Simple SummaryDifferent types of endoparasites are present in small ruminants, especially in sheep. One of the most important parasites are the gastrointestinal nematodes (Trichostrongylidae). Emaciation, anorexia, decreased productivity, and diarrhea are observed in infected animals. These infections cause great economic damage to sheep owners through production losses and treatment costs. In connection with the frequent treatment of sheep against endoparasite infections, anthelmintic resistance has developed. Due to the global anthelmintic resistance situation, the principles of parasite control have changed. New alternatives have been sought, and one of the most researched alternatives is phytotherapy. Different plants with anthelmintic activity would be valuable for the control of ruminant helminths. Phytotherapy would reduce the use of synthetic drugs and reduce treatment costs.AbstractDue to the high prevalence of gastrointestinal nematodes in sheep, the growing anthelmintic resistance, and the development of organic farming systems, sustainable alternatives are being sought. One such method is phytotherapy. This study aimed to evaluate the in vitro ovicidal and larvicidal activity of extracts of tansy (Tanacetum vulgareL.) growing in Latvia on gastrointestinal nematodes (Trichostrongylidae) in sheep. The leaves and flowers of the tansy were extracted separately in 70%, 50%, and 30% ethanol and acetone. Six concentrations were prepared from each extract 500 mg/mL, 200 mg/mL, 100 mg/mL, 50 mg/mL, 20 mg/mL, and 10 mg/mL. In vitro egg hatching test and micro-agar larval development test were performed. Extracts of tansy have strong larvicidal activity. The highest percentage of larvae inhibition for most of the extracts was 100%, but for egg inhibition, it was 95.8% for the 200 mg/mL concentration of 50% acetone and 93.3% for the 500 mg/mL concentration of 50% ethanol leaf extracts. All tansy extracts had ovicidal and larvicidal activity against Trichostrongylidae in sheep.