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Basic
Vaccinology Pt.2 of 2
Randall
J Berrier, DVM
Staff Veterinarian
Technical Service
Traditionally
there have been two types of vaccines through the years; live
or modified live vaccines and killed vaccines.
Modified live vaccines infect host cells and undergo
replication. These
infected cells then process endogenous (inside the cell)
antigen. In this
way the vaccine antigen triggers an immune response.
One of the hazards with this approach is the live
vaccine organisms may themselves cause disease or persistent
infection (residual virulence).
For this reason the virulence of live vaccines must be
reduced so that they will create a protective immune response
without creating disease.
The process of reducing virulence is called
attenuation. Attenuation
of live vaccines can be achieved in a number of different
ways. One way is
adapting organisms to growth in unusual conditions so that
they lose their adaption to their usual host.
Genetic manipulation of the organism is another
attenuation technique. Growing
viruses in cells or species to which they are not naturally
adapted. Prolonged
growth in tissue culture in cells that the virus is not
adapted to is a common attenuation technique for viral
modified live vaccines. Advantages
to using live vaccines include fewer inoculating doses
required, adjuvants are unnecessary, less chance of
hypersensitivity and, induction of interferon.
Killed
or inactivated vaccines are the other common type of vaccine.
If organisms are to be killed for use in vaccine, it is
important that they remain as antigenically similar to the
living organism as possible. One common way of inactivating is
the use of formaldehyde.
Alkylating
agents such as ethylene oxide, ethyleneimine,
acetylethleneimine, and beta-propiolactone are also used in
veterinary medicine as inactivating agents.
The advantages of using killed vaccines are; 1.) They
are stable in storage. 2.)
They will not cause disease through residual virulence.
3.) They
are unlikely to contain contaminating organisms.
The disadvantages to using killed vaccines include the
tendency to induce a lesser immune response then live vaccines
and therefore adjuvants are commonly added to killed vaccines
to increase the immune response.
Common adjuvants are Aluminum phosphate, Aluminum
hydroxide, Freund’s incomplete and complete adjuvant,
saponin and many others. Other
disadvantages to killed vaccines are the local reactions that
the adjuvants can cause and the need for multiple dosing which
increases the risk of hypersensitivity reactions.
The
most common route of administering vaccines is through
injection subcutaneously or intramuscularly.
Some modified live vaccines can be given intra-nasally
to more closely simulate the natural route of infection and
stimulate a quicker, local immune response (interferon).
Research using oral vaccines is ongoing and has
actually worked with some success in raccoons with an oral
rabies vaccine in an edible vehicle.
When
a farm or ranch encounters an organism that is unique or a
variant that there is no commercial vaccine for, an autogenous
vaccine may be useful. Autogenous
vaccines are made from a sample of organisms taken from
infected animals on a given farm and is made specifically for
use at that particular location.
Although
killed and modified live vaccines have been very successful
over the years, modern vaccine technology is an exciting and
continuously growing field.
Modern genetic techniques can produce new, improved and
safe vaccines. The
USDA classifies these new vaccines into 3 categories; Category
1.) Antigens generated by genetic engineering.
Category 2.) Genetically attenuated organisms.
Category 3.) Live recombinant organisms.
The
decision to use vaccines for the control of any disease must
be based on considerations that the risks of vaccination do
not exceed the risk of contracting the disease itself.
In most cases vaccination is the way to go.
As mentioned in the beginning of this article, vaccines
have provided a great service to people and animals for
decades. They
have helped to eradicate or control many diseases that
previously caused epidemics.
Vaccines help the rancher to provide a safe end-product
on the grocery store shelves while increasing profitability
for the rancher by eliminating diseases.
Vaccines have been so successful in fighting disease
through the years that the general public has taken the
benefits from vaccinating for granted.
The slight risks of adverse reactions following
vaccination should always be viewed in light of the great
numbers of animals that have been protected from deadly and
costly diseases from vaccination.
Stop and think what the world would be like without
vaccines.
Ref.:
Tizard, Veterinary Immunology, 6th edition.
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