Dealing with disease – 1

Four main approaches are used to prevent disease in livestock.

Most articles in this column deal with specific livestock diseases. This week, we’ll have a look at the basic principles of disease prevention. Knowledge of the following approaches, as described by Dr Danie Odendaal of the Veterinarian Network, will help farmers to reduce the risk of disease on the farm. There are four basic approaches to the prevention of disease:

  • To increase general resistance;
  • To increase specific resistance;
  • To decrease exposure;
  • To prevent exposure.

In most cases a combination of two or more are employed.

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General disease resistance
This is influenced by many factors, such as the breed, sex and age of the animal, as well as its stage of production and nutritional status. In a dairy cattle population, for example, each animal will have a different level of resistance to a specific disease, even if not exposed to the disease before (see Graph 1). The number of animals that become ill will be influenced by two factors: increased exposure or reduced disease resistance.

Increased exposure
Graph 2 shows how an increase in exposure can have a dramatic effect on the number of animals that will become susceptible. In this example, it increased from 215 to 370 animals, due to an increase in exposure to the disease-causing organisms. An example of how this occurs in practice is if a few animals in an unvaccinated herd become ill, the disease-causing organisms multiply in these animals and cause a very high exposure to the others.

A decrease in the general resistance can be due to factors such as underfeeding, especially during critical stages (for example, calving) in the production cycle. General disease resistance plays a greater role in certain diseases, such as endometritis. With others – lumpy skin disease, for example – it is not as significant. Increased exposure can move the cut-off point in Graph 2 to the right, meaning a larger portion of the population becomes susceptible to the disease.

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More than one approach
As mentioned, preventing a disease usually needs a multi-pronged approach. This is the case with Staphylococcus areus mastitis in a dairy herd. It requires the following:

  • Firstly, improve the general disease resistance of the herd through effective parasite control and good nutritional management, especially during the dry period and after calving.
  • Secondly, vaccinate against the disease. A new vaccine for this type of bovine mastitis is due to be launched soon; this promises better results than earlier vaccines.
  • Thirdly, decrease the level of exposure by identifying and culling chronically affected carrier animals and using effective dry cow mastitis remedies prescribed by the herd vet.

Next issue we’ll look at disease prevention.

Source: Based on information supplied by Dr Danie Odendaal of the Veterinarian Network to the Livestock Health and Production Group.