Virus Disease in Populations and Individual Animals

The Nature of Virus Reservoirs

What are virus reservoirs?

Is the reservoir always a living organism?

What traits/abilities must a virus possess that uses a nonliving source as its reservoir?

Examples of Living Reservoirs

Humans

Other vertebrate animals (dogs, horses, squirrels, foxes, deer mice, etc.)

Arthropods (mosquitoes, leaf hoppers, asphids, plant hoppers, thrips)

Examples of Nonliving Reservoirs

Desk top Contaminated clothing Soil Water Fecal material What is the ultimate source of a viral

pathogen in nature?

List some viruses that have a living reservoir.

List some viruses that have a

nonliving reservoir.

What is a zoonosis? What is a vector? Is the vector always a living organism? List some living vectors. List some nonliving vectors. List some viruses that use a living

vector. List some viruses that use a nonliving

vector.

Even when the vector is living it can also

serve as a passive vector. The virus does not replicate in the vector.

Arthropod vectors often mediate viral zoonoses and serve in a mechanical and passive manner.

Which is most efficient, a virus that replicates in the arthropod that will serve as the vector or one that does not replicate in the arthropod?

Give some examples of viruses that do not replicate in the arthropod vector.

Give some examples of viruses that replicate in the arthropod vector.

When the virus replicates in the arthropod vector, the arthropod vector serves as a second reservoir, allows the virus to amplify to a higher titer, and increases the efficiency of spread to a human host.

Viruses in Populations

In order for a virus to spread effectively in a population: There must be a sufficient # of people

who are not immune to the virus. The virus must be able to replicate itself

in another reservoir population if it induces protective immunity or it must spread through the population in waves.

What happens if the majority of the members of the population is immune to a virus?

What is the result of herd immunity, if the virus is able to continue to exist?

Figure 3.2 in small populations, the rate of virus spread is limited by the generation of herd immunity. The virus becomes extinct or is reduced to low numbers in the population.

What does the term “the index case” mean?

What is a nosocomial infection?

In large populations, the rate of virus spread must be greater than the generation of herd immunity.

What is the best way to reduce human to human spread of virus in a population?

What is the best way to reduce arthropod to human spread of a virus in a population?

Factors Affecting the Control of Viral Disease in Populations

The generation of lasting immunity Effective vaccination programs Influenza (Flu) virus variants arise by

genetic mixing of human and animal strains

HIV remains associated with lymphatic tissue in infected individuals even when drugs eliminate virus replication.

Factors Affecting the Control of Viral Disease in Populations, con’t.

Herpesviruses establish latent infections and reactivate at a later date making an effective vaccine difficult to generate.

Economic factors – it costs a lot of money to develop, produce, and deploy a vaccine.

Factors Affecting the Control of Viral Disease in Populations, con’t.

Pharmaceutical corporations interested in profit.

Lack of political will and insight

Animal Models to Study Viral Pathogenesis

Why are animal models necessary for the study of viral pathogenesis?

Koch’s rules The same pathogen must be able to be

cultured from every individual displaying the symptoms of the disease in question.

The pathogen must be cultivated in pure form.

The pathogen must be able to cause the disease in question when inoculated into a suitable host.

It is unethical to inoculate a human host with an agent suspected of causing serious or life-threatening disease.

Examples of unethical studies: Nazi Germany The Tuskegee syphilis study Studies in the US that used volunteer prisoners

Is it ever appropriate to use human subjects to study a disease and its therapy?

What are the conditions under which it would be ethical to use human subjects to study a disease and its therapy?

Animal Models

Mouse models Rabbit models Guinea pig models

Reasons for Using Animal Models

Can gather a lot of information using simple experiments, (i.e. observations) Ability of the virus to cause symptoms Location of virus replication Passage of the virus through the body

during various time periods post infection

Limitations of Animal Studies

Cannot accurately reflect the course of the disease in humans Must have uniformity of animals in study Amount of virus inoculated controlled Site of inoculation controlled

Must be adapted to replicate effectively in the test animal

Simple Experimental Methods

Dissection of various organs for gross and microscopic observation

Measurement of virus levels in organs thought to be the site of virus replication

In situ hybridization to detect viral RNA in tissue

Course Pox Infection in Mice

Rabbit Model for HSV Mice cannot clinically reactivate HSV Rabbits can spontaneously reactive

HSV; therefore, rabbits can be used to study the physiological process of reactivation.

The reactivation can also be induced by iontrophoresis of epinephrine.

HSV-2 cannot be studied in mice nor rabbits.

Guinea Pig Models

Used for the study of HSV-2 Infected vaginally HSV-2 Latent infections will spontaneously

reactivate. Reactivation rates can be determined. Reactivation cannot be induced.