Careful inspection of newly purchased or other introduced sheep for lice is a critical part of a good lice biosecurity program. Regular monitoring of mobs for lice and, in particular, careful inspection before shearing, is essential to enable early detection of new infestations so that sheep can be treated, if appropriate, or managed to minimise further spread.
Lice are extremely difficult to find when present in low numbers. This is one of the reasons why lice have been so hard to eradicate and why quarantining new purchases and other introduced sheep is so important. It is also the reason that many producers mistake an infestation detected in sheep in long wool as a new infestation whereas more often it is an indication of a population that was suppressed but not eradicated by a previous treatment.
The best way to monitor for lice is to look for rubbing sheep. However, because rubbing can be due to a number of other causes, it is critical to actually see lice, or at least rule out other causes before deciding to treat the mob.
Lice can cause sheep to rub when present in very low numbers, so you may have to inspect many fleece partings on many rubbing sheep to find lice even if they are the cause of rubbing.
Images above: (L) Parting the wool to the skin to inspect for lice (R) Sheep that have been rubbing should be inspected for lice first. Source: Peter James
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