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|Title: ||Factors affecting formulation and efficacy of a sheep ectoparasiticide|
|Authors: ||Hackett, Scott Leigh|
|Issue Date: ||30-Jun-2015|
|Publisher: ||University of Sydney|
Faculty of Veterinary Science
|Abstract: ||This research thesis, FACTORS AFFECTING FORMULATION AND EFFICACY OF A SHEEP ECTOPARASITICIDE used six experimental formulations containing a range of excipients and a novel ectoparasiticide, AHC-2013.
The project was conducted at Yarrandoo R&D Centre, Novartis Animal Health Australasia
Pty Limited (NAH), Kemps Creek, Australia. The project utilized resources and personnel
approved by NAH management, e.g. animals, animal facilities, scientific analytical equipment
and technical support.
Three in vivo studies in sheep were conducted to investigate efficacy, drug migration in the
fleece and around the body (from the site of deposition), and the scourability of an
incorporated dye from the wool – each of these associated with potential production losses of
meat from louse infestation or of wool from residues.
The efficacy study found no formulation fully controlled the louse populations on the sheep
for the 20 week period required for regulatory approval (Holdsworth et al., 2006). Despite
this, several formulations displayed efficacy above 95%; these formulations, FD 0184-sol-31,
FD 0184-sol-32, FD 0184-sol-33 should be the preferred formulations considered for
optimization and evaluation at equivalent and higher doses. At the evaluated dose it was
demonstrated that even at the application site with the highest chemical concentrations there
were small residual populations of lice. Larger populations of lice were found as
concentrations declined rapidly away from the application site along the backline. Mean
louse counts within the control group were reduced by 31.8% 14 days after shearing, 52.2%
28 days from shearing and peaking at 52.5% on day 56 post-shearing. The decline in mean
louse count started to stabilize by day 42 (51.5%) with further subtle declines up to day 56.
This trend of population decline stopped at day 70 post-shearing when an increase in the
mean louse population was observed.
It is probable this reversal in trend was due to a resumption of the surviving parasite’s
breeding cycle, and an increase in wool length and therefore the louse habitat coupled with a
possible reduction and/or dilution of the overall chemical residues.
Although the spray-on application method used in the evaluation of these formulations has
many advantages to the end consumer it is not the only option available for application and
alternatives such as higher volume spray-on and jetting solutions should also be considered
in optimizing drug migration. The drug residue concentrations in wool were considerably
higher on the backline close to application site (location A), as compared to the flank
(locations B) and belly (location C). Although there was some variation of drug
concentrations between groups for the same location and time points, there was a trend for
the highest drug levels being closest to the application site and reducing significantly with
distance from this area. Drug migration was found to be relatively poor with only one
treatment group (FD 0184-sol-030) having drug movement over 1% (Concentration B/A*100)
from location A to location B in a 42 day period. In all other groups the total concentration
migration was less than 1%. Drug migration from location A to location C was less than 1% in
all groups over 42 days however drug migration from location B to C varied markedly with
migration of up to 63% (Concentration C/B*100; FD 0184-sol-032) down to 25% (FD 0184-
sol-030). Although there was relatively good migration in concentration between the two
locations, overall migration was poor given the relatively low levels of active being found at
location B and C in relation to the drug concentrations found at the application area, i.e.
Residue depletion was slow (e.g. from days 28 to 42) so only limited conclusions can be
drawn with regard to this parameter. Although a relatively slow drug residue depletion is
desirable for persistent efficacy it could have potential environmental, and health and safety
implications, which impact the Wool Harvesting Interval, Wool Rehandling Interval and
potentially Export Slaughter Interval.
Each parameter is assessed by the APVMA in the course of product registration. This low
drug depletion rate should be a consideration in the design of future residue studies and
particularly the timing of wool collections.
Modelling of drug concentrations in wool and relative efficacy could assist in determining
dose however, as seen in the efficacy and residue studies although there is a clear
relationship between louse population and drug concentration in wool within treatment group
and in comparison to the untreated controls. Yet the response effect was quite variable
between treatment groups and would therefore be difficult to achieve without further in vivo
evaluation; particularly extended residue depletion data beyond 42 days post-treatment.
As drug concentration in wool data is only available for the first 42 days from treatment and
the overall depletion is slow it is difficult to see an efficacy response over time to drug
concentration within group.
All formulations were found to be scourable to AS 4054 (Australian Standards, 2003) and
therefore commercially viable. The colouring agent used was the same in all formulations. It
could be assumed from the consistent scourability that the active ingredient and excipients
used did not influence binding of the colouring agent to the wool fibre. This is an important
consideration for commercial formulations and given this finding it would be prudent to use
the same colouring agent in any future formulations.|
|Type of Work: ||Masters Thesis|
|Type of Publication: ||Master of Science in Veterinary Science M.Sc.Vet.Sc.|
|Appears in Collections:||Sydney Digital Theses (Open Access)|
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