Disease transmission

Definition

Disease transmission is the means by which contagious, pathogenic microorganisms are spread from one person to another.

Description

There are four major pathways by which pathogenic organisms may be spread to an individual: contact transmission, airborne transmission, vehicle transmission, and vector transmission.

Contact transmission

Infectious particles may be spread through direct or indirect contact with an infected individual, animal, or inanimate object. Direct contact refers to instances in which there is direct person-to-person spread of a disease. This occurs in the case of sexually transmitted diseases (STDs), when the genitals of one individual come into direct contact with the genitals of a person suffering from an STD such as herpes simplex virus (HSV). Contact of an uninfected person with infected wounds or blood may result in direct transmission, such as in the case of human immunodeficiency virus (HIV), if he/she is not protected by universal precautions .

A very common form of direct contact transmission occurs when infectious particles in the form of airborne droplets are expelled by an infected individual by sneezing or coughing and inhaled by another individual. Expelled droplets may travel for several meters before falling and therefore are easily spread to surrounding individuals. Examples of diseases that are caused by inhalation of infectious droplets are influenza (caused by the influenza virus) and tuberculosis (caused by the bacterium Mycobacterium tuberculosis).

Indirect contact results in the transmission of infectious particles from person to person via an intermediary carrier. Inanimate objects such as eating utensils, medical instruments, or equipment used in food preparation may become contaminated with a pathogenic microorganism and in turn spread the disease to other individuals. Infected needles may be the source of indirect transmission of pathogens between intravenous drug users. The hands of a health care worker may also cause the transmission of infectious particles from patient to patient if adequate handwashing practices are not enforced.

Organ transplantation presents a unique mode by which disease may be transmitted. Microorganisms that have disseminated to the donor organ may be transplanted to the recipient and subsequently cause disease. For example, hepatitis C virus (HCV) may be transmitted by liver transplantation if the donor organ is infected. Similarly, transmission of cytomegalovirus (CMV) from infected donor kidneys may occur.

KEY TERMS

Cleaning —The physical removal of microorganisms from an object, usually with water with or without a detergent.

Disinfection —Treatment which destroys most microorganisms from an object.

Droplet nuclei —Tiny particles on which microorganisms may be carried in airborne transmission.

Sterilization —The complete removal of all living microorganisms from an object or fluid.

Universal precautions —An infection-control technique designed to prevent the transmission of blood-borne diseases. These guidelines were initially developed by the Centers for Disease Control in 1987.

Airborne transmission

Airborne transmission differs from droplet transmission in that infectious particles are carried on tiny particles called droplet nuclei and may remain suspended or carried in the air for hours or days. These aerosolized particles may be widely dispersed before settling, therefore increasing the chance that they will be inhaled. Organisms that survive well under dry conditions (e.g. Staphylococcus aureus) are often spread by the airborne route.

There are many ways in which infectious microorganisms remain airborne. Droplet nuclei may be dispersed by ventilation and respiratory equipment, such as nebulizers, humidifiers, and air conditioners. S. aureus may be shed on skin scales from contaminated skin. Mycobacterium tuberculosis may be transmitted by droplet or airborne transmission and may survive for years in dust particles. Other organisms may be transmitted by disrupted soil or dried fecal matter.

Vehicle transmission

The spread of infectious agents through a common reservoir such as food or water supply is referred to as vehicle or common supply transmission. Food products may become contaminated after being handled by unwashed hands or processed by unclean equipment. Cattle infected with bovine spongiform encephalopathy (BSE, also known as mad cow disease) may cause Creutzfeldt-Jakob disease in humans who consume the cow's nervous system tissue. Water may be contaminated by infected feces, urine, or other body fluids, and diseases can be spread by drinking or bathing in this contaminated water. For example, the diarrheal disease cholera (caused by the bacterium Vibrio cholerae) is often spread by drinking water contaminated by infected fecal matter.

The blood supply is another means of vehicle transmission. Donated blood may be infected with any number of microorganisms (HIV, HCV, etc.) that could be transmitted upon transfusion. Contaminated drugs or intravenous fluid supplies at hospitals may also result in vehicle transmission of disease.

Vector transmission

Vector transmission occurs when an insect, arthropod, or rodent is the source of infection . Often the source of infection is an animal reservoir and the vector serves as an intermediate in the chain of infection. In the case of Lyme disease, the vector is a deer tick that transmits the spirochete Borrelia burgdorferi from deer to humans. Malaria is spread by the bite of female Anopheles mosquitoes infected with the protozoan Plasmodium. Rodents may also act as a vector—hantavirus infects many species of wild mice without causing disease but it causes disease in humans who inhale virus particles in aerosolized feces or urine.

Role in human health

There are numerous practices that can be adopted in a home, business, or health care setting in order to reduce the risk of disease transmission. These include:

• Handwashing: Good hand hygiene is the simplest and most effective method for preventing hospital-acquired (nosocomial) infections. Antiseptic agents that have both detergent and antimicrobial effects are recommended.
• Food preparation: Common causes of food contamination are Salmonella, Staphylococcus aureus, and Clostridium perfringens. To reduce the risk of food-borne illness, it is recommended that food be cooked to a temperature of 74°C (165°F) or greater and then stored at 4°C (40°F) or cooler.
• Waste disposal: Serious enteric (intestinal) diseases are common in less developed countries where adequate drainage and treatment of sewage is not available. For example, cholera is often spread in areas where there is no clean water supply or sanitary disposal of sewage.
• Sterilization: An object is sterile when it is free of living organisms. There are multiple techniques that can be used to sterilize an object or fluid, including heat (moist or dry), irradiation, chemical treatment, or filtration. It is almost always recommended that critical items (those that enter normally sterile areas of the body) be sterilized.
• Cleaning and disinfecting: Cleaning is the physical removal of microbes from an object, while disinfection is defined as treatment to destroy microorganisms (although this does not normally lead to sterile conditions). Semicritical or noncritical items (those that do not enter sterile areas of the body) may be cleaned or disinfected based on the degree of risk of infection.
• Protective barriers: The Centers for Disease Control and Prevention (CDC) recommend that universal precautions be used by all health care workers in contact with the body fluids of patients. These precautions include the following: wearing gloves when in contact with the blood, body fluids, mucous membranes, or non-intact skin of patients; wearing masks and/or protective eyewear during procedures that are likely to generate droplets of blood; and placing all disposable sharp items (needles, scalpel blades, etc.) in puncture-resistant containers for disposal.

Resources

BOOKS

Ayliffe, G. A. J., J. R. Babb, and Lynda J. Taylor. Hospital-acquired Infection: Principles and Prevention. Woburn, MA: Butterworth-Heinemann, 1999.

Greenwood, David, Richard C. B. Slack, and John F. Peutherer. "Epidemiology and Control of Community Infections." In Medical Microbiology. Edinburgh, UK: Churchill Livingstone, 1997.

Palmer, Sue, Jean Giddens, and Darwin Palmer. Infection Control Outline. El Paso, TX: Skidmore-Roth Publishing, Inc., 1996.

Rhinehart, Emily. Infection Control in Home Care. Gaithersburg, MD: Aspen Publishers, Ltd., 1999.

Schaechter, Moselio, and Barry I. Eisenstein. Mechanisms of Microbial Disease. Philadelphia: Williams and Wilkins, 1999.

PERIODICALS

Fishman, Jay, and Robert Rubin. "Infection in Organ-transplant Recipients." New England Journal of Medicine (June 1998): 1741-1751.

Pittet, Didier. "Improving Adherence to Hand Hygiene Practice: A Multidisciplinary Approach." Emerging Infectious Diseases (March-April 2001).

ORGANIZATIONS

Centers for Disease Control and Prevention. 1600 Clifton Road, Atlanta, GA 30333. (800) 311-3435. <http://www.cdc.gov>.

OTHER

Centers for Disease Control and Prevention. "Universal Precautions for Prevention of Transmission of HIV and Other Bloodborne Infections." <http://www.cdc.gov/ncidod/HIP/blood/universa.htm>.

Stéphanie Islane Dionne