The life cycle of the different louse species is similar, consisting of eggs, three nymphal stages and adults (Figure 1). All stages take place on cattle. The female louse lays individual eggs that are ‘glued’ to the hair fibres. Depending on species, the eggs take between 5 and 18 days to hatch and the entire cycle from egg to egg takes 20-40 days. In cooler temperatures this time period can increase. The period until eggs hatch is important because most treatment products do not kill the eggs. Any eggs that hatch after the protective effect of the treatment has worn off can re-establish the infestation. Read the label carefully to determine whether a repeat treatment is required.
Lice have three nymphal stages which are similar in form to adult lice but smaller and usually paler in colour. It is notable that with the five sucking lice species there are approximately equal numbers of males and females, but often with cattle biting lice very few males are found and female lice are able to produce fertile eggs without mating (parthenogenesis).
Generally lice build up during winter and spring, particularly on cattle that develop a winter coat. Numbers then fall away to undetectable levels when the winter coat is shed and lice are exposed to high levels of solar radiation during summer. Low numbers of lice may survive over summer in sites on the body protected from the effects of high solar radiation. For example, lice may persist under the neck and on the dewlap and it has been shown that low numbers of at least one species of cattle louse (short nosed sucking louse) can persist through summer in the ears of cattle. These lice form the basis for population build up during winter.
Lice that infest cattle are largely specific to cattle and will not survive on other animal species in Australia, with the possible exception of the two tropical lice species the tail-switch louse and buffalo louse which may survive on camels and buffalos. Therefore transfer of lice from other animal species to cattle is unlikely to be a significant source of new infestations.
Cattle lice do not survive more than a few days away from cattle so indirect transfer, for example from pastures, fences or feed bunks, is unlikely to cause infestations. Although there have been reports of cattle lice being carried on buffalo flies, this is probably rare and is unlikely to be a significant reason for new infestations. Nearly all infestations begin from other infested cattle. This can be from purchased, stray or agisted stock, cattle that have missed treatment or where treatments were poorly applied, or where lice have carried over summer on susceptible ‘carrier’ animals within the herd.
Transmission of lice within herds is by direct contact between cattle or from cows to their calves. When cattle are in close contact, lice rapidly move to the surface of the hair coat and can quickly transfer from infested hosts to other cattle in the herd. Poor nutrition or stress from other disease can increase the susceptibility of cattle to lice and the heaviest infestations are often seen in drought-affected cattle or sick cattle.