Lecture 7- Disease causes, determinants, transmission

agent factors in epidemiology triad

o Infectivity

o Pathogenicity

o Virulence

o Immunogenicity

o Antigenic stability

o Survival

role of agent in the epidemiological triad

all may have varying inherent factors influencing their ability to cause disease

what are the limitations of Koch's postulates

o Do not apply for inapparent infections or carrier states

Cholera and typhoid fever

o Some organisms are tough (or impossible) to grow in pure culture

M. leprae

o Rarely apply to viral infections/diseases

o Do not apply to multi-factorial etiologies

Describe epidemiology triad (Host, Agent, Environment)

all play a role in disease potential

define infectivity

a measure of the ability of a disease agent to establish itself in the host

o Usually referred to qualitatively (low, medium, high)

define pathogenicity

epidemiological term used to describe the ability of a particular disease agent of known virulence to produce disease in a range of hosts under a range of environmental conditions

define virulence

a measure of the severity of a disease caused by a specific agent

Describe host factors

• Species

• Breed

• Sex

o Hemophilia A (Factor VIII deficiency)isthe most common inherited bleeding disorder in dogs

o Usually, females carry the gene for the disease without showing any signs, while males do show signs

• Age

• Conformation

• Genotype

• Nutrition Status

• Physiologic status

• Pathologic status

environment factors relevant to disease transmission

• Weather

• Housing

• Geography

• Management practices

• Noise

• Water Quality

• Air Quality

• Food

• Chemicals

• Others

what are Koch's postulates?

• For a disease agent to be infectious, it must fulfil these postulates:

o The agent should be present in all cases of the disease

o It can be isolated and grown in pure culture

o It should be capable of producing the disease when inoculated into healthy animals

o The same organism should be recovered from the diseased animal

define direct contact transmission

the physical contact between an infected individual and a susceptible individual, and the physical transfer of microorganisms

direct contact transmission includes-

Includes touching an infected individual, licking, sexual contact, contact with oral secretions, transplacental, body secretions or contact with body lesions

Direct transmission- airborne

Airborne diseases include any that are caused by pathogens and transmitted through the air

o FMD virus

Airborne transmissions have been recorded up to 50 km over land

More than 200 km over water

Direct transmission- droplet

primarily respiratory (coughing, sneezing)

what is a fomite?

any object or substance capable of carrying infectious organisms, such as germs or parasites, and hence transferring them from one individual to another

define indirect transmission

situations where a susceptible person is infected from contact with a contaminated surface or material (bedding, feeders, etc.)

Foodborne disease

A disease caused by consuming contaminated food or drink

Indirect transmission- Foodborne

o Numerous microbes and toxic substances can contaminate foods

o There are numerous known foodborne illnesses

o The majority are infectious and are caused by bacteria, viruses, and parasites

Indirect transmission-vector borne

Transmission via insect or animal vector

define vertical transmission

Transmission from parent to offspring via ovum, sperm, placenta or milk

define horizontal transmission

disease is spread through a population from one infected individual to another

CDC definition of zoonosis

any disease or infection that is naturally transmissible between vertebrate animals and humans

• A zoonotic disease is a disease that can be passed between animals and humans (CDC)

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