Transmission routes

The pathway of causative agents from a source to infection of a susceptible host is called the 'transmission route'. The characteristic of the transmission route depends mainly on the characteristics of the causative agent and those of the host. Some microorganisms are restricted to limited transition routes, whereas others can follow many different pathways to infect their hosts. It is useful to have detailed knowledge about the specific transmission routes of pathogens since this gives practical information of effective control measures by interrupting the spread of the infection within the population.

Direct transmission

This means the direct and immediate transfer of infectious agents to a susceptible host. This may be through direct contact such as touching, biting, kissing or sexual intercourse, or by the direct projection of droplets (droplet spread) spraying onto other people's eyes, nose or mouth during sneezing, coughing, spitting, singing or talking. Droplet spread is usually limited to short distances, such as 1 meter or less).

Direct transmission routes are linked to behaviour, and most interventions that target this particular transmission usually aim to educate people to reduce risk behaviour (e.g. condom use, using facial masks while contacting patients, sneezing in handkerchiefs or sleeves, etc.)

Vertical transmission

A specific form of direct transmission occurs between mother and child during pregnancy or childbirth.

Indirect transmission

When transmission of infectious organisms occurs from a source through objects (vehicles) or insects (vectors), we call this indirect transmission. Transmission through vehicles is usually linked to processes such as food production, food handling, cleaning procedures in daycare centres, hygiene procedures in medical facilities etc.

Vehicle-borne

Infectious agents can reach susceptible hosts through transport on inanimate objects (fomites) such as toys, handkerchiefs, soiled clothes, bedding, medical instruments, food, water, blood products or any other substance that can be contaminated. Some vehicles allow multiplication of the infectious agent (e.g. salmonella in food), though this is not always the case. Intervention measures to reduce the risk of vehicle-borne transmission aim to enhance the safety of procedures (safe food production, education of food handlers, hand washing protocols in health care, etc).

Vector-borne

When insects transfer infectious agents to susceptible hosts, they act as 'vectors' of the infection. Intervention measures to reduce the risk of vector-borne transmission aim to control the size of the vector population. This may include spraying of insecticides and reducing the breeding spaces for insects.

Mechanical:

This includes simple mechanical carriage by crawling or flying insects, and does not require multiplication of the micro organisms.

Biological:

When the micro organisms multiply within the vector and / or undergo developmental cycles within the insect, then this is part of biological vector-borne transmission of infectious diseases. In such cases, an incubation period is required (starting from the moment of introduction of the infectious agent into the vector) before the vector itself becomes infective. In such situations, infected insects may transmit the pathogen (vertically) to the next generations of offspring.

Airborne transmission

Microbial aerosols are suspensions of particles (fluid or solid) in the air consisting partially or wholly of microorganisms. They may remain suspended in the air for prolonged periods (as opposed to droplets, that are too large in diameter and fall to the ground relatively fast). This transmission route works particularly efficient for viruses such as the measles virus (a coughing patient may produce an infectious aerosol in a corridor that can remain suspended to infect others passing by several minutes or longer after the patient has left).

References:

• David L. Heymann (ed.). Control of Communicable Diseases Manual. 19th Edition.
• Patrick L. Remington, MD; William N. Hall, MD, MPH; Irving H. Davis, PE, CIH; Anita Herald, MD; Robert A. Gunn, MD, MPH. Airborne Transmission of Measles in a Physician's Office. JAMA. 1985;253(11):1574-1577. doi: 10.1001/jama.1985.03350350068022