Worm eggs are detected by mixing faeces with a salt solution so they float and can be counted under a microscope. The worm egg count (WEC) is expressed as the number of eggs per gram of faeces.
Worm egg counting can be used for any class of livestock and is especially important in sheep, where good worm control is essential to prevent disease and maintain efficient production.
Worm egg counts have several purposes including:
- determining whether drenching is needed to treat or prevent disease or production loss caused by worms
- monitoring the effectiveness of planned control programs
- testing for drench effectiveness
- testing individual sheep for selection for worm resistance.
A veterinarian or sheep adviser should be consulted to assist in planning a WEC monitoring schedule and to recommend action on the basis of results.
Use of faecal worm egg counts
Monitoring worm burdens for disease or potential production loss
Worm egg counts will determine whether signs of worms (such as scouring, ill-thrift or weakness) are due to worms and whether immediate treatment is needed. WECs also indicate if smaller worm burdens have reached the point where they are likely to be causing unrecognised production losses. Pre-emptive treatment reduces this loss and prevents further worm egg contamination of paddocks which may lead to further worm problems. Monitoring is most important during the ‘wormy seasons’ of winter and spring.
A WEC will indicate whether a drench is needed, can be avoided or delayed, or if a further WEC should be taken in the near future to confirm a trend. The exact figures for these decisions vary depending on several factors including the time of year, seasonal conditions, geographical location and class of sheep. While very high or low counts may give a straightforward indication, in many cases assistance will be needed to interpret the result.
Monitoring for potential pasture contamination
Worm egg counts during autumn will determine whether there is significant worm egg contamination of pastures to lead to significant worm problems later in the year. A drench given at this time, as indicated by the worm egg counts, may be needed to reduce the risk of problems in winter or spring.
The most important flocks to check are weaners and hoggets. This is especially important if the drench type used for summer drenches is not known to be fully effective, as resistant worms surviving that treatment will become the main source of future worm populations.
Worm egg counts at this time will also increase the efficiency of control programs, by checking whether a routine treatment is in fact necessary. For example, adult sheep flocks which would usually be drenched in autumn would in some cases have counts that have not reached significant levels so this treatment can be safely avoided. (See Sheep worms – summer-autumn worm control.)
Drench effectiveness testing
Worm egg counts are the basis of drench resistance testing. Several drench types can be tested at the same time by treating different groups of sheep with different products. The level of resistance for each drench is indicated by the percentage reduction in egg count compared to an untreated control group. For more details on how to conduct a drench resistance test see Sheep worms – testing for drench resistance and effectiveness.
Worm egg counts can also be used to give a rapid assessment of the effectiveness of a particular drench after a treatment has been given. Ideally, a worm egg count is taken when the sheep are in the yards for drenching and then again 10–14 days later (from the paddock, see Sampling guidelines) to indicate the percentage reduction in egg count. Even a single count taken 10–14 days after drenching can be useful, as it will show whether or not treatment was sufficiently effective for the purpose.
Selecting worm resistant sheep
Ram breeders are encouraged to include worm resistance in their selection index. Using rams of a low worm egg count index will, over time, lower worm burdens on the property. Worm egg counts vary considerably between individual sheep in any flock and comparing the average worm egg counts of groups of lambs from ewes mated to different rams indicates the relative worm resistant status. This information can be used in a breeding index, along with production and product quality indices.
Sheep worms - breeding for worm resistance in sheep provides more information. It is recommended that a geneticist is consulted to plan testing and analyse the results.
How many samples?
To determine whether a drench is necessary, we recommend that 20 samples are collected from each mob. The minimum sample number to give a useful result is 10.
Until recently the recommended sample number per mob was 10 but analysis shows that the preferred level of accuracy requires more samples. Worm egg counts within a flock follow an unusual pattern, with a small number of animals having relatively high counts but most having counts well below the average figure. As the interpretation of the significance of the count is based on the average, it is important that there is a reasonable representation of the less-common high counts. Taking 20 samples is a good compromise between the effort involved in sample collection and confidence that the average count is a good estimate.
For drench resistance testing, 10 samples per drench group included are sufficient. This number is also sufficient when checking whether a drench was effective after a mob treatment, as the intent is that the post-drench count is close to zero.
Which laboratory test?
Worm egg counting can be conducted either on individual sheep samples or on samples bulked together from a number of sheep. Individual testing is preferred where the diagnosis of a disease outbreak requires between-animal worm burden estimates and for some drench groups in a drench resistance test. Where barber’s pole worm is likely to be a problem, individual counts may also be useful, as the range of counts can diagnose its presence. However, for routine monitoring purposes the ’bulk worm egg count’ procedure is recommended. This provides a single average worm egg count figure and is based on mixing all samples from a flock and conducting counts on a small number of representative sub-samples.
There is little difference in precision between the average WEC figure from the individual or bulk counting method, however much less time is needed for the latter. This means that the cost of a bulk WEC on 20 samples is less than the cost for 10 individual counts. The better estimate of the average mob WEC is from the larger sample number and is therefore at no greater cost.
Worm species identification
In some situations it is important that the types of worms present are identified. Identification of barber’s pole worm may indicate that a specific narrow-spectrum drench would be more appropriate for worm control (see Sheep worms - barbers pole worm in sheep). An increased frequency of worm egg count monitoring is also necessary in barber’s pole zones, and pasture management and planning should focus on reducing the risk of sheep losses. The identification of the worms present is essential in drench resistance tests.
The identification of worm species (a ‘larval differentiation’) requires specific technical skills and is usually conducted only in parasitology laboratories. The procedure involves culturing sheep faeces from a mob or treatment group so that the worm eggs hatch and develop through various larval stages. The different species of worm eggs cannot be differentiated. Larval culture takes a week and the worm larvae can then be identified by skilled operators.
Larval culture and differentiation is time-intensive and therefore incurs a cost. It is recommended that advice is sought regarding the need in different situations.
It is essential that samples are collected from individual sheep and placed in separate containers (not pooled together). If samples are to be sent away for testing, they should be taken early in the week to allow sufficient time for samples to arrive before the weekend.
Samples can be collected by several methods:
- directly from the rectum with a gloved finger
- from freshly-passed deposits when sheep are yarded (with care to ensure that samples are likely to have been passed by different sheep and are not covered in debris)
- from the paddock, the sheep should be ‘mobbed up’ and stand quietly for a few minutes, then allowed to walk away. Dung samples that appear to have been freshly-passed are then collected from the ground.
Sample size: for each animal, 5-10 separate pellets or equivalent is the minimum required. If using plastic bags (sandwich bags are ideal), the air should be excluded as this helps to prevent eggs from hatching. Samples should be kept cool (fridge temperature - not frozen) before and during dispatch to the laboratory. (This is especially important if using jars or bottles.)
Your details and details of the flock should be provided with the samples including: owner name and contacts, sheep age, class, body condition, pasture type and when last drenched.
For planning worm egg count monitoring, drench resistance testing and worm resistant testing, it is recommended that a veterinarian, sheep adviser or Department of Primary Industries and Regional Development animal health staff member is consulted. The WormBoss website also has relevant information.