Addressing frequently asked questions on external
parasites in sheep

Keeping sheep free of external parasites is a challenge, but cleaning them up after an infestation can be even more challenging. Understanding the life cycle of sheep lice, mode of transmission and effective treatments are important to prevent and eradicate infestations.

Lice types:
B. ovis  – biting lice

Bovicola ovis (B. ovis), perhaps the most prominent sheep lice, are pale yellow in appearance with 1.5- to two-millimeter long brown stripes on their abdomen. Unlike cattle lice, sheep lice eggs will not hatch unless females have been mated. 

Females lay one to two eggs every three days. These eggs are generally cemented on wool fiber no more than  one-half inch from the skin surface. 

Eggs usually hatch after nine to 10 days, and adolescent nymphs molt three times over 21 days to reach maturity at 34 to 36 days of age. The major mode of transmission is between sheep in close proximity. 

Transmission at lambing is from ewe to lamb contact and from facilities and shearers. Unlike the sheep ked, sheep lice can survive longer off the sheep under warmer temperatures and different environments for up to three weeks. 

Optimum temperatures for lice range from 98.6 to 102.2 degrees Fahrenheit, and temperatures outside this range inhibit their ability to reproduce. However, their ability to migrate to or away from the skin surface allows them to thermoregulate. Complete saturation of lice and drowning has been reported but is unlikely in production settings.

Lice types: foot and face lice – sucking lice

Less common, but still  problematic, are the sucking lice species Linognathus pedalis, also know as foot lice and Linognathus ovillus, commonly referred to as face lice. 

Foot lice are found on hairy portions of a leg from fetlock to hock and can also be found on the scrotum or belly. Foot lice generally predominate in the winter and spring. Affected sheep don’t always show clinical signs but have a pungent-sour odor. Backline pours versus dipping is a possible cause of resurgence from Australian perspectives. 

Face lice are not as common but occur at the junction of the skin and the wool around the face. Treatments for most lice species are similar with a major difference being specific areas of infestation should be prioritized for treatment.

Economic losses

B. ovis is a chewing lice and feeds on skin flakes, lanolin, sweat secretions and skin bacteria, resulting in economic damage to all phases of the wool production cycle. Reductions in grease fleece weight ranging from one-half pound to two pounds have been reported in finer-wool type breeds, and 0.20 pound reduction with coarse-wool sheep breeds (James et. al., 2011).

Clean yield losses of two to six percent have been reported with additional losses at the top, making phase of seven percent (Wilkenson et. al., 1982). 

Research trials have reported significant yellowing of lice-infested wool compared to non-infested controls. Additionally, pelt defects such as discolored lumps have been reported by pelt processors.

Detection of lice

Rubbing, scratching and a characteristic odor are the most recognizable clinical signs of lice but aren’t immediately visible when sheep are first exposed. The build-up of lice infestations is gradual and usually peaks almost a year after initial exposure. 

Diagnosing degrees of infestation is challenging, and using a magnifying glass and lying a sheep on its side will allow a more complete investigation.  

Part the wool a minimum of three-inches long at 15 to 20 places along the side of the sheep. A rule of thumb is more than eight lice per parted location indicates a high degree of infestation. Adolescent nymphs and adult lice are generally observed on the wool within one-quarter inch of the skin surface. 

Remember, a low level of infestation is very hard to detect. Examining the sides of the sheep is the most beneficial location. 

After shearing, 30 to 50 percent of lice are mechanically removed, and the remaining lice generally migrate to under the neck, lower britch area and shoulders. Cemented eggs, adolescent nymphs and adult lice can especially remain on areas where longer wool remains after shearing.

Prevention

An ounce of prevention is worth a pound of cure with lice. Unfortunately, like many animal health issues, purchased animals are the mode of introduction for sheep lice. Quarantine procedures for purchased rams and ewes should include lice treatment. 

Pre-shearing may not always be feasible but will help improve efficacy of treatment. Paying attention to sheep rubbing during quarantine periods can help determine if more aggressive treatments are needed. 

In instances of shared grazing allotments, it’s beneficial to make sure all sheep have been treated for sheep lice at shearing to avoid re-introduction of sheep lice.

Recent research by Crawford et al. (2000) found adults and nymphs can survive up to 24 to 29 days once removed from the sheep. In Australian shearing sheds, spring lice survived up to 14 to 16 days in the facilities. 

Specifically, researchers found lice were able to survive on shearing moccasins for up to 10 days. Microwaving each shearing moccasin for five minutes effectively killed all lice and should be a consideration for shearing contractors after shearing infested flocks. 

Treatment methods 

Dipping is only effective with less than two inches of wool growth due to insufficient wetting of skin. Recommendations abroad indicate running sheep twice through a dip 30-feet long. 

Jetting or pressurized application of insecticide as the sheep work through the race has also been utilized. However, proper calibration of spray nozzles in addition to ensuring insecticide reaches the skin surface is problematic. Considering the size of many U.S. sheep flocks, in addition to expense of dipping, back-line or spraying treatments are more feasible for many operations in the U.S.

Backline and spraying applications should be done immediately after shearing as staple lengths exceeding one inch prevent contact of active ingredients with the skin surface. Since all external parasites irritate and thrive in close proximity to skin surface – especially in cold weather – treatment strategies should be guided by ensuring insecticide reaches skin surface and the entirety of the body. 

Depending on the insecticide used a second time – two to three weeks after the initial administration – will target the nymph and hatched eggs that may have survived the first treatment.

An important guiding principle in effective treatment of sheep lice is applying treatments to shorn sheep no later than 30 days after shearing.

When ambient temperatures are low, lice will move to the skin surface, and during warmer temperatures, will migrate to the tip of the wool. Back-line treatment has been improved in recent formulations due to greater surface area coverage because of increased volume of product applied and nozzle width providing more coverage at the skin surface. 

Unfortunately, B. ovis resistance to permethrins has been well documented in Australian flocks but resistance in U.S. flocks is not widely known. Tea tree oil in a one to two percent solution has been effective at reducing B. ovis infestations when sheep were treated via dipping or high-pressure spraying (James and Callander, 2012). However, this has not been approved for use in the U.S.

Dr. Whit Stewart is a University of Wyoming associate professor and Extension sheep specialist. He can be reached at whit.stewart@uwyo.edu.