Sucking lice often take on a darker appearance than chewing lice as they become engorged with blood. Female lice lay their eggs by attaching them to hairs with a strong glue to prevent them from falling off. Lice, in contrast to some other livestock pests, are permanent parasites that spend their entire lives on the host animal. All four types of lice cause extreme annoyance to the host animals. Milk production declines in heavily infested cattle, and the animals' preoccupation with rubbing leads to hair loss, reduced feed conversion efficiency, and general unthriftiness.
Infested animals become irritable and difficult to work with, especially during milking.
People working around lousy animals are exposed to greater risk of injury and are also annoyed by stray lice they pick up from infested animals during handling. Although louse problems are generally perceived as being most severe during the fall and winter months, animals of different age groups show distinct differences in the seasonality of infestation. Lice are most common on mature cows in December through March, with peak populations found in March. In contrast, calves housed inside barns show high levels of infestation throughout the year, with peak populations in June. This difference may be due to the fact that cows are placed on pastures in the spring, where exposure to direct sunlight heats the skin to levels lethal for most lice.