This topic is a bit of a minefield, and we can’t stress enough that faecal egg counts are a GUIDE to the number of worms inside animals. You must interpret them in light of all other information available! The TREND in FECs can often be as helpful as the absolute numbers. Repeat FEC monitoring of a mob of animals often gives better information than one count in isolation
Things that can affect FEC results
FEC is pretty well correlated to number of worms on board in lambs, but less so in calves and adult sheep. FECs are a poor guide to parasite burden in cattle over about 9 months of age.
Animals that have been off feed (e.g yarded for shearing) can have FECs that make them look more wormy than they really are. The small amount of feed left in the gut and slowed gut movement concentrates worm eggs in a low volume of faeces. Animals with very high feed intakes can have lower FECs for the opposite reason; lots of feed moving through quickly dilutes the worm eggs.
Animals with a scour can have lowered FECs because of the dilution effect of the extra moisture in the faeces.
- In the first 3 weeks (at least) since a drench treatment, FECs should be near zero.
- About 3 weeks after the previous worms were killed, any newly-established young worms can start to lay eggs.
- In lambs and calves with little worm immunity, FECs may rise quite quickly.
- This will depend on the feed they are grazing. * Lambs or calves grazing crop, new grass or other 'low worm challenge' feed, may go for many weeks after drenching before FECs rise again, whereas those on 'wormy' pasture could have high FECs again after 4–5 weeks.
I’ve got the FEC results – what do they mean?
The answer to this question usually starts with ‘It depends....’
When faecal egg counts are very low, or very high, its usually pretty straightforward to determine what to do. It’s that middle range that can be tougher to interpret. The tables below are there to give you some guidelines, but NEVER interpret FECs in isolation, ensure you also have the following information:
| Animals | Feed |
|
Age and management mob (e.g. 'Main line' ewes versus 'B flock' ewes). How the animals are looking. How well grown they are. Any breakdown in immunity they may face. For example lambing, weaning (lambs), other diseases (e.g. BVD, Johne's disease, respiratory disease). |
Feed quantity. Feed quality. Likely level of worm contamination – feed grazed now and feed they’re going onto. Time of year can be important – is worm challenge rising or falling? What worm species are likely to be about?
|
Interpreting sheep FECs
Note: don't over-interpret FECs – the results represent a range, not a fixed number.
Possible range of individual FEC values represented by different FEC results.
| Lamb FEC | 95% of repeat counts will fall within this range |
| 0 | 0–150 |
| 100 | 0–350 |
| 200 | 50–500 |
| 300 | 150–650 |
| 400 | 200–750 |
| 500 | 250–900 |
| 600 | 300–1,050 |
| 700 | 400–1,150 |
| 800 | 450–1,250 |
| 900 | 550–1.400 |
| 1,000 | 650–1,500 |
Adapted from: Brambell (1963) J. Helminthol. 37. 1–10.
Individual sheep FECs – interpretation
| FEC range | Interpretation | Action |
| 0–300 | Low | Don't treat, continue to monitor |
| 300–800 | Moderate | May or may not treat depending on other factors* |
| 800–2,000 | High | Treat now |
| 2,000+ | Very high | Treat yesterday |
* Feed quality and quantity, sheep age, condition, and other heath challenges. Source: Manawatu Veterinary Services staff training notes 2003. G Dodunski.
What do I do if my FEC results don't all fall into one range?
- It is quite normal for FECs to vary over a wide range.
Examples of this:
- Ewes – 80% of FECs are zero or low, and 20% are moderate to high
- Hoggets in winter – 50% of FECs are zero or low, 50% are moderate to high
In these cases you need to consider all your management information alongside the FEC data, to decide the next step. Depending on this, you may elect not to treat, to treat a proportion of the mob, or not treat at all.
Total worm count versus FEC (lambs)
| Individual lambs with FEC <500 epg generally have total worm counts <4,000. | Individual lambs with FEC 600–2,000 generally have total worm counts from 4,000–10,000. | Individual lambs with FEC >2,000 generally have total worm counts >10,000 (usually more damaging species). |
Source: McKenna PB. The diagnosis of gastro-intestinal parasitism in cattle and sheep. Proc 11th Annual Seminar: Society of Sheep & Beef Cattle Veterinarians NZVA, 1981 .
Composite sheep FECs – interpretation
| FEC range | Interpretation | Action |
| 0-200 | Low | Don't treat, continue to monitor. |
| 200-500 | Moderate | May or may not treat depending on other factors*. |
| 500-800 | High | Treat now. |
| 800+ | Very high | Treat yesterday. |
Interpreting cattle FECs
FECs are quite useful for cattle up to about 9 months of age. They do need to be interpreted with caution - low counts may be misleading; they can mask a significant worm burden, especially beyond about 6 months of age.
From this time the animals’ immune system can limit the number of eggs being produced by resident worms – those worms can still be causing health and production issues in the face of a low egg count.
Individual calf FECs: interpretation
| FEC range | Interpretation | Action |
| 0–100 | Low | Don't treat, continue to monitor |
| 100–300 | Moderate | May or may not treat immediately depending on other factors* |
| 300–700 | High | Treat now |
| 700+ | Very high | Treat yesterday |
* Feed quality & quantity, calf age, condition, other heath challenges .
Source: Manawatu Veterinary Services staff training notes 2003. G Dodunski.