**5. Anthelmintic resistance of GIT nematodes**

Resistance development against anthelmintics consists of a certain phase, i.e., during first phase, number of parasites developing resistance against specific anthelmintics is less; there is a gradual increase, and heterozygous parasites develop resistance and lead to the final phase where individuals become resistant against those anthelmintics, and the population becomes homozygous parasites population. It is also observed that parasite resistance against a specific anthelmintic also brings resistance against some other anthelmintics groups [14].

Resistance is a drug tolerance ability of a worm and survives in the recommended doses of anthelmintics that are normally an effective dose [15]. Parasitic resistance was first described in 1957, and firstly studied anthelmintic agents were organophosphates, phenothiazine, rafoxanide, thiabendazole, and macrocyclic lactones [16]. Recently different GIT parasites especially *H*. *contortus* resistance are studied against different anthelmintics groups, i.e., rafoxanide, macrocyclic lactones, phenothiazine, organophosphates, levamisole, ivermectin, and thiabendazole in small ruminants [17]. It is also noted that resistance development started after a few years of drug development especially in *H*. *contortus* [18]. But, the resistance of the parasites against a broad spectrum of anthelmintics is increasing gradually within days; multiple factors are involved in developing resistance such as excessive and repeated use of the same anthelmintic, underdosing, poor management, etc. [19, 20]. Resistance of some GI parasites, specifically of *H. Contortus* against diverse groups of drugs, namely rafoxanide, organophosphates, phenothiazine, macrocyclic lactones (ivermectin), thiabendazole, and levamisole in small ruminants, has been reported worldwide [18]. Currently, numerous tests are available for the detection of anthelmintic resistance of GI parasites including *in vitro* egg hatch assay, fecal egg count reduction test, *in vivo* anthelmintic efficiency assay (AEA), and tubulin binding assay (TBA) [21].


**Table 1.**

*The renowned anthelmintic classes (with drug examples) reported resistance [3, 22].*

The prominent anthelmintic classes reported for resistance of *H*. *contortus* in sheep [3, 22] have been presented in the **Table 1**.
