Genetic parameters for traits associated with resistance to Haemonchus contortus in South African Dohne Merino sheep

 

M.A. Snyman1, W.J. Olivier1 & A. Fisher2

1 Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900, South Africa

Willem Olivier (Corresponding author)

2 Queenstown Provincial Veterinary Laboratory, Private Bag X7093, Queenstown, 5320, South Africa

 

 

Summary

Resistance of Haemonchus contortus to anthelmintics in South Africa is a well-documented problem. In some areas, farming with animals resistant to nematode infestation seems to be the only solution in the long run. The farm Wauldby, in the Stutterheim district of South Africa, has a well-documented history of heavy Haemonchus contortus challenge and of Haemonchus resistance to all five major anthelmintic groups on the market prior to 2011. At the end of 2011, a project aimed at goal oriented selection for resistance to Haemonchus contortus was implemented in the Dohne Merino stud at Wauldby. Data on faecal egg counts (FEC), Famacha© score (FAM), body condition score (BCS) and log-transformed faecal egg counts (LFEC), recorded on the 2011- to the 2015-born lambs were analysed in this study. Between 10 and 12 two-weekly recordings of FAM, BCS and FEC were done each year. The number of individual data records available per year for these recorded resistance traits varied between 2365 and 3003 for a total of 13648 records. Various univariate, multivariate and repeatability animal models using the ASReml program were fitted to estimate genetic parameters for these resistance traits. The most suitable model of analyses for FAM, BCS, FEC and LFEC averaged over all recordings per year, included only direct additive genetic effects. Direct heritabilities of 0.20 ± 0.06, 0.32 ± 0.07, 0.15 ± 0.05 and 0.22 ± 0.06 were estimated for FAM, BCS, FEC and LFEC respectively. Direct heritability increased for all traits when data from all 12 available recordings were included. Various combinations of FEC taken at the different recordings were analysed in an effort to obtain the most suitable recordings for inclusion in a protocol for selection against resistance to Haemonchus. The combination of the 1st (January), 6th (March) and 9th (May) recordings yielded the most promising results. Genetic correlation among these three recordings for BCS (0.89 to 0.98), FEC (0.40 to 0.92) and LFEC (0.58 to 0.96) were high and positive. Univariate heritability estimates obtained for FEC and LFEC, averaged for the 1st, 6th and 9th recordings, compared well with those obtained under univariate analyses where all available data were averaged (0.16 vs. 0.15 for FEC and 0.20 vs. 0.22 for LFEC). However, it was much lower for FAM and BCS (0.02 vs. 0.20 for FAM and 0.23 vs. 0.32 for BCS). These results were used to develop protocols for selection for resistance / resilience to Haemonchus contortus in sheep under South African conditions.

 

Keywords: faecal egg count, Famacha©, body condition score, heritability

 

Introduction

Resistance of internal parasites to anthelmintics has become a widespread problem, with resistance of Haemonchus contortus in South Africa one of the most evident (Van Wyk et al., 1997). In some areas, farming with animals resistant to nematode infestation seems to be the only solution in the long run. Genetic variation in resistance to nematode infestation in sheep, based on faecal egg count as criterion, have been reported for various breeds (Morris et al., 1997; Nieuwoudt et al., 2002; Khusro et al., 2004; Yadav et al., 2006; Cloete et al., 2007). Successful breeding programs for resistance have been reported for Australian (Pocock et al., 1995; Greeff et al., 2006) and New Zealand (Morris et al., 2005) sheep. No reports on active breeding programs for resistance in South African sheep could be found.

 

The objectives of this study were to estimate genetic parameters for resistance / resilience to Haemonchus contortus and to use these parameters for the development of protocols for selection for resistance / resilience to Haemonchus contortus in South African sheep.

 

Materials and methods

The farm Wauldby, in the Stutterheim district of South Africa, has a well-documented history of heavy Haemonchus contortus challenge and of Haemonchus resistance to all five major anthelmintic groups on the market prior to 2011. In 2011, a Haemonchus resistant line was established in the Dohne Merino stud at Wauldby. Faecal egg counts (FEC), Famacha© score (FAM) and body condition score (BCS) of all lambs were recorded annually from January until July for the 2011- to the 2015-born lambs (± 250 to 280 lambs per year). FAM was recorded weekly and FEC and BCS every 14 days. Lambs were only drenched when they had a FAM of 2.5 or more, in conjunction with a BCS of less than 1.5. Replacement lambs for the resistant line were selected from those that did not receive any anthelminthic treatment. Selection was based on a selection index incorporating FEC, FAM and BCS.

 

Data on FEC, log-transformed FEC (LFEC), FAM and BCS recorded on the 2011- to the 2015-born lambs were analysed. Between 10 and 12 two-weekly recordings of FAM, BCS and FEC were done each year. The number of individual data records available per year for these recorded resistance traits varied between 2365 and 3003 for a total of 13648 records. Various univariate, multivariate and repeatability animal models using the ASReml program (Gilmour et al., 2009) were fitted to estimate genetic parameters for the resistance traits.

 

Results and discussion

Variance components and genetic parameters for FAM, BCS, FEC and LFEC averaged over all recordings per year are presented in Table 1. The most suitable model of analyses for all the resistance traits included only direct additive genetic effects.

 

Table 1. Variance components and genetic parameters for FAM, BCS, FEC and LFEC averaged over all recordings per year over all recordings per year

Component / Ratio

FAM

BCS

FEC

LFEC

 

 

 

 

 

σ2a

0.007

0.007

564961

0.025

σ2e

0.030

0.015

3095490

0.090

σ2p

0.037

0.022

3660500

0.115

h2a

0.20 ± 0.06

0.32 ± 0.07

0.15 ± 0.05

0.22 ± 0.06

 

As it will be impractical and expensive to record FEC every second week under commercial farming conditions, various combinations of FEC taken at the different recordings were analysed in an effort to obtain the most suitable recordings for inclusion in a protocol for selection against resistance to Haemonchus. The combination the 1st (January), 6th (March) and 9th (May) recordings yielded the most promising results. Heritabilities for and genetic and phenotypic correlations among the resistance traits collected at the 1st, 6th and 9th recordings, estimated with multivariate animal models, are summarised in Table 2. Except for FAM, genetic correlation among these three recordings for BCS, FEC and LFEC were high and positive.

 

Table 2. Heritabilities (on diagonal) for and genetic (above diagonal) and phenotypic (below diagonal) correlations among resistance traits collected at the 1st, 6th and 9th recordings

Trait

FAM1

FAM6

FAM9

 

 

 

 

FAM1

0.05 ± 0.04

-0.56 ± 0.65

-0.29 ± 0.45

FAM6

0.04 ± 0.03

0.04 ± 0.04

0.92 ± 0.44

FAM9

0.11 ± 0.03

0.11 ± 0.03

0.10 ± 0.05

 

BCS1

BCS6

BCS9

BCS1

0.11 ± 0.05

0.98 ± 0.10

0.89 ± 0.18

BCS6

0.47 ± 0.02

0.21 ± 0.06

0.93 ± 0.10

BCS9

0.36 ± 0.03

0.46 ± 0.02

0.14 ± 0.05

 

FEC1

FEC6

FEC9

FEC1

0.10 ± 0.01

0.40 ± 0.00

0.73 ± 0.00

FEC6

0.11 ± 0.03

0.07 ± 0.01

0.92 ± 0.00

FEC9

0.11 ± 0.03

0.21 ± 0.03

0.07 ± 0.01

 

LFEC1

LFEC6

LFEC9

LFEC1

0.17 ± 0.01

0.75 ± 0.00

0.58 ± 0.00

LFEC6

0.09 ± 0.03

0.09 ± 0.01

0.96 ± 0.00

LFEC9

0.10 ± 0.03

0.19 ± 0.03

0.07 ± 0.01

 

Variance components and genetic parameters for FAM, BCS, FEC and LFEC averaged over the 1st, 6th and 9th recordings, estimated with univariate animal models are presented in Table 3. Heritability estimates obtained for FEC and LFEC compared well with those obtained under univariate analyses where all available data were averaged (0.15 for FEC and 0.22 for LFEC). However, it was much lower for FAM and BCS (0.20 for FAM and 0.32 for BCS).

 

Table 3. Variance components and genetic parameters for FAM, BCS, FEC and LFEC averaged for the 1st, 6th and 9th recordings, estimated with univariate animal models

Trait

FAM

BCS

FEC

LFEC

 

 

 

 

 

σ2a

0.019

0.007

1156490

0.040

σ2e

0.085

0.023

6072540

0.159

σ2p

0.104

0.030

7229030

0.199

h2a

0.02 ± 0.03

0.23 ± 0.06

0.16 ± 0.05

0.20 ± 0.05

 

 

Conclusions

Data collected on FAM, FEC and BCS at the beginning, middle and towards the end of the Haemonchus season can be used for selection of sheep resistant to Haemonchus contortus. The developed protocols need to be validated on various farms before it could be implemented on a wider scale.

 

Acknowledgements

The following people / institutions are acknowledged for their contribution to this project:

 


References

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Published

Proc. 11th Wrld. Congr. Gen. Appl. Livest. Prod., Auckland, New Zealand, 11-16 February, 288