Fomites are nonliving objects that facilitate the indirect transmission of pathogens. Contaminated doorknobs, towels, and syringes are all common examples of fomites. The term vehicle transmission refers to the transmission of pathogens through vehicles such as water, food, and air. Water contamination through poor sanitation methods leads to waterborne transmission of disease. Waterborne disease remains a serious problem in many regions throughout the world. The World Health Organization WHO estimates that contaminated drinking water is responsible for more than , deaths each year.
Figure 3. Food is an important vehicle of transmission for pathogens, especially of the gastrointestinal and upper respiratory systems. Notice the glass shield above the food trays, designed to prevent pathogens ejected in coughs and sneezes from entering the food. Meade Public Affairs Office. Dust and fine particles known as aerosols , which can float in the air, can carry pathogens and facilitate the airborne transmission of disease. For example, dust particles are the dominant mode of transmission of hantavirus to humans.
Hantavirus is found in mouse feces, urine, and saliva, but when these substances dry, they can disintegrate into fine particles that can become airborne when disturbed; inhalation of these particles can lead to a serious and sometimes fatal respiratory infection. Although droplet transmission over short distances is considered contact transmission as discussed above, longer distance transmission of droplets through the air is considered vehicle transmission.
Unlike larger particles that drop quickly out of the air column, fine mucus droplets produced by coughs or sneezes can remain suspended for long periods of time, traveling considerable distances. In certain conditions, droplets desiccate quickly to produce a droplet nucleus that is capable of transmitting pathogens; air temperature and humidity can have an impact on effectiveness of airborne transmission.
Tuberculosis is often transmitted via airborne transmission when the causative agent, Mycobacterium tuberculosis , is released in small particles with coughs. Because tuberculosis requires as few as 10 microbes to initiate a new infection, patients with tuberculosis must be treated in rooms equipped with special ventilation, and anyone entering the room should wear a mask.
After identifying the source of the contaminated turduckens, the Florida public health office notified the CDC, which requested an expedited inspection of the facility by state inspectors. Inspectors found that a machine used to process the chicken was contaminated with Salmonella as a result of substandard cleaning protocols.
Inspectors also found that the process of stuffing and packaging the turduckens prior to refrigeration allowed the meat to remain at temperatures conducive to bacterial growth for too long. The contamination and the delayed refrigeration led to vehicle food transmission of the bacteria in turduckens.
Based on these findings, the plant was shut down for a full and thorough decontamination. All turduckens produced in the plant were recalled and pulled from store shelves ahead of the December holiday season, preventing further outbreaks.
Diseases can also be transmitted by a mechanical or biological vector , an animal typically an arthropod that carries the disease from one host to another. Mechanical transmission is facilitated by a mechanical vector , an animal that carries a pathogen from one host to another without being infected itself. For example, a fly may land on fecal matter and later transmit bacteria from the feces to food that it lands on; a human eating the food may then become infected by the bacteria, resulting in a case of diarrhea or dysentery Figure 4.
Figure 4. Biological transmission occurs when the pathogen reproduces within a biological vector that transmits the pathogen from one host to another Figure 4. Arthropods are the main vectors responsible for biological transmission Table 1. Most arthropod vectors transmit the pathogen by biting the host, creating a wound that serves as a portal of entry. The pathogen may go through part of its reproductive cycle in the gut or salivary glands of the arthropod to facilitate its transmission through the bite.
Biological insect vectors include mosquitoes , which transmit malaria and other diseases, and lice , which transmit typhus.
Other arthropod vectors can include arachnids, primarily ticks , which transmit Lyme disease and other diseases, and mites , which transmit scrub typhus and rickettsial pox. Biological transmission, because it involves survival and reproduction within a parasitized vector, complicates the biology of the pathogen and its transmission. There are also important non-arthropod vectors of disease, including mammals and birds.
Various species of mammals can transmit rabies to humans, usually by means of a bite that transmits the rabies virus. In , an epidemic of the Zika virus was linked to a high incidence of birth defects in South America and Central America.
As winter turned to spring in the northern hemisphere, health officials correctly predicted the virus would spread to North America, coinciding with the breeding season of its major vector, the Aedes aegypti mosquito.
The range of the A. Because these same mosquitoes serve as vectors for other problematic diseases dengue fever , yellow fever , and others , various methods of mosquito control have been proposed as solutions.
Chemical pesticides have been used effectively in the past, and are likely to be used again; but because chemical pesticides can have negative impacts on the environment, some scientists have proposed an alternative that involves genetically engineering A. This method, however, has been the subject of some controversy. Figure 5. The Zika virus is an enveloped virus transmitted by mosquitoes, especially Aedes aegypti.
The range of this mosquito includes much of the United States, from the Southwest and Southeast to as far north as the Mid-Atlantic. The range of A. One method that has worked in the past to control pests, with little apparent downside, has been sterile male introductions. This method controlled the screw-worm fly pest in the southwest United States and fruit fly pests of fruit crops.
In this method, males of the target species are reared in the lab, sterilized with radiation, and released into the environment where they mate with wild females, who subsequently bear no live offspring. Repeated releases shrink the pest population. A similar method, taking advantage of recombinant DNA technology, [5] introduces a dominant lethal allele into male mosquitoes that is suppressed in the presence of tetracycline an antibiotic during laboratory rearing. The males are released into the environment and mate with female mosquitoes.
Unlike the sterile male method, these matings produce offspring, but they die as larvae from the lethal gene in the absence of tetracycline in the environment. The use of genetically modified organisms GMOs to control a disease vector has its advocates as well as its opponents.
In theory, the system could be used to drive the A. Opponents are also wary of the program because it is being administered by a for-profit corporation, creating the potential for conflicts of interest that would have to be tightly regulated; and it is not clear how any unintended consequences of the program could be reversed.
There are other epidemiological considerations as well. Aedes aegypti is apparently not the only vector for the Zika virus. Aedes albopictus , the Asian tiger mosquito, is also a vector for the Zika virus. Many other mosquitoes have been found to harbor Zika virus, though their capacity to act as vectors is unknown.
Finally, the Zika virus can apparently be transmitted sexually between human hosts, from mother to child, and possibly through blood transfusion. All of these factors must be considered in any approach to controlling the spread of the virus. Clearly there are risks and unknowns involved in conducting an open-environment experiment of an as-yet poorly understood technology. The term zoonosis refers to an infectious disease that is transmissible under natural conditions from vertebrate animals to humans.
Zoonoses newly emergent in North America include West Nile encephalitis birds , and monkeypox prairie dogs. Environmental reservoirs. Plants, soil, and water in the environment are also reservoirs for some infectious agents.
Many fungal agents, such as those that cause histoplasmosis, live and multiply in the soil. Outbreaks of Legionnaires disease are often traced to water supplies in cooling towers and evaporative condensers, reservoirs for the causative organism Legionella pneumophila.
Portal of exit is the path by which a pathogen leaves its host. The portal of exit usually corresponds to the site where the pathogen is localized. For example, influenza viruses and Mycobacterium tuberculosis exit the respiratory tract, schistosomes through urine, cholera vibrios in feces, Sarcoptes scabiei in scabies skin lesions, and enterovirus 70, a cause of hemorrhagic conjunctivitis, in conjunctival secretions.
Some bloodborne agents can exit by crossing the placenta from mother to fetus rubella, syphilis, toxoplasmosis , while others exit through cuts or needles in the skin hepatitis B or blood-sucking arthropods malaria. An infectious agent may be transmitted from its natural reservoir to a susceptible host in different ways.
There are different classifications for modes of transmission. Here is one classification:. In direct transmission, an infectious agent is transferred from a reservoir to a susceptible host by direct contact or droplet spread. Direct contact occurs through skin-to-skin contact, kissing, and sexual intercourse. Direct contact also refers to contact with soil or vegetation harboring infectious organisms.
Hookworm is spread by direct contact with contaminated soil. Droplet spread refers to spray with relatively large, short-range aerosols produced by sneezing, coughing, or even talking. Droplet spread is classified as direct because transmission is by direct spray over a few feet, before the droplets fall to the ground.
Pertussis and meningococcal infection are examples of diseases transmitted from an infectious patient to a susceptible host by droplet spread. Indirect transmission refers to the transfer of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects vehicles , or animate intermediaries vectors. Airborne transmission occurs when infectious agents are carried by dust or droplet nuclei suspended in air.
Airborne dust includes material that has settled on surfaces and become resuspended by air currents as well as infectious particles blown from the soil by the wind. Droplet nuclei are dried residue of less than 5 microns in size.
In contrast to droplets that fall to the ground within a few feet, droplet nuclei may remain suspended in the air for long periods of time and may be blown over great distances. Vehicles that may indirectly transmit an infectious agent include food, water, biologic products blood , and fomites inanimate objects such as handkerchiefs, bedding, or surgical scalpels.
A vehicle may passively carry a pathogen — as food or water may carry hepatitis A virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin — as improperly canned foods provide an environment that supports production of botulinum toxin by Clostridium botulinum. Vectors such as mosquitoes, fleas, and ticks may carry an infectious agent through purely mechanical means or may support growth or changes in the agent.
Examples of mechanical transmission are flies carrying Shigella on their appendages and fleas carrying Yersinia pestis , the causative agent of plague, in their gut. In contrast, in biologic transmission, the causative agent of malaria or guinea worm disease undergoes maturation in an intermediate host before it can be transmitted to humans Figure 1.
The portal of entry refers to the manner in which a pathogen enters a susceptible host. The portal of entry must provide access to tissues in which the pathogen can multiply or a toxin can act.
Often, infectious agents use the same portal to enter a new host that they used to exit the source host. For example, influenza virus exits the respiratory tract of the source host and enters the respiratory tract of the new host. Other portals of entry include the skin hookworm , mucous membranes syphilis , and blood hepatitis B, human immunodeficiency virus. Figure 1. The final link in the chain of infection is a susceptible host. For example, persons with sickle cell trait seem to be at least partially protected from a particular type of malaria.
Specific immunity refers to protective antibodies that are directed against a specific agent. Such antibodies may develop in response to infection, vaccine, or toxoid toxin that has been deactivated but retains its capacity to stimulate production of toxin antibodies or may be acquired by transplacental transfer from mother to fetus or by injection of antitoxin or immune globulin. Nonspecific factors that defend against infection include the skin, mucous membranes, gastric acidity, cilia in the respiratory tract, the cough reflex, and nonspecific immune response.
Factors that may increase susceptibility to infection by disrupting host defenses include malnutrition, alcoholism, and disease or therapy that impairs the nonspecific immune response. Knowledge of the portals of exit and entry and modes of transmission provides a basis for determining appropriate control measures. In general, control measures are usually directed against the segment in the infection chain that is most susceptible to intervention, unless practical issues dictate otherwise.
For some diseases, the most appropriate intervention may be directed at controlling or eliminating the agent at its source. A patient sick with a communicable disease may be treated with antibiotics to eliminate the infection. An asymptomatic but infected person may be treated both to clear the infection and to reduce the risk of transmission to others. In the community, soil may be decontaminated or covered to prevent escape of the agent.
Some interventions are directed at the mode of transmission. Interruption of direct transmission may be accomplished by isolation of someone with infection, or counseling persons to avoid the specific type of contact associated with transmission. Vehicleborne transmission may be interrupted by elimination or decontamination of the vehicle. To prevent fecal-oral transmission, efforts often focus on rearranging the environment to reduce the risk of contamination in the future and on changing behaviors, such as promoting handwashing.
For airborne diseases, strategies may be directed at modifying ventilation or air pressure, and filtering or treating the air. To interrupt vectorborne transmission, measures may be directed toward controlling the vector population, such as spraying to reduce the mosquito population. Some strategies that protect portals of entry are simple and effective. The specifications, in addition to those above, include a private room and the use of masks, gowns and gloves for all persons entering the room.
Special ventilation requirements with the room at negative pressure to surrounding areas are desirable. In addition to the basic requirements, a private room is indicated, but patients infected with the same pathogen may share a room.
Masks are indicated for those who come close to the patient, gowns are indicated if soiling is likely and gloves are indicated for touching infectious material. In addition to the basic requirements, masks are indicated for those who come in close contact with the patient; gowns and gloves are not indicated.
Specifications include use of a private room with special ventilation and closed door. In addition to the basic requirements, respirator-type masks are used by those entering the room. Gowns are used to prevent gross contamination of clothing. Gloves are not indicated. In addition to the basic requirements, specifications include use of a private room if patient hygiene is poor. Masks are not indicated; gowns should be used if soiling is likely and gloves are to be used for touching contaminated materials.
A private room and masking are not indicated; in addition to the basic requirements, gowns should be used if soiling is likely and gloves should be used for touching contaminated materials.
Morbidity rate —An incidence rate q. The population may be limited to a specific gender or age group, or to those with certain other characteristics. Mortality rate —A rate calculated in the same way as an incidence rate q. A total or crude mortality rate utilizes deaths from all causes, usually expressed as deaths per 1, A disease-specific mortality rate covers deaths due to only one disease and is often reported on the basis of , persons.
The population base may be defined by gender, age or other characteristics. The mortality rate must not be confused with case-fatality rate q. Synonym: death rate. Nosocomial infection — An infection occurring in a patient in a hospital or other healthcare facility in whom it was not present or incubating at the time of admission; or the residual of an infection acquired during a previous admission.
Includes infections acquired in the hospital but appearing after discharge, and also such infections among the staff of the facility. Synonym: hospital-acquired infection. Pathogenicity —The property of an infectious agent that determines the extent to which overt disease is produced in an infected population, or the power of an organism to produce disease.
Prevalence rate —The total number of persons sick or portraying a certain condition in a stated population at a particular time point prevalence , or during a stated period of time period prevalence , regardless of when that illness or condition began, divided by the population at risk of having the disease or condition at the point in time or midway through the period in which they occurred.
QuarantIne —Restriction of the activities of well persons or animals who have been exposed to a case of communicable disease during its period of communicability i. See Isolation. It may be designed to accommodate particular situations.
Examples are exclusion of children from school, exemption of immune persons from provisions applicable to susceptible persons, or restriction of military populations to the post or to quarters. It includes: personal surveillance , the practice of close medical or other supervision of contacts to permit prompt recognition of infection or illness but without restricting their movements; and segregation , the separation of some part of a group of persons or domestic animals from the others for special consideration, control or observation; removal of susceptible children to homes of immune persons; or establishment of a sanitary boundary to protect uninfected from infected portions of a population.
Report of a disease —An official report notifying an appropriate authority of the occurrence of a specified communicable or other disease in humans or in animals. Diseases in humans are reported to the local health authority; those in animals, to the livestock, sanitary, veterinary or agriculture authority.
Some few diseases in animals, also transmissible to humans, are reportable to both authorities. Each health jurisdiction declares a list of reportable diseases appropriate to its particular needs see Communicable Disease Reporting. Reports should also list suspected cases of diseases of particular public health importance, ordinarily those requiring epidemiologic investigation or initiation of special control measures.
When a person is infected in one health jurisdiction and the case is reported from another, the health authority receiving the report should notify the jurisdiction where infection presumably occurred, especially when the disease requires examination of contacts for infection, or if food, water or other common vehicles of infection may be involved. In addition to routine report of cases of specified diseases, special notification is required of all epidemics or outbreaks of disease, including diseases not listed as reportable.
See Epidemic. Reservoir of infectious agents —Any person, animal, arthropod, plant, soil or substance or combination of these in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host. Resistance —The sum total of body mechanisms that interpose barriers to the invasion or multiplication of infectious agents, or to damage by their toxic products.
Inherent resistance—an ability to resist disease independent of immunity or of specifically developed tissue responses; it commonly resides in anatomic or physiologic characteristics of the host and may be genetic or acquired, permanent or temporary. See Immunity. Synonym: Nonspecific immunity. Source of infection —The person, animal, object or substance from which an infectious agent passes to a host. Source of infection should be clearly distinguished from source of contamination, such as overflow of a septic tank contaminating a water supply, or an infected cook contaminating a salad.
See Reservoir. Surveillance of disease —As distinct from surveillance of persons see Quarantine, 2 , surveillance of disease is the continuing scrutiny of all aspects of occurrence and spread of a disease that are pertinent to effective control.
Included are the systematic collection and evaluation of. A report summarizing the above data should be prepared and distributed to all cooperating persons and others with a need to know the results of the surveillance activities. The procedure applies to all jurisdictional levels of public health from local to international. Serologic surveillance identifies patterns of current and past infection using serologic tests.
Susceptible —A person or animal not possessing sufficient resistance against a particular pathogenic agent to prevent contracting infection or disease when exposed to the agent.
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