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Ovine Observer

Body condition score as a selection tool for worm control using Targeted Selective Treatment

Meghan England, DAFWA WA; C. Jacobson, Murdoch University WA; B. Besier, Brown Besier Parasitology, Albany WA

Corresponding author: Meghan.England@agric.wa.gov.au

Introduction

The effectiveness of worm control is increasingly compromised because of increasing and widespread resistance to anthelmintics (drenches). On-going investigations into sustainable worm control have focused on the “refugia” strategy. This technique works by ensuring the survival of sufficient worms with genotypes susceptible to anthelmintics within in the total population on a property, to dilute resistant individuals surviving anthelmintic treatment.

‘Targeted selective treatment’ (TST) is a type of refugia-based strategy by which anthelmintic treatments are restricted to animals judged likely to suffer significant production loss or health effects if not treated, while treatment to others in the group is withheld. It is based on the theory that the individual animals who exhibit greater resilience to parasites, seen as fewer signs of ill-health or increased efficiency, can be exploited by TST strategies to ensure that a proportion of a worm population remains in refugia from anthelmintic exposure.

A key factor that has delayed utilization of TST for trichostrongylid worms other than H. contortus (barbers pole), is the absence of a convenient and accurate method for identifying animals that would benefit most from treatment. Approaches used in other investigations have been based on repeated measurements of production indices (for example body weight, worm egg count, ocular membrane inspection) in animals under parasite challenge as an indicator of resilience. However these require investment in labour and/or equipment that may limit their application on a large scale.

Body condition score (BCS) is a commonly used on-farm measure, requiring no technological investment. BCS is accepted as an indicator of general condition and body reserves, and therefore may act as an indicator of resilience to worm infections. This report summarises the findings of a paper investigating the relationship between BCS and worm burdens, and the practical use of BCS in TST programs on farm.

Differences in body condition score for treated and untreated ewes

In order to evaluate the usefulness of BCS in on-farm TST programs, an investigation was conducted to determine the relationship between BCS and production losses due to worm burdens. We hypothesised that mature sheep in poorer body condition would suffer greater production loss due to worm infections than sheep with higher body condition, and therefore BCS would provide a suitable selection tool.

Materials and Methods

Adult merino ewes at two sites were separated in to four different groups on the basis of their body condition score prior to lambing. Ewes from each of these groups were then separated into two equal sub-groups and either treated for worms (treated) or not (untreated). The body condition score and WEC’s of all ewes were monitored from 3 weeks pre-lambing until 28 weeks post lambing. The predominant worm species found in this experiment were Trichostrongylus spp., Teladorsagia circumcincta and Chabertia ovina.

Results

In general, ewes in poorer starting body condition showed a greater relative BCS response to treatment than those of higher starting BCS. The untreated ewes lost more condition than the treated ewes in the two lowest BCS groups; ≤2.5 and 2.7 compared to the two higher BCS groups; 3.0 and >3.0 (Table 1). This suggests that pre-lambing BCS does offer promise as a selection index for identifying Merino ewes most likely to benefit from anthelmintic treatment in TST-based worm control programs.

The response was found to be more consistent at Farm A, which was characterised by poorer nutritional conditions (pasture availability), lower average flock body condition and higher average flock worm egg counts (WEC) in untreated ewes, compared with the same measures at Farm B.

 

Table 1 Change in body condition score (average ± standard error) in ewes in different worm treatment groups and different groups of initial condition score

Initial BCS

Farm A

Farm B

Treated

Untreated

P value

Treated

Untreated

P value

≤2.5

-0.42 ± 0.05

-0.71 ± 0.04

<0.001

0.31 ± 0.06

0.02 ± 0.06

0.001

2.7

-0.71 ± 0.04

-0.86 ± 0.06

0.044

0.19 ± 0.04

0.00 ± 0.06

0.014

3.0

-0.95 ± 0.05

-1.05 ± 0.04

ns

-0.05 ± 0.04

-0.10 ± 0.04

ns

>3.0

-1.18 ± 0.08

-1.24 ± 0.07

ns

-0.28 ± 0.06

-0.39 ± 0.05

ns

ns = not significant (P>0.05)

Relative risk for untreated ewes

Regardless of treatment, the risk of sheep falling below BCS 2.0 during the experiment was increased for ewes in poorer BCS before lambing.

Table 2 shows the relative risk of the different groups of ewes falling below a BCS of 2 after lambing relative to that of ewe with a BCS of 3 or greater pre-lambing. The ewes in poorer body condition (BCS <3.0) pre-lambing were more than three times more likely to fall below BCS 2.0 after lambing. Furthermore, untreated ewes in very poor condition (BCS <2.0) were over 230 times more likely to have BCS <2.0 after lambing at Farm B (P<0.001; Table 2). These results further support the notion that BCS can be used to identify sheep more likely to benefit from treatment.

 

Table 2 Number of times more likely that ewe BCS will fall below 2.0 after lambing relative to ewes with an initial BCS ≥3.0 pre-lambing

Pre-lambing BCS

All ewes

Treated ewes only

Untreated ewes only

Farm A

Farm B

Farm A

Farm B

Farm A

Farm B

<2.0

*

62.4

*

ns

*

231.0

≤2.5

9.8

18

5.6

ns

*

31.7

<3.0

4.2

9.3

3.6

ns

5.5

16.1

*All sheep fell below BCS 2.0 after lambing
ns = not significant (P>0.05)

The results of this experiment suggest that only treating ewes in a poor BCS could be a viable tactic to allow worm burdens to remain in the well-conditioned animals in the flock. This is unlikely to affect production as treating well-conditioned individuals with anthelmintics did not significantly affect the change in BCS.

Take home messages

This experiment supported the hypothesis that ewes in poorer body condition prior to lambing are more likely to benefit from anthelmintic treatment than well-conditioned ewes. Better conditioned ewes were also less likely to fall to a critically low body condition level, below BSC 2 where welfare and production are likely to be compromised. The findings from these flocks therefore suggest that under a TST strategy, pre-lambing treatments could be given to ewes in poorest BCS, leaving ewes in better body condition (BCS > 3.0) as a source of refugia.