**2. Resistance to antihelminthic drugs**

The development of resistance to antihelminthic drugs is an increasing problem that compromises livestock productivity and threatens the success of treatment in humans [21]. The intensive use of drugs in the livestock industry has led to widespread resistance to all current antihelminthic drugs [22]. Notably, resistance to antihelminthic drugs occurred rather quickly after their introduction to animals. The first widely administered antihelminthic, phenothiazine, was introduced into the market in the 1940s, and resistant populations were reported by 1957. In 1961, thiabendazole was released by 1964, resistance to this compound had been reported. Similar trends occurred with the release of levamisole in 1968, ivermectin in 1981, and moxidectin in 1991 [21]. Resistance to these was reported in 1979, 1988, and 1995, respectively [23]. Parasite populations are genetically heterogeneous, and this genetic diversity leads to a variable response to drugs. Although the impact of parasitic diseases could be reduced dramatically by improved sanitation for humans and pasture control in domestic animals, such methods are not sufficient to eradicate helminths [21], which are treated with a variety of drugs, i.e., macrocyclic lactones, benzimidazoles, imidazothiazoles, and praziquantel [24].
