Drench resistant worms in sheep – how to test for worm burdens

Sheep, farming, husbandry, health, disease, worms, internal parasites, testing methods

By Dr Clive Dalton

Faecal Egg Count (FEC)

• A good general indicator of a worm burden is the number of eggs passing out of the animal in the faeces.
• This is the basis of a ‘Faecal Egg Count’ or FEC that can be done through a vet clinic or on the farm if you have a microscope and the equipment.
  
• Ask your vet for details, or you can buy a commercially available kit with everything you require plus technical support when needed from FECPAK International Ltd, Box 5057, Dunedin.
• The costs through a veterinary clinic can vary depending on quantity. Some farmers with a FEC kit share the work and cost with neighbours.
• A FEC shows the number of eggs per gram of faeces (epg) and there are accepted ‘trigger levels’ after which you should take action by drenching – or do nothing.
• You cannot tell the worm species from the eggs so to identify species, the eggs have to be incubated in the lab which takes about 10 days and consequently costs more.
• However, the eggs of Nematodirus are larger and very distinctive compared to all other strongyloides worm eggs.
  
• They are always counted separately as Nematodirus worms are not great egg producers so animals can harbour heavy worm burdens and have low (less than 500) epg. This often occurs in lambs.
  

Limitations of a FEC

• A FEC is only a snapshot of the animal’s worm burden judged by egg output at that time of sampling. It’s really a measure of what was going on inside the sheep 3 weeks beforehand.
• With the standard FEC used in the past, the biggest limitation was that samples were not corrected for the Dry Matter intake of the animal or the moisture content of the faeces.
• Despite the limitations of FEC (and still plenty of academic criticism of its interpretations) it’s a very cost-effective tool and in any case it’s the best we have at the moment.
  

Sampling tips

• The easiest way to get a sample is to pick up fresh faeces from the paddock.
• Follow a few sheep around for a while or go where they have been camping and make up a composite sample from at least 5 heaps and better still 10. Mix them well.
• Using composite samples (mixed from a number of sheep) keeps the costs to a minimum. When the lab gets a composite sample it should give it a further good mix before sub sampling, but check that this will be done.
• The more accurate way for the lab to do a composite FEC it for individual samples to be sent to the lab and an equal weight of sample is taken from each, and a special composite FEC carried out to a sensitivity of 10 epg per pooled sample.
  

FEC for genetic selection programmes

Accuracy at the laboratory is very important for
accurate genetic selection decisions

• Here you need FEC samples from individual sheep to make accurate selection decisions.
  
• To do this you can either extract a sample with your finger directly from the sheep’s rectum (wearing a rubber glove), or use a plastic probe which is much faster and more comfortable for the sheep. Contact me for details.
• You can organise a series of small individual pens that you can put sheep in so accurate ID of their faeces can be made after they have passed them naturally.
• Fasting reduces FEC output so epg will be increased; so avoid yarding sheep too early before sampling so their guts are full.
  
• Only bring in enough animals that you can do in 3-4 hours to make the job easier for both human and sheep.
• Also don’t sample too early in the morning – give them a chance to have a good feed before bringing them in. Sample males and females separately.
• Only collect one faecal sample per container, and keep them in a cool place - and not in the family fridge! The farm ‘beer fridge’ is ideal, where they can stay until you go to town. They can remain in the fridge for up to 3 weeks before there is any deterioration. Check that it is set to 4-5ºC.
  

What ‘trigger levels’ should be used?

• A trigger level is the number of eggs per gram of faeces (epg) at which you have to act.
• Above the trigger level you drench, and below it you don’t bother.
  
• The problem is to decide what the trigger level is, and this is not helped by many consultants who dive for cover at the question because of the variables involved in an answer.
• But you have to make a decision, so consider levels below 500 epg to be low, 600-2000 epg to be moderate, and above 2000 epg to be high.
• Key Point: You need to have a different FEC trigger for fattening works’ lambs to those kept for flock replacements where the aim is to put selection pressure on worm resistance.
  
• If you drench lambs with anthelmintics every time they get to 500 epg you’ll certainly make it hard to identify natural (genetic) immunity for worms that needs time and a worm challenge to do this.
  
• For works lambs using 500 epg as trigger is no problem, especially if the drench gives them a lift in growth.
• For breeding decisions, use the FEC of the rams you have used as a reference point. Classify them into groups of below 500, 500-1000, 1000-2000, 2000-3000, 3000-5000 epg, and so on, and see how many are in each group. This would certainly let you see where you were and the size of the challenge to improve things.
  
• It’s no good setting a level of 500 epg if most of your rams are around 10,000 epg. If you don’t know what the rams are, use 1000 epg as a short-term trigger and work to get it down.
  

Faecal Egg Count Reduction Test (FECRT)
This is used to test for drench resistance. It’s a ‘drench check’ where you first drench sheep and then see if the product worked. Some vets recommend that it’s best to sample for FECRT from January – April and not from May – September.

The test is done through a veterinarian using a standard protocol. Here’s one version of it:

• Select a mob of appropriate animals – usually lambs.
• Run them on what you think will be your most contaminated pasture until they have from 500 to 700 epg. Check 2-3 to see when they reach this level.
• Randomly select a group of 10 and individually identify them.
• Do an individual FEC on each of the ten.
• Drench them according to weight with a well-maintained drench gun. Check its accuracy before use.
• The drench used will be from the family you are most concerned about.
• After 7-10 days each animal is re-sampled for FEC.
• If the drench worked, there should have been a reduction of more than 95% in FEC.
• Any surviving worm eggs then need to be cultured to establish their species as they are resistant and dangerous.
  

This is a more costly exercise than just a straight FEC because of the laboratory time and work involved in culturing larvae.

Check with your vet beforehand to ensure you get maximum value from the information and especially advice on the correct drench product and programme to fix the problem – if there is one. The FECRT like all tests is not foolproof, so keep this in mind.


Correcting FEC for consistency (moisture content)

• This is a very important issue so consider it carefully. The research of Dr Leo LeJambre at CSIRO, Armidale in Australia has sorted it out.
  
• He studied the moisture content of faeces, which controls faecal formation and the speed with which faeces pass through the digestive system.
• He reminds us that when FEC was invented by the CSIRO scientists, drying the faeces in the lab was a standard part of the technique.
• It was tedious and slowed up the process so it was gradually dropped over time, and now nobody seems to remember - or have conveniently forgotten!

LeJambre's technique

• It’s very simple. The more moisture in the faeces, then the quicker they pass through the gut, and the more diluted the eggs/gram will be in the faecal sample.
• So unless you correct the FEC for faecal consistency (moisture level), you will make errors in selection.
• A healthy sheep produces faecal “marbles” even when on lush pasture. These are formed in the hind gut where water is extracted and the motions of the gut wall (peristalsis) form the marbles. It’s an amazing bit of physiology.
  

Advantages of faecal marbles to the sheep

Sheep don't get up at nights to go to the toilet - they lie and let it flow! On popular overnight camps they return to the same spot each night to rest. So if they produce marbles, no dung sticks to their wool - so no dags form.

• Marbles don’t stick to wool and form dags. This is especially important during the night when the sheep is lying down and at greater risk of soiling her rear end. They don’t get up to go to the loo!
• Faecal marbles are a sign of an efficient digestive system so such sheep must be converting their feed efficiently.
• If their digestive system is working well, then they are healthy and their immune system must be working well too.
• Spherical faecal marbles have a much greater surface area than the same volume of sloppy dung. It’s about six times greater. So the resident worm larvae die faster though desiccation and radiation – all at no charge to the farm.
• The clusters of marbles soon separate when they dry out, and their dark colour absorbs more UV radiation.
• So faecal marbles are the key to healthy sheep and reducing the larval population on the paddock at the same time, where remember, over 90% of the worm problem resides.
  

How to assess faecal consistency – the Faecal Consistency Score (FCS)
It’s very easy to record the state of the sheep’s faeces when sampling. The Australian researchers have used numerical scores but the following perhaps rather ‘ un-scientific’ descriptions are suggested below (see photographs) as they are easier to remember than numerical scores:

• Photo 1 ‘Marbles’

• Photo 2 ‘Hand grenades

• Photo 3 ‘Plops’

• Photo 4 ‘Slops’

• Photo 5 ‘Scour’

Correction factors used
The corrections used are very simple and again are easy to remember. Multiply the FEC by 1 to 5 as shown below:

• Marbles x 1
• Hand grenades x 2
• Plops x 3
• Slops x 4
• Scour x 5
  

Practical implications of the FCS

• Correcting a FEC for faecal consistency has major implications to improve the accuracy of FEC interpretation, especially when selecting individual animals in breeding programmes.
• For example: When selecting ram hoggets as future sires, if you decided to keep a ram with a FEC of less than 500 epg, you would be fairly happy that he was resistant to worms. But that would only be correct if he was passing marbles.
• If he was scouring, then multiplying his FEC of 500 x 5 would give a 2,500 epg count and you certainly would not want to keep him as a future sire – even though he may be classed as resistant under the old method.
  
• He should be on his way to the dog tucker freezer. The FCS has changed the decision drastically – and surely for the better.
• So not correcting for faecal moisture could lead to a very wrong genetic assessment of replacements (especially rams) for the flock. You could easily end up with high-index daggy sheep that would pass those genes on. That’s not progress!
  

Industry implications of the FCS

• Everybody knows that dags are caused by scouring (faeces with high moisture content) and are the last thing the industry needs with all the extra costs involved of removing them, flystrike, shed-stained wool and contamination of works lambs.
• So by correcting the FEC with the FCS, it’s possible that the confusion caused by retaining low-FEC daggy sheep can be avoided.
  
• The combined FEC and FCS will certainly provide a more accurate selection tool to identify resistant animals in all flocks – stud or commercial. It could be as simple as that!
  

Using FCS to screen ewe lambs/hoggets

• What could be simpler for selecting ewe lambs/hoggets as replacements than keeping those that passed marbles or hand grenades and certainly culling any with loose (high moisture) faeces or are scouring? This simple process could be done any time from June onwards.
• This selection needs to be carried out after the lamb’s immune development has taken place as during this process faecal formations may vary.
  
• By May-June (8-9 months of age) the ewe hoggets that will be dag free in later life should all be producing marbles or hand grenades.
  

What would be the benefits?

• If nothing else, the marbles would reduce larval survival (especially L3) on the pasture through desiccation and radiation.
• So the high pollution rates blamed on lambs, hoggets and two-tooths would be greatly reduced.
• The selected animals would be free of dags and free of costs to remove them.
• The selected sheep would have a high possibility of being resilient or resistant (or both) to worms based on FEC corrected for FCS.
• It would allow large numbers of lambs/hoggets to be checked with no extra work or lab costs as a first initial screening.
  

Some practical challenges

• The idea of screening ewe hoggets or two-tooths on Faecal Consistency Score (FCS) to select worm resistant replacements would have most farmers taking off to the back paddock, and shepherds finding an urgent need to clean under the woolshed!
  
• Here’s three things to consider if you really want to get serious and fix things.

1. You could check each individual animal’s FCS – when it defaecated naturally. This would be a very time-consuming job with a large number of sheep. You’d have to put them on a solid floor, or put some old carpet over the woolshed grating. Put it in the ‘daft idea’ file.
2. Insert your finger into each sheep’s rectum. You don’t need to bring out any faeces to look at it, just remove your finger and inspect the end. This method has been used by NZ Romney breeder Melvin Forlong, and here’s what he does:

• Marbles you can easily feel. Score that FCS 1
• If you can’t feel anything in the hind gut, a hand grenade has just left! Score that FCS 2.
• If your finger end only has a bit of dry green stain on it, score that FCS 3.
• If you finger has more green stain on it and it’s wet, score FCS 4.
• If your finger is covered in green gravy, then score FCS 5

3. You can use a plastic faecal probe which on withdrawal shows the folds of marbles or hand grenades, which contrast with the soft plops, slops or scour. You can do one sheep a minute and it’s a lot less stressful for the sheep and operator. (Contact me for details).

Cheap and quick

• These suggestions are no way near as accurate as doing a proper FEC but they are quick, don’t cost money and would be worth a try as an initial screening.
• Supporting these ideas is the experience of farmers who have always culled daggy sheep (that produce soft faeces) and they’ve made progress in reducing dagging and drenching.
• Culling on FCS could speed up this process as it identifies the problem of dags nearer the source!
  
• Don’t wait for ‘peer reviewed’ research to test these suggestions – you’ll be long gone before anyone gets round to it. Try the ‘probe-it-and-see’ approach.
  

Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.