Lice affect sheep and goats worldwide. Prevalence in a given region depends rather on the abundance of sheep, herd management and breeds, and less on climatic or ecological conditions. As a general rule, sheep lice tend to be more abundant during the cold season.
Most lice species affecting sheep and goats are species specific, and consequently there is no risk of transmission from one species to the other (e.g. from sheep to cattle, from dogs to cats or humans, etc.). However, sheep lice may survive on goats and vice versa, but usually do not reproduce off their specific host.
Infestations of animals with lice are medically called pediculosis.
Biology and life cycle of lice on sheep
Lice are small (1 to 5 mm), wingless insects that live on the surface of their hosts, close to the skin. Some species suck blood, other species feed on skin and hair debris.
Lice undergo an incomplete metamophosis. The life cycle takes about 1 month to complete. Each female deposits 20 to 50 eggs (nits) during her lifetime. She glues them one by one to single hairs in the hair coat of the host. Incubation lasts 4 to 20 days. Young nymphs look like adults but are smaller. Adult life lasts 2 to 6 weeks, depending on the species. Off the host most lice survive only for a few day, and exposed to sunlight only for a few hours. But under ideal conditions some species may survive up to one month off the host.
Lice are obligate parasites that spend their whole life, including development on the host. Transmission from sheep to sheep is by contact, typically from mothers to their offspring before weaning. Within a flock, transmission is very quick: after introduction of a few infested sheep in a clean flock >50% sheep can become infected within one month. In regions with cold winters (Europe, North America, Northern China, etc.) lice are typically a winter pest in sheep kept indoors during the winter: this is explained by the easier spread through close contact due to overcrowding, by the longer wool and by weakening due to poorer food. In regions with tropic or subtropical climate lice populations depend more on wool length, time of shearing (long wool favors lice development) and climatic conditions: e.g. by heavy rains many lice may drown in water saturated wool and eggs may not hatch.
Click here to learn more about the general biology of insects.
Main species of lice on sheep
Bovicola ovis (=Damalinia ovis = Trichodectes ovis), the sheep body louse is a chewing louse that occurs worldwide and is the most damaging louse of sheep. Adult lice are 1.5 to 2 mm long and have a brownish to yellow color. They are found only on those parts of the body covered by wool, i.e. not in the face, the udders or the legs. The feed on skin and wool debris, scurf, gland secretions and body fluids exudating from injuries they produce with their mouthparts. Lice do not move a lot within the wool, but tend to be close to the skin by hot and sunny weather, and closer to the fiber tip by colder or cloudy conditions. This is relevant to understand that transmission between animals is more likely to happen in the afternoon or early morning when the temperature is lower, because lice are closer to the tip of the fibers and thus can easier pass from one sheep to another one when they come close to each other, e.g. for drinking. By short wool lice tend to concentrate in the neck, by long wool on the sides.
Linognathus pedalis, the sheep foot louse is another important louse species that affects sheep worldwide, although less frequent than the body louse. It is found mostly on legs and feet free of wool, but also on the belly. It is a bloodsucking louse that pierces the skin of the host to get access to blood.
Linognathus ovillus, the face louse, is another blood sucking louse that is found typically on the face, but also on other parts of the body, excepting those without hair. It certainly occurs in Europe, Australia and New Zealand and probably in many other countries.
Damage, harm and economic importance of lice on sheep
Lice bites are very irritating for the hosts skin. Sheep and goat react scratching and rubbing intensively against objects and licking or biting the affected parts. This behavior can cause skin injuries susceptible of infection with secondary bacteria. Wool loss and reduced weight gain can be considerable. However, deaths due to lice infestations are extremely uncommon.
In countries where both quantity and quality of wool is a substantial economic factor, lice control is essential since they can reduce not only the amount of wool produced, but also substantially diminish wool quality (diameter, length, strength, color, staining or clotting, etc.).
Lice infestations can also affect hide and leather quality resulting in reduced income. Milk production of dairy sheep and goat can also drop due to heavy lice infestations.
Linognathus ovillus can transmit Eperythrozoon ovis, a blood parasite of sheep.
Prevention and control of sheep lice
Shearing is one of the best measures to knock down lice populations. A huge number of lice (close to 90%) are eliminated with the clip and most lice remaining on the shorn animals won't survive without the humid and warm microclimate of long wool. However, a few lice usually manage to survive in less exposed areas of the body (e.g. the ears) and will restart the infestation. If possible all sheep in a flock should be shorn at the same time: this will have the strongest knock down effect on the lice population. If it is not possible, shorn sheep should be kept separated from unshorn ones.
An important measure to avoid infestation of clean flocks is ensuring proper fencing to keep away potentially infested stray or neighboring sheep.
Stock introduced in a farm (including rams) must be quarantined or closely inspected for lice and, if positive, they must be shorn and treated with a lousicide. Remember that lice are difficult to detect in shorn animals!
Where flocks are routinely treated with insecticides and/or acaricides for tick and/or mites control it can be expected that lice will be controlled as well, since most tickicides and acaricides are also effective against mites. In countries where compulsory treatment of sheep against sheep scab was abandoned in the last decades (e.g. UK, Uruguay), the incidence of lice has reportedly increased.
Where specific lice control measures are needed a large number of lousicides are available in the market, although specific brands and application methods vary from country to country.
Concentrates for dipping, spraying, or jetting containing classic parasiticides such as organophosphates (e.g. diazinon), synthetic pyrethroids (e.g. cypermethrin, deltamethrin), amidines (mostly amitraz) or mixtures are still available in many countries. They are usually the most economic option for treating large flocks. Nevertheless, after increasing concerns about operator safety and environmental risks, and following more stricter regulatory requirements the use of such products has strongly diminished in several countries (e.g. Australia and UK) and some of these products have been even prohibited or are being phased out. New generation parasiticides containing ivermectin and spinosad, alone or mixed with other parasiticides are also available as concentrates for jetting or spraying.
Ready-to-use pour-ons (= backliners) have filled the gap and are becoming more popular everywhere, also because they are easier to use and more convenient to handle than concentrates. Today's pour-ons for lice control contain mainly synthetic pyrethroids (e.g. cypermethrin, deltamethrin), insect development inhibitors (diflubenzuron, triflumuron), ivermectin, spinosad and imidacloprid, alone or in mixtures.
Insect development inhibitors do not kill adult lice but interrupt the development of immature stages that die during the next molt. Consequently it may take several months until all adult lice die "naturally" and the flock becomes lice free.
Most products do not only cure existing infections more or less quickly, but also protect for a few weeks up to several months against re-infestation, i.e. they have a long residual activity. However, most products with a long residual activity are likely to have longer withholding periods too.
Macrocyclic lactones (e.g. ivermectin, moxidectin) are also available as injectables or drenches for sheep in several countries. Lice are not their primary target parasite of macrocyclic lactones, although lice may be included in the label, sometimes as "aid to the control of...". As a general rule, such systemic products are usually not reliable lousicides because sheep lice are not obligate bloodsuckers and may not be sufficiently controlled. Such products also have a protection period of only 1 to 3 weeks, if ever, because they are excreted or metabolized from the sheep blood within two to four weeks.
Depending on the country, some products may be recommended or approved only for off-shears treatment, others only for short and/or long wool treatments, etc. Product labels must anyway be carefully read before selection to ensure that it satisfies the own needs.
Besides efficacy and length of protection against lice (and price...), other product features play an important role when deciding which brand is best for the own operation. The most relevant features are the following ones:
- Spectrum of efficacy: i.e. efficient or not against other pests that may affect sheep and need to be treated as well, e.g. blowfly strike, mites, ticks, etc.
- Withholding periods for meat and wool, and Export Slaughter Intervals (in some countries).
- Resistance of lice to several active ingredients (especially organophosphates, pyrethroids and insect development inhibitors)
Where dairy sheep and goats are important (e.g. for cheese production, or for human consumption), most of the above products may not be approved. Many products leave too high residues in milk (e.g. macrocyclic lactones and insect development inhibitors). And for economic reasons many manufacturers just don't investigate milk residues in dairy sheep and goat because they represent only a very small part of the louisicides market in the large sheep countries like Australia, New Zealand, UK, Ireland, Argentina or Uruguay. As a consequence, for dairy sheep farmers the number of available louisicides may be rather small.
For the time being there are no vaccines that will protect sheep by making them immune to lice. There are no repellents, natural or synthetic that will keep lice away from sheep. And there are no traps for catching csheep lice, for the simple reason that they spend their whole life on the animals and therefore there are no stages in the environment searching or waiting for a host.
So far there are no effective biological control methods against lice on sheep. Learn more about biological control of insects.
Click here if you are interested in medicinal plants for controlling lice and other external parasites of livestock and pets.
|If available, follow more specific national or regional recommendations or regulations for sheep lice control.|
Resistance of sheep lice to parasiticides
Resistance of the sheep body louse (Bovicola = Damalinia ovis) to organochlorines, organophosphates, synthetic pyrethroids and insect development inhibitors is now widespread in Australia and New Zealand. There are also reports of resistance to various lousicides in the UK and South Africa, especially to synthetic pyrethroids.
As a general rule, once pyrethroid resistance appears in a farm or a region, it spreads and increases quickly, and reaches such high levels that products with such compounds completely fail to protect sheep against lice.
A practical consequence of this is that if a particular product does not achieve the expected control of sheep lice, it is reasonable to consider a resistance problem, especially if that product has been used for several consecutive years without rotation. However, experience shows, that many cases of product failure are due to the use of inadequate products or to incorrect administration, and not to resistance.