Dr. James Evermann
Lecture 2: Transmission and Control
Environment -The environment may be considered a source of infection when the disease agent multiples in nature, not requiring any animal host for its continued survival, e.g. Histoplasma capsulatum, a fungus that resides in the soil.
Animal Reservoir - Animals may serve as a reservoir for infections including carrier animals which are (animals with unapparent infections that are also transmitting the agent). Animals may also serve as intermediate hosts and vectors. Animal reservoirs are the most common source of infectious agents.
Iatrogenic - Also referred to as nosocomial infections. These infections are acquired through human intervention e.g. the use of common needles that become contaminated with an infectious organism.
Transmission: Distinguishing the mode of transmission from the route of infection.
Modes of transmission may be broadly classified as horizontal (animal A to animal B) or vertical (animal A to its offspring). Within horizontal transmission there are direct, indirect and airborne. Vertical transmission is an infection that is spread from parent to offspring. Vertical transmission includes in utero (congenital) infection and infection via colostrum.
Route(s) of infection. The route of infection refers to the manner by which an infectious agent gains access to the animal. Routes of infection (and exit of an organism from the body) include alimentary, respiratory, urogenital, anal, skin, and conjunctival.
"Ping-pong" transmission – A form of horizontal transmission amongst susceptible animals.
Diseases are spread from one animal to another by several modes of transmission.
Introduction to the Science of Epidemiology
Populations of Animals versus Individual Animal
Epidemiology is the study of health and disease in populations
Epidemiologists study disease in its natural habitat, not in a controlled laboratory environment.
Herd Health/Preventative Medicine is a form of epidemiology which combines quantitative (analytical; data gathering; statistics; etc.); ecological (agent-host-environment triad); etiologic (causal relationship; "shoe leather" fact finding); and clinical (disease symptoms) epidemiology.
Objective of Herd Health/Preventative Medicine – To design optimal management and effective disease control strategies. Economic considerations frequently determine which strategy is most effective. Strategies are expressed as either cost effectiveness or cost-benefit.
Prevalence – The proportion of sampled animals that display a condition of interest at a given point in time. It is a single measurement of each animal in a group. It is usually a static measure in which the time is short (1-7 days).
Incidence – The proportion of sampled animals that develop a condition of interest over a defined period of time. It is the measure of the occurrence of disease over time in a defined group of susceptible animals. Incidence can also be applied to measuring the occurrence of new cases of disease in an ever-changing population over time.
Attack Rate – The proportion of a defined population affected during an outbreak. Since the attack rate is based only on new cases of disease, it is comparable to incidence.
Risk Assessment and Prevention
Risk factors are those factors associated with an increased likelihood of an accident
and/or disease occurring. Risk factors for many animal diseases are poorly understood and
only come to light through systematic study of naturally or spontaneously occurring cases.
Risk factors include:
Cleaning and Disinfection Terminology
disinfectant: germicidal chemical that destroys microorganisms and the potential infectivity of a material
antiseptic: a mild disinfectant used on living tissues
sanitizer: disinfectant of low toxicity used to reduce microbial contamination of food handling equipment
sterilize: destroy all microorganisms; usually not possible with chemical disinfectants, and requires physical disinfection
bacteriostat: inhibits or retards bacterial growth
bacteriocide: kills bacteria
virucide: kills viruses
fungicide: kills fungi
parasiticide: kills parasites
Disinfectants act in a variety of ways, but in general, coagulate, precipitate or otherwise denature protein components in the cell wall, protoplasm or enzyme systems of microorganisms. Disinfectants vary in their activity and efficacy. The phenol coefficient is one measure of their power, and may be present on the product label. The coefficient is a measure of how many more times the disinfectant can be diluted versus phenol and still retain equivalent activity. Tests in hard water and in the presence of protein/organic matter are realistic tests; the results of such tests also may be on the label.
General Points About Disinfectants:
READ THE LABEL OF THE DISINFECTANT BEFORE YOU BUY AND BEFORE YOU USE
Cleaning and disinfecting a building
Generally, 90% of microorganisms are removed by physical cleaning. 6-7% are subsequently removed by disinfection and 1-2% subsequently killed by fumigation
Physical methods of disinfection and sterilization include:
Examples of chemical disinfectants