General principles of infectious diseases (epidemiolgy)

epidemiology is the study of

disease in populations and of factors that determine its occurence

epidemiology

distribution and determinants of health and diseases, morbidity, injuries, disability, and morality in populations

epidemiologic studies are

applied to the control of health problems in populations

distribution

the occurrence of diseases and other health outcomes varies in population

determinants

any factor that brings about change in a health condition or other defined characteristic

determinants are related to all risk factors but

not only infectious

determinants of health can be associated with

host, agent or the environment

population

all the organisms of the same group or species, which live in a

particular geographical area/habitat

infectious disease

Illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the environment

exposed individual

one that has met with an infectious agent in a way that we know from experience may cause disease

epidemic

the occurrence in a community or region of cases of an illness, specific health related behavior, or other health related events clearly in excess of normal expectancy

is the flu an epidemic

no because we expect it to come in the cold months

endemic

disease occurs at expected frequency (often predictable) is always present in a population usually at a stable low level

epidemic or endemic

endemic

epidemic or endemic

epidemic

pandemic

an epidemic occuring worldwide or over a very wide area crossing international boundaries and usually affected a large number of individuals

example of a pandemic

bluetongue virus and covid

patterns of infectious diseases

sporadic

sporadic

disease occurs irregularly and haphazardly; appropriate circumstances have occurred locally that produce small localized outbreaks

example of a sporadic disease

bovine spongiform encephalopathy

goal of infectious agents

maintain their existence by replicating and spreading within and among ecological niche including animal populations

what are some mechanisms infectious agents have to succeed

compete with other flora (commensals) and adapt to changing environments (use of different antibiotics)

generation times of infectious agents

short generation times compared to animal hosts

goal of veterinary epidemiologists

determine the disease origin where the cause is known and control it

investigate and control a disease where the cause is unknown

acquire information on the ecology and natural history of a disease

determine the disease origin where the cause is known and control it example

determining why salmonellosis in calves occurred (contaminated feed?)

investigate and control a disease, whose cause initially is

unknown example

contagious bovine pleuropneumonia was eradicated from the US before knowing the causing agent was mycoplasma mycoides; knew the infectious nature of the disease

acquire information on the ecology and facts of disease example

host is the animal infected with an infectious agent

hosts and agents live in certain environments that live with others

ecology

study of ecosystems

environment of an ecosystem affects

the survival rate and spread of infectious agents

horizontal transmission

infections transmitted from any segment of a population to another

direct horizontal transmission

when a susceptible host contracts an infection, by physical contact with an infected host or its infectious secretions/discharges

indirect horizontal transmission

involves an intermediate vehicle that transmits infection between infected and susceptible hosts

vector

an animate transmitter of infectious agents

vector example

invertebrate animals usually anthropods that transmit infectious agents to vertebrates (Lymes)

fomites

inanimate carriers of agents

fomite example

transportation trucks contaminated with salmonella species

vertical transmission

infections transmitted from one generation to the next by infection of the embryo or fetus while in utero (in mammals) or in ovo (in birds, reptiles, amphibians, fish and arthropods); some consider colostrum as well

reservoir

Any animal species, insect, soil or substance in which the infectious agent normally lives and multiplies so that it can be transmitted to a susceptible animal

reservoir example

raccoons are reservoirs for rabies

factors influencing disease risk of the host

age, breed, sex, previous exposure, vaccine status, production status (lactating vs not), passive immune status

factors influencing disease risk of the agent

dose, infectivity, toxicity, survival in environment, replication in the environment

factors influencing disease risk of the environment

housing, management, cleaning, nutrition, airflow, temperature

infectivity

capacity of an agent to enter and multiply in a susceptible host and produce infection or disease

pathogenicity

the capacity of the agent to cause overt disease in the infected host or not

what kind of trait is pathogenicity

qualitative trait (not quantification)

how is pathogenicity meaured

by the ratio of the number of individuals with clinically apparent disease to the number exposed to the infection

virulence

Refers to an agent’s capacity to induce different extent of pathology in the host (degree of pathology)

what kind of trait is virulence

quantitative

how is virulence measured

Measured by the ratio formed by the number of total cases with overt infection divided by the total number of infected cases

if virulence is fatal, use

case fatality risk (CFR)

resistance

all the body’s defenses against infection

immunity

particular type of resistance associated with cells or antibodies of the immune system

active immunity

Immunity acquired by previous exposure to the antigens of the agent (by previous infection or vaccination); short duration to life long

passive immunity

Immunity acquired through transfer of antibodies, such as from colostrum, transplacental transfer or administration of hyperimmune serum; short lived

herd immunity

Resistance of a group of animals to an infectious disease due to large percentage of the group being immune

what does infectiousness depend on

the duration of the period when an animal is contagious and the relative amount of an infectious agent that an animal can transmit

latent period

a period of time lapse between infection and the ability to transmit the agent

incubation period

period of time between infection and the development of clincal signs (not infectiousness)

carrier

an individual who harbors the organism but without clincal signs (subclinical)

latent period example

Animals may have rabies virus in their saliva and be able to transmit virus a short period of time (reported up to 8 days) before clinical signs start

incubation period

rabies incubation period is 3-8 weeks and subclinical infections have a latent period but do not have an incubation period

carrier could be

fully infectious or have lower infectiousness during that period

subclinical cases are often

more frequent than clinical cases

subclinical disease often drives

the infection transmission and economic impact of a disease