The study of the international literature revealed that 90% of the Italian flocks were infected with Haemonchus contortus, in Turkey the rate of infection of small ruminants with gastrointestinal trichotrollis was found to be 83% for goats and 87.5% for sheep. In India it was found that the infection rate of sheep with roundworms was 61.6%. Even in central Africa, and more specifically in Nigeria, it was found that 60% of sheep and 52% of goats were infected with at least one type of gastrointestinal nematode, while in Ethiopia 43.5% of animals were found to be infected with gastrointestinal nematodes.

The systematic and reckless use of antiparasitic drugs to treat these gastrointestinal nematode parasites has contributed to the development of parasite resistance to them. More specifically, in 1997 in Greece a rate of 7% of herds resistant to benzimidazoles was found (5.9% of sheep and 8.9% of goats), while more recent data (2012) indicate that 82.8% of parasites are heterozygously sensitive and 17, 2% homozygous resistant to benzimidazoles In Spain, it was found that 18.07% (15 out of 83) of flocks were resistant and 81.93% susceptible (68 out of 83 flocks) to benzimidazoles. Also in Slovakia, a benzimidazole resistance rate of 6.9% was detected in sheep flocks.

However, in addition to resistance to benzimidazoles there are reports of strains resistant to the other anthelmintic groups. More specifically in South Zealand, strains of the parasite Teladorsagia circumcincta resistant to macrocyclic lactones are reported. Resistant strains of T. circumcincta in sheep were also found in Scotland, while in Australia a modification of the larval migration test detected strains of Haemonchus resistant to macrocyclic lactones.

Even in the Netherlands, ivermectin-resistant strains of Haemonchus and Teladorsagia were found. In Brazil, strains of Haemonchus and Trichostrongylus resistant to both macrocyclic lactones and imidazothiazoles (levamisole) were detected in sheep flocks. Finally, strains of Trichostrongylus, Teladorsagia and Cooperia resistant to imidazothiazoles have been reported in sheep flocks in the UK.

It is evident that the problem of resistance threatens modern sheep farming worldwide. For this reason, alternative ways of combating them (e.g. creating genetically resistant sheep) are necessary to find in order to reduce the dependence on antiparasitic drugs with various beneficial effects (reduction in the use of antiparasitic drugs, increase in animal yields, well-being, reduction of working hours etc.).