Dynamics of Disease Transmission

What is the epidemiological triad and what does it connect?

• model used to explain how disease occurs through the interaction of three factors: agent (pathogen), host, and environment

What is host susceptibility to an agent (pathogen) determined by?

• biological characteristics - genetics, nutrition, immune response (ex. immunocompromised)

• demographics - age, gender, race, reproductive status

• socioeconomic factors - crowding, povertu

• behavior - ex. multiple sexual partners, cannibalism, needle sharing

Why are vector-borne diseases more complicated to combat?

• have to manage vector AND pathogen

• more impacted by environmental conditions

• vectors can spread efficiently between many hosts

What disease transmission dynamics affect concern over widespread transmission?

• agent factors - type of infectious agent (bacteria, protozoan, etc.)

• pathogenecity and severtiy of disease

• environmental factors, including cultural

• vector factors - mode of transmission, aggressive vs nonaggressive feeding behavior, longevity, infection rate, seasonality of vector activitry

Describe the four stages that determine the risk of pathogen spillover from animals to humans

1. presence in animal population - impacted by reservoir host distribution/density, prevalence and intensity of infection

2. transmission to human - impacted by shedding rate, harvest rate, pathogen survival, biting rate of vector on host, vector survival

3. human exposure to pathogen - impacted by human behavior, preparation of meat, biting rate of vector on human

4. host susceptibility- innate immune responses and compatability, structural barriers

What are barriers to successful disease spillover?

• reservoir host distribution

• reservoir host density

• pathogen prevalence

• infection intensity

• pathogen release from reservoir host

• pathogen survival and spread

• human exposure

• strucutral barriers

• innate immune response and molecular compatability (HUGE BARRIER)

• replication cycle

if any barrier is impenetrable, spillover CAN NOT occur

What are the most and least limiting barriers for pathogens to successfully transition from animal to human infection?

• most - pathogen release

• least - reservoir density and distribution

What are the modes of transmission categories of pathogens?

• direct

• indirect

What are the direct modes of pathogen transmission?

• physical contact with infected person

• droplet - short distance spread through sneezing, coughing, etc. LARGER

• vertical - transplancental (mother to fetus)

What are the indirect modes of pathogen transmission?

• airborne - pathogen in air as aerosol SMALLER

• vehicle-borne - inanimate objects (fomites) - ex. medical devices, water, soil, food, blood - may have to be ingested first

• vector-borne - either via relationship with pathogen or mechanical (fly lands on horse poop, picks up pathogen, lands on cupcake)

What are common pathogen portals of entry into the body?

• skin

• respiratory tract

• GI tract

• genitourinary tract

• transplacental (mother to fetus)

What are common pathogen portals of exit from the body?

• skin

• blood

• secretions

• urine

• feces

How do pathogens taht enter through the respiratory tract exit the body?

secretions

How do modes of transmission impact the chain of disease infection?

• link portal of entry and exit

• determine how easily a disease can spread

• determines which control measures can break chain of infection

Airborne transmission is associated with what portal of entry and exit?

• entry - respiratory tract

• exit - respiratory secretions

ex. flu

GI tract portal of entry is associated with what form of transmission and portal of exit?

• transmission - food (indirect)

• exit - feces

Mosquito-borne transmission is associated with what portal of entry and exit?

• entry - skin (via mosquito bite; indirect)

• exit - skin (via mosquito bite; indirect)

What are the major components of the chain of infection?

• infectious agent

• reservoir

• portal of exit

• mode of transmission

• portal of entry

• susceptible host

What is biological and mechanical vector transmission

• biological - pathogen develops/replicated within a vector

• mechanical - vector physically carries pathogen without pathogen developing (ex. bacteria on fly foot)

What is vertical transmission?

transmission from mother to offspring

What is zoonotic disease transmission?

transmission of a pathogen from animals to humans

What happens if one spillover barrier is impenetrable?

spillover can not occur

How does reservoir host density affect spillover risk?

greater reservoir density can increase opportunities for pathogen maintanance, mutation, transmission, and human exposure

How does pathogen shedding affect spillover risk?

greater shedding increases the amount of pathogen released into the environment and opportunities for transmission

What type of transmission involves contaminated food or water?

indirect; vehicle-borne

What type of transmission involves a contaminated object?

indirect; vehicle-borne

A pathogen leaves an infected host in feces, contaminates food, and enters another person through the GI tract. Identify the chain.

• portal of exit - feces

• transmission - indirect; vehicle-borne via food

• entry - GI tract