March 5, 2010

Breeding sheep to eliminate dags and worms. 8.

By Dr Clive Dalton


Life cycle of main species of sheep worms

  • The main species are 'roundworms', but there are also tape worms, lung worms and liver flukes. Below are Barber's Pole worms (Haemonchus contortus)
  • Mature worms live in the sheep's gut where they mate and produce eggs.
  • These eggs then pass out in dung on to the pasture.
  • When conditions are right (warm and moist), they hatch into larvae and go through three main stages - L1 to L3.
  • The L3 stage is the critical one as they crawl up the pasture plants to be eaten by another sheep to continue the cycle as mature worms inside the sheep.
  • It takes about 21 days for ingested larvae to develop into adults capable of laying eggs.
  • These larvae are aquatic creatures and need moisture to survive and travel. Dessication kills them.
  • Larvae need a minimum of 2-6 weeks of warm moist conditions to develop successfully.
  • Some larvae can live for 6-8 months in cooler temperatures (surviving frosts) and in warmer conditions can die after 2-3 months.
  • They can survive droughts by going down 10-20cm.
  • KEY POINT: Around 90% of the worm cycle is on the pasture where you can't do much about them - other than get other species of animals to eat them with no ill effects.
  • If you want to kill any worms, you have to do it with chemicals when they are inside the sheep. Some drenches only kill the worms and others kill both worms and eggs.

Faecal Egg Counting (FEC)
  • A FEC is 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’ 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, New Zealand.
  • The costs through a veterinary clinic can vary depending on quantity.
  • To check the quality control of the veterinary laboratory, split some samples and send them in with different numbers. The sub-samples should not vary more than a few hundred eggs per gram.
  • 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 their eggs, so to identify species the eggs have to be incubated in the lab which takes about 10 days and consequently costs more.
Larvae being incubated in lab

  • 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 can only be 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 some 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.
  • Whatever its many shortcomings, it's the most cost effective at this time.

FEC sampling tips

Paddock (composite) samples
  • The easiest way to get a dung sample for FEC 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 keeps the lab costs down.
  • 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 is 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.

Finger sampling
  • Use your index finger to draw a sample of faeces from the rectum.
  • Rubber gloves are recommended.
  • The sheep will not like this, and it can take a good push to gain entry if the animal has not been scouring.
  • You need at least 5 g of faeces so it usually needs a second finger insertion. The sheep won't like this either.
  • With many sheep to do, this can be tiring on your fingers so you may have to change hands occasionally.
Probe rectal sampling

  • You can use a probe made from half inch plastic water pipe to draw out the sample.
  • The probe is carefully inserted, and given a 180 degree twist before removal.
  • You will normally get more than the 5g needed by one insertion of the probe.

Faecal Consistency Score (FCS)
  • Australian research from Armidale in NSW has shown that when you get the FEC, you should correct it for moisture content. They showed that the easiest way to judge this was from the form of the faeces.
  • The more liquid the faeces, the quicker they pass through the digestive system, and hence the more diluted the 'eggs per gram of faeces' will be when they come out the rear end.
  • A healthy sheep with an efficient digestive system passes round ‘marbles’ – even when on lush pasture and crops.
  • These are formed in the hind gut where water is removed, and the movement of the gut wall (peristalsis) produces the marbles. It’s an amazing bit of physiology!
When to do it?
  • It should always be done when collecting a FEC sample and the score built into the animal ID number.
  • Or it can be done at any other time.

Scores used

  • The Australian researchers used numerical scores but the following descriptions are used below as they are easier to remember:
Photo 1: ‘Marbles’

Photo 2: ‘Hand grenades’

Photo 3: ‘Plops’

Photo 4: ‘Slops’

Photo 5: ‘Scour’

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

A quick and easy method

If you don't want to pull out a sample of faeces from the rectum, then all you do is insert your finger and feel what's in there.
  • Marbles - you will feel these easily
  • Hand grenades - you will feel an empty gut as the handgrenade has just been voided.
  • Plops - you will see just a green stain on the end of your finger.
  • Slops - your finger will come out quite green.
  • Scour- your finger will be covered in green soup!

Advantages of marbles

An overnight pile of marbles from a sheep that has defeacated 2-3 times without getting up.
She has got up and no dung has stuck to her wool causing dags.

Faecal marbles have big advantages for sheep, and big disadvantages for larvae:
  • Marbles don’t stick to wool and produce dags.
  • They fall to the ground and many separate and spread out.
  • If the lamb’s tail has not been docked too short, causing damage to the supporting tissue around the anus, when the sheep wags it’s tail during defaecation the marbles are projected away from the body.
  • Single marbles or small clusters dry out quickly on the ground leading to larval death through desiccation.
  • The surface area of marbles is about six times greater than if the same volume of dung was in a large wet heap.
  • The dark green to black colour of the marbles absorbs more UV radiation adding to larval death.
Conclusion: Having sheep that pass marbles will be a practical and effective way to reduce the larval population on the paddock.

Breeding implications of FCS correction
Correcting a FEC for FCS has major implications to improve the accuracy of FEC interpretation, when selecting individual animals in breeding programmes.

  • When selecting ram hoggets or two tooths 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 (FCS 1).
  • If he was scouring (FCS 5), then multiplying his FEC of 500 x 5 would give a 2,500 epg count and you would certainly not want to keep him as a future sire – even though he may be classed as resistant using FEC alone.
  • He should be on his way to the dog tucker freezer. The FCS has changed the decision drastically – and for the better.
  • So not correcting for faecal moisture can 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 will pass those genes on to their progeny, and you would be bending over their rear end and removing dags for the next five years.. That’s not progress!

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