Chp 19 Epidemiology

Transmission - vectors

Are living organisms that can carry a disease causing microbe. Most common are arthropods, vector may carry organism internally or externally. Control of vector-borne disease is directed at controlling the arthropod population

Zoonotic infections

Disease often more severe in humans than in normal animal. Infection in humans is accidental.

Communicable disease

Disease transmitted from one host to another. In order for disease to spread, a chain of events must occur. Pathogen must have suitable environment in which to live.

Endemic

Diseases that are constantly present

Respiratory system infections

Encompass an enormous variety of illnesses: trivial to fatal. Divided into infections of upper respiratory and lower respiratory

Trasmission - direct contact

Hands are main vehicle of contact transmission. Han washing is an important preventative measure. Pathogens that do not survive for extended periods in the environment are usually spread by direct contact.

Fomites

Indirect contact involves transfer of pathogens via inanimate objects such as clothing, table-tops, doorknobs, and drinking glasses.

Human reservoir

Infected humans are the most significant reservoirs. In some cases humans are the only reservoir, in this case disease is easier to control. Human reservoirs can be symptomatic infections or asymptomatic carriers. Asymptomatic carriers potentially more dangerous.

Transmission - indirect contact

Involves transmission of pathogens via inanimate objects or fomites. Organisms on hands or fingers of carrier can be transferred to objects and picked up by another individual. Hand washing is an important control measure.

Portals of exit

Microbes must leave on host in order to be transmitted to another.

Lower respiratory

More serious, can be life threatening

Normal flora

Nasal cavity, nasopharynx, and pharynx colonized by numerous bacteria: many classes of organisms are present from aerobes to anaerobes. Other sites are sterile. Conjunctiva commonly have no bacteria: microbes are swept into nasolacrimal duct, tear duct, and nasopharynx.

Reservoir

Natural habitat of a pathogen

Food can become contaminated in different ways

Organisms can originate with animal, organisms can be inadvertently added during food preparation. Cross-contamination, cutting boards and knives.

How would the following organisms exit the host?

Organisms inhabiting intestinal tract are shed in feces. Organisms inhabiting respiratory tract are expelled in respiratory droplets of saliva. Organisms of the skin are shed with skin cells as they slough off. STDs are shed in seamen and vaginal secretions.

Portal of exit

Pathogen must leave reservoir to be transmitted to susceptible host. Organisms leaves animal through portal of exit.

Transmission - food and water

Pathogens can be transmitted through contaminated food and water. Food can become contaminated in different ways

Mortality

Rates reflect population that dies from disease

Morbidity

Rates reflects the number of cases of illness in a population

Outbreak

Refers to a cluster of cases in a brief time affecting a specific population

Incidence

Reflects number of new cases per specific time period

Prevalance

Reflects total number of existing cases

Reservoirs of infectious disease

Reservoir of a pathogen affects the extent and distribution of a disease. Recognizing the reservoir can help protect population from disease. Reservoirs can be human, non-human animal, and environmental

Transmission - air

Respiratory droplets can be transmitted through the air. Smaller droplets dry in the air leaving one or two organisms attached to dry material - creates droplet nuclei. Airborne transmission is difficult to control, HEPA filters effective at removing airborne organisms.

Droplet nuclei

Smaller droplets dry in the air leaving one or two organisms attached to dry material - creates droplet nuclei. Droplet nuclei can remain suspended indefinitely in the presence of light air currents.

Environmental reservoirs

Some pathogens have environmental reservoirs, which can include water and soil. These pathogens are difficult or nearly impossible to eliminate.

Epidemiology

Studies the cause and distribution of health states, positive and negative, in populations

Upper respiratory

Uncomfortable but generally not life threatening

Epidemic

Unusually large number of cases in a population

Food and water - waterborne disease outbreaks

Waterborne disease outbreaks can involve large numbers of people - due to the fact that municipal water is distributed to large areas. Prevention of waterborne disease requires: chlorination and filtration of public water sources, proper disposal of sewage.

Pandemic

When epidemic spread worldwide

Non-human animal reservoirs

Zoonotic infections

Transmission - droplet transmission

microbe-laden respiratory droplets generally fall to the ground within 3 feet of release. People in close proximity can inhale infected droplets. Considered direct transmission because of the close range required for transmission.