ESTIMATION OF GENETIC PARAMETERS FOR RESISTANCE TO HAEMONCHUS CONTORTUS IN A SOUTH AFRICAN DOHNE MERINO SHEEP FLOCK

 

M.A. Snyman1 I.D. Mashinini1#, & A. Fisher2

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

2 Queenstown Provincial Veterinary Laboratory, P/Bag X7093, Queenstown, 5320, South Africa

#Corresponding author: Ignatious Mashinini

 

 

Background:

Internal parasites are one of the major disease problems affecting grazing livestock worldwide and thus negatively impact on sustainability of livestock production. Expensive and unsuccessful treatment with anthelmintics, production losses and mortality due to severe infestations cost sheep farmers millions each year. Resistance of internal parasites to anthelmintics has become a widespread problem, with resistance of Haemonchus contortus in South Africa one of the most extreme. For some areas, farming with animals resistant to nematode infestation seems to be the only solution in the long run. At the end of 2011, a project aimed at selection for resistance to Haemonchus contortus was implemented on a farm in the Stutterheim district, known for its history of heavy Haemonchus contortus challenge and Haemonchus resistance to all anthelmintic groups.

 

Aim: The aim of this study was to estimate genetic parameters for resistance to Haemonchus contortus in a South African Dohne Merino sheep flock.

 

Methodologies:

Data on faecal egg counts (FEC), Famacha© score (FAM) and body condition score (BCS) recorded from 2011 to 2015 during the parasite resistance trial done on the Wauldby Dohne Merino stud were analysed with various univariate, multivariate and repeatability animal models using the AsReml programme. Between 10 and 12 two-weekly recordings of FAM, BCS and FEC were done over the years. 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.

 

Results: The most suitable model of analyses for the average FAM, BCS, FEC and LFEC (Log transformed FEC) over all recordings estimated with univariate models included only direct additive genetic effects. Direct heritabilities of 0.20 ± 0.06 (FAM), 0.32 ± 0.07 (BCS), 0.15 ± 0.05 (FEC) and 0.22 ± 0.06 (LFEC) were obtained. Heritability increased with an increase in the number of data recordings included in the analyses. Repeatability model heritabilities of 0.03 ± 0.01 (FAM), 0.11 ± 0.03 (BCS), 0.03 ± 0.01 (FEC) and 0.05 ± 0.01 LFEC) were obtained. Genetic correlations between the first (January), sixth (March; peak) and ninth (May) FEC recordings were as follow: FEC1 and FEC6 = 0.40 ± 0.00; FEC1 and FEC9 = 0.73 ± 0.00; FEC6 and FEC9 = 0.92 ± 0.00. Direct heritability obtained under univariate analyses for FEC and LFEC averaged for the 1st, 6th and 9th recordings were 0.16 ± 0.05 for FEC and 0.20 ± 0.05 for LFEC.

 

Discussion: The more recordings included for estimation of genetic parameters under univariate analyses, the higher the heritability. Heritabilities, as well as the permanent animal effects obtained with repeatability models were very low. The highest heritability for FEC and LFEC were obtained for univariate analyses on the average FEC and LFEC over all recordings, and for FEC and LFEC averaged for the 1st, 6th and 9th recordings. Genetic correlations among these FEC and LFEC traits were also significant and high.

 

Conclusions: Recording of the resistance traits, FAM, BCS and FEC, at the beginning (January), peak Haemonchus season (March) and towards the end of the season (May) could be used as a basis for selection against Haemonchus contortus in this flock.

 

Published

Proc. 50th Congr. S. Afr. Soc. Anim. Sci. Port Elizabeth, September 2017