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Friday, 12 July 2013

Microbiology VI. TRANSMISSION OF INFECTIOUS AGENTS

A. Infectious agents are found in a number of different environments (reservoirs).
1. Humans are reservoirs for diseases that are obligate human pathogens. When humans are the reservoir for the disease, they are said to be carriers.
a. Asymptomatic carriers harbor an infection but have no symptoms. Some asymptomatic carriers carry the infectious agent as part of their normal flora (e.g., S. pyogenes).
b. Symptomatic carriers have obvious signs and symptoms of disease.
 2. An animal reservoir exists when the primary host is an animal. Such animals may be wild (e.g., foxes, raccoons) or domesticated (e.g., livestock and pets). A zoonosis is an animal disease that can be transmitted to humans. 
3. Environmental reservoirs include soil, lakes, and plants.

B. The transmission of infectious agents depends on the source of infection. Many but not all diseases attributable to microbes are communicable (i.e., spread from person to person). For example, the common cold is communicable; botulism and tetanus are not. 
Diseases originating from other individuals can be acquired by a number of different mechanisms.
1. Contact
a. Direct contact requires physical contact between an infected individual and a susceptible individual (e.g., sexually transmitted diseases). Person-to-person spread of infection can occur from those infected with an obvious illness; those who are incubating the illness, but have not yet developed symptoms; those with asymptomatic or subclinical infections; and, in some cases, individuals with latent infections.
b. Indirect contact involves a susceptible individual coming in contact with a contaminated surface. Fomites are objects that can transfer infectious agents when contaminated (e.g., doorknobs, tabletops and other surfaces, computer keyboards, toys).
2. Droplet transmission. Infected droplets or aerosols are formed when an infected individual coughs or sneezes. The infected droplets transmit the disease to a susceptible individual when they come in contact with the mucous membranes of the individual’s nose, mouth, or eyes.
3. Airborne transmission. When small, contaminated dust particles or small infectious aerosol droplets (droplet nuclei) remain suspended in the air for long periods, they can transmit disease (e.g., influenza, tuberculosis). These small nuclei can infect both the upper and lower respiratory tracts.
4. Food and water contamination. Food or water contaminated with bacteria or viruses from human or animal infectious material can lead to transmission of disease through the fecal–oral route (e.g., cholera, polio).

C. Vectors are living entities capable of transmitting diseases. The most common vector for infectious diseases is the mosquito (e.g., malaria, West Nile virus), although other insects (fleas, ticks, and flies) and mammals (dogs, mice, and rats) can also be vectors for infections.

D. An infection is epidemic if it occurs with more frequency than usual, pandemic if the epidemic is distributed worldwide. An infection that occurs with regular frequency is endemic.

E. Stages of infectious disease
1. Incubation period is the time between infection and appearance of symptoms.
2. Prodrome is the period when nonspecific symptoms occur; not all infectious diseases have a prodromal period.
3. Specific illness is when the characteristic features of the illness are present.
4. Recovery is the period when the symptoms resolve and health is restored. This period may include a convalescent phase, where the agent associated with the disease is no longer detectable, but health is not yet restored.
5. After the recovery phase, some infections are associated with latent infections, where the infection remains in the body in an inactive form, or a chronic infection, where the infectious agent continues to be produced.







REF;
Comprehensive Pharmacy Review
Wikipedia

pictures in this article are found by google search;

http://toomuchviroismad.files.wordpress.com/2010/01/stages-of-infectious-disease1.jpg
http://www.mayomedicallaboratories.com/images/articles/communique/2009/11fig1.jpg
http://www.microbiologyonline.org.uk/themed/sgm/img/slideshows/3.3.4_transmission_2.png
http://www.microbiologyonline.org.uk/themed/sgm/img/slideshows/3.3.2_immune_3.png
 

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