The purpose of giving a vaccine is to sensitize an animal’s immune system to a specific bacteria or virus, without actually causing the disease. Under natural circumstances, a disease-causing pathogen (bacteria or virus) will enter the body, attack cells, and eventually cause the outward signs of the disease. The immune system will produce antibodies that are specifically designed to recognize and control the pathogen. If the animal survives the pathogen attack, its immune system will retain a ‘memory’ of that particular organism and will be able to respond more rapidly if it is encountered again.
The strength of the immunity and length of time that it lasts depends on the type of pathogen and the overall health of the animal (i.e. poor nutrition and other stresses weaken the immune system). Manufactured vaccines mimic the effects of natural infection by exposing the immune system to controlled amounts of a disabled pathogen. The immune system reacts as it would to the disease, but the animal does not become ill. To maintain a high level of immunity, vaccines need to be given to sheep at regular intervals. If directly exposed to a disease-causing pathogen, even a vaccinated animal may show signs of the disease. However, the severity of the attack should be reduced, as the immune system will able to respond quickly to the pathogen.
Vaccines designed to mimic viruses may contain live viruses (modified to not cause disease) or killed viruses. Bacterial vaccines contain inactivated bacterial cultures (bacterin) or non-toxic derivations of bacterial toxins (toxoids). Antitoxins are available to reduce the effects of some bacterial diseases in unvaccinated animals (i.e. antitoxin for tetanus may be given if the animal receives a deep puncture wound, etc.).
Before using vaccines, read the manufacturer’s label carefully for information regarding administration and dosage recommendations for animals of different ages. Some vaccines are packaged in two parts; a dry component and a liquid component. The vaccine must be reconstituted (liquid portion mixed with dry) before administration. The product label will have instructions detailing the reconstitution and storage of the vaccine (e.g. refrigerate, expiry date). Vaccines that are not stored properly or are used after the expiration date may not provide the proper level of immunity. Consult with a veterinarian if the information on the label is not clear or if sheep are not included in the species listed on the label. Vaccines are most often administered by a subcutaneous injection. A lump will often form at the injection site as part of the normal reaction to the vaccine.
Some of the more common vaccines used for sheep are listed below. If you are just starting your sheep operation, contact your veterinarian to help determine which vaccines are important for your area. The ‘vaccination schedules’ provided below are suggestions only. Always follow your veterinarian’s advice and/or label instructions for administration.
