Transmission (medicine)

In medicine and biology, 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.[1] Sometimes transmission can specifically mean infection of a previously uninfected host.

The term usually refers to the transmission of microorganisms directly from one person to another by one or more of the following means:

Transmission can also be indirect, via another organism, either a vector (e.g. a mosquito or fly) or an intermediate host (e.g. tapeworm in pigs can be transmitted to humans who ingest improperly cooked pork). Indirect transmission could involve zoonoses or, more typically, larger pathogens like macroparasites with more complex life cycles.

Horizontal and vertical

Disease can be directly transmitted in two ways:

(vectors or fomites that allow the transmission of disease without physical contact).[3]

Transmission, virulence and survival

Pathogens must have a way to be transmitted from one host to another to ensure their species' survival. Infectious agents are generally specialized for a particular method of transmission. Taking an example from the respiratory route, from an evolutionary perspective a virus or bacteria that causes its host to develop coughing and sneezing symptoms has a great survival advantage – it is much more likely to be ejected from one host and carried to another. This is also the reason that many microorganisms cause diarrhea.

The relationship between virulence and transmission is complex, and has important consequences for the long term evolution of a pathogen. Since it takes many generations for a microbe and a new host species to co-evolve, an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can get passed along to another host. However, this cost may be overwhelmed by the short term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious aerosols).

Locus

In transmission, a locus is the point on the body where a pathogen enters.

Routes of transmission

Droplet contact

Further information: Airborne disease

Also known as the respiratory route, and the resultant infection can be termed airborne disease. If an infected person coughs or sneezes on another person the microorganisms, suspended in warm, moist droplets, may enter the body through the nose, mouth or eye surfaces. Diseases that are commonly spread by coughing or sneezing include:

Viral droplet nuclei transmission

Droplet nuclei are an important mode of transmission among many infectious viruses such as Influenza A. When viruses are shed by an infected person through coughing or sneezing into the air, the mucus coating on the virus starts to evaporate. Once this mucus shell evaporates the remaining viron is called a droplet nucleus or quanta. The mucus evaporation rate is determined by the temperature and humidity inside the room. The lower the humidity, the quicker the mucus shell evaporates thus allowing the droplet nuclei to stay airborne and not drop to the ground. The low indoor humidity levels in wintertime buildings ensure that higher levels of droplet nuclei will survive: droplet nuclei are so microscopic that they are able to stay airborne indefinitely on the air currents present within indoor spaces. The Wells-Riley equation predicts the infection rates of persons who shed quanta within a building and is used to calculate indoor infection outbreaks within buildings.

An infected person can release viruses by talking, sneezing, coughing and breathing, though less are released by just breathing. Some of viruses will become droplet nuclei. If these droplet nuclei are breathed into nose or mouth of an uninfected person (known as a susceptible)[5] – then the droplet nuclei may penetrate into the deep recesses of their lungs. Viral diseases that are commonly spread by coughing or sneezing droplet nuclei include (at least):

Fecal-oral transmission

Main article: Fecal-oral route

Direct contact is rare in this route for humans in developed countries, but relatively common for humans in developing countries, especially those living in urban slums without access to adequate sanitation. More common in developed countries are the indirect routes; foodstuffs or water become contaminated (by people not washing their hands before preparing food, or untreated sewage being released into a drinking water supply) and the people who eat and drink them become infected. In developing countries only few people are connected to sewer systems, and where they are, most sewage is discharged into the environment without treatment. However, a bigger problem in developing countries is open defecation which leads to disease transmission via the fecal-oral route, especially for children. Even in developed countries there are periodic system failures resulting in a sanitary sewer overflow. This is the typical mode of transmission for the infectious agents of (at least):

Sexual transmission

This refers to any disease that can be caught during sexual activity with another person, including vaginal or anal sex or (less commonly) through oral sex (see below). Transmission is either directly between surfaces in contact during intercourse (the usual route for bacterial infections and those infections causing sores) or from secretions (semen or the fluid secreted by the excited female) which carry infectious agents that get into the partner's blood stream through tiny tears in the penis, vagina or rectum (this is a more usual route for viruses). In this second case, anal sex is considerably more hazardous since penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating.

Some diseases transmissible by the sexual route include (at least):

Oral sexual transmission

Sexually Transmitted Diseases such as HIV and Hepatitis B are thought to not normally be transmitted through mouth-to-mouth contact, although it is possible to transmit some STDs between the genitals and the mouth, during oral sex. In the case of HIV this possibility has been established. It is also responsible for the increased incidence of herpes simplex virus 1 (which is usually responsible for oral infections) in genital infections and the increased incidence of the type 2 virus (more common genitally) in oral infections.

Oral transmission

Diseases that are transmitted primarily by oral means may be caught through direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette.

Diseases that are known to be transmissible by kissing or by other direct or indirect oral contact include all of the diseases listed above as transmissible by droplet contact and also (at least):

(Notice these are all forms of herpes virus.)

Transmission by direct contact

Further information: Contagious disease

Diseases that can be transmitted by direct contact are called contagious (contagious is not the same as infectious; although all contagious diseases are infectious, not all infectious diseases are contagious). These diseases can also be transmitted by sharing a towel (where the towel is rubbed vigorously on both bodies) or items of clothing in close contact with the body (socks, for example) if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms.

Some diseases that are transmissible by direct contact include:

Vertical transmission

This is from mother to child, often in utero or during childbirth (also referred to as perinatal infection). It occurs more rarely via breast milk.

Infectious diseases that can be transmitted in this way include: HIV, Hepatitis B and Syphilis.

Iatrogenic transmission

Transmission due to medical procedures, such as injection or transplantation of infected material.

Some diseases that can be transmitted iatrogenically include:

Vector-borne transmission

Further information: Vector (epidemiology)

A vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another.[6]

The route of transmission is important to epidemiologists because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as cholera. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that polio is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route.It can be minimised to a certain extent if we all consider being sure of what we do.

Vectors may be mechanical or biological. A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a housefly, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly. In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious blood-borne diseases, such as malaria, viral encephalitis, Chagas disease, Lyme disease and African sleeping sickness. Biological vectors are usually, though not exclusively, arthropods, such as mosquitoes, ticks, fleas and lice. Vectors are often required in the life cycle of a pathogen. A common strategy used to control vector borne infectious diseases is to interrupt the life cycle of a pathogen by killing the vector.

Infectivity

The term infectivity describes the ability of an organism to enter, survive and multiply in the host, while the infectiousness of a disease indicates the comparative ease with which the disease is transmitted to other hosts.[7] Transmission of pathogen can occur in various ways including physical contact, contaminated food, body fluids, objects, airborne inhalation, or through vector organisms.[8]

See also

References

  1. ↑ Bush, A.O. et al. (2001) Parasitism: the diversity and ecology of animal parasites. Cambridge University Press. Pp 391-399.
  2. ↑ Horizontal Disease Transmission, online-medical-dictionary.org.
  3. ↑ Routes of transmission of infectious diseases agents from Modes of Introduction of Exotic Animal Disease Agents by Katharine M. Kurkjian & Susan E. Little of The University of Georgia College of Veterinary Medicine
  4. ↑ Vertical transmission (definition -- medterms.com)
  5. ↑ Interim Guidance on Infection Control Measures for 2009 H1N1 Influenza in Healthcare Settings, Including Protection of Healthcare Personnel. July 15, 2010. CDC.
  6. ↑ Pathogens and vectors. MetaPathogen.com.
  7. ↑ "Glossary of Notifiable Conditions". Washington State Department of Health. Retrieved 2010-02-03.
  8. ↑ Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 0-8385-8529-9.