Chain of Infection

Interventions are directed at:

-Controlling or eliminating agent at source of transmission -Protecting portals of entry -Increasing host's defenses

direct transmission

-Direct contact −Droplet spread

Factors that may increase susceptibility to infection by disrupting host defenses include:

-Malnutrition -alcoholism, and -disease or therapy that impairs the nonspecific immune response

Typhoid Mary

A historical example (early 1900's) of a woman who was a chronic carrier of Salmonella typhi. Over the course of her infection, she was responsible for at least 10 outbreaks in New York. she infected 47 people, three of whom died from the disease. • Her notoriety is in part due to her vehement denial of her own role in spreading the disease, together with her refusal to cease working as a cook. • She was forcibly quarantined twice by public health authorities and died in quarantine. It is possible that she was born with the disease, as her mother had typhoid fever during her pregnancy.

Carriers (asymptomatic)

Human reservoirs that commonly transmit disease because they do not realize they are infected • consequently take no special precautions to prevent transmission

indirect transmission

Indirect transmission refers to the transfer of an infectious agent from a reservoir to a host by suspended air (airborne) particles, inanimate objects (vehicles), or animate intermediaries(vectors) − Air-borne − Water-borne − Vehicle-borne − Vector-borne (mechanical or biologic)

zoonotic diseases include

Long recognized zoonotic diseases include: • brucellosis (cows and pigs) • anthrax (sheep) • plague (rodents) • trichinellosis/trichinosis (swine) • tularemia (rabbits) • rabies (bats, raccoons, dogs, and other mammals) -West Nile encephalitis (birds), • monkeypox (prairie dogs) HIV/AIDS • Ebola infection and • SARS are thought to have emerged from animal hosts, although those hosts have not yet been identified

human reservoirs

Many common infectious diseases have human reservoirs •Diseases that are transmitted from person to person without intermediaries include: the sexually transmitted diseases • Measles • Mumps • Streptococcal infection • Many respiratory pathogens

animal reservoirs

Many infectious diseases also have Animal reservoirs • Humans are also subject to diseases that have animal reservoirs • Many of these diseases are transmitted from animal to animal, with humans as incidental hosts

environmental reservoir

Plants, soil, and water in the environment are also reservoirs for some infectious agents Many fungal agents live and multiply in the soil• E.g. Histoplasma capsulatum that cause histoplasmosis Some bacteria live and multiply in water supplies in cooling towers and evaporative condensers • E.g. Legionella pneumophila that cause Legionnaires disease outbreaks

host

The final link in the chain of infection is a susceptible host • Susceptible host: is a member of a population who is at risk of becoming infected by a disease • Susceptibility of a host depends on: • genetic or constitutional factors• specific immunity, and • nonspecific factors that affect an individual's ability to resist infection or to limit pathogenicity • An individual's genetic makeup may either increase or decrease susceptibility • For example, persons with sickle cell trait seem to be at least partially protected from a particular type of malaria

immunity

When there is little to no immunity within a population, the disease spreads quickly

Passive Immunity (natural passive)

acquired through transplacental transfer of a mother's immunity to diseases to the unborn child (also via breastfeeding) • can also come from the introduction of already produced antibodies into a susceptible case

zoonosis

an infectious disease that is transmissible under natural conditions from vertebrate animals to humans

Vector

an organism that transmits a pathogen from reservoir to host • E.g. 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

Active Immunity

body produces its own antibodies • can occur through a vaccine or in response to having a similar disease • Similar to acquired

incubatory carrier

exposed to and harbors a disease and is in the beginning stages of the disease, showing symptoms, and has the ability to transmit the disease

intermittent carrier

exposed to and harbors disease and can intermittently spread the disease

passive carrier

exposed to and harbors disease causing organism, but has no signs or symptoms

convalescent carrier

exposed to and harbors disease- causing organism (pathogen) and is in the recovery phase but is still infectious

Healthy carrier

exposed to and harbors pathogen, has not shown any symptoms

active carrier

individual exposed to and harbors a disease-causing organism. May have recovered from the disease

Modes of transmission

means by which an organism (infectious agent) transfers from one carrier to another (susceptible host) by direct or indirect transmission

Acquired Immunity

obtained by having had a dose of a disease that stimulates the natural immune system or artificially stimulating immune system

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 • E.g. infectious mononucleosis ("kissing disease") and gonorrhea are spread from person to person by direct contact 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 • E.g. Influenza and meningococcal infection

Carrier

one that spreads or harbors an infectious organism

Carriers (symptomatic)

persons who are aware of their illness • may be less likely to transmit infection because • they are either too sick to be out and about • Take precautions to reduce transmission, or • receive treatment that limits the disease

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

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 Infectious agents may use the same portal to enter a new host that they used to exit the source host • E.g. influenza virus exits the respiratory tract of the source host and enters the respiratory tract of the new host In contrast, many pathogens that cause gastroenteritis follow a so-called "fecal-oral" route • they exit the source host in feces, are carried on inadequately washed hands to a vehicle such as food, water, or utensil, and enter a new host through the mouth Other portals of entry include:• the skin (e.g. hookworm, schistosomiasis, malaria)• mucous membranes (syphilis), and• blood (hepatitis B, human immunodeficiency virus)

Vehicles of indirect transmission

that may indirectly transmit an infectious agent include: Food Water Biologic products (blood), and Fomites (inanimate objects e.g. 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.

reservoir

the habitat in which the agent normally lives, grows, and multiplies humans, animals, environment -The reservoir may or may not be the source from which an agent is transferred to a host E.g. Clostridium botulinumReservoir: SoilSource of most botulism infections: improperly canned food containing C. botulinum spores

portal of exit

the path by which a pathogen leaves its host or •the route of escape of the pathogen from the reservoir e.g. -Respiratory tract (e.g. mouth, nose) •Intestinal tract (e.g. feces) •Urinary tract • Blood •Other body fluids

herd immunity

the resistance a population or group (herd) has to the invasion and spread of an infectious disease

airborne

transmission occurs when infectious agents are carried by dust or droplet nuclei suspended in air • 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 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 • E.g. Measles has occurred in children who came into a physician's office after a child with measles had left, because the measles virus remained suspended in the air

In contrast, in biologic transmission, the causative agent of: • Malaria • Schistosomiasis, or • Guinea worm disease

undergoes maturation in an intermediate host before it can be transmitted to humans

Diseases for which vaccines are used

• Antrhax• Chicken pox• Cholera• Diphtheria• German measles (rubella)• Hepatitis A & B• Influenza• Malaria (in process)• Measles• Menigitis• Mumps• Plague • Pneumonia• Polio• Rabies• Small pox • Spotted fever• Tetanus• Tuberculosis• Typhoid Fever• Typhus • Whooping Cough• Yellow Fever

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