housing and manure production characteristics, the emission and distribution routes
for veterinary medicines are essentially similar. Manure or slurry will typically be
stored before it is applied to land. During this storage time it is possible that residues
of veterinary medicines will be degraded. A number of studies have therefore
explored the persistence of a range of substances in different manure/slurry types
(e.g. Loke et al. 2000 ; Teeter and Meyerhoff 2003 ; Blackwell et al. 2005 ; Kolz et al.
2005 ). For example, macrolides andb-lactam antibiotics have been shown to be
rapidly dissipated in a range of manure types whereas avermectins and tetracyclines
are likely to persist for months. Available data indicate that the dissipation of
veterinary medicines in manure or slurry can be very different from their dissipation
behaviour in soils (e.g. Blackwell et al. 2005 ), possibly due to differences in the
degradation mechanisms. Degradation in manure and slurry may be anaerobic
whereas degradation in soils is most likely to be due to aerobic organisms. For
many substances, while the parent compound appears to disappear, it may not in
fact be degraded; rather it becomes associated with the manure or slurry and
becomes non-extractable. There is currently considerable discussion over the nature
of these non-extractable residues, as well as about the implications of these bound
residues on environmental health.
Drugs administered to grazing animals or animals reared intensively outdoors
may be deposited directly onto land or surface water in dung or urine, exposing soil
organisms to high local concentrations (McCracken 1993 ; Strong and Wall 1994 ;
Halling-Sørenson et al. 1998 ).
Another significant route for environmental contamination can be the release of
substances used in topical formulations. Various substances are used externally on
animals and poultry for the treatment of external or internal parasites and infections.
Sheep in particular are host to a number of external insect parasites for which
treatment and protection can be obligatory. The main methods of external treatment
include plunge dipping, pour-on formulations and the use of showers or jetters.
With all externally applied veterinary medicines, both diffuse and point source
pollution can occur. Sheep dipping activities provide several routes for environ-
mental contamination. In dipping practise, chemicals may enter watercourses
through inappropriate disposal of used dip, leakage of used dip from dipping
installations and from excess dip draining from treated animals. Current disposal
practises rely heavily on spreading used dip onto land. Wash-off, of the chemicals
from the fleeces of recently treated animals into the soil, water and hard surfaces
may occur on the farm, during transport or at stock markets. Medicines washed off,
excreted or spilt onto farmyard hardsurfaces (e.g. concrete) may be washed off to
surface waters during periods of rainfall.
2.3 Aquaculture
Veterinary medicines used for aquaculture are commonly administered as medi-
cated feed, by injection or, in the case of topical applications, as a bath formulation.
294 A.B.A. Boxall