40 CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 



Treatment. — None of the dips recommended for use in treating 

 horses can be depended on to eradicate lice at one dipping. The first 

 dipping, if properly done, will probably kill all the lice, but it may 

 not destroy all the eggs. The nits or eggs which survive dipping often 

 hatch, forming a new generation of lice. This new generation should 

 be destroyed by a second dipping after hatching is completed and 

 before the young lice become mature and begin depositing eggs. Since 

 these two periods overlap somewhat and vary greatly, and because 

 the periods from hatching to egg laying are different for the sucking 

 and biting lice, it is impossible on this basis to calculate the exact 

 period that should elapse between dippings. Young lice appear on 

 dipped animals as early as the third day after the first dipping, and 

 since the average period of maturation is about 12 days it seems evi- 

 dent that the second dipping should be given in about 2 weeks. 

 Experience has shown that two dippings with an interval of from 

 14 to 16 days usually can be depended on to control both sucking and 

 biting lice. 



The dips commonly used for destroying lice on horses, mules, and 

 asses, named in the order of their effectiveness, are arsenical solution, 

 coal-tar creosote, and nicotine. The arsenical solution is poisonous, 

 and unless handled with due precaution injury to man and animal may 

 result. When properly used, however, it is the most dependable known 

 dip for destroying lice. The coal-tar creosote dip is effective when 

 used in reasonably good water. Two dippings, from 14 to 16 days 

 apart, in either of these dips can usually be depended on to eliminate 

 horse lice. Two dippings in nicotine solution will ordinarily free 

 horses from lice. On account of the difficulties in dipping in cold 

 weather, and the unsatisfactory nature of hand treatments, most of 

 them only palliative, it is advisable to dip for lice, when they are 

 present, in the fall of the year before the onset of cold weather. 



Applications of dusting powders or sprays are often of value for 

 holding lice in check. Dusting powders may be applied with a dust 

 gun, a shaker, or by hand, and mixtures of wettable sulfur, talc, or 

 pyrophyllite containing approximately 10 percent of any of the fol- 

 lowing insecticides are efficacious; namely, finely ground derris or 

 cube powder containing from 3 to 5 percent rotenone, DDT powder, 

 finely ground sabadilla seed, or benzene hexachloride containing at 

 least 10 percent of the gamma isomer. Such powders should be used 

 sparingly, and not allowed to get into the eyes, nose, mouth, or into 

 the feed of the animals. Biting lice can be destroyed with sodium 

 fluoride. Care should be taken not to apply sodium fluoride too freely 

 around the natural body openings or where the skin is very thin and 

 hairless, and not to rub it into the skin. 



Some of the newer insecticides which appear to be particularly 

 efficacious against lice, notably DDT, benzene hexachloride, and cer- 

 tain of the thiocyanates, are marketed in forms that are suitable for 

 application as sprays. These should be used according to directions 

 accompanying them. 



Fumigating horses with the fumes from burning sulfur (sulfur 

 dioxide) will kill horse lice, but many of the eggs survive, and two 

 treatments 2 weeks apart are necessary to effect eradication. The ad- 



