Genetic Parameters for Resistance to Haemonchus Contortus in Merino Sheep in South Africa

a INTRODUCTION If uncontrolled, infection with internal nematode parasites may lead to serious production losses and even death of animals. Over the last few decades new and better drugs have been developed, and these have often been used as the only means of control against worms in sheep. The continued use of these anthelmintic treatments has now brought the sheep industry to the point where parasitic nematodes have developed resistance to all of the main classes of anthelmintics and there are very limited management options available to control these parasites. Surveys in South Africa have indicated that about 90 % of sheep farms harbour strains of nematodes that are resistant to drugs from at least 1 of the 5 available anthelmintic groups. In at least 2 cases, resistance to all 5 anthelmintic groups was demonstrated 21. In most sheep-producing areas where internal parasites are a chronic problem, there is increasing pressure for alternative control


INTRODUCTION
If uncontrolled, infection with internal nematode parasites may lead to serious production losses and even death of animals.Over the last few decades new and better drugs have been developed, and these have often been used as the only means of control against worms in sheep.The continued use of these anthelmintic treatments has now brought the sheep industry to the point where parasitic nematodes have developed resistance to all of the main classes of anthelmintics and there are very limited management options available to control these parasites.Surveys in South Africa have indicated that about 90 % of sheep farms harbour strains of nematodes that are resistant to drugs from at least 1 of the 5 available anthelmintic groups.In at least 2 cases, resistance to all 5 anthelmintic groups was demonstrated 21 .In most sheep-producing areas where internal parasites are a chronic problem, there is increasing pressure for alternative control strategies such as breeding parasite-resistant animals.This means that, in order to ensure their own survival, sheep farmers will have to modify selection for production traits to also increase the ability of their sheep to withstand worm challenge, in addition to using other methods aimed at reduced selection for resistant worms.
Studies on breeding for resistance to internal parasites in sheep have been in progress in Australia 25 , New Zealand 14 , Kenya 3 , France 13 , the UK 6 , the USA 9 and Indonesia 17 for a number of years, and the results predict a fair amount of success for similar programmes in South Africa.This report covers part of an investigation aimed at examining the heritability of resistance of the South African Merino population to infestation with parasitic nematodes as a first step towards consideration of the trait in commercial breeding programmes in South Africa.

Animals
Two different flocks of Merino sheep were included in the study.From February 1995 to July 1996 data were collected from all lambs weaned from the OTK Merino Stud on the farm Wildebeesfontein near Hendrina in Mpumalanga province.This comprised 1 group (born during spring 1994) monitored from February 1995 to January 1996, a 2nd group (born during autumn 1995) from September 1995 to April 1996, and a 3rd group (born during spring 1995) from February 1996 to July 1996.These animals were grazed alternatively on natural and artificial pastures as well as harvested maize fields and sexes were separated at weaning.
The 2nd flock consisted of 5 rams from the Wildebeesfontein flock mated to Merino ewes kept at Irene in Gauteng province.Weaners from this flock (born during spring 1997 and spring 1998) were monitored from March 1998 to January 1999 and March 1999 to January 2000 respectively.This flock was also separated according to sex, but grazed on similar kikuyu pastures in adjacent camps, with additional hay fed from May to October.Pedigree records were available for all animals in both flocks.

Trial protocol
Resistance to nematode parasites was assessed on the basis of faecal egg counts (FECs) and packed cell volumes (PCVs) following natural infection of weaners.Faecal egg counts were performed using a modified McMaster method 19 , with counts expressed in eggs per gram (epg) and a sensitivity of 100 eggs per egg counted.Packed cell volumes were determined by micro-centrifugation of blood taken from the jugular vein and expressed as percentage red blood cells in whole blood 8 .Although FEC gives a more direct indication of levels of worm infection in individual animals, PCV indicates levels of anaemia caused by blood-sucking parasites.It therefore also serves as an indirect indication of especially severe infections of parasites such as Haemonchus contortus.
A subsample of 50 animals from each group was screened weekly for FEC until a mean of 2500 epg was reached, when all lambs in the group were sampled for FEC and PCV and then drenched.Wildebees-fontein animals were also weighed at this stage.Pooled faecal samples were used to prepare larval cultures from which larvae were identified to determine the generic composition of nematode populations.The entire procedure was repeated over the periods mentioned above for each group of lambs.

Statistical analysis
Faecal egg counts were logarithmically transformed (loge) to normalise the skewed distribution and a value of 1 was substituted for each nil value when no eggs were counted from a sample.Data obtained from the 3 groups of weaners at Wildebeesfontein and 2 at Irene were accumulated.At Wildebeesfontein, sires were common between spring 1994 and autumn 1995, and dams between spring 1994 and spring 1995 , while 5 of the male offspring from Wildebeesfontein were used as sires at Irene, thus creating genetic links between all the groups.
Non-genetic effects were tested in an analysis of variance, using SAS 18 .Sire was treated as a random effect, while the contemporary group year-week (year and week of the year when sampled), sex and birth type (twin/single) were treated as fixed effects.Live mass was not recorded in the Irene flock and birth dates were not recorded in the Wildebeesfontein flock.Although birth dates were not included in the analysis, all lambs per group were born within a period of 60 days and were considered to be of the same age.
Variance components and resulting heritabilities for logFEC and PCV were estimated by means of VCE4 12 using a bivariate animal model with repeated records and fitting the significant effects identified from the analyses of variance.
The following model was fitted: y = xb + za + e where y = vector of observed traits of animals; b = vector of fixed effects (year-week and sex); a = vector of direct additive genetic effects; x1 z = corresponding incidence matrices relating the effects to y, and e = vector of residuals.
The genetic correlations between logFEC, PCV, and live mass (in the case of Wildebeesfontein) were estimated.

RESULTS
In total, data for 583 animals and 1722 records (3-5 repetitions per group) were analysed.The numbers per group of lambs, as well as the average logFEC's and PCV's for every group of lambs at every sampling time, are presented in Table 1.From the analysis of variance, year-week and sex (P ≤ 0.001) and live mass (P ≤ 0.05) were shown to be significant sources of variation for PCV, while only sex and year-week (P ≤ 0.001) caused significant variation in logFEC.These data were used to estimate heritabilities (h 2 ) for PCV and logFEC, as well as the genetic correlations between traits, as is shown in Table 2. Maternal genetic and permanent environmental effects were not included in the current study.As shown by the log likelihood values, the design was probably inadequate to provide a robust test of these effects.

DISCUSSION
The sex difference indicated by the analysis of variance was confounded by nutrition and management in both flocks, as ram and ewe lambs were grazed separately after weaning.It was therefore not possible to separate these effects from the recorded data.Romjali et al. 17 , however, found in their study that sex had a significant effect on both PCV and FEC, while Albers et al. 1 found no differences between sexes after artificial infection.In both these studies H. contortus was the parasite species concerned.
The estimated heritability for logFEC was 0.236 ± 0.022.This result confirms that resistance to H. contortus (represented by FEC) is a heritable trait and suggests that South African Merinos should respond favourably to selection.This heritability estimate is also similar to those from other studies, which range on average between 0.2 and 0.4 [1][2][3]7 . Ina number of Australian studies 1,24,26,27 , an artificial infection of animals was preferred to a natural challenge from infected pastures, as was the case in other studies 3,6,7,10 and the present study.The mode of infection, however, had no significant influence on the results of these studies.Selection based on these estimations under both natural and artificial parasite challenge has led to successful reductions in levels of internal parasite infections in flocks of sheep in Australia and New Zealand, where resistant lines have been bred against Trichostrongylus colubriformis 10,22,23 and H. contortus 27 .
The heritability for PCV was estimated at 0.139 ± 0.022.Other studies have shown heritabilities for PCV in the order of 0.4 1 and 0.24 ± 0.16 4 .According to Baker 2 , the average heritability for a single PCV measurement from 14 Australian and New Zealand studies was 0.35, which is similar to that for FEC (0.32) from the same studies.Romjali et al. 17 concluded that both FEC and PCV are good indicators of naturally-acquired nematode infections.The lower heritability for PCV for the current study is probably due to relatively low infections of wireworm that resulted in limited variation in PCV values.
A strong negative genetic correlation (-0.835 ± 0.059) existed between logFEC and PCV.Larval identifications indicated Haemonchus spp.(wireworm, a bloodsucking parasite) as the only or main nematode species present in the flocks.Infection by blood-sucking parasites leads to anaemia that is indicated by lower PCV, which explains the negative correlation.The correlation further suggests a common genetic control for the 2 traits and implies that scoring of anaemia on the colour of eye mucous membranes can be used as an indication of levels of Haemonchus infection as proposed by Van Wyk et al. 20 .Similar high genetic correlations of -0.86 ± 0.28, -0.99 and -0.87 were reported between FEC and PCV by Albers et al. 1 , Baker et al. 4 and Woolaston and Piper 26 respectively.
The genetic correlation between PCV and live mass for the Wildebeesfontein flock was low (0.28 ± 0.09), while that between logFEC and live mass was negative, although not significantly different from zero (-0.13 ± 0.09).This is in agreement with Baker et al. 5 , who found that correlations between logFEC and live-mass gain in sheep on farms in New Zealand did not differ significantly from zero.In this case Trichostrongylus and Ostertagia were the most prevalent parasitic genera.Negative correlations between logFEC of Ostertagia circumcincta and live mass, ranging from -0.63 ± 0.32 to -0.90 ± 0.28 were reported by Bishop et al. 6 .Romjali et al. 17 found that weaning mass of lambs decreases with increasing FEC (negative correlation) and increases with increasing PCV (positive correlation) where H. contortus was the main parasite species present.Watson et al. 22 and Bisset et al. 7 also reported negative correlations of -0.43 and -0.48 ± 0.21 respectively between FEC and live mass gain in lambs.In both these studies Trichostrongylus was the main parasite concerned.
There have been some reports of unfavourable correlations between resistance to internal parasites and production traits.Morris et al. 16 compared FEC from 5 Romney selection lines to controls while being run together at a single site.The average FEC for 1 of the 3 lines selected for body mass was 63 % higher, although the other 2 were not significantly different from the controls.In the case of the 2 lines selected for fleece mass, the average FEC was significantly greater than that of the controls by 35 and 46 % respectively.In addition, Morris et al. 15 found that single-trait selection for reduced FEC in Romney and Per rendale sheep is associated with reduced greasy fleece mass in 3 selection lines in New Zealand.McEwan et al. 14 , on the other hand, found strong positive correlations between reduced FEC and live mass gain (0.95) and wool growth (0.41) in New Zealand Romney sheep.Greeff et al. 11 found that selection for low faecal egg count did not result in any unfavourable changes in hogget body mass, clean fleece mass, or fibre diameter in the Rylington Merino selection line in Australia.
Preliminary evidence suggests that breeding for resistance to internal parasites in sheep might be one of the few remaining options for roundworm control.The results obtained so far from the present study also indicate that it may be feasible to include resistance to internal parasites in a multi-trait breeding objective under South African conditions.It should, however, be kept in mind that selective breeding will only yield longterm results, while the state of resistance in the country requires fast action in addition.Breeding for resistance to internal parasites should therefore be considered as part of a broader integrated control programme.

Table 1 : Average logFEC and PCV for the different groups of lambs used in the study, presented according to locality, group, sampling time and sex.
WBF = Wildebeesfontein; Spr = Spring; Aut = Autumn; YRWK = year and week when sampled.