But clinical parasitism - deaths, dags and obvious illthrift - is just the tip of the iceberg. About 80% of the cost of sheep worms in Australia comes from production losses. Most of this is subtle, well-nigh invisible.
Those farmers who frequently measure worm burdens - and drench efficacy - by way of worm egg count monitoring (WormTesting) will have a good handle on these subtle but significant losses. The rest could quietly lose anywhere between 'just' a couple of dollars to over $10 per sheep per year depending on location, season and commodity prices.
There is one more thing to ponder. Cold weather doesn't stop worms dead in their tracks. Sure, temperature senstive worm eggs like those of barber's pole worm and, to a lesser extent, some types of black scour worm, won't be able to complete their life cycle in the winter months in cooler areas, eg the NSW tablelands. But their infective larvae (and the infective stages of liver fluke too), possibly produced in large numbers up until late autumn, can survive on pasture over winter. Some will die along the way, but others will make it through to spring, providing a launch pad for worms when warmer weather comes along.
Add to this the worms left behind inside sheep due to unwitting use of drenches affected by drench resistance.
The bottom line? You really have to get into a program of regular WormTesting. Don't just look at the cost and hassle of WormTesting: consider the bigger picture. It really is time and money very well spent.
Additional comment – Arthur Le Feuvre:
Surveys show that around 90% of sheep producers in Australia have no factual information on the efficacy of the drenches they use. This means that when a sheep producer purchases a drench, he/she is buying it on the assumption it is effective – and that is likely to be an expensive mistake. Unseen mostly, but still very expensive!!