Infestation with lice can affect all sheep and cause production losses due to reduced feeding time. The spread of infection between infested and clean animals is slow. Transfer of lice between animals requires close contact, such as yarding, transport or housing, especially in cool cloudy weather.
The entire life cycle of the sheep chewing louse, Bovicola ovis, is spent on its sheep hosts, and transmission is by close contact.
The reproductive rate of chewing lice is slow when compared with mite parasites, and it takes several months for heavy infestations to develop.
Chewing louse infestations have increased in UK sheep flocks, following the withdrawal of compulsory plunge dipping for the control of sheep scab and greater use of systemic endectocide injections, which are ineffective against chewing lice that do not feed on systemic body fluids.
Animals with heavy chewing louse burdens are often ill thrifty, but there is little evidence to show that louse infestations per se cause significant weight loss. Instead, the lice probably exploit the fact that their hosts are already in poor body condition.