Worm resistance in Victoria

Dr Steve Cotton – Dynamic Ag Hamilton

Part of a solid worm control program is to understand what level of resistant worms you have on your property and to what extent these worms have resistance to the various chemical groups on the market. While rotating drenches is important, we must be rotating actives rather than brand names because quite often, the different brand name products contain an active from the same chemical group (for example Abamectin versus Moxidectin) or worse still, the same active (e.g., Triguard versus Trifecta).

The level of drench resistance from year to year can change markedly on the same farm. You can see in Table 1 how the efficacy of the same active changes over time. This is dependent on what level of selection pressure is applied to the worms (drench frequency, time of year) and through your pasture and grazing management (level of worm larvae and pick up on pastures).

A drench resistance test should be conducted every 3 years. Knowing what chemical actives are working against the worms on your farm allows you to make informed management decisions. At times, it can demonstrate that your worm program is on track but it can also demonstrate a need to reduce your reliance on chemical control and look at alternative options. You can easily conduct your own ‘mini’ drench trial from time to time by re-testing a mob 10–14 days after drenching. Compare the post drenching worm egg count to the initial count pre drenching. Ideally the drench will have greater than 95% efficacy but preferably greater than 98% .

For example, you collect samples from a mob and they test at 1900epg. You drench these sheep, and re-test 10–14 days later and your count comes back at 60epg.

A simple calculation will determine the effectiveness of that drench:

60/1900 = 0.0316
1 – 0.0316 = 0.9684
0.9684 x 100
= 96.8%

In the above example, the drench has been effective. For that drench to be 98% effective, the post drenching worm egg count would need to be around 30epg. You should also keep in mind that the different laboratories have different methods for preparation of samples and 1 egg counted under the microscope could represent anywhere from 20epg to 40epg depending on the dilutions used in the sample preparation.

Traditionally, drench resistance has usually developed around 10 years after a new active has been released onto the market. Table 2 shows the level of drench resistance from multiple properties across Victoria over the past 8 years. Approximately 90% of the properties in this data set are from western Victoria, so care should be given to interpretation of these results.

Conducting a drench resistant test is easy. For a comprehensive test, you will require between 100–120 lambs. Weaning is an ideal time to select lambs for a drench resistance test because usually they have a high worm egg count and they still have little to no immunity to worms. You require a worm egg count of 300epg or higher to get an effective result. Ideally select a mob that did not receive a drench at marking time.

Once your count is high enough, you will drench groups of lambs (usually 12–15) with different drench actives and identify the different groups with ear tags or coloured spray marker on their head. Make sure you use a clearly visible means of identification that won’t wash off with a rainfall event in the short term. You then run each of the drench groups plus a control group (animals not drenched) together in a paddock for 10–14 days until each animal is sampled and counted.

This period ensures the only worm eggs that are in the sheep’s faeces are ones that have survived the drench and not new pick up from the pasture. The easiest way to collect samples is to draft the lambs into their drench groups and get individual samples from ten in each group.

Your laboratory will also conduct larval cultures which will identify what worm species were still present at 10–14 days (the resistant ones!)

Contact a Paraboss accredited laboratory near you to discuss the specific requirements of drench resistance testing. Keep in mind that some laboratories are just that, laboratories. If you need help understanding or interpreting your results or you would like a drench management report and plan, speak to your consultant or animal health care provider.

Key messages

• Drench resistance of most of the single active drenches in Victoria is quite high on properties tested.
• Knowing and managing your property’s resistance to chemical groups will give you better protection and control to worm infestations, particularly in challenging years (like last year).

More information

Note: In the last (autumn edition) edition of SheepNotes we included a list of accredited laboratories from the WormBoss/ParaBoss website.

For more information on conducting a worm egg resistance test visit the Testing Drench Effectiveness with a DrenchTest or WECRT page.