How to decide which animals to treat?

In Australia, cattle kept under paddock, pen or feedlot conditions are likely to carry some parasitic worms. Cattle and sheep differ in the development of their immune systems with cattle developing the ability to manage their own parasite burdens in certain stock classes. Cattle should only be treated when needed as a small population of gastrointestinal worms is considered normal because they:

  1. Are present in most environments and eradication is rarely feasible.
  2. Small burdens cause little economic harm and they ‘prime’ the immune system which helps cattle to develop natural immunity by the time they reach about 15 - 18 months of age.
  3. Excessive treatments lead to the development of drench resistance, which reduces the effectiveness of drenching when needed.

Figure 1. Observing animals with souring may be a sign of worms. Image courtesy of Jenny Cotter

However, worm numbers can rapidly build up to dangerous levels, particularly where susceptible cattle are kept in highly contaminated paddocks and under conditions that favour worm development e.g. mild temperatures and some moisture, or under nutritional stress. For this reason, it is important to monitor cattle to assess whether and when treatment may be needed, to avert production loss or treat disease. This can be done by: