Sheep worm counts and drench decision aids

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When to treat sheep for worms is a common question.

Various decision aids have been used over the years. For example,a traditional approach has been to simply combine drenching with other management events, including joining (tupping or mating in other countries), off-shears, pre-lambing, at lamb marking, and at weaning.

In the pastoral areas such as the western division of NSW, where sheep are less frequently mustered, sheep may be drenched when they are mustered for other reasons.

In the Haemonchus – endemic areas of northern NSW and SE Qld (higher rainfall areas, eg > 700 mm, with summer rainfall dominance), drenching of young sheep in summer was largely a monthly affair prior to the 1982 release of the long-acting anthelmintic, closantel.

As well as considering the calendar (for management events and the seasonality of different types of worms), producers and advisers consider indications of worm burdens (typically worm egg counts (WECs)), the condition of sheep, the age and class of sheep, and their nutritional status.

As an aid in making decisions, skilled producers and advisers have always considered a number of factors, as outlined above. Drench decision aids merely facilitate this process and can take the form of charts, graphs and tables, or interactive computer programs or websites.

Following is a bird’s eye view of various decision aids.

The Brain

The brain has been a popular decision aid among hominins over the 2 million years or so of its development. However, despite their name, Homo sapiens have sometimes been less than wise in the use of this decision aid.

Seriously though, as intimated above, skilled operators use a combination of logic and intuition, weighing up various factors when deciding whether to drench or not. In essence, this is what all decision aids do. While superficially decision aids may look quite different, they are fundamentally similar, being built on similar foundations.

Image credit: Simpsons Trivia.

WormKill – 1980s

The original WormKill (1984 to early 1990s) was a simple, prescriptive strategic program. The WormKill table in effect was a decision aid.

Here is an example of the WormKill table from July 1986:

There are two further things to note.

Firstly, promotion of grazing management as part of worm control is not new.

Secondly, although WormKill (1980s versions) was a prescriptive program, regular worm egg count (WEC) monitoring was done on a number of properties. A WEC of 500 strongyle eggs per gram of faeces (epg) was used as a benchmark, with drenching considered advisable when WECs significantly exceeded this level.

WormTest for livestock and guide to egg counts – Primefact, I&I NSW

This Primefact (2003 and 2007 editions) discusses various factors in interpreting WECs. Two tables provide guidance:

Source: Primefact 480.

The following table has been prepared for the revised editions of Primefact 480, and also ‘WormKill – the basics’ (in preparation).

The Rendell Matrix

David Rendell is a sheep veterinary consultant in western Victoria. He developed this matrix some years ago: the version below is from 2002. He has tested it in the field among his clientele.

Source: The Weekly Times 18.9.2002.

Remember that decision aids are applicable only for the areas for which they have been developed. The WECs (aka FECs) in the table above come mainly from black scour worm and brown stomach worm, with little from barber’s pole worm, apart from farms close to the Victorian coast.

And the make up of ‘scour worms’ can vary from one area to another. In the NSW northern tablelands Trichostronglyus colubriformis tends to be more common than another type of black scour worm, Trichonstronglyus vitrinus, which tends to be more pathogenic and produces fewer eggs. Also, the further one goes north in summer rainfall areas, small brown stomach worm (Ostertagia (Teladorsagia) circumcincta) – a relatively poor egg layer – becomes less common, to the point of being uncommon in Queensland. As one moves south into the non-seasonal and especially the winter rainfall areas of south eastern Australia, T. vitrinus and Ostertagia become more important.

Vet Lab Manual – Industry & Investment NSW (formerly Dept. Primary Industries)

Here is a guide to worm egg counts in sheep from the I&I NSW Vet Lab Manual.

These WEC benchmarks may seem alarmingly high: that’s because this table is not really a drench decision aid, but rather a guide to WECs that may be associated with clinical disease.

By the time parasitism is clinically obvious, productivity and economic returns have already taken a substantial hit, hence the lower benchmarks used in most drench decision aids.

In round figures, internal parasites cost the Australian sheep industry $400 million a year, and about 80% of this is from production losses, most of which is not obvious. This is why objective measurement (usually by way of worm egg count monitoring) is an important part of worm control.

Ask the Boss

Ask the Boss is the decision aid in WormBoss.

The user interacts with Ask the Boss, and a worm egg count result is assumed.

The user is then presented with recommendations.

Decision Aid – IPM-S project (summer rainfall)

This is one of the more recent drench decision aids. It was developed by Lewis Kahn and others (UNE / Sheep CRC) as part of the AWI Integrated Parasite Management-Sheep (IPM-S) Project.

The aid was developed for and tested on several IPM-S project properties in the summer rainfall zone of north eastern NSW and south-eastern Queensland.

The aid’s ‘interface’ can be computer-based (interactive Excel spreadsheet) or hard copy (a table / matrix). Like other aids it aims to take into account a number of factors including WEC, condition of sheep and nutrition.

Below are snapshots (computer-based and hard copy versions):

More information on IPM-S: