Sampling guidelines
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.
Sample collection
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.
Further information
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.