Nearly all new infestations begin from contact with another infested sheep. Major sources of lice are:

• Infested sheep that stray onto a property.
• Sheep which stray from the property, come into contact with a lousy sheep, and later return to the mob.
• Purchased or agisted sheep and sheep brought in from other properties.

However, infestations also result from sheep missed at muster and therefore left untreated in the previous year or sheep not effectively treated at their previous shearing.

Note that sheep lice do not breed on animals other than sheep (with the possible exception of goats, in very rare instances). They are not carried by birds and they do not remain in wool rubbed onto fences, trees or other structures, so this is not likely to be a source of infestation.

A well-designed lice biosecurity plan which avoids contact with infested sheep is critical to keeping a property lice-free. The linked farmnote on Lice biosecurity, provides guidelines towards developing an efficient lice biosecurity program.