8. Prevention, Control, and Eradication of Viral Diseases

Which of the following are the terms associated with virus variation?

Serotype

Genotype

Which of the following are the terms associated with vaccination?

Live attenuated

Inactivated

Subunit

Adjuvant

DIVA

Which of the following are associated with population immunity?

Passive

Herd

Endemic stability

Which of the following are terms associated with disease occurrence?

Endemic

Exotic

What are neutralizing antibodies?

Can prevent infection of susceptible cells

What is cell-mediated immunity?

Important for eliminating virus-infected cells

What is immunological memory?

Allows a faster and stronger response following a re-exposure

What are maternal antibodies?

Provide early protection but may interfere with vaccination

What does effective protection against viruses involve?

Humoral and cell-mediated immunity

What is herd immunity?

When a sufficiently large proportion of a population is immune to an infectious agent

How does herd immunity typically happen?

Vaccination or previous infection

What does an increase in population immunity mean in regards to herd immunity?

Reduction in the number of susceptible animals available for infection

What does a reduction in the number of susceptible animals available for infection do?

Reduce transmission and provides indirect protection to susceptible animals

The level of immunity required in order to establish herd immunity is the same no matter the virus.

False

What does the level of immunity required depend on?

How efficiently the virus is transmitted

What is endemic stability?

A situation in which infection is continuously present within a population

When are animals usually exposed in endemic stability?

Young, often while protected by maternal immunity

How does exposure present within endemic stability?

Immunity before the animals reach an age when severe disease is more likely

With endemic stability, clinical disease is common.

False

Infection may be common while clinical disease remains relatively uncommon within endemic stability.

True

What is endemic stability classically associated with?

Vector-borne infections

What is passive immunity?

Transfer of pre-formed antibodies to a non-immune animal

What does passive immunity provide?

Rapid although temporary protection

How is natural passive immunity acquired?

Maternal antibodies transferred through colostrum, transplacental transfer, or transfer via egg yolk

How is artificial passive immunity acquired?

Administration of specific immunoglobulin or antiserum

What do you need to understand about the virus to provide effective control?

Survival

Transmission

Reservoirs

What must you understand about the host in order to practice effective control?

Susceptibility

Immunity

Vaccination status

What must you understand about the environment to provide effective control?

Management

Vectors

Opportunities for transmission

What do you need to understand about the epidemiology to practice effective control?

Where is the infection occurring?

When is the infection occurring?

How is the infection occurring?

What is the pathway for control measures that are targeted?

Prevent introduction → reduce transmission → increase host resistance → detect and remove infection

What are examples of well-proven approaches to the prevention, control, and eradication of important viral diseases?

Exclusion

Vaccines

Hygiene and sanitation

Arthropod vector control

Test-and-removal programs

Border regulations, quarantine, testing

Disease surveillance activities

What are examples of good management and biosecurity?

Maintaining closed herd/flocks where possible

Quarantine and assess newly introduced animals

Separate sick animals from susceptible animals

Minimize contact between groups with different infection risks

Maintain appropriate stocking density and ventilation

Ensure good nutrition and minimize unnecessary stress

Ensure adequate colostrum intake in neonates

Apply appropriate cleaning, disinfection, and PPE procedures

How do you practice hygiene and disinfection properly?

Cleaning first

Removal of feces, mucus, bedding, and other organic material

Selection of a disinfectant against the virus concerned

Use of the correct concentration and contact time

Ensure the disinfectant reaches all contaminated surfaces

Consider the temperature, pH, and presence of organic material

How would you damage the lipid envelope with disinfectants?

Detergents and solvents disrupt the viral membrane

How would you damage the viral proteins with disinfectants?

Oxidizing agents, acids, and alkalis can denature proteins

How would you damage nucleic acids with disinfectants?

Chemical damage

How should you choose the disinfectant?

Based on the virus, environment, and amount of organic contamination

How can vector control reduce transmission?

Reduces contact between infected vectors and susceptible hosts

What are some common approaches of vector control?

Reduction of vector breeding sites

Insecticides and acaricides

Housing animals during periods of peak vector activity

Physical barriers (screens, netting, etc)

Movement restrictions during high-risk periods

Surveillance for vectors and virus activity

Measures to reduce movement of vectors between regions

What are the limitations of vector control?

May occur over very large geographical areas

Insecticide resistance can develop

Environmental and non-target effects

Complete vector elimination is rarely achievable

Why is vector control often not enough when used in isolation?

May be widely distributed

May disperse over considerable distances

Difficult to eliminate

Can involve wildlife reservoirs and complex transmission cycles

Vaccination of susceptible hosts can be a major control measure where effective vaccines are available.

True

What are examples where effective vaccines are a major control measure?

Bluetongue

African horse sickness

West Nile disease in horses

What does control of arboviral diseases often require an integrated approach of?

Vaccination

Surveillance

Vector control

Movement management

What is quarantine?

The separation and restriction of movement of animals that may have been exposed to an infectious agent

What happens during quarantine?

Animals are monitored for clinical signs and may be diagnostically tested

How is the duration of the quarantine determined?

Incubation period and epidemiology of the disease

What does quarantine prevent?

Introduction of infection into a susceptible population

How is quarantine used in preventing introduction?

Importation of live animals

Combined with pre- and/or post-entry testing

When is quarantine particularly important with preventing introduction?

When a disease is not present in the importing country

How is quarantine used to control outbreaks?

Restriction of animal movements

Isolation of exposed groups or premises

What are the limitations of quarantine?

It cannot completely prevent disease introduction

How can viruses still enter through the limitations of quarantine?

Illegal movement or smuggling of animals

Contaminated animal products

Infected arthropod vectors

Wildlife movement

Fomites, vehicles, and equipment

Infected animals that are difficult to detect

When is quarantine most effective?

When combined with border controls, biosecurity, surveillance, and diagnostic testing

What does vaccination aim to do?

Induce protective immunity before exposure to the virus

What are all the benefits that can come from vaccination but are not guaranteed?

Reduce susceptibility to infection

Prevent or reduce clinical disease

Reduce virus replication and shedding

Reduce transmission within a population

Contribute to disease control, elimination, or eradication

What are the traditional vaccines?

Live attenuated

Inactivated

What are some newer vaccine technologies?

Recombinant protein/subunit

Viral-vectored

Gene-deleted

DNA

RNA

Why is it important to have different vaccine platforms?

Generate different types and durations of immune response

What are the reasons we vaccinate animals for their health and welfare?

Prevent or reduce clinical disease

Reduce morbidity and mortality

Improvement

What are the reasons that we vaccinate animals for production?

Reduction of losses

Improve herd/flock health

Why do we vaccinate animals at a population-level disease control?

Reduce virus circulation

Support disease control and eradication programs

Why do we vaccinate animals from a public health view?

Reduce transmission of zoonotic viruses

Reduce secondary bacterial disease and the need for antimicrobial treatment

Which group does veterinary vaccination benefit?

All of these

What can be seen as population immunity increases?

Infected animals encounter fewer susceptible hosts

Transmission becomes less efficient

Susceptible animals receive indirect protection

Virus circulation may eventually be interrupted

What does the proportion of animals that must be immune depend on?

Transmissibility of the virus

Effectiveness of the vaccine

Duration of immunity

Population structure and mixing

Presence of reservoirs

What does virus stimulate within the vaccinated animal?

Adaptive immunity and immunological memory

What does protection generally include?

Neutralizing antibodies

Cell-mediated immunity

Memory B and T cells

How does a neutralizing antibody work?

Binding to viral particles → prevent attachment and/or entry into susceptible cells

How does cell-mediated immunity work?

Cytotoxic T cells recognize and destroy infected cells

How do memory B and T cells work?

Generate a faster and stronger response following subsequent exposure

What are types of whole-virus vaccines?

Live attenuated and inactivated

What are the types of recombinant/antigen-based vaccines?

Subunit/recombinant protein and gene-deleted

What are the types of vector-based vaccines?

Recombinant viral-vectored vaccines

What are the types of nucleic acid vaccines?

DNA and RNA

How do the types of vaccines differ?

Safety

Immunogenicity

Duration of protection

Ability to mimic natural infection

What is attenuation?

Reduction in the virulence of a virus

How does an attenuated vaccine work?

Virus remains replication competent

What is inactivation?

Treatment of virus so that it can no longer replicate

How does an inactivated vaccine work?

Antigens must remain sufficiently intact to stimulate protective immunity

How are attenuated vaccines traditionally achieved?

Repeated passage in cell culture

Embryonated eggs

Non-natural hosts

How are inactivated vaccines achieved?

Chemical or physical methods

What are the advantages of using live attenuated viruses?

Stimulate strong humoral and cell-mediated immunity

Often produce relatively long-lasting immunity

Usually require fewer doses

Generally, does not require an adjuvant

Some can be administered by mucosal routes (intranasal, intraocular, oral/drinking water)

What are the disadvantages of using attenuated vaccines?

Potential for residual virulence

Possibility of reversion to increased virulence

Vaccine virus may replicate and sometimes be shed

May be unsuitable for pregnant or immunocompromised animals

More susceptible to loss of viability during storage and transport

Often require a reliable cold chain

Maternal antibodies may interfere with vaccine replication

What are the advantages to inactivated vaccines?

Cannot revert to virulence

Cannot spread between animals

Generally have a good safety profile

Often suitable where use of a replication vaccine would be undesirable

What are the disadvantages of inactivated vaccines?

Usually induce weaker immune responses

Tend to favor antibody responses rather than strong cell-mediated immunity

Often require adjuvants, multiple doses, and booster vaccines

Immunity may be shorter

What do we need to balance to select vaccines?

Immunogenicity

Safety

Practicality

What are recombinant technology approaches?

Recombinant protein (subunit)

Recombinant viral-vectored

Gene-deleted

What is a recombinant protein vaccine?

A proactive viral antigen is produced in an expression system

What is a recombinant viral-vectored vaccine?

Another virus is engineered to express and antigen from the target virus

What is a gene-deleted vaccine?

When specific viral genes are deleted to reduce virulence and/or provide a marker for DIVA

What may recombinant protein (subunit) vaccines include?

Viral surface proteins

Capsid proteins

Recombinant proteins

What are the advantages of recombinant protein (subunit) vaccines?

No infectious virus present

Good safety profile

Antigens can be specifically selected

What are the limitations of recombinant protein (subunit) vaccines?

May generate a narrower immune response than whole-virus vaccines

Often require an adjuvant

Booster vaccinations may be required

What happens after something is vaccinated with a viral-vectored vaccine?

Vector enters cell → antigen is produced → immune responses develops

What are the advantages of viral-vectored vaccines?

Antigen is produced within host cells

Can stimulate both humoral and cell mediated immunity

No need to administer the disease-causing virus itself

What is an example of a viral-vectored vaccine?

Canarypox expressing rabies virus glycoprotein

What can gene-deleted vaccines potentially do?

Reduce viral virulence

Produce a safer live vaccine

Remove an antigen that can be used as a diagnostic marker

What is an example of a gene-deleted virus?

Pseudorabies

How does a DNA vaccine work?

DNA enters host cells → antigen is produced → immune response develops