Chapter 1 & 2: Introduction to Zoonotic & Vector-borne Diseases

Zoonotic Diseases

Zoonotic diseases are diseases transmitted from animals to humans.
The term "zoonotic" is derived from the Greek roots "zoo" (animal) and "noses" or "notic" (disease).
Transmission occurs through direct or indirect contact with an infected animal.
There are many zoonotic diseases, but this module focuses on five: Lyme disease, Powassan virus, West Nile virus, Triple E (Eastern Equine Encephalitis), and La Crosse encephalitis.
Understanding the cycling of these five diseases provides a foundation for understanding other zoonotic diseases.
All zoonotic diseases share similar components and cycles, differing primarily in their specific pathogen, reservoir, and vector.

Cycling of Diseases: The Venn Diagram Approach

Any zoonotic disease can be understood using a Venn diagram that illustrates the interactions between:
- Agent (Pathogen): The causative agent of the disease (e.g., virus, bacteria, fungus, prion).
- Host: An organism that can harbor the pathogen, allowing it to infect cells and reproduce.
- Environment: Conditions that allow the host, pathogen, and other components to interact.
All components must be present in the same time and place for the disease to occur.
Removing any one component disrupts the cycle and prevents the disease.

Pathogens

A pathogen is any disease-causing agent.
Zoonotic diseases are multifactorial, influenced by interactions between various elements.

Prions

Prions are misfolded proteins that can cause other proteins to misfold, leading to diseases.
Prions are naturally occurring proteins in mammalian neurological systems that have become misshapen
A prion is the same amino acid sequence, the same number of amino acids, everything is exactly the same except that the normal functioning protein in the neurological system has shape a. If that gets misshapen in the shape b, shape b is now a protein.
Stanley Krusner won a Nobel Prize for discovering that this is the same protein as this one.
Prions lead to spongiform encephalopathies, causing neural tissue to become spongy and develop holes.
Prions spread through consumption of infected tissue, as seen in mad cow disease.

Vector-Borne Diseases

Vector-borne diseases are a subset of zoonotic diseases where a vector is required to transmit the pathogen from a reservoir to a new host.
A vector is an organism (typically an arthropod) that picks up the pathogen from a reservoir and physically transmits it to a new host.
Without the vector, transmission to a new host cannot occur.
Removing any component, including the vector, collapses the system and prevents disease transmission.

Germ Theory

Zoonotic and vector-borne diseases align with germ theory, which states that specific agents cause specific diseases.

Koch's Postulates

Robert Koch (called Bob in the transcript) was a medical scientist and founder of bacteriology who sought to determine the cause of anthrax in farm animals.
Koch converted his home into a medical laboratory with a single microscope.
He hypothesized that a gram-positive bacillus bacterium, Bacillus anthracis, was the causative agent of anthrax.

Koch's Four Postulates to Confirm Pathogen Causation:

The pathogen must be isolated from animals that died from the disease.
The pathogen must be grown in a pure culture and isolated as the only thing in that culture.
The pathogen from the pure culture must be injected into a healthy animal, producing the same disease.
The pathogen must be re-isolated from the newly infected animal, regrown in pure culture, and identified as the same pathogen.
- These steps demonstrate that the pathogen is the sole common component between all infected organisms, confirming its role in causing the disease.

Limitations and Exceptions to Koch's Postulates

Difficulty in Isolating and Growing Pathogens: Some pathogens are hard to isolate and grow in pure culture.
Asymptomatic Carriers: Some infected individuals may not show symptoms, leading to false conclusions.
Multiple Pathogens: Some diseases may be caused by multiple pathogens, not just one.
Pathogen Survival: Some pathogens may not survive outside the host.
Acquired Immunity: Prior exposure and acquired immunity can prevent a healthy animal from developing the disease after inoculation.
Vector-borne Disease Exception: malaria violates the postulates because it is requires a host and cannot be grown in pure culture

Vector And Disease Components

Vectors

A vector is an organism, often an arthropod, that transmits a pathogen from a reservoir to a host.
Vectors are typically blood-drawing organisms, such as mosquitoes, ticks, and flies.
Vectors can be classified as:
- Primary Vectors: Highly effective and efficient at transmission.
- Secondary Vectors: Less effective but still capable of transmission.
- Tertiary Vectors: May transmit the pathogen, but not reliably.
- Low Competency Vectors: Has it. You may get it, etcetera

Sylvatic Cycle

The sylvatic (or sylvanic) cycle is the cycling of a pathogen in nature, involving vectors and reservoirs.
This cycle keeps the pathogen available in nature, ensuring its persistence and potential to infect new hosts.

Reservoirs

A reservoir is a long-term host of the pathogen.
Reservoirs maintain the pathogen in nature, often without showing severe symptoms themselves (subclinical).
Good reservoirs acquire the pathogen and house it for a long, long time.
Mice and rats are reservoirs for hantavirus, housing the pathogen in nature for extended periods.
The organisms are typically subclinical with the pathogen and asymptomatic.

Fomite Transfer

Fomite transfer is the transmission of a pathogen via inanimate objects (e.g., infected medical equipment), bypassing the need for a vector.

Overlapping Interactions

Understanding where those letters are is crucial for understanding disease transmission.
All components must work together to continue the spread of disease and infection of new hosts.

Disease Presentation

Clinical Disease: Visible and detectable symptoms in an individual that a physician can easily detect, they get tested for, and it comes back positive.
Subclinical Disease: No visible symptoms in a host despite having the pathogen, also known as the super spreader.
Incubation Period: The time between initial pathogen exposure and the onset of symptoms.
Pandemic vs. Epidemic : Pandemic is is wide and large area while the epidemic is in a smaller area, and is more specific.
Proper environment that is very specific will result in an epidemic. There also needs to be a proper worldwide environment that results in an pandemic.

Severity and Duration of Disease

Acute Disease: Symptoms develop rapidly and tend to be more severe. Symptoms are very rapid and cause fast acute.
Chronic Disease: Symptoms develop slowly, are typically less severe, but last for a long time. Individual will have the illness for an extended amount of time.
Subacute Disease: Runs in between the acute and chronic. Not a rapid onset and does not result in extended lengths.
Latent Disease: The disease has an onset and is typically acute, but the patient will recover (convalescence), then a pathogen may reinitiate after recovery which would cause the individual to have reoccurring symptoms.

Mosquitoes as Vectors

The number one killer of diseases on the planet is from mosquitoes.
Mosquitoes are in the order Diptera (two wings).
Mosquitoes are in the flies family.

Mosquito Characteristics

Mosquito mouth parts are shaped like a needle, and it extends way out from their head.
Mosquitoes also have very long legs, as their legs are longer than their body.
Have veiny wings that also have scales.
The antennae are super long, more than twice the length of the head.
Two primary species for vector diseases that we will focus on are aedes and culex

Mosquito Species

There are about 3,000 to 3,500 species of Mosquitoes on Earth.
In the United States, there are about 70 species.
And in Ohio, there are about 60.