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Veterinary surgeon
Kenneth Wilson
XLVets practice
Donald S McGregor
and Partners
KENNETH WILSON,
DONALD S McGREGOR AND PARTNERS
WORKING
TOGETHER
FOR A HEALTHIER FUTURE...
23
LIVESTOCK MATTERS
B IOSECUR I TY FEATURE
Getting the best out of
sheep vaccinations
Vaccination is a routine activity in the vast majority of sheep flocks. It is
seen as a cornerstone of preventative medicine and flock health control
and has allowed farmers to significantly improve the health of their flocks,
reducing losses from a wide range of infectious diseases.
The aim of this article is to provide
an overview of the science of
vaccination, and explore how
effective practical implementation
of vaccination protocols on farm
can be achieved.
What is a vaccine?
Whenever an animal is infected by a foreign
organism (antigen) it is either overwhelmed by
the infection, killing the animal or an immune
response is mounted to eliminate and kill the
organism. This immune response involves
molecules called antibodies. The role of
antibody is to recognise foreign agents, and
attract the attention of the immune system to
these invaders. Once alerted, the immune
system can then home in on the invading
organisms and kill them. This is known as the
antibody/antigen response.
Vaccines aim to induce this protective response
without causing disease to develop. Vaccines
either contain killed forms of an infectious
agent or live, attenuated (weakened) agents.
Agents used in attenuated vaccines have been
altered in the laboratory so that they stimulate
the immune response without developing into
full blown disease. It should be noted that
some live vaccines will cause mild clinical
signs of disease in some animals. Some
vaccines also contain an adjuvant. This is a
chemical which is designed to irritate the
immune system so as to alert it to the presence
of the vaccine antigen and prompt a much
stronger immune response.
Variation in response
to vaccination within
populations
Studies have shown that even in flocks with
identical husbandry and ages, the response
to vaccines varies significantly within groups
of animals. This is due to individual animals
mounting differing responses to invading
pathogenic organisms.
By vaccinating whole populations of
animals, those sheep which do not develop
such a strong immunity are protected to
some degree by the strength of the overall
flock immunity following vaccination. This
in itself will significantly reduce the chance
of an outbreak of disease.
It must be noted, however, that in some cases
even when strong immunity exists (natural or
vaccine induced), an overwhelming infection
can still break through.