How accurate are worm tests?
| How accurate are equine worm tests? The faecal worm egg count is a common procedure for investigating intestinal parasitism in horses. A sample of faeces is taken, suspended in liquid and the eggs in a small amount of the liquid are counted. Unless the worm eggs are distributed evenly throughout the faeces, then the result may not accurately reflect the egg output of the horse as a whole. |  |
Five stabled horses were tested over a week. Faeces samples were collected morning and afternoon .
A McMaster slide was used for counting the eggs. Normally the result is calculated after counting the eggs in only one or two chambers. For this study, Denwood recorded the eggs present in ten chambers.
Two horses were also tested more intensively. A single faecal ball and a faecal pat were each tested five times.
Denwood found considerable variation between tests on the same horse. For example, one animal gave counts ranging from 100 eggs per gram (epg) to 850epg (average 454epg) from the same pile of faeces. If only a single test had been used to determine if the egg count were high enough to require anthelmintic treatment, it would have been wrong 25% of the time.
Although he did not specifically test for changes in worm egg count throughout the day ( there were not enough samples) he found no evidence to suggest there was a diurnal variation.
Most of the variability (around 74%) was due to differences between animals. Differences between samples (from the same horse) accounted for around 13% of the variation. The counting process itself was responsible for only around 4% of the variability - when the result was based on counting10 McMaster chambers. However this rose to around 11% when only one chamber was counted.
Denwood concludes that repeatability of faecal egg count tests appears to be poor because of variability between animals and between samples .
How can you make the faecal egg count more closely reflect the actual worm egg output of the horse? He suggests counting more chambers and reducing the variability of the sample by including more homogenised faeces in the sample examined.
Source: British Equine Veterinary Association Congress. Liverpool. September 2008
Reproduced with kind permission of Mark Andrews BVM&S CertEP MRCVS
© Copyright Mark Andrews - Equine Science Update 2008
Added on: 30/09/08.
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