Vaccines

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Why Vaccinate?

• To protect against infectious diseases

• Where there is no effective treatment once infected
  • E.g. FeLV, FIV

• Where disease is life-threatening
  • E.g. Canine Parvovirus

• To prevent the spread of disease
  • E.g. Rabies, FMDV

How do we vaccinate?

• Usually by subcutaneous injection for systemic protection (IgG)

• For mucosal immune response, intranasal administration is best (IgA)

What do we vaccinate with?

Passive Immunisation - Copyright nabrown RVC

Passive immunisation

Advantages

• Immediate protection

Disadvantages:

• Short duration of action
  • Temporary protection by the administration of preformed antibody from another individual of the same or of a different species
  • The aquired antibodies are used in combination with antigen and catabolised by the body meaning protection is gradually lost
• Injection of antiserum may cause an allergic response
• Antiserum contains many antibodies not just the specific antibodies needed

Types of antibodies administered:

• Maternally-derived antibodies in colostrum
• Antiserum (artificial)
  • The antibodies are used in combination with antigen (and often an adjuvant) is injected into the host animal
  • The immune system of that animal synthesised antibodies
  • Repeated injections at intervals increases the total antibody production
  • The immunised animal is bled and the serum collected which contains the newly made antibodies. The serum is called antiserum.
  • The serum can then be injected into a different animal to confer passive immunisation

• Example of when passive immunisation is used:
  • Suspect tetanus

Passive Immunotherapy with Antibody

INFECTION	HUMAN SOURCE OF ANTIBODY	EQUINE SOURCE OF ANTIBODY	USE
Tetanus Diptheria	Used	Used	Prophylaxis treatment
Botulism	Not used	Used	Treatment
Venomous bite	Not used	Used	Treatment
Rabies	Used	Not used	Post-exposure to vaccine

Active Immunisation - Copyright nabrown RVC

Active immunisation

• Administer antigen so the patient develops its own antibodies to protect against disease
  • Living organisms
  • Dead organisms
  • Toxoids
  • Subunit antigens
  • DNA

Advantages

• Long duration of action
  • Once antibody is produced against the antigen, memory cells are formed which continue circulating in the body
  • For further information on memory cells click here

Disadvantages

• Delay in protection
  • The host's immune system needs to evoke an immune response against the antigen which can take a few days
  • For further information on the antibody response click here

• Often needs two or more doses
  • The first dose initiates the priming reaction where antibody production ceases after a few weeks, but the second and subsequent doses creates memory cells which remain in the circulation for a much longer period of time
  • For further information on the T cell independent and dependent responses click here

What antigen(s) do we use in the vaccine?

Whole Organism

• Live attenuated organism
  • Virulent organisms cannot be used as vaccines as they would cause disease
  • Virulence is reduced by growing the organism in altered conditions so that it is less able to replicate when introduced to the host and therefore less likely to cause disease
  • Produces a superior response to disease than using killed organisms as the dose of antigen is larger and more sustained
  • Response takes place at site of natural infection producing a greater local response than with killed organism vaccines
  • E.g. The current vaccine for Tuberculosis (called BCG) contains an attenuated form of a mycobacteria
  • E.g. Vaccines for Leishmaniasis

• Killed inactivated organism or toxin (toxoid)
  • Virulent and toxic organisms cannot be used as vaccines as they would cause disease
  • Organisms can be killed using radiation or chemicals so that they still possess the antigens to stimulate an immune response, but the organisms are unable to replicate inside the host
  • Toxins are inactivated to produce a toxoid which will still have the antigens needed to produce an immune response but will not be harmful to the host
  • Needs two doses (for an explanation on the T cell response click here)

Subunit Vaccine (part of the organism)

• Purified protein
  • From lysed organisms

• Recombinant or synthetic protein
  • The gene for the antigen required is inserted into a virus vector or cloned into bacteria
  • Small antigens, such as peptides can be synthetically produced
  • E.g. being developed constantly to fight the Inflenza viruses

• DNA coding for proteins (antigens)
  • Can be vaccinated directly into the host

Adjuvants

• Used with vaccines containing inactivated organisms which alone only stimulate a weak immune response

• Some create a depot of antigen at the injection site allowing a steady flow of antigen into the afferent lymph

• Some stimulate the immune system to amplify the adaptive immune response to antigens
  • E.g. Pathogen-associated molecular patterns (PAMPs)
  • E.g. PAMP-like adjuvants which assist naive T cell priming

• Different subtypes of T helper cells are stimulated by different adjuvants
  • E.g. Aluminium salts generate bias T helper II responses for antibody-mediated immunity
  • E.g. Killed mycobacteria generate IL-12 producing good cell-mediated immunity

• Adjuvants decrease the number of injection needed and the amount of antigen administered

Which type of vaccine is used for each disease?

• The life-cycle of the organisms needs to be understood to ascertain the best type of immune response for fighting the particular infection

• A vaccine can be created to provide specific immunity which is best suited for fighting the specific infection

Immunity to Virus Infection

• The virus life cycle consists of an extracellular phase, a replicative intracellular phase and another extracellular phase spreading viral particles to other cells to begin the life cycle again

• Immunity for the extracellular phase requires neutralising antibody
  • B cells needed
  • T helper type II cells needed (for the MHC class II pathway)
  • Live vaccine can be used
  • Killed vaccine can be used
  • Subunit vaccine can be used

• Immunity for the intracellular phase requires CD8+ cytotoxic T cells
  • MHC class I pathway
  • Only live vaccine can be used to get into cells (entering via the endogenous pathway)

Immunity to Bacterial Infection

• Extracellular bacterial infection need antibody production for opsonisation and to activate the complement pathways
  • B cells needed
  • T helper type II cells needed

• Vesicular infections can only be cured by organisms being destroyed inside macrophages
  • T helper type I cells needed

When do we vaccinate?

• Usually when animals are young

• Breeding females so immunity is passed to offspring via the colostrum
  • Protects neonates for the first 8-12 weeks of life

• Vaccination of young animals should be when the natural passive immunity decreases below the threshold for providing protection. Active immunity should then be stimulated so that the animal has constant protection. The vaccination should not be given too early, as the natural immunity can interfere with immunisation by binding and neutralising the vaccine antigens.

• 2 vaccines are usually given to allow for differences between neonates as the point where natural immunity decreases and active immunity needs to be stimulated, will differ between littermates and between different animals

Dog Vaccinations

Diseases covered by Vaccination

• Canine Parvovirus

• Canine Distemper

• Canine Infectious Hepatitis

• Leptospirosis

• Canine Parainfluenza virus

• Kennel Cough

• Rabies

When to Vaccinate

• Puppies are usually first vaccinated between 6 to 8 weeks of age
  • A second vaccination is needed 2 weeks later

• Adult dogs need booster vaccination regularly (depending on the specific vaccination)

Cat Vaccinations

Diseases covered by Vaccination

• Feline Infectious Enteritis

• Feline Infectious Respiratory Disease 'Cat Flu'
  • Feline Herpesvirus
  • Feline Calicivirus

• Feline Leukaemia virus

• Feline Infectious Viraemia virus

• Feline Chlamydophilosis

When to Vaccinate

• Kittens are usually vaccinated around 9 weeks old
  • A second vaccination is needed 3 weeks later

• Adult cats need booster vaccination regularly (depending on the specific vaccination)

Rabbit Vaccinations

Diseases covered by Vaccination

• Viral Haemorrhagic Disease

• Myxomatosis

When to Vaccinate

• Rabbits can be vaccinated against Myxomatosis from 6 weeks of age

• HVD from 2½ to 3 months of age

• Booster vaccinations are given every 12 months. In areas at high risk of myxomatosis, it is recommended to give myxomatosis boosters at six-monthly intervals.

Links

• Myxomatosis Clinical Case

• Viruses A to Z

References

• Ivan Roitt: Essential Immunology, Ninth edition

• Dr Brian Catchpole BVetMed PhD MRCVS

Creators

Natalie Brown

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