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.