10.4C: Chain of Transmission

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Learning Objectives

Differentiate between the various types of transmission: air-borne, common vehicle, vector borne, direct and indirect contact transmission

The drug-resistant Gram-negative bacteria, for the most part, threaten only hospitalized patients whose immune systems are weak. They can survive for a long time on surfaces in the hospital and they enter the body through wounds, catheters, and ventilators.

The most important and frequent mode of transmission of nosocomial infections is by direct contact. Transmission occurs when droplets containing microbes from the infected person are propelled a short distance through the air and deposited on the host’s body; droplets are generated from the source person mainly by coughing, sneezing, and talking, and during the performance of certain procedures, such as bronchoscopy.

Dissemination can be either airborne droplet nuclei (small-particle residue {5 µm or smaller in size} of evaporated droplets containing microorganisms that remain suspended in the air for long periods of time) or dust particles containing the infectious agent. Microorganisms carried in this manner can be dispersed widely by air currents and may become inhaled by a susceptible host within the same room or over a longer distance from the source patient, depending on environmental factors; therefore, special air-handling and ventilation are required to prevent airborne transmission. Microorganisms transmitted by airborne transmission include Legionella, Mycobacterium tuberculosis and the rubeola and varicella viruses.

Common vehicle transmission applies to microorganisms transmitted to the host by contaminated items, such as food, water, medications, devices, and equipment. Vector borne transmission occurs when vectors such as mosquitoes, flies, rats, and other vermin transmit microorganisms.

Figure: **Cross-transmission**: Contaminated surfaces increase cross-transmission.

Contact transmission is divided into two subgroups: direct-contact transmission and indirect-contact transmission.

Direct-contact transmission involves a direct body surface-to-body surface contact and physical transfer of microorganisms between a susceptible host and an infected or colonized person, such as when a person turns a patient, gives a patient a bath, or performs other patient-care activities that require direct personal contact. Direct-contact transmission can also occur between two patients, with one serving as the source of the infectious microorganisms and the other as a susceptible host.

Indirect-contact transmission involves contact of a susceptible host with a contaminated intermediate object, usually inanimate, such as contaminated instruments, needles, or dressings, or contaminated gloves that are not changed between patients. In addition, the improper use of saline flush syringes, vials, and bags has been implicated in disease transmission in the US, even when healthcare workers had access to gloves, disposable needles, intravenous devices, and flushes.

Key Points

Contact transmission is divided into two subgroups: direct-contact transmission and indirect-contact transmission.
Direct-contact transmission involves a direct body surface-to-body surface contact and physical transfer of microorganisms.
Indirect-contact transmission involves contact of a susceptible host with a contaminated intermediate object, usually inanimate.

Key Terms

microorganisms: A microorganism or microbe is a microscopic organism that comprises either a single cell (unicellular), cell clusters, or multicellular relatively complex organisms.
susceptible: likely to be affected by something; here, sensitive to growth inhibition by an antimicrobial drug.
transmission: Transmission is the passing of a communicable disease from an infected host individual or group to a conspecific individual or group, regardless of whether the other individual was previously infected.