transmission

Any mechanism by which an infectious agent is spread from a source or reservoir to a person. These mechanisms are:

Direct Transmission

Direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place. This may be by direct contact as by touching, biting, kissing or sexual intercourse, or by the direct projection (droplet spread) of droplet spray onto the conjunctiva or onto the mucous membranes of the eye, nose or mouth during sneezing, coughing, spitting, singing or talking (usually limited to a distance of about 1 meter or less).

Indirect Transmission

Vehicle-Borne

Contaminated inanimate materials or object (fomites) such as toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, surgical instruments or dressings (indirect contact); water, food, milk, biological products including bood, serum, plasma, tissues or organs; or any substance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry. The agent may or may not have multiplied or developed in or on the vehicle before being transmitted.

Vector-Borne

(a) Mechanical: Includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis, or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism.

(strong) Biological: Propagation (multiplication), cyclic development, or a combination of these (cyclopropagative) is required before the arthropod can transmit the infective form of the agent to man. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (trans-ovarian transmission); trans-stadial transmission indicates its passage from one stage of life cycle to another, as nymph to adult. Transmission may be by injection of salivary gland fluid during biting, or by regurgitation or deposition on the skin of feces or vomitusl capable of penetrating through the bite wound or through an area of trauma from scratching or rubbing. This transmission is by an infected.non-vertebrate host and not simple mechanical carriage by a vector as a vehicle. However, an arthropod in either role is termed a vector.

c) Airborne:

The dissemination of microbial aerosols to a suitable portal of entry, usually the respiratory tract. Microbial aerosols are suspensions of partially or wholly of microorganisms. They may remain suspended in the air for long periods of time, some retaining and others losing infectivity or virulence. Particles in the 1- to 5 micrometer range are easily drawn into the alveoli of the lungs and may be retained there. Not considered as airborne are droplets and other large particles which promptly settle out (see Direct Transmission, above). Droplet nucleiUsually the small residues which result from evaporation of fluid from droplets emitted by an infected host (see above). They also may be created purposely by a variety of atomizing devices, or accidentally as in microbiology laboratories or in abattoirs, rendering plants or autopsy rooms. They usually remain suspended in the air for long periods of time. DustThe small particles of widely varying size which may arise from soil (as, for example, fungus spores separated from dry soil by wind or mechanical agitation), clothes, bedding, or contaminated floors.

See Also:

• transmission (Chapter 9b)