Defining A "Contact"

According to the Merriam-Webster dictionary, a contact is defined as "a union or junction of surfaces; direct experience through the senses" (m-w.com). However when it comes to the transmission of infectious diseases, a contact can mean so much more. From the Medical Dictionary (medical-dictionary.thefreedictionary.com), a contact is defined in two ways:

  • A mutual touching of two bodies or persons
  • An individual known to have been sufficiently near an infected person to have been exposed to the transfer of infectious material

    The latter definition is further broken down into:

  • Direct contact - The contact of healthy person with a person having a communicable disease, the disease being transmitted as a result
  • Indirect contact - That achieved through some intervening medium, as prolongation of a communicable disease through the air or by means of fomites

    Which leads to:

  • Fomite - An inanimate object or substance, such as clothing or furniture, that is capable of transmitting infectious organisms from one organism to another.

    From an epidemiological perspective, infectious diseases can be broken down into direct and indirect transmissible diseases. Direct transmissible diseases can be spread only via direct physical contact between individuals, or by contact with bodily secretions, and transmission usually occurs between family members or close friends. Examples of direct transmissible diseases include MRSA (methicillin-resistant Staphylococcus aureus), HPV (human papillomavirus), and HIV(human immuno-deficiency virus), and are generally modeled in a mathematical sense via pairwise interactions between individuals.

    In contrast, indirect transmissible diseases are spread via contact with a contaminated environmental reservoir, surface or object, or via contact with vectors, and are further subdivided into droplet contact transmission, airborne transmission, fecal-oral transmission, and vector-borne transmission classifications. Droplet contact transmission includes respiratory spread when an infected person sneezes or coughs, sending infectious droplets into the air. This type of disease (for example, influenza) requires fairly close proximity between individuals in order for transmission to take place, and is typically modeled as direct interactions between individuals (in an agent- or individual-based model) or groups of individuals (in a population level model) which mix homogeneously or heterogeneously based on group characteristics.

    Organisms spread via airborne transmission travel on much smaller droplet nuclei than those spread by droplet contact transmission. They must be capable of surviving outside the body for long periods of time, be resistant to drying and a wide range of environmental conditions, and can often travel for considerable distances through the air. Mathematically, airborne transmissible diseases can be modeled similar to droplet transmission (i.e. as relatively close contacts between individuals and groups, as in the spread of smallpox) or as plumes traveling through the air, as with an airborne release of inhalational anthrax.

    Fecal-oral transmissible microorganisms enter the body through the ingestion of contaminated food or water, and can be shed from infectious persons to continually re-contaminate environmental reservoirs unless proper hygienic and sanitation procedures are in place. These diseases are frequently modeled via interactions between susceptible individuals and the external contaminated reservoir, with transmission being a function of the infectious dose for the appropriate microorganism. However, some of these infections may also be spread via direct household contacts, thus the mathematical model system can become a hybrid of the two transmission types.

    Finally, vector-borne transmission occurs via vector animals such as mosquitoes, flies, mites, fleas, ticks, rats, dogs, etc., which transfer the disease indirectly from one individual host to another. The mobility of vectors has the potential to dramatically increase the transmission range of a disease, as was observed with the rapid spread of West Nile Virus across the United States following its introduction in 1999, or the newly expanding range of the mosquitoes which are vectors for dengue into Texas and southern Arizona. Dynamic mathematical models of vector-borne diseases require an additional layer of complexity, in which disease progression within the vector population is modeled explicitly in addition to that within the host population. Transmission occurs via interactions between the two separate species, as opposed to simply via contact between susceptible and infectious individuals within the same population. Prevention of vector-borne diseases typically focuses on minimizing or avoiding these between-species interactions.

    Thus the definition of an effective infectious disease contact can range widely from standing right next to an infected person, to being yards, miles or even hundreds of miles away. If your organization is interested in exploring how the mode of transmission can impact the spread of disease and its prevention, contact MathEcology to learn more about how mathematical modeling can help!

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