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Life after naphthalophos (Rametin): a good time to test drench efficacy

Figure 1. Rametin, the organophosphate drench, will no longer be available.
Figure 1. Rametin, the organophosphate drench, will no longer be available.

Rametin® will no longer be produced as the manufacturer, Bayer, has been unable to overcome ingredient supply issues. 

Naphthalophos, the active ingredient in Rametin, is an organophosphate and has been a highly valuable drench active against worms.

Despite Rametin being in production for over 30 years, it has generally remained effective on most farms, especially against barber’s pole worm.

Over the years, a number of other organophosphate products (in production for relatively short times compared to Rametin) have also fallen by the wayside, leaving no products currently available in this drench group.

The last supplies of Rametin are expected in late December 2016.

Bayer's letter to customers is reproduced below this article.

Life after naphthalophos

With no organophosphate drenches, the number of effective drench groups on many properties will be fewer, as drench resistance to older drench actives is widespread.

If you have not done a DrenchTest in the last 2–3 years, your sheep or goats could be suffering significant production penalties if ineffective drenches are unknowingly being used.

These penalties have a real impact, but are almost impossible to see during day-to-day management.  Work conducted in Australia and New Zealand has demonstrated that using ineffective drenches results in lower levels of production and this has a high dollar cost for sheep producers.

Until you next do a DrenchTest, carry out a DrenchCheck after each different drench.

How is a DrenchCheck done?

A DrenchCheck, is a fast, easy and inexpensive way to get a good indication of how well a particular drench is working. This test does not replace the more accurate DrenchTest (read about the two tests here: DrenchCheck and DrenchTest) but is a good starting point.

When you are next planning to drench, do a mob worm egg count (WEC or WormTest) beforehand.

A larval culture should also be done on the sample. To save an unnecessary culture (if the test shows the animals do not need drenching), request that the culture be done on this “before drenching” test only if the WEC is equal to or greater than a particular figure—which should be the figure that would trigger you to drench the stock.

If you don’t know the level at which you should drench, consult the WormBoss Your Program and Drench Decision Guide for your region.

If your WormTest indicates drenching is required, do so within 10 days of the test, so it is still considered reasonably current.

At exactly 14 days after the drench is given, collect dung samples again from the drenched mob (these can be collected from fresh dung piles in the paddock).

Ensure that samples are kept separate for individual animals, that is, do not mix or bulk the samples. Send them to the laboratory and this time, ask that a larval culture is done regardless of the worm egg count (even if the WEC is zero, larvae can sometimes be cultured).

DrenchCheck results

A DrenchCheck gives an estimate of drench effectiveness.

You can calculate the effectiveness this way:

Firstly, calculate how much the worm egg count has been lowered by the drench:

For example:

Before-drench WEC average = 800 epg; After-drench WEC average =  200 epg

200     =     0.25


then, 1 – 0.25  = 0.75  (or 75% effectiveness)

However, this doesn’t tell you what is going on with each worm type. For example, you may find a particular drench may be fully effective against scour worms, but may have only poor to moderate effectiveness against barber’s pole worm, which should influence how and when you use this drench again.

The larval culture enables you to see how the drench is affecting particular worm types.

Calculate your results for each individual species by multiplying the WEC by the % of the worm type in the sample, like so:

Before-drench WEC average = 800 epg; Culture: 14% black scour worm and 86% barber’s pole worm

After-drench WEC average = 200 epg; Culture: 2% black scour worm and 98% barber’s pole worm

For black scour worm

200 X 0.02   =    4    =   0.04

800 X 0.14       112    


1 – 0.04 = 0.96   (96% effective)

For barber’s pole worm

200 X 0.98   =   196   =   0.28

800 X 0.86        688    


1 – 0.28 = 0.72   (72% effective)


In this example, the drench was 96% effective against black scour worm, but only 72% effective against barber’s pole worm.

You should always have the after-drench test done on individuals (although the samples can be collected from discrete piles of dung in the paddock).

An individual result that stands out as being extremely high compared to the others after drenching should be viewed with suspicion. The animal may have been mis-drenched or been missed during drenching. Calculate the mob average without the very high result.

Plan to do a DrenchTest each 2–3 years

DrenchTests are considered more accurate as they are able to test all drench groups at the same time using a side-by-side comparison of drenched and undrenched animals. The side-by-side “control group” animals give a real-time comparison of how WEC results change during the 2-week course of the test—the WECs of the mob could be rising or falling naturally—which is not captured with a before and after DrenchCheck.

You can test all drench groups (as single actives) in a DrenchTest and then use the Combination Drench Efficacy Calculator to predict the efficacy of combination products using your single active tests results.

DrenchChecks and DrenchTests provide valuable information to identify effective drenches and help you to more profitably and effectively control worms in your livestock.

Figure 2. Bayer's letter to customers about the cessation of supply of Rametin.
Figure 2. Bayer's letter to customers about the cessation of supply of Rametin.