A drench check vs a FECRT

Drench resistance is rising and increasing in importance in New Zealand. Have you got drench resistance on your farm? There are two ways to tell; a “drench check” and a faecal egg count reduction test (FECRT). The difference between the two is important to know as they tell you different things.

What is a drench check?

A drench check is a faecal egg count 7-14 days after drenching. (A faecal egg count FEC is also known as worm egg count WEC since it is not the faeces that lay the eggs).

By this time, the eggs that were already laid when the adult worms were killed by the drench will have passed out in the poo. There won’t have been enough time for larvae that were eaten just after the drench to have developed into adults, mated and laid eggs of their own. So a WEC at this time should always be zero.

If it’s not zero that means some worms have survived the drench.

A drench check only tells you whether some worms are surviving the drench. We don’t know which worms, or why.

A non-zero egg count is an indication the drench is ineffective, but a zero egg count doesn’t necessarily mean it is effective.

Unless we know what the WEC was before drenching, we can’t say how ineffective the drench was. In other words, a WEC after drenching of 100 eggs/gram when the WEC before drenching was 5000 e/g is not as big a deal as a WEC of 100 e/g when the WEC before drenching was 100 e/g.

We generally recommend all farmers do drench checks twice a year – once near the beginning of their summer lamb drenching and once in the autumn. The first will make sure the drench you are planning to use for the summer is effective. The second to check for resistance that may have developed through the summer. A drench check after a quarantine drench may also be useful.

A FECRT

A faecal egg count reduction test (FECRT) is the only test that will tell you which worm species are surviving which drench families on your farm. It gives you a lot more information than a drench check.

Unfortunately the eggs of all the major worm species we worry about look identical down a microscope. For the past 10 years we have been promised that a PCR test to identify worm species from the eggs is two or three years away. There still isn’t one available. The only way to tell which worm species are present is to let them develop into larvae and identify the larvae. This is called a larval culture, and takes about two weeks.

For a FECRT you do a starting (predrench) egg count and larval culture. The larval culture is very important and we try to time our FECRTs to when a variety of worm species are likely to be present. Then lambs are drenched to their exact weight with different drenches. At a second visit 10-14 days later we collect faecal samples from each treated lamb and do larval cultures on these samples.

Now you know the number of eggs at the start and the worm species that laid them, and the number of eggs and worm species that have survived the drench. From this we can work out how effective each drench family is against each worm species that was there at the start. For example, we could say that levamisole is 70% effective against Trichostrongylus worms, but 100% effective against Haemonchus.

A FECRT is very important because your parasite management plan will be different depending on the amount of resistance present (drench efficacy), how many drenches or drench families are ineffective, and which worms are resistant to which drenches.

Because of the higher cost of a FECRT ($1500-2000) we recommend these are done every three years. You may choose to do them more often if you are concerned that you may have bought in resistance, if you have made major management changes, or if a drench check shows worms surviving a drench that the FECRT previously showed was effective.

• Sara Sutherland is a veterinarian for Veterinary Services Wairarapa.