Disease Transmission: Infectious Disease Epidemiology II

Infectious disease results from what

the invasion of a host by a pathogenic organism.

Disease causation is considered what

multifactorial, & infectious diseases result from the interaction of the agent, host and the environment.

What do we mean by the portal of exit

Infectious disease transmission results when the agent leaves its reservoir or host through a portal of exit.

Portal of exit often corresponds to the site where

the pathogen is localized e.g., respiratory viruses exit via the respiratory tract.

Portal of exit may include what parts of th body

nose, mouth, anus, body surface including wounds, urogenital tract, eyes, mammary glands.

The pathogen exits through an appropriate medium depending on what

depending on the portal of exit e.g., from the nose via nasal secretions, from the mouth via saliva, from the anus via feces etc.

What are Reservoirs?

• A primary habitat in which an infectious agent is typically found.

A reservoir host is one in which is what

in which an infectious agent normally lives, grows and multiplies, and is therefore a common source of infection for other animals.

The word reservoir is also used to mean

any substance that is a common source of infection e.g., soil is a reservoir for anthrax (Bacillus anthracis), Burkholderia pseudomallei.

What is an Animal reservoirs:

Animals may be important reservoirs of infection for other animals and humans e.g., bats , raccoons, foxes, skunks may be reservoirs of rabies virus for other animals & humans.

What is a Human reservoirs:

humans are reservoirs of many infectious diseases of humans, that can also be transmitted from people to humans e.g., human alphaherpesvirus-1 transmitted from humans to ring-tailed lemurs.

What is an environmental reservioirs

Plants, soil & water are also reservoirs of some infectious agents e.g., fungal agents that live & multiply in soil.

What is the mode of transmission (method of transmission):

Upon exit, the agent is conveyed to a susceptible host via an appropriate mode of transmission and enters through an appropriate portal of entry.

The mode of transmission may be either:

• Horizontal (lateral)

• Vertical

What is the defintion of Horizontal transmission:

Involves the transmission of the pathogen from one segment of the population to another. E.g., canine influenza from one dog to another in a household, bovine respiratory disease from one cow to others in the feedlot.

What is the definition of Vertical transmission:

Involves transmission of infections from one generation to the next via infection of the embryo or fetus in utero (in mammals) or in ovo (in birds, retiles, fish, amphibians & arthropods.

Vertical transmission involves what

transplacental or perinatal transmission from mother to her offspring (e.g., Neospora caninum in cows.

Horizontal transmission may be either

direct or indirect

What do we mean by Close contact:

transmission requires close contact between two individuals e.g., bovine tuberculosis.

What do we mean by Casual contact:

transmission is through less intimate contact (e.g., respiratory disease in feedlots or dog shelters)

What do we mean by Sexual transmission:

requires sexual contact (or artificial insemination) e.g., Tritrichomous fetus)

What do we mean by Droplet spread:

Transmission occurs through the spraying of relatively large-short range aerosols produced by sneezing, coughing or barking (dogs), talking (in people)

What is Contaminative transmission:

agent survives for prolonged periods of time in the environment & exposure of susceptible animals occurs from environmental exposure to soil, or vegetation harboring the infectious agent e.g., in anthrax.

Diseases that are spread from individual-to-individual are called

contagious disease (or communicable diseases in humans)

How is airborne transmission

long distances is often defined as indirect, but it is correctly defined as direct because no intermediate vehicle is involved.

In airborne transmission, infectious agents are carried by

dust or droplet nuclei suspended in air.

Airborne dust includes:

• material settled on surfaces and become resuspended by air currents e.g., pathogen spread in feedlots spread in bare ground (dirt based) feedlots.

• infectious particles blown from the soil by the wind.

What do we mean by Indirect transmission:

Involves the transfer of an infectious agent from an infected host to a susceptible host through an intermediate vehicle (living or inanimate).

Indirect transmision occurs through

animate intermediaries (vectors), inanimate objects (vehicles or fomites), or suspended air particles.

What is Vector-borne:

often by insect vectors e.g., mosquitoes, ticks, fleas etc.

What is Vehicle-borne:

transmission through vehicles such as food, water, biological

products like blood, fomites like surgical equipment e.g., scalpels, farm tools and equipment, needles etc.

What is Reservoir species:

this involves transmission of an agent by another species e.g., rabies transmission from an infected bat to cattle.

There are two methods of vertical transmission: what are they

• Hereditary:

• Congenital:

What is Hereditary transmission

hereditarily transmitted diseases are carried within the genome of either parent or both parents.

What is Congenital transmission

These are present at birth. Transmission occurs at various stages of embryonic development.

What is Germinative transmission:

infection of the superficial layers of the ovary or infection of the ovum itself e.g., in avian salmonellosis or chicken leukosis virus.

Transmission to the embryo via the

placenta (transplacentally) or via fetal circulation through the placenta to the fetus.

Where does Ascending infection from the

lower genital canal to the amnion & placenta e.g., staphylococcus & streptococcus infection.

Infection at parturition acquired from the

lower genital canal at birth e.g., Human herpes simplex infection.

What is the Portal of entry:

This refers to the path through which an agent enters a susceptible host.

The portal of entry provides access to

tissues in which the pathogen can multiply or the toxin it produces can act.

Infectious agents often use what

the same portal they used to exit the source host to enter a susceptible host

Routes of infection are basically the site or sites of what

through which a pathogen gains entry into a host, and by which it leaves the host

Routes of infection are basically the portals of

exit & entry.

one of the more common routes of entry is what

Oral route

Infectious agents are excreted in feces of the source host,

contaminate food or water which then act as a vehicle/fomites, or are carried on inadequately washed hands, entering the new host via the mouth, Which route is this

Fecal-oral route:

What is Respiratory route:

another common route of transmission. Likely to occur in locales with high population density e.g., crowded barns, & where ventilation is poor.

The six common modes/methods of transmission

• Ingestion

• airborne (aerial) transmission of infectious agents via contaminated air

• Contact

• Inoculation-actually a form of contact transmission.

• Iatrogenic transmission-by the doctor (transmission through surgical & medical practice.

• Coitus

The mode of transmission dictates the

epidemic picture

Agents that are transmitted by the airborne (e.g., COVID-19) or fecal-oral route tend to

produce sudden explosive epidemics

Diseases spread primarily through coitus spread how fast

spread slowly over a long period of time

Appropriate disease prevention & control measures are determined based on

knowledge of the chain of infection/transmission (portals of exit & entry, modes of transmission.

When designing practical disease prevention/control measures, direct your efforts to the

segment in the infection chain that is susceptible to the chosen interventions.

Some disease prevention/control efforts that are directed at the mode of transmission include:

• Interruption of direct transmission through quarantine & isolation

• Interruption of vehicle borne transmission through decontamination & cisinfection practices

• Interruption of fecal-oral transmission through proper storage of feed, foodstuffs & promoting hand washing.

• Interruption of airborne transmission through proper ventilation, appropriate stocking densities in barns, masks, social distancing.

• Vector population control to interrupt vector borne transmission e.g., spraying against ticks, mosquitoes, use of vector nets for cows, bed nets for people.

Three factors are important for the transmission of infection

• Characteristics of the host

• Characteristics of the pathogens

• Effective contact

what is the Characteristics of the host:

• Host susceptibility and infectiousness determine the ability to transmit infection.

• Susceptibility may vary by age, or breed or other host factors.

what is the Characteristics of the pathogens

• Infectivity, virulence & stability are pathogen characteristics that affect transmission.

• Some pathogens require low doses to cause disease.

• Pathogen stability is the length of time it can remain infective outside its host

• Some survive for only short periods; others are more resistant.

What is the effective contact

• Effective contacts are contacts that are likely to result in transmission

• They may be short e.g., in seasonally transmitted disease or vector borne diseases or long e.g., anthrax spores in soil.

Probability of transmission refers to the likelihood of an

• infected individual passing the disease to an uninfected individual

• It is the proportion of contacts which result in transmission if one of the individuals is infectious & the other is susceptible.

Basing on the factors associated with the spread discussed previously, the likelihood of spread from an infected to an uninfected will depend on:

• The characteristics of the pathogen

• The nature of contact-was the contact between the two really adequate for transmission to occur?

• Infectiousness of the infected animal/person

The goal of an outbreak response is

always to end the outbreak.

• This may mean stopping transmission completely or the disease occurrence returns to the expected level.

The three main targets of outbreak control are:

• Reducing infectious contacts through quarantine & isolation, social distancing, proper housing to prevent vector entry.

• Reducing the per contact transmission probability e.g., through pre-exposure or post exposure prophylaxis, proper sanitation in premises.

• Reducing the number of susceptible individuals through vaccination-This may lead to herd immunity.