Worm egg counts

The number of worm eggs passed in the faeces (faecal worm egg count, or WEC) is the most common way to assess roundworm burden in the gastrointestinal tract of livestock and is reported as the number of eggs per gram (epg). Although the relationship between the number of worms in the animal and worm eggs in the faeces is not clear-cut, in general, high egg counts indicate a heavy infection with worms. Low egg counts, however, do not necessarily indicate low infection levels, because a major effect of the immunity that cattle develop is to suppress the production of worm eggs, even when high numbers of some worm types are present. This is especially evident in cattle in comparison to sheep or goats, and in older age groups (cows older than about 18 months rarely have high worm egg counts). Some worm species, such as the small brown stomach worm (Ostertagia ostertagi), produce many fewer eggs for the same number of worms than some other species. In general, bulls carry more worms than females.

Compared to WECs in sheep or goats, counts in cattle are less easily related to worm burdens or visible signs. For weaners under 18 months they can be a reliable indicator of worm burden. For older animals and other classes, WECs must be interpreted in the light of many factors, including animal age, class, signs of disease, body condition or weight (and changes), likely ‘worminess’ of the pasture, and the time and type of recent worm treatments. It may also be necessary to identify the worm species involved. Interpreting WECs in cattle requires considerable experience and is best handled by veterinarians or animal health advisers. In many instances, where the signs are not easily attributed to worms (and especially if a drench has not led to an improvement), you should seek expert advice.

What does the laboratory do?

Faecal samples from individual animals should be submitted to a laboratory for screening. To do the count, a defined amount of faeces is mixed with saturated salt solution to float the worm eggs. A subsample of the flotation solution is then transferred into a counting chamber and all of the eggs under the grid of the chamber are counted. Results are reported as the number of worm eggs per gram (epg) of dung. Modern counting devices (e.g. Mini-FLOTAC) provide higher sensitivity than traditional methods, it is now possible to detect down to 5 epg. The advantage of these more sensitive WEC methods is that the effectiveness of drenches at killing worms (drench efficacy) can be calculated with far greater precision on animals with lower starting WEC. A larval culture or DNA test follows on from a WEC, to determine which types of worms are present.

What should you sample?

  • Collect the sample from a fresh pile of dung.
  • Very soft or runny dung can be collected with a plastic spoon (2 tablespoons per pat) or if bags are used, insert your hand in the bag, grasp a golf ball sized amount of dung through the bag and then invert the bag around the dung.
  • Exclude as much air as possible from samples if they are in plastic bags.
  • Complete the laboratory submission form.
  • Keep samples in a cool place, but do not refrigerate or freeze them, until they are delivered or collected for express postage/couriering (do not send over a weekend).
  • Worm egg count results will generally be sent back within 24 hours of the laboratory receiving the samples.

Larval culture or DNA results will be reported in 7–10 days.

Figure 1. A high powered microscope is used to view worm eggs. Image courtesy of Jess Morgan