Timing is everything

Vaccination of ewes and lambs is one of the more common management practices performed by sheep producers each and every year.   Some producers will vaccinate their flock for “everything”, while others choose a more conservative approach.   Irrespective of which approach is followed, knowing how vaccination programs affect the immune status of flocks can give a better understanding of why and when to vaccinate for the optimum effect.

The process of vaccination  simulates a natural challenge without the risk of exposure to the actual disease-causing organisms.   Achieving adequate immunity usually requires two doses of vaccine to be administered about 3 to 6 weeks apart.   The initial vaccine dose stimulates the immune system to respond to the challenge, but it is the second dose which is important for raising the immunity to a protective level.

The above figure schematically depicts the sequence of immunological events that occurs in a pre-lambing vaccination program utilising two initial doses of vaccine followed by a single annual pre-lambing booster.

The graph shows that protective immunity tends to decline with time, and requires an annual booster dose to maintain immunity at an adequate level.   Annual pre-lambing vaccination of the ewe flock is the usual mechanism for maintaining flock immunity.   While this annual booster could be administered at any time during the year, from the ewe’s point of view the pre-lambing vaccination will contribute towards protecting the newborn lamb.  

Vaccination produces what is known as active immunity and typically results in long-term protection.   In contrast, passive immunity refers to the short term protection received from another individual – for example, from the antibodies received by a lamb from the ewe’s colostrum.   The passive immunity a lamb receives from colostrum must be absorbed from the lamb’s intestine during the first 6 to 12 hours after birth.   This 12 hour oral “window of opportunity” is the only time the gut is capable of moving immunity from the ewe’s colostrum across the gut wall and into the circulatory system of the lamb.  

The immune response to a booster dose is typically 3-4 weeks.   It is important, therefore, that boosters are administered at least three weeks before the period when maximum protection is required.

Ewes that are vaccinated pre-lambing with, for example, a 7-in-1 clostridial and pasteurella vaccine, will provide a level of passive protection to their offspring.

The diagram above highlights the importance of pre-lambing vaccination of the ewe to protect lambs against pasteurella and the clostridial diseases that could occur in the first few weeks of life.   Once lambs reach three weeks of age, it is possible to start to provide them with their own active immunity.   Again, it is important that a booster dose is given 4-6 weeks after the initial dose to ensure an adequate level of protection.

Further information is available from Intervet UK Limited 01908 685685.

Article first published 02/02/06