membranes passively, including anti-parasiticides, some antibacterials, steroid hor-
mones, and growth promoters. These agents are administered to the animal once and
the effects can persist for weeks, similar to a constant rate intravenous infusion.
Dips, sprays, pour-on and spot-on delivery formulations are of real practical and
widespread use in veterinary medicine, in contrast to transdermal patches (Walters
and Roberts 1993 ). Hydrophilic drugs including peptides and DNA are, however,
difficult to develop into SR systems for either oral or non-parenteral delivery in
human or veterinary medicine, as they generally do not cross cell membranes
un-aided, and are labile and expensive to produce in large quantities.
Despite these challenges, considerable research effort continues to be directed
towards the development of suitable formulations for delivery of biotechnology
derived molecules in human medicine. These include buccal insulin sprays contain-
ing mixed micelles (Bernstein 2008 ), the temporary marketing of a dry powder
inhaled insulin formulation (Mitri and Pittas 2009 ), and the delivery by injection of
bioresponsive polymer-based small interfering RNA (Schaffert and Wagner 2008 ).
Systemic delivery of nebulised insulin was in fact also recently achieved in cats;
most subjects had a 50% reduction in blood glucose after 15 min (DeClue et al.
2008 ). Besides achieving improved drug efficacy in animals through use of SR
delivery devices and formulations, additional clinical benefits accrue as clients use
more convenient dosing schedules resulting in better compliance, for example, by
reducing dosing to once-a-day in companion animals or to once in an entire
growing season for cattle. For production animals, improved convenience and
compliance are linked to the practical and economic benefits of reduced labour
costs, less risk of injury to farm workers, and reduced potential for damage to
animal muscle from adverse reactions at the injection site.
3 Exogenous Regulation: Devices and Formulations
Routes of delivery used in veterinary medicine are summarised in Table 2 .This
section highlights devices and formulations using specific examples relating to
avermectin delivery.
3.1 Oral Formulations
As an example of the differing requirements for device design in different species,
intraruminal (i.r) cattle devices rely on a single administration to release anti-
parasitic agents at relatively constant rate for an entire growing season, while at
the same time remaining functional and intact within the confines of the rumen over
that period. In contrast, gastroretentive devices for monogastric species including
humans have a simpler physiological environment to cope with for a shorter period
and the aim is to gradually release drug from a degradable short-term system to
Drug Delivery Systems in Domestic Animal Species 83