Small Ruminants

It cannot be assumed that cattle withdrawal times can be applied to sheep and goats.

When extrapolating pharmacokinetic data between species, one should be aware that (1) this is not necessary a linear process; (2) target tolerances or MRLs may vary across species and (3) clinical outcomes can vary. For these reasons, estimation of a withdrawal interval of a drug across species should be approached with extreme caution and the veterinarian should consult with FARAD. Below are examples of suboptimal clinical outcomes:

Different tolerance and/ or marker residues for the same drug in different species
Metabolic differences
- There is some evidence that antihelmintics compounds are more rapidly metabolized and excreted in goats compared to sheep. (Aksit, et al. 2015)
- " The greater ability to detoxify exogenous compounds, including anthelmintics, has been attributed to the specific feeding behaviour of goats, since the feeding behaviour of goats is quite different compared with that of sheep. Sheep are known as grazers, preferring to feed on grass and forbs, whereas goats are described as ingesting substantial amounts of browse (woody plants, vines and brush). Thus, goats are better adapted to tolerate and detoxify plant toxins and exogenous compounds compared with sheep" (Aksit, et al. 2015)
Milk Production differences
- Small ruminants don't produce as much milk, so there is a potential for higher drug concentrations in milk compared to cows if using per cattle label.
Morphological differences
- Digestive retention and absorption times can differer between species.
Diseased state of the animal
- Pathophysiological states (fever, diarrhea, renal disease) have been shown to have an effect on the pharmacokinetics of a drug. (Schwark. 1992)

Age can also be a factor in the elimination of drugs.

In particular oral medications eliminate at a different rate in young animals then older animals.
- oral drugs tend to be eliminated slower in neonates and pre-ruminating animals when compared to ruminating adult animals.
Kidneys are minimally functional at birth and there are differences in the rate of maturation of renal function between mammalian species. (Schwark. 1992)