Comparative and Veterinary Pharmacology

many pesticides), the difference in sorption of a given compound in different soils
cannot be explained by variations in soil organic carbon. These large differences in
sorption behaviour are explained by the fact that many veterinary medicines are
ionisable with pKa values in the pH range of natural soils. Medicines can therefore
occur in the environment as negative, neutral, zwitterionic and positively charged
species (e.g. Ter Laak et al.2006a,b). Depending on the chemical species, interac-
tions with soil can occur through electostatic attraction, surface bridging, hydrogen
bonding or hydrophobic interactions (Ter Laak et al.2006b). The sorption behaviour
is also influenced by the properties of the soil including pH, organic carbon content,
metal oxide content, ionic strength and cationic exchange capacity (e.g. Jones et al.
2005 ; Sassman and Lee 2005 ; Strock et al. 2005 ; Ter Laak et al.2006b). The
complexity of the sorbate–sorbent interactions means that modelling approaches
developed for predicting the sorption of other groups of chemicals (e.g. pesticides
and neutral organics) are inappropriate for use in veterinary medicines. Manure and
slurry may also alter the behaviour and transport of medicines. Studies have demon-
strated that the addition of these matrices can affect the sorption behaviour of
veterinary medicines and that they may affect persistence (e.g. Boxall et al. 2002 ;
Thiele Bruhn and Aust 2004 ). These effects have been attributed to changes in pH or
alterations in the nature of dissolved organic carbon in the soil/manure system.

non persistent

(DT50 < 5 d)

slightly persistent

(DT50 5-21 d)

moderately persistent

(DT50 22-60 d)

persistent

(DT50 >60 d)

very mobileKoc <15 sulfamethazine

chlorsulon

sulfadimethoxine metronidazole florfenicol

mobile

Koc 15-74

olaquindox ceftiofur

moderately

mobile

Koc 75-499

eprinomectin

diclazuril

oxfendazole

tylosin

Slightly mobileKoc 500-4000 emamectin

Non-mobile deltamethrin

Koc >4000

albendazole

cypermethrin

danofloxacin

doramectin

erythromycin

ivermectin

moxidectin

oxytetracycline

selamectin

sarafloxacin

Increasing persistence

Decreasing mobility

Fig. 2Distribution of reported mobility (based on sorption coefficients) and persistence data for
veterinary medicinal products (adapted from Pope et al. 2009 ).Koc¼organic carbon-normalised
sorption coefficient in soil, DT50¼the time taken for degradation/dissipation of 50% of the
amount of drug originally present

296 A.B.A. Boxall