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Individual or bulk worm egg counts: is there a difference?

Figure 1. Worm eggs under the microscope
Figure 1. Worm eggs under the microscope

by Lewis Kahn, ParaBoss Executive Officer

October 2017

 

When considering whether the individual or bulk WormTest method is better for determining worm egg count (WEC), it’s important to keep in mind that either method is better than not testing!

Summary

The individual WormTest method must be used for DrenchTests, sampling to determine Australian Sheep Breeding Values for WEC and for post treatment DrenchCheck.  When WEC are required to determine the need for tactical treatments the bulk method will generally give a better estimate of the true mob average WEC but will not display the variation in WEC among animals.

A quick refresher about WEC

WormTests identify the number of worm eggs in faeces (WEC), which is a good indication of the worm burden of sheep and goats. Some laboratories can also perform a ‘Larval Culture’ (also called a ‘Larval Differentiation’) to identify the types of worms present and their proportion, which is useful in some locations for interpreting the WormTest and deciding on treatment.

A number of laboratories and veterinarians supply WormTest kits and the procedure described by the testing laboratory should be followed.

Individual or bulk method

Regardless of method, faeces must be collected from freshly deposited dung piles or from individual sheep; the latter is necessary if the samples are being used to establish Australian Sheep Breeding Values (ASBV) for WEC. 

Individual WormTests
  • Require faeces from individually-deposited faecal piles to be collected into separate containers that are then analysed individually in the laboratory.
  • Typically, 10 containers are provided in WormTest kits and the report will provide the individual WEC of those 10 sheep (though the identity is unknown because samples are collected from deposited piles) and the average of the 10 WEC values.
  • Faeces are collected from individual sheep (not from faeces on the ground) when the purpose is to provide WEC for the calculation of ASBVs.
Bulk WormTests
  • Require faeces from individually-deposited faecal piles to be collected into a single bulk container which is then thoroughly mixed and then 4–5 subsamples analysed in the laboratory.
  • Bulk WormTests require samples from 20–40 faecal piles (akin to 20–40 sheep). Twenty is sufficient except where barber’s pole worm is an issue or if the mob or herd has over 200 animals.
  • Collect the same amount per pile. For adults, about three pellets from each pile—or the equivalent amount if soft or runny—or five pellets for weaners.
  • Reports differ among laboratories and may provide the WEC of the subsamples but definitely the average of WEC values.

Table 1. Strengths and weaknesses of individual versus bulk worm egg counts.

 

Individual method

Bulk method

Strengths

  • Shows the variation in WEC among animals in a mob
  • Essential for determining ASBVs
  • Essential for DrenchTests
  • Essential for post treatment DrenchCheck
  • Samples a larger proportion of the mob
  • Results more likely to reflect the true mob average

Weaknesses

  • Samples a smaller proportion of the mob
  • Results may not reflect the true mob average
  • Does not show variation in WEC among animals in a mob
  • Cannot be used for determining ASBV
  • Cannot be used for DrenchTests
  • Should not be used for post treatment DrenchCheck

Reflecting the true mob average WEC

The importance of WEC for determining the need for tactical* treatments, highlights the need for sampling that provides a good estimation of the true group average.  With the typical individual WormTest method the collection of 10 samples from a mob of 200–1000 sheep will account for 1–5% of the mob.  Is this a large enough sample size?

(*a tactical or therapeutic treatment is to prevent further ill-effects in the sheep, wheras a strategic treatment is to remove any worms to reduce worm larval contamination of a pasture for the benefit of the whole mob/flock)

This question was tested to check if 10 faecal samples provided an accurate and precise estimate of the true group average WEC.

  • Accuracy represents how closely the WEC from the 10 samples matched the ‘true’ mob average. 
  • Precision represents how much variation in WEC would arise if a different group of 10 animals had been sampled.

To test the question, faecal samples were collected on four occasions from all sheep in a single mob of 500 adult Merino ewes in the Northern Tablelands of NSW, Australia.  Ewes were treated with naphthalophos and levamisole at recommended rates in between the second and third sampling occasions.  Data were entered into a spreadsheet.

For each sampling occasion, the requisite number of animals to achieve sampling of 4% (20 sheep), 8% (40) and 12% (60) was randomly chosen from the data in the spreadsheet.  This process was repeated a further four times to provide five random selections from each sampling occasion.  The accuracy and precision of the WEC estimate for each group (i.e. 4, 8 or 12%) was calculated by comparing to the group average WEC at each occasion from all 500 ewes.

Group mean WEC were 563, 1612, 2605 and 805 epg respectively for the four sampling dates with barber’s pole worm contributing 43, 54, 70 and 74%.  The performance of each sampling group is apparent from the figure below.  An example is provided to help with interpreting the figure.

At the fourth sampling occasion (WEC 4), the average WEC of the entire mob was 805 epg.  If 4% of the mob were randomly sampled five times, the lowest WEC average (of those five times) was about 600 epg and the highest about 1600 epg.  This means that the using an individual method (and in reality this sample size of 20 sheep is twice that typically used) could have returned an average WEC value of between 600–1600 epg. Increasing the sample size to 8% of the mob reduced this range to 700–1400 epg.  A further increase to 12% reduced the range even further to 800–1200 epg.

Of course the important issue is, did this change the decision about the need for treatment?  The answer to this depends on the region and the threshold WEC values provided in the WormBoss Drench Decision Guides for tactical treatments. As barber’s pole worm were 74% of the larval culture, and the true average WEC were 805 epg, a suggestion of no treatment and further monitoring (assuming pasture and sheep were in good condition and there were no looming threats) would be made for these ewes in the Northern Tablelands of NSW. It was much more likely that this decision would have been made from sampling the larger group size.

For those interested in the accuracy and the precision in relation to the true group mean. The accuracy of WEC estimates increased from 69 to 73 and to 81% as sampling increased from 4 to 8 and then 12% of the mob.  The precision improved (a reduction in variation is an improvement in precision) from 19 to 17 and to 12% respectively. Increased level of sampling, provided by the bulk method, is more accurate and precise.


Figure 2: Average worm egg counts from the entire mob of 500 ewes (100%) on four sampling occasions, with the range in average values from the 4, 8 & 12% sampling groups.  The columns indicate the lower and upper average WEC values from the five random selections.