Table 3.

Factors influencing the emergence of anthelmintic resistance in livestock.

Factor	Description	Livestock	Humans
Treatment frequency	The greater the frequency, the greater the drug pressure and risk for resistance	5–10 treatments per year (61)	1–3 treatments per year (62, 63). Selection for resistance in goats and sheep at these treatment frequencies (64, 65)
Refugia	Proportion of the parasite population free from being exposed to the drug (66)	Can delay emergence of anthelmintic resistance by leaving some animals untreated (1–4% of adult stock) (67)	MDA only targets children, with coverage less than 80%. Adults have the highest worm burdens. Treatment is given during dry seasons, when the proportion of living parasites in the soil is low, for logistical reasons, thereby reducing refugia. As treatment coverage increases for the 2020 goal, the proportion of children treated rises (68).
Under-dosing	Specific dose regimens have different effects on resistance allele frequency, depending on prior frequency of resistance allele in pre-drug parasite population.	Drug efficacy is very high (ERR >99%). Dosing is well-controlled.	Sub-optimal efficacy, never achieve 100% cure. Drugs are often shared among poor families, produced at substandard qualities, and even sold past their expiry date. This could either aid the development of AR (allow for the survival of resistant strains) or delay it (lower drug pressure) (68).

ERR, egg reduction rate; AR, anthelmintic resistance.