Oorsig/Review
Increasing Specific Resistance through
vaccination
The most specific way
of increasing the
resistance of an animal population is to vaccinate
the animals against that specific disease. It
effectively changes the level of resistance of the
whole herd by moving most animals to the right
of the cut off-point.
The level of resistance in a vaccinated herd
is depicted by the graph on the right, versus
the non-vaccinated herd on the left. It must
be remembered that in most cases, a single
vaccination will not protect all the animals in
the herd. Usually there are individual animals
that do not reach the required level of disease
resistance (immunity) after only one vaccination.
Vaccination of replacement heifers against
Brucellosis is a good example to remember.
Vaccination can have a dramatic effect on
decreasing the number of susceptible animals.
In this example it decreased from 240 to 10
animals. All vaccinations are not equally effective
and the level of immunity (resistance) acquired
depends on the type of vaccine, the timing of
vaccination, and the potential of the individual
animal’s own immune system to react to the
antigenic stimulation provided by the vaccine.
Increased Disease Challengeon a
vaccinated herd
Although vaccination moves the majority of the
animals to the right of the cut-off point, meaning
that the majority of the herd is protected, this
protection is not absolute.
If there is a very high increase of exposure in the
vaccinated herd, for example the buying in of
disease carrying animals, a larger number of the
vaccinated herd will become susceptible because
the immune system of cattle with weak immunity
will be overwhelmed.
BVD is a good example of a disease where
carrier animals (persistently infected animals)
are frequently bought in.Even though a few
more animals in the vaccinated herd are now
susceptible to the disease the effect is not as
dramatic as it would be in an unvaccinated herd.
In this example the increase in susceptible
animals was as follows:
•
•
Susceptible
animals
32
Resistant
animals
Unvaccinated herd – From 240 to 350
animals
Vaccinated herd – From 10 to 25 animals
This shows that vaccination is still the most cost-
effective way of reducing the risk of disease
outbreaks spreading through a herd on an un-
controlled basis.