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From March 1998 until May 1999 a field trial in Schleswigholstein was carried out in order to test three anthelmintic substances of the group macrocyclic lactones with regard to their efficacy and the period of efficacy against worm infections in horses. 223 horses from four different stud farms were split into three groups: adults, yearlings and foals. All of them were tested by examinations of faeces with a flotation method from Mc Master. There were used Ivermectin ( Ivomec P . ) and Moxidectin ( Equest . ), which were orally given to the horses, and further Doramectin ( Dectomax . ) as a non registered active substance for the horse treatment which was injected subcutaneously. At the beginning of the trial, all yearlings and adult horses were infected by small strongylides.Tapeworms and ascarids occured occasionally. Provided that 50% of a horse therapy group reached an epg of 200 or more, a new treatment was carried out. For the yearlings as well as for the adults a treatment interval of 16 weeks was determined so for Ivermectin and Doramectin, and about 22 weeks for Moxidectin. Because of organisational reasons all adult horses (including the Moxidectin-group) were treated together every 16 weeks in one stud farm. The horses which were treated with Moxidectin achieved in this stud farm epg-rates of maximum 30 in the middle.Only 16 weeks after the first treatment, small strongyles and ascarids appeared in larger quantities at the foals. By that moment they were between 4,5 and 6 months old. During the research there was no occurrence of Strongyloides westeri. The foals got the first anthelmintic therapy when they were between 10 days and 7 weeks old, after that they got it every 8 weeks for all substances, regardless of the quantity of worm eggs in the faeces. The foals which were treated with Moxidectin showed epg-rates between 77 and 142 in the middle, starting from the 16. week of the trial. Foals which were treated with Ivermectin and Doramectin showed an epg-rate between 223 and 686 in the middle. These rates were two times, partly up to eight times higher than the rates of the Moxidectin-group. All three substances showed a good efficacy against worms in the control examination 2 weeks after the therapy. A reduction of efficacy during the trial was not recognized. The trial showed that Ivermectin and Doramectin had an comparable anthelmintic effect, but Moxidectin has a clearly longer period of efficacy to control strongyles and ascarids. Where as a therapy for older horses with Ivermectin and Doramectin is recommended three or four times a year, it seems to be sufficient to use Moxidectin twice or three times a year. Yearlings should be treated four times a year with Ivermectin and Doramectin, while three treatments a year should be enough for Moxidectin. A therapy for the foals is not necessary when they are younger than three months. Thereafter they should be treated with Ivermectin and Doramectin every 8 weeks and with Moxidectin every 10 weeks. All three anthelmintics were easy to handle and also well tolerated, except for individual cases with low side effects.