in the saliva, blood and other body secretions.

Hepatitis B and hepatitis C are classic examples of diseases that may manifest with or without symptoms, and thus, the clinician may be faced with either a convalescent or an asymptomatic carrier of hepatitis B or C virus. Note: a convalescent carrier can be identified from the history of infection, as opposed to an asymptomatic carrier who cannot be diagnosed in this way. In the case of hepatitis C, a carrier may not be identified for 20-30 years when they present to their doctor with liver disease or liver cancer, but during that period they may have unwittingly transmitted their infection to many other people.

Standard infection control precautions

From the foregoing, it is clear that it is impossible to ascertain whether the patient who attends for dental treatment is a carrier of infectious agents. Therefore, all patients should be treated as if they were reservoirs of pathogens. The infection control procedures involved in such treatment are termed standard infection control precautions (SICPs; previously termed 'universal precautions'), and all clinical procedures performed on any patient should be conducted using SICPs. The corollary of this is that no additional infection control precautions should be necessary when a patient who is a carrier of infection such as hepatitis C or human immunodeficiency virus (HIV) attends the clinic. The importance of this concept cannot be overemphasized and should be noted by all who practise dentistry.

Evolution of universal precautions, standard infection control precautions and additional precautions (or transmission-based precautions)

The first set of recommendations on infection control in dentistry, issued in the late 1980s, focused primarily on the transmission of bloodborne pathogen transmission in dental care and other clinical settings and was termed universal precautions. These recommendations emphasized the need to treat blood and other bodily fluids contaminated with blood from all patients as potentially infectious.

However, the realization that moist body substances, that is, secretions and excretions such as semen, saliva, tears, breast milk are equally important in disease transmission led to the development of SICPs in the mid-1990s. Thus SICPs are similar to universal precautions as they are designed to reduce the risk of infection transmission from both recognized and unrecognized sources of infection to patients and clinicians. SICPs apply to contact with:


 all body fluids, secretions and excretions except sweat, regardless of whether they contain blood

 non-intact skin

 mucous membranes.

For the overwhelming majority of infectious diseases, including those possibly encountered routinely in dental settings, the application of SICPs will arrest disease transmission.

However, in special situations where a known infection with a high transmission potential is suspected or encountered, additional precautions or transmission-based precautions have to be implemented. These include situations dealing with patients either having or suspected to be infected with virulent pathogens that are transmitted through:

 air or droplets (e.g., tuberculosis, influenza, chickenpox, mumps)

 indirect or direct contact with contaminated sources (e.g., methicillin-resistant S. aureus (MRSA) or multidrug-resistant Mycobacterium tuberculosis).

These so-called transmission-based precautions include patient isolation, adequate room ventilation, respiratory protection of workers and postponement of non-emergency dental care procedures. It should however be realized that in routine dentistry, application of standard precautions would be the norm. However, additional precautions have to be implemented in special situations, such as in hospital settings where such patients are treated or during epidemics, such as during outbreaks of Middle Eastern respiratory syndrome (MERS), pandemic influenza, Zika, and Ebola, or when confronted with patients harbouring multidrug-resistant organisms.

A note on the management of potential carriers of transmissible spongiform encephalopathy or prion diseases

The regulations in the USA state that standard infection control measures have to be modified when treating such cases, as prions cannot be destroyed using the routine sterilization protocol. Hence when transmissible spongiform encephalopathy (TSE) patients, for example, patients with variant Creutzfeldt-Jakob disease (vCJD), are treated, special sterilization procedures are required, or alternatively all instruments need to be disposable (see Chapter 4). However, according to the British guidelines, special precautions for patients with TSE are not required for routine dentistry but strict adherence to national standards of decontamination and SICPs are adequate. However, national guidelines specify special caveats for instruments that are in contact with cranial nerves or lymphoid tissue such as the tonsil that is in a higher risk category for transmission of prions, for example, vCJD than the oral tissues. In this context, note that endodontic instruments are deemed single-use only as the dental pulp is supplied by branches of the trigeminal nerve.

Mode of transmission

Transmission of infection may occur by:

 direct contact of tissues with secretions or blood; this is the least common mode (e.g., an ungloved practitioner with a cut on the finger performing an extraction) or during an exposure-prone procedure where the gloved hand of the clinician is not completely visible and is pierced by a sharp instrument or tooth resulting in bleed-back into the patient's oral cavity or open wound.

 droplets containing infectious agents

 contaminated sharps and instruments that have been improperly sterilized (Fig. 36.1).

Some of the infectious agents of concern in dentistry and their possible routes of transmission are given in Table 36.1.

Table 36.1 Some infectious agents of concern in dentistry and their routes of transmission


Major transmission route




Hepatitis viruses


Hepatitis B


Hepatitis C


Delta hepatitis (hepatitis D)


Herpes simplex virus types 1 and 2

Direct contact

Human immunodeficiency virus (HIV)


Measles and mumps viruses


Respiratory viruses


Influenza virus






Rubella virus



Neisseria gonorrhoeae


Treponema pallidum (syphilis)


Mycobacterium tuberculosis


Streptococcus pyogenes


Airborne infection

Airborne infective organisms in the form of infectious aerosols may be inhaled, causing diseases such as influenza, the common cold and tuberculosis. When aerosols are created, for example, by high-speed instruments, different sizes of droplets are produced. Their fate depends on their size. Droplets greater than 100 μm in diameter are called splatter (or spatter) and settle very quickly on surfaces as a result of gravitational pull; they contaminate whatever is immediately in front of and below the patient within a 1-2-m radius. Droplets or particles between 20 and 100 μm in size fall from airborne suspension within seconds. Usually generated during coughing or sneezing, the latter contain numerous microbes. True aerosols, by contrast, comprise very small particles less than 5 μm or droplet nuclei (fluid droplets that evaporate rapidly and shrink to less than 5 μm) that remain suspended or entrained in the air for many hours and travel long distances on air currents. Small droplets of less than 100 μm in diameter account for the majority of droplets created in the dental surgery (Table 36.2).

Table 36.2 Characteristics of aerosols produced by high-speed instrumentation


Droplet nuclei

Diameter >100 μm

<100 μm

Time spent airborne Minutes


Penetration into Unlikely respiratory tract


Possible mode of Direct contact transmission or from dust


Droplet nuclei, which consist of dried salivary, sputum or serum secretions and any organisms they may contain, eventually fall to the ground or are inhaled into the alveoli of the lungs with the potential to cause respiratory infections. In practical terms, this underscores the importance of adequate ventilation of the clinical environment, and wearing face masks, particularly during the use of aerosol- creating instruments and the routine disinfection of surgery surfaces.

Infection via sharps and needlestick injuries

The major route of cross infection in the dental surgery is through the skin or mucosa due to accidents involving sharps or needlestick injuries (Fig. 36.1). There is evidence that hepatitis B, hepatitis C and HIV transmission from patient to dentist and vice versa has occurred by this means.

Mode of entry

Transmission of the pathogen to the new host is sometimes by direct contact but is more often an indirect process involving various vehicles of infection, dealt with above. Once the organism has approached the new host, it may gain ingress via a number of portals:


 inoculation or injection

 ingestion (e.g., diarrhoeal diseases, see Chapter 26)

 transplacental (e.g., congenital syphilis, congenital Zika syndrome or HIV acquired in utero).

Inhalation, inoculation and, rarely, direct contact are the modes by which the pathogens gain access to the host tissues in the dental clinic environment.

Infection control procedures

From the foregoing, it is evident that the number of infectious diseases that dental personnel may be exposed to during the working day could be fairly substantial. Several measures are available to dental personnel (dentists, dental hygienists, dental surgery assistants, school dental nurses, dental laboratory technicians and radiology technicians) to break this chain of cross infection. These may be categorized as:

 patient evaluation

 personal protection

 sterilization and disinfection

 safe disposal of waste

 clinical and laboratory asepsis.

These subjects are dealt with in detail in the next chapter.

Key facts

 Cross infection may be defined as the transmission of infectious agents between patients and staff within a clinical environment.

 The animate (e.g., insects, humans) and inanimate sources (e.g., blood, saliva) that carry and transmit infection are called vectors and fomites, respectively.

 Transmission of infection from one person to another requires a source of infection (the index case), a mode or vehicle of transmission (e.g., vectors and fomites) and a route of transmission (e.g., inhalation, percutaneous).

 Transmission of infection in dentistry could occur by direct contact, airborne spread or via contaminated sharps.

 The sources of infection in clinical dentistry are mainly humans and constitute those (1) with overt infections, (2) in the prodromal stage of infections and (3) who are healthy carriers of pathogens.

The infective agents may gain entry into the body by inhalation, inoculation (or injection) or ingestion.

Healthy carriers of pathogens are of two types: convalescent carriers and asymptomatic carriers.

Standard infection control precautions uphold the concept of treating every patient as a potential carrier of infectious disease and that all body fluids except sweat are potentially infectious. All patients in dentistry, irrespective of whether they carry apparent infections or not, should be treated under a standard infection control protocol.

Review questions (answers on p. 367)

Please indicate which answers are true, and which are false.

36.1 Which of the following statements related to cross infection are true?

A. blood and saliva are regarded as fomites with respect to infection transmission

B. viral infections are unlikely to spread during the prodromal stage

C. convalescent carriers are different from asymptomatic carriers in that asymptomatic carriers have a history of infection

D. prions are resistant to conventional sterilization methods

E. droplet nuclei less than 100 pm in diameter are entrained in the air for many hours

36.2 Which of the following statements are true?

A. the first person that is traced to have begun an infection is called the index case

B. overt infection refers to a situation where the carrier is unaware that he/she is having a specific infection

C. convalescent carriers of infection harbour the infectious agent for an extremely long period

D. standard infection control precautions are applied when dealing with blood, body fluids, sweat and saliva

E. inhalation is a major route through which infections are transmitted in dental surgery

Further reading

Centers for Disease Control and Prevention. (2016). Summary of infection prevention practices in dental settings: Basic expectations for safe care. Atlanta, GA: Centers for Disease Control and Prevention, US Department of Health and Human Services.

Department of Health. (2013). Decontamination: Health technical memorandum 01-05. Decontamination in primary care dental practices (2nd ed.). Available from: publications/decontamination-in-primary-care-dental-practices.

Goering, R., Dockrell, H., Zuckerman, M., et al. (2012). Hospital infection, sterilization and disinfection. In Mims' medical microbiology (5th ed.). Philadelphia: Saunders/Elsevier. Chapter 36.

Pankhurst, C. L., & Coulter, W. A. (2017). Basic guide to infection prevention and control (2nd ed.). Chichester: Wiley-Blackwell.

Samaranayake, L. P. (1989). Cross infection prevention in dentistry. Part I: General concepts and surgery attire. Dental Update, 16, 58-63.

Samaranayake, L. P., Scheutz, F., & Cottone, J. (1991). Infection control for the dental team. Copenhagen: Munksgaard.

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