Review/Oorsig Volume 22, Issue 02 - Page 31

Volume 22 • Issue 02 • 2018 Increased exposure for example can move the cut-off point to the right, meaning that a larger part of the population becomes susceptible to a specific disease. Cut off point An increase in exposure can have a dramatic effect on the extra number of animals that will become susceptible. In this example it increased from 240 to 370 animals, due to an increase in exposure to the disease-causing organisms. Cumulative number of susceptible animals Low High Cumulative number of susceptible animals Impact of Increased Exposure or Decreased Resistance An example of how this occurs in practice is if a few animals in an unvaccinated herd become sick, the disease-causing organisms multiply in these animals and cause a very high exposure to the rest of the animals in the herd. The number of animals that become sick can to a very large extent be influenced by two factors: 1. Increase in the number of disease-causing organisms that the animal is exposed to. 2. A decrease in the general resistance due to factors like underfeeding, especially during critical stages in the production cycle like just after calving. The level of general disease resistance will play a larger (e.g. “dirty cows” = endometritis) or smaller (e.g. lumpy skin disease) role depending on the type of disease-causing organism or factor involved. Susceptible animals Increase in cumulative number of susceptible animals Increase in cumulative number of susceptible animals Resistant animals 31