Full text of DOI:10.1080/00480169.2002.36253

70
New Zealand Veterinary Journal 50(2), 70-76, 2002
Scientific Article
Production and immunological responses associated with controlled-
release-capsule vs 5-drench preventive anthelmintic programmes for
parasite control in lambs
DM Leathwick*§, CM Miller*, DS Atkinson*, AE Brown*, RS Green† and IA Sutherland*
Abstract
Introduction
AIMS: To determine whether: a) using a controlled-release
anthelmintic capsule (CRC) instead of a programme of 5 oral
drenches administered at 3–4 week intervals, would delay the
development of anti-parasite immunity in lambs; b) the use of
ivermectin instead of albendazole, administered either as a CRC
or as a programme of 5 oral drenches, would delay the develop-
ment of anti-parasite immunity in lambs; c) lambs treated with
CRCs would have higher liveweight gains than lambs drenched
orally 5 times at 3–4 week intervals, and; d) delayed onset of
anti-parasite immunity is associated with reduced liveweight
gains in the period following anthelmintic treatment.
The use of CRCs has been shown to significantly impair the devel-
opment of immunity to T. colubriformis in sheep housed indoors
(Sutherland et al 1999) or grazing in the field (Williamson et al
1995). It appears that the prolonged absence of resident worms
from the gut removes a component of the antigenic stimulation
required for the development and/or maintenance of immunity.
However, it has been shown (Sutherland et al 1998, 1999) that
the ingestion of L3 larvae alone, even in the presence of a CRC, is
sufficient for animals to develop a substantial, albeit partial, im-
mune response to these parasites. Numerous trials have demon-
strated that CRC use in lambs can result in substantial productiv-
ity gains over no treatment at all (Barton et al 1990; Gogolewski
et al 1997; Rehbein et al 1999). However, only a few studies have
compared CRC use with locally typical oral drenching regimes
(Barger et al 1992; van Houtert et al 1995) and few data are avail-
able on the benefits of using a CRC compared with the New Zea-
land practice of routinely drenching lambs at 3–4 week intervals
with an oral anthelmintic. If production gains from CRC use are
made at the expense of host immunity, then increased susceptibil-
ity to parasitism following the treatment period could result in a
reduction in the net benefits of CRC use. Indeed, cattle receiving
intensive chemoprophylactic treatments in their first year of life
can subsequently be more susceptible to parasitism (Jacobs et al
1989; Claerebout et al 1998).
METHODS: Three field trials were conducted, 1 on a research
farm and 2 on commercial sheep farms, in which groups of 30
lambs were treated with either a CRC containing albendazole, a
CRC containing ivermectin, 5 oral drenches with albendazole,
or 5 oral drenches with ivermectin, administered at 3–4 week
intervals. Liveweights and faecal nematode egg counts (FECs)
were recorded in all trials. Immunoglobulin-G (IgG) antibody
levels to Ostertagia circumcincta and Trichostrongylus colubri-
formis adult and larval antigens were measured in Trials 1 and 3,
and fleece weights and resistance of animals to nematode chal-
lenge infection were measured in Trial 1.
RESULTS: CRC-treated lambs had higher levels of antibodies to
O. circumcincta infective-stage larvae (L3) than orally drenched
lambs in Trial 3, but no other immunological differences due
to mechanism of delivery were detected. Antibody levels were
lower in lambs treated with ivermectin than albendazole, as a
CRC or oral drench in Trial 1, but this was not associated with
any measurable effects on FEC or productivity. No significant
differences (p>0.05) were detected between drench types (al-
bendazole vs ivermectin) or delivery mechanisms (CRC vs oral
drenching) in any of the production parameters measured, in
any of the trials. Albendazole-CRCs failed to control FECs in
all 3 trials.
It has also been reported that anthelmintics themselves may
directly affect immune responsiveness. In fact, all 3 of the cur-
rently available broad-spectrum anthelmintic families have been
implicated as having adverse effects on host immunity (Cabaj
et al 1994, 1995; Stankiewicz et al 1994, 1996a,b; Niezen and
Robertson 1997), although some have also been reported to be
immuno-stimulatory (Quinn 1990).
There is currently no indication as to whether the above findings
have any practical implications regarding the use of anthelmintics
or CRCs for parasite control under New Zealand conditions. This
study aimed to compare the on-farm performance and immune
status of lambs treated with either ivermectin or albendazole, each
administered either as a CRC or through a structured programme
CONCLUSIONS: Although some differences between treat-
ments in antibody levels were detected these were not associated
with measurable differences in level of parasitism or productiv-
ity of lambs. CRC use did not appear to offer substantial gains
in productivity over a structured programme of 5 oral drenches
administered at 3–4 week intervals.
KEY WORDS: Anthelmintic capsule, parasite control, immu-
nity, liveweight gain, sheep
CRC
Controlled release capsule
ELISA Enzyme-linked immunosorbent assay
epg
FEC
Eggs per gram of faeces
Faecal egg count
FECRT Faecal egg count reduction test
* AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand
† AgResearch Wallaceville, PO Box 40063, Upper Hutt, New Zealand
§ Author for correspondence. Email: Dave.Leathwick@agresearch.co.nz
IgG
L3
immunoglobulin-G
infective third-stage larvae

Leathwick et al
New Zealand Veterinary Journal 50(2), 2002
71
of 5 oral drenches. Specifically, the aims of this work were to
determine whether: a) using a CRC instead of a programme of
5 oral drenches administered at 3–4 week intervals, would delay
the development of anti-parasite immunity in lambs; b) the use of
ivermectin instead of albendazole, administered either as a CRC
or a programme of 5 oral drenches, would delay the development
of anti-parasite immunity in lambs; c) lambs treated with CRCs
would have higher liveweight gains than lambs drenched orally 5
times at 3–4 week intervals and; d) delayed onset of anti-parasite
immunity is associated with reduced liveweight gains in the pe-
riod following anthelmintic treatment.
leased nearly all their contents over the preceding 100 days. On
the same day, 6 animals from each treatment group were drenched
with an albendazole/levamisole combination anthelmintic (AR-
REST, Ancare NZ Ltd) to remove their parasites, and relocated
indoors where they were maintained on a diet of lucerne pellets,
chaffed hay and an ad libitum supply of fresh water. Seven days
after being moved indoors each of these animals was dosed with
10,000 L3 of T. colubriformis and 10,000 L3 of O. circumcincta.
Once infections were patent, faecal samples were collected twice
(22 and 26 days post-infection) for the measurement of FEC.
Monitoring of the animals in the field continued for 6 months
for liveweight (Weeks 4, 6, 10, 17 and 25 after the 5th drench),
and FEC (Weeks 1, 2, 4, 6, 10, 17 and 25 after the 5th drench);
dag weights (crutchings) were recorded in May and fleece weights
were recorded in September. An unscheduled sample for FEC
was collected 6 days after the last scheduled (5th) drench, both
to check the efficacy of the oral treatments and monitor rising
FEC in the albendazole-CRC-treated lambs. Due to high FECs
in this sample, the albendazole-CRC-treated group was given an
additional drench with oral albendazole 6 days later; 12 days after
expiry of the CRCs. On 15 May 1998 an additional anthelmintic
treatment was administered to all lambs when group mean FECs
exceeded 1500 eggs per gram of faeces (epg). On this occasion,
lambs were drenched with either albendazole or ivermectin to
maintain consistency with their earlier treatments.
Materials and Methods
Properties
Field trials were run on 3 properties: Ballantrae Hill Country
Research Station in the southern Hawke’s Bay in 1997–98, and
on 2 commercial sheep farms, near Marton and near Mt Stewart,
in the Manawatu region in 1998–99. All trials followed a similar
protocol.
Experimental protocol
On each property, 120 replacement ewe-lambs were randomly
allocated at weaning to 1 of 4 treatment groups (n = 30 lambs/
group) on the basis of liveweight and, where possible, FEC. The
treatments were:
Blood samples for antibody analysis were collected from the
jugular vein of the same 10 animals in each treatment group on 5
occasions (Days 0, 42, 70, 98 and 139). On another 5 occasions
(Days 21, 42, 98, 127 and 139), surplus faeces collected from
each group for determining FECs were cultured to determine
the parasite genera present. In response to increasing FECs in the
albendazole-CRC treated lambs, a faecal egg count reduction test
(FECRT; McKenna 1990) was conducted on Day 42 on non-
trial lambs running with the trial animals, in order to determine
whether resistance to this family of drenches was present on the
property.
a) A priming oral drench with albendazole (Albendazole Sheep,
Ancare NZ Ltd) and a CRC releasing albendazole at approxi-
mately 21 mg/day for 100 days (albendazole-CRC; Extender
Junior, Nufarm NZ Ltd).
b) A priming oral drench with ivermectin (Ivomec liquid for
sheep and goats, Merial NZ Ltd) and a CRC releasing iver-
mectin at approximately 0.8 mg/day for 100 days (ivermectin-
CRC; Ivomec Maximizer, Merial NZ Ltd).
c) Five oral drenches with albendazole administered at 0, 21, 21,
28 and 28 day intervals.
d) Five oral drenches with ivermectin administered at 0, 21, 21,
28 and 28 day intervals.
Trial 2
The Marton trial commenced on 15 December 1998 and fin-
ished with the last drench on 29 March 1999. The animals were
all Romneys with a mean liveweight at weaning of 31.0 (SE 0.27)
kg. The farmer had drenched all lambs with ivermectin 14 days
prior to weaning so animals were randomised to treatment based
on liveweight only (as no pre-treatment FEC data were available).
Growth rates in this trial were low, due principally to a very dry
summer and consequent lack of feed. The farmer was in the
habit of removing dags (faecal-soiled breech wool) whenever the
animals were in the yards and so no dag scores were recorded on
this property. Blood samples for antibody assessment were not
collected at this site.
All oral drenches were administered at the manufacturer’s recom-
mended dose rate based on the liveweight of the largest animal
in the mob. All animals were weighed and faecal sampled on 5
occasions, on the dates that the oral drenches were administered
to treatment groups (c) and (d). Additional data were collected as
outlined below. Throughout the trials all lambs were run together
as part of a larger mob of replacement animals and, apart from the
anthelmintic treatments, all management was at the discretion of
the owner. All non-trial animals were treated with oral ivermectin
as per group (d) above. All procedures involving the experimental
use of animals were approved by the Crown Research Institutes’
Animal Ethics Committee (Palmerston North, New Zealand).
Trial 3
The Mt Stewart trial commenced on 23 December 1998 and fin-
ished 1 month after the last drench on 27 April 1999. The lambs
were a mix of ½ and ¾ East Friesian x Romney and their mean
liveweight at weaning was 27.3 (SE 0.39) kg. Blood samples were
collected from the same 10 lambs in each treatment group on 2
occasions, one at the start of the trial and another on Day 99 at
the end of the drenching programme.
Trial 1
The Ballantrae experiment commenced on 17 December 1997
and finished at shearing on 19 September 1998. The trial animals
were all Romneys with a mean liveweight at weaning of 24.1 (SE
0.17) kg. One hundred days after administration of the CRCs (27
March 1998), 5 animals (2 ivermectin-CRC and 3 albendazole-
CRC) were slaughtered and the capsules recovered to determine
the extent of their release. In all cases the capsules contained only
a small residue of active ingredient, indicating that they had re-
Parasitology and serology
FECs were determined using a modified McMaster method in

72
New Zealand Veterinary Journal 50(2), 2002
Leathwick et al
2500
which 1 egg counted equated to 50 epg. Blood samples were
collected directly into 10ml plain vacutainer tubes. After coagula-
tion, samples were centrifuged and aliquots of serum frozen at
–20°C for storage. Serum IgG levels to larval and adult antigens
of T. colubriformis and O. circumcincta were assayed using an
ELISA after the method of Douch et al (1994).
a
2000
1500
1000
500
0
Statistical analysis
0
3
6
9
12
15
18
Data from each trial were analysed separately. FECs were trans-
formed using Ln(x+1) prior to analysis to equalise variances. All
data except for serum IgG levels from Trial 1 were analysed using
ANOVA factored by drench-type (albendazole vs ivermectin) and
delivery mechanism (oral vs CRC), including the drench-by-de-
livery interaction. Trial 3 data were also factored by sheep breed.
When analysing liveweight gain, weaning liveweight was included
as a covariate in the model. Serum IgG levels from Trial 1 were
analysed by fitting quadratic regression curves against time to the
data, and comparing the intercepts and slopes of the fitted lines
corresponding to the main effects (i.e. drench type and delivery
mechanism). These data were also Ln transformed to equalise
variances before analysis.
500
400
300
200
100
0
b
0
3
6
9
12
15
18
500
400
300
200
100
0
c
Results
0
3
6
9
12
15
18
FECs
Weeks after weaning
Over the 100-day period of anthelmintic treatment in all 3 trials,
treatment with ivermectin (averaged across CRC and oral delivery
mechanisms) resulted in significantly lower FECs than treatment
with albendazole (p<0.01; Figure 1) on nearly all sampling dates,
the only exception being Day 21 in Trial 1. Anthelmintic treatment
by CRC (averaged across albendazole and ivermectin) significantly
reduced FECs compared with oral drenching on all sampling dates
(p<0.05), except for Days 21 and 42 in Trial 2, and Days 70 and 98
in Trial 3. However, on Days 42, 70 and 98 in Trials 1 and 3, and
Days 70 and 98 in Trial 2, the drench-type by delivery-mechanism
interaction was significant (p<0.05). This largely reflected the failure
of the albendazole-CRC (in contrast to the ivermectin-CRC) to ad-
equately control FECs in all 3 trials. Treatment with oral albendazole
at the time of capsule administration ensured that FECs were initially
low, but these increased steadily in CRC-treated lambs over the 100
days of drug release, culminating in Trial 1 in a group mean FEC for
the albendazole-CRC-treated lambs of 673 epg on Day 98. These
animals were ‘salvage’ drenched with oral albendazole 12 days later,
at which time their group mean FEC was 2,185 epg. Maximum
FECs in albendazole-CRC-treated lambs in Trials 2 and 3 were 448
and 124 epg, respectively. In contrast to the albendazole-CRC, the
ivermectin-CRC generally maintained FECs at low levels (<80 epg),
although counts were seldom zero. Thus, the ability of the 2 CRCs
to limit increases in FEC over the 100-day period of release differed
markedly. In Trial 1, where FECs were monitored for 6 months after
the end of the anthelmintic treatment period, there were no further
significant differences in FECs between groups.
Figure 1. Group mean faecal nematode egg counts in eggs per gram
(epg) for (a) Trial 1, (b) Trial 2 and (c) Trial 3, where: [ ] =albendazole-
CRC administered at weaning; [ ] =ivermectin-CRC administered at
weaning; [ ] =5 drenches with albendazole-oral administered 0, 3, 6,
10 and 14 weeks after weaning and; [ ] =5 drenches with ivermectin-
oral administered 0, 3, 6, 10 and 14 weeks after weaning.
was conducted on non-trial lambs running with the trial animals
on this property, reduction in FEC 10 days after treatment with
albendazole was only 90% (488 and 50 epg for pre- and post-
treatment samples, respectively). In this latter test, albendazole
was administered at the manufacturer’s recommended dose rate
based on individual animal liveweights.
Faecal cultures from Trial 1 indicated that all the major parasite
genera expected were present in all treatment groups throughout
the study period (data not shown). While there was a general trend
for Ostertagia spp to become more prevalent, and Haemonchus
spp less prevalent over time, there was no obvious change in the
generic composition of larvae cultured from the different groups.
Similarly, in Trials 2 and 3, all the major parasite genera were pres-
ent, although Haemonchus spp were less common and Cooperia
spp were more common than in Trial 1 (data not shown).
Productivity
Liveweight gains, fleece weights, dag weights and dag scores,
are presented in Table 1. There were no significant differences
(p>0.05) between drench types (i.e. albendazole vs ivermectin) or
delivery mechanisms (i.e. oral vs CRC administration) in any of
the production parameters measured in any of the trials.
In Trial 1, when oral albendazole was administered at a dose rate
based on the heaviest lamb in each mob, control of FEC was
satisfactory. For example, FECs for the albendazole-oral group
on Days 0 and 21 were 1190 and 47 epg, respectively, indicating
a reduction in FEC following the first albendazole treatment of
96%. Similarly, FECs in this group on Days 98 and 104 were
468 and 17 epg, respectively, indicating a reduction following
the 5th drench of 96%. However, when on Day 42 a FECRT
Antibody levels
Serum antibody levels from Trial 1 are presented in Figure 2.
Levels of all 4 of the antibodies measured were lower in the iver-
mectin-treated groups than in the albendazole-treated animals

Leathwick et al
New Zealand Veterinary Journal 50(2), 2002
73
burdens (Figure 1). The ivermectin-CRC maintained FECs at
a low level although some worms did establish and develop to
maturity.
(p<0.007). There was no significant difference between delivery
mechanisms (CRC vs oral) for 3/4 antibody types measured
(p>0.35), but levels of anti-T. colubriformis-adult antibodies were
significantly (p=0.011) higher in CRC-treated than in orally-
drenched lambs. This largely reflected the fact that antibody
levels were lower in the ivermectin-oral group than in the other 3
groups (Figure 2). Because of this, the interaction between drench
type and delivery mechanism was highly significant (p=0.001) for
anti-T. colubriformis-adult antibody. In Trial 3, in which only 1
post-treatment blood sample was taken, there was no difference
between drench type in any of the antibodies measured (p>0.8).
However, there were higher levels of anti-O. circumcincta L3 an-
tibody in the CRC-treated animals than in those treated orally
(p=0.015). This difference was not evident for the other 3 anti-
body types measured (p>0.7).
Failure of the albendazole-CRC to prevent parasite establish-
ment compromised somewhat the comparison of CRCs and oral
treatments. Based on previous work (Sutherland et al 1999), a
difference in immune status between the ivermectin-CRC and
oral-treatment groups might have been expected due to differing
levels of antigenic stimulation. This, however, was not observed
in the present study. In general, antibody levels were not different
in CRC- and orally-treated animals. In Trial 1, a consistent and
significant reduction in antibody levels was measured in those an-
imals treated with ivermectin irrespective of delivery mechanism.
This is consistent with results from a previous study, suggesting
that ivermectin may interfere with the development of anti-para-
site immunity (Stankiewicz et al 1996a). However, no differences
in parasitological or productivity measures could be attributed to
the observed difference in antibody levels. Based on the FECs of
the lambs that were relocated indoors, there was no difference
between groups in the establishment of challenge infections.
Also, there were no significant differences in FECs or growth
rates following the end of the 100-day anthelmintic treatment
period. Thus, while treatment with ivermectin had some effect on
antibody levels, this did not appear to translate into meaningful
differences in levels of parasitism which might influence the use
of this product on-farm. Similarly, there was no detectable effect
on the development of immunity associated with CRC use com-
pared with a structured programme of oral drenches.
Establishment of challenge infection
Group mean FECs of Trial-1 lambs relocated indoors and challenged
with T. colubriformis and O. circumcincta larvae were 1096, 775, 738
and 792 epg for treatment groups albendazole-CRC, ivermectin-
CRC, albendazole-oral and ivermectin-oral, respectively, which were
not significantly different (SE 178; p=0.47).
Discussion
Each of the treatment protocols used in this study was intended to
control parasites in lambs for a period of approximately 100 days.
In the interval between oral drenches, ingested L3 larvae are able
to establish infections in lambs and, although only a proportion
develop fully to become egg-laying adults, substantial numbers of
immature worms can accumulate. In contrast, the continuous re-
lease of anthelmintic from CRCs should maintain both adult and
immature worm populations at low levels (Barton et al 1990).
This was not the case for the albendazole-CRC-treated animals in
this study. Following the initial removal of worms by the prim-
ing oral drench, FECs in the albendazole-CRC-treated lambs
increased over time, indicating a progressive increase in worm
Another objective was to establish whether lambs treated with
CRCs would have higher growth rates than orally-treated ani-
mals and whether this might be compensated for in the period
following anthelmintic treatment. Our data did not indicate any
production advantage from using CRCs over a programme of 5
oral drenches for either drench family. However, the full produc-
tion benefit from using a CRC is likely to be realised only when
treated animals are grazed separately from other animals (Barger
et al 1992). This is because an effective CRC should suppress
Table 1. Lamb liveweight gain, fleece and dag weights and dag scores from trials comparing controlled release capsules with a programme of 5
oral drenches administered at 3–4 week intervals. Treatments were split with half the animals receiving albendazole and half ivermectin. Data are
presented as means (and 95% confidence intervals); n=30 lambs/group.
Albendazole
Ivermectin
Trial
Measurement
Oral
Capsule
Oral
Capsule
a
1
Liveweight gain (kg)
6.2 (5.45–6.90)
7.5 (6.55–8.53)
2.34 (2.20–2.47)
107 (95–119)
6.1 (5.34–6.79)
8.2 (7.14–9.18)
2.32 (2.17–2.47)
96 (83–109)
6.2 (5.45–6.93)
8.7 (7.72–9.71)
7.1 (6.38–7.86)
7.9 (6.96–8.94)
2.55 (2.41–2.69)
107 (95–120)
b
Liveweight gain (kg)
c
Fleece weight (kg)
2.37 (2.23–2.50)
100 (88–112)
d
Dag weight (g)
a
2
3
Liveweight gain (kg)
1.2 (0.3–2.0)
1.8 (0.9–2.7)
1.4 (0.5–2.2)
1.6 (0.8–2.4)
a
Liveweight gain (kg)
10.6 (9.3–11.9)
0.84 (0.46–1.22)
12.1 (10.8–13.5)
0.54 (0.16–0.91)
12.2 (10.9–13.5)
0.64 (0.26–1.02)
12.7 (11.3–14.1)
0.4 (0.02–0.78)
e
Dag score (0–4)
a
liveweight gain over the 100 days of anthelmintic treatment
b
c
d
e
liveweight gain over the 176 days following the period of anthelmintic treatment
Measured 168 days after the end of the 100-day anthelmintic treatment period
Measured 34 days after the end of the 100-day anthelmintic treatment period
Measured 2 days before the end of the 100-day anthelmintic treatment period

74
New Zealand Veterinary Journal 50(2), 2002
Leathwick et al
FEC to low levels for an extended period of time, resulting in
low levels of pasture contamination; if CRC-treated animals are
grazed with other animals which are contaminating pastures to a
greater extent, then the CRC-treated animals will be disadvan-
taged and the full production benefit from CRC use will not be
realised. In the current trials, the objectives relating to immune
status required that all animals be subject to equal larval chal-
lenge and nutritional conditions, thus making separate grazing
impractical. It is possible that by running all animals together and
exposing the CRC-treated lambs to higher levels of contamina-
tion originating from the orally-treated lambs, the current trials
produced only a conservative estimate of the production response
likely from CRC use. However, this is by no means certain. Un-
like trials in which animals treated with CRCs have been grazed
together with untreated animals (Barton et al 1990; Gogolewski
et al 1997; Rehbein et al 1999), in the current experiments, CRC
use was compared with a preventive programme of oral treat-
ments designed to minimise autumn pasture contamination by
repeated treatment of lambs at intervals only slightly greater than
the pre-patent period of the major parasites of concern (Brunsdon
and Vlassoff 1982). Both treatment regimes would therefore be
expected to result in low levels of pasture contamination over the
treatment period, so the possibility that production benefits as-
sociated with CRC use may have been underestimated remains
uncertain.
compared with a single oral treatment at weaning in 3 on-farm
trials in which all animals grazed together. No production benefits
were recorded from the use of CRCs; in fact on 2 farms, carcass
weight at slaughter was significantly lower for CRC treated
lambs (Webb Ware et al 2000). In New Zealand, Gogolewski
et al (1997) showed in a series of trials that lambs treated with
ivermectin-CRCs grew significantly faster than untreated lambs.
While these, and similar comparisons overseas (Barton et al 1990;
Rehbein et al 1999), may demonstrate the benefits of effective
parasite control in lambs, they do little to help farmers or vet-
erinarians evaluate CRC use as an alternative to the more usual
practice of repeated treatments with an oral anthelmintic (Bruns-
don and Vlassoff 1982). Macchi et al (2001) conducted trials on
5 farms on which resistance to benzimidazole anthelmintics had
been diagnosed. The performance of lambs treated orally every
28 days with levamisole or levamisole + oxfendazole (the latter
was shown to be an effective treatment) was compared to that of
lambs treated with 2 albendazole-CRCs (the second administered
90 days after the first) plus 3 additional oral treatments with mox-
idectin. The moxidectin treatments were included in that study
because the albendazole-CRCs failed to adequately control FECs
on 2/5 farms. Over the 5-month duration of those trials, the aver-
age benefit from the latter regime was 1.58 kg liveweight (Macchi
et al 2001). The design of those trials did not allow for a direct
evaluation of the economic benefits associated with CRC use and,
again, animals from the various treatment groups were grazed
together, confounding possible differences in levels of pasture
contamination. However, given that successive treatments with
2 CRCs plus 3 treatments with moxidectin gave a production
benefit of only 1.58 kg over a 5-month period (approximately
10 g/day) compared with monthly treatments with an effective
oral anthelmintic, it is likely that the advantage associated with
the use of a single CRC would have been less. Williamson et al
CRC use has twice been compared with the Australian “worm-
kill” strategic lamb-drenching programme under conditions in
which treatment groups grazed separate pastures, and in both
cases there were no significant differences in lamb performance
between groups treated with CRCs and those given a programme
of effective oral treatments (Barger et al 1992; van Houtert et al
1995). In southern Victoria, Australia, albendazole-CRCs were
a
b
1.6
1.6
1.2
0.8
0.4
0
1.2
0.8
0.4
0
0
4
8
12
16
20
24
0
4
8
12
16
20
24
c
d
1.6
1.2
0.8
0.4
0
1.6
1.2
0.8
0.4
0
0
4
8
12
16
20
24
0
4
8
12
16
20
24
Weeks after weaning
Weeks after weaning
Figure 2. Serum immunoglobulin-G levels against (a) O. circumcincta-larvae, (b) O. circumcincta-adult, (c) T. colubriformis-larvae and
(d) T. colubriformis-adult antigens from Trial 1 (Ballantrae), where: (
) = capsule treatments, (
) = oral treatments, ( — ) = albendazole treatments
and ( − − ) = ivermectin treatments.

Leathwick et al
New Zealand Veterinary Journal 50(2), 2002
75
(1995) compared lambs given CRCs with animals only treated
to prevent clinical parasitism and found production responses to
be variable and on some occasions the CRC-treated animals grew
significantly slower than untreated animals. Our results are thus
consistent with other published reports showing that production
benefits associated with the use of CRCs compared with repeated
treatments with an oral anthelmintic are unlikely to be substan-
tial.
levels were lower in ivermectin-treated than in albendazole-
treated lambs, whether the anthelmintic was administered as oral
drenches or as a CRC, although differences in antibody levels did
not translate into measurable differences in productivity or FEC
that might influence the choice of product.
Acknowledgements
In all 3 of the trials reported here, albendazole-CRCs failed to
control FECs to the expected level. Despite this, oral albendazole
appeared to be effective. On the property used for Trial 1, when
animals not involved in the trial were dosed with albendazole ac-
cording to individual animal liveweights, a 90% reduction in FEC
was achieved, but when trial animals were dosed according to the
weight of the heaviest animal in the group efficacy was 96%.
Thus, while a low level of resistance was detected, the oral treat-
ments given to trial animals were clearly achieving acceptable con-
trol. In contrast to the efficacy of the albendazole-oral treatment,
the albendazole-CRC treatment failed to control parasitism in
Trial 1 to the point where a ‘salvage’ drench was necessary to pre-
vent clinical disease from occurring in these lambs. Moreover,
while FECs were generally similar for the albendazole-CRC
and albendazole-oral-treated groups (Figure 1), the pattern
of FECs in relation to time differed markedly between these
groups. FECs in the orally-treated animals would have been
reduced to low levels by the oral treatments every 3–4 weeks
and would only have climbed to the measured values follow-
ing maturation of post-treatment infections. In contrast, egg
production from the albendazole-CRC-treated animals would
have been more or less continual over time, resulting in a
considerably greater contribution to pasture contamination
than those from the orally-treated lambs. Thus, somewhat
ironically, the most likely reason for any excessive pasture
contamination that may have influenced the productivity of
lambs treated with the ivermectin-CRC was the failure of the
albendazole-CRC to control FEC, especially in Trial 1.
We gratefully acknowledge the assistance of the farmers who al-
lowed us to run the trials on their properties. Alex Vlassoff and
Allen Heath made helpful comments on an earlier draft of this
manuscript.
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Accepted for publication 18 January 2002