Preliminary evidence for genetic resistance to endoparasites in Menz and Horro lambs in the highlands of Ethiopia

J.E.O. Rege,¹ S. Tembely,¹ E. Mukasa,¹ S. Sovani¹
D. Anindo,¹ A. Lahlou-Kassi¹ and R.L. Baker²

¹ International Livestock Centre for Africa (ILCA)
P.O. Box 5689, Addis Ababa, Ethiopia
² ILCA, P.O. Box 46847, Nairobi, Kenya

Abstract
Introduction
Materials and methods
Results
Discussion
References

Abstract

The Menz and Horro highland sheep breeds are being evaluated at the ILCA Debre Birhan research station in Ethiopia for between-and within-breed genetic variation for resistance to internal parasites. The experimental design involves mating, for each breed, 150 ewes with 10 rams to lamb at the beginning of the dry season (October/November) and the wet season (June/July). This paper reports data collected from birth to weaning on lambs from the first three lamb crops (i.e. born October/November 1992; June/July 1993; October/November 1993). The traits analysed were birth weight (BWT), weaning weight (WWT), packed red cell volume (PCV) recorded at weaning, logarithm transformed faecal egg count (LFEC) recorded at weaning and lamb survival from birth to weaning. The effect of both breed and lambing group was significant (P) for all traits analysed. Horro lambs were heavier at birth and at weaning, had lower PCV, higher LFEC and lower survival rate from birth to weaning than Menz lambs. There was a significant interaction of breed by lambing group for all traits except BOOT. For lamb survival, WWT and PCV the interaction took the form of varying magnitude of differences between the breeds in different lambing groups rather than any change in breed ranking. For LFEC the breed ranking changed in different groups with the Menz having higher LFEC than Horro lambs in the first lambing group, while the reverse was true in the other two lambing groups. There was no evidence from these data that the breed difference in lamb survival was due to resistance to endoparasites. There was significant additive genetic direct and maternal variance for BWT and WWT. For BWT the estimate of the direct (h²_A) and maternal (h²_M) heritability were 0.07±0.06 and 0.34±0.06, respectively, while for WWT the corresponding estimates were 0.04±0.05 and 0.43±0.07. For PCV and LFEC h²_M was not significant and the overall heritability estimates were 0.34±0.14 and 0.32±0.13, respectively.

Introduction

Helminthiasis is of considerable significance in a wide range of agro-climatic zones in sub-Saharan Africa and constitutes one of the most important constraints to small ruminant production (ILCA 1991; Over et al 1992). The widespread occurrence of infection with internal parasites in grazing animals, the associated loss of production, the costs of anthelmintics and death of infected animals are some of the major concerns. There are also increasing environmental concerns which may influence anthelmintic usage through consumer demand for animal products and pastures free of chemical residues.

Current control methods for internal parasites outside Africa focus on reducing contamination of pastures through anthelmintic treatment and/or controlled grazing. In Africa, these control methods are limited by the high cost of anthelmintics, their uncertain availability, increasing frequency of drug resistance and limited scope in many communal pastoral systems for controlled grazing (Waller 1991; Mwamachi et al 1995; Tembely et al this conference). It appears unlikely that new broad-spectrum anthelmintics will be available in the near future because of the major costs associated with the development of new products, and to date only a few commercial vaccines are available including the lungworm-irradiated vaccine. Alternative approaches to control internal parasites are therefore being considered. One such approach is utilisation of host genetic variation for resistance.

Evidence of genetic variation in sheep and goats for resistance to or tolerance of gastro-intestinal nematodes was first documented 40-50 years ago and this subject has been comprehensively reviewed by Gray (1991), Gray and Woolaston (1991), Baker et al (1992) and Gray et al (1995). The best documented examples of sheep breeds showing resistance to endoparasites in Africa are the Red Maasai sheep in East Africa and the Djallonké sheep in West Africa (Preston and Allonby 1978; Preston and Allonby 1979; Baker et al 1993; Baker et al 1994; Baker 1995).

There are about 23 million sheep and 17 million goats in Ethiopia (FAO 1987). Three-quarters of the national sheep flock is located in the highland regions which receive more than 700 mm of rainfall per year and accommodate three-quarters of the human population. Gebrekiros Asegede (1990) compared four breeds of indigenous Ethiopian sheep for their resistance to endoparasites (predominantly Haemonchus contortus) at Awassa in southern Ethiopia. The breeds evaluated were the Adal and Blackhead Somali originating from the semi-arid lowland regions, and the Horro and Arsi from the humid highlands. The Blackhead Somali were the most susceptible to endoparasites while the Arsi were the most resistant.

This paper presents some preliminary results from an evaluation of the resistance of Menz and Horro lambs to endoparasites in the highlands of Ethiopia. This research is part of an ILCA Pan-African research programme to investigate and characterise genetic resistance to endoparasites in some indigenous sheep and goat breeds in sub-Saharan Africa (ILCA 1991; ILCA 1993).

Materials and methods

Experiment site

The Debre Birhan experiment station is located in the central Ethiopian highlands about 120 km NE of Addis Ababa at an altitude of 2780 m above sea level. The climate is characterised by a long rainy season (July-September), a short rainy season (February-March) and an extended dry season (October-February). Annual rainfall averages 920 mm. Air temperatures range from a near freezing low of 2.4°C in November to 23.3°C in June. The pasture at the station is dominated by Andropogon grass (Andropogon longipes) with variable proportions of Trifolium spp. A detailed presentation of relevant epidemiological data, including predominant parasites at the experimental station is reported elsewhere (Tembely et al this proceedings).

Experimental animals and management

Menz (indigeneous to the study area) and Horro (introduced from a slightly lower highland area) ewes were oestrous synchronised with progestron sponges left in place for 10 to 12 days to permit for dry and wet season lambings. Single sire mating occurred in night pens for 30 days and ewes were allowed to graze together during the day. After mating, all ewes received concentrate supplement (200 g/head per day). Female lambs were separated from the male lambs after weaning, but exposed to the same grazing paddocks in a rotational grazing system until the end of the experiment at the age of 12 months. The mating schedule used to produce the lamb crops analysed is presented in Table 1. Grazing management and health intervention programmes are described elsewhere (Tembely et al this conference).

Table 1. Mating and lambing schedules for the three lamb crops analysed.

Group	Mating period	No. of rams		No. of ewesa)		Lambing season	No. of lambs born
		Horro	Menz	Horro	Menz		Horro	Menz
1	May 1992	11	11	276(20)	284(20)	Oct/Nov 1992	124	137
2	January 1993	10b)	11	286(20)	274(20)	June/July 1993	116	233
3c)	May 1993	11	11	233(20)	224(20)	Oct/Nov 1993	144	201

^a) Number of unmated control ewes are in parenthesis.

^b) One ram died before the second mating season.

^c) For each breed eight new rams were introduced and three previous ones (groups 1 and 2) retained.

Data recorded

Live weights were recorded on all lambs at birth (BOOT) and at weaning (WWT) at 3 -4 months of age. Blood and faecal samples were collected from all lambs at 1 and 2 months of age and at weaning. Packed red cell volume (PCV) was determined using a haematocrit centrifuge. Faecal egg counts (FEC) were analysed using the modified McMaster method (MAFF 1977). Additionally, faecal samples were bulked by breed and sex and cultured for larvae identification (Hansen and Perry 1990). Though FEC and PCV measurements were taken on all lambs at 1 and 2 months of age and at weaning (at 3 months of age), only the data on weaners is reported in this paper. Individual lambs with FEC greater than 2000 eggs per gram (epg) and/or PCV less than 15% at the 2-month sampling were drenched.

Statistical analysis

Analyses of fixed effects were performed by least squares (Harvey 1990). The fixed effects model fitted included lamb breed (Menz and Horro), lambing group (3 levels), lamb sex (male and female), lamb birth type (single or twin born), dam parity (first, second and later) and the interaction of breed and lambing group. In preliminary analyses, parity was only significant for BWT and WWT and was excluded from subsequent models for the other traits. Birth date was included in the model as a linear covariate when analysing BWT. Lamb age was included as a linear covariate when analysing WWT, PCV and FEC. Preliminary analyses identified that most other first-order interactions were not significant (P>0.05), except for birth type x parity for WWT and lamb age x lambing group for WWT and FEC. Survival from birth to weaning was analysed as a binomial variable (0 = dead, 1 = alive) with the same fixed effect model as for the other traits but with no covariate or dam parity effect in the model. FEC was analysed both as an untransformed variable and using a logarithm transformation, log_e (FEC + 10), to normalise the distribution of this trait.

Heritabilities were estimated by Restricted Maximum Likelihood (REML) using an animal model applying the DFREML software of Meyer (1991) and fitting the significant fixed effects identified in the fixed model analyses of variance.

Results

Tests of significance from analyses of variance, overall means and residual standard deviations for the six traits analysed are presented in Table 2. Differences between breeds and lambing groups were significant (P) for all traits analysed, and, with the exception of BOOT, the interaction of breed by lambing group was also significant (P). The skewed, non-normal distribution of FEC is clearly reflected in the high coefficient of variation (CV) of 111 % which is reduced to 23.5% when the logarithm transformation is applied (i.e. LFEC).

The least squares means for breed, lambing group and the interaction of breed by lambing group are presented in Table 3. Horro lambs were heavier at birth and weaning, had lower PCV, higher LFEC and lower survival from birth to weaning. Compared to the two lamb crops which were born at the beginning of the dry season (groups 1 and 3), the wet season lambing (June/July 1993) produced heavier lambs at birth and weaning and had the highest survival rate from birth to weaning. However, in terms of the parasitological parameters at weaning, the group 2 lambs had the highest LFEC and an intermediate PCV.

Table 2. Analysis of variance, overall mean, residual standard deviation and coefficient of variation (CV) for birth weight (BWT), weaning weight (WWT), packed cell volume (PCV), faecal egg count (FEC), logarithm transformed FEC (LFEC) and lamb survival from birth to weaning (SURV).

Source of variation		BWT (kg)	WWT (kg)	PCV (%)	FEC (epg)	LFEC	SURV (%)
Breed		***	***	***	**	*	***
Lambing gp (LG)		***	***	***	***	***	***
Breed x LG		ns	***	***	***	***	*
Sex		***	**	*	*	*	ns
Birth type (BT)		***	***	***	*	*	***
Dam parity (DP)		***	*	-	-	-	-
Covariate
	Birth date	**
	Lamb age		***	**	***	***
Lamb age x LG			*	*	***
BT X DP			***
Total records		954	783	781	733	733	954
Overall mean		2.42	9.2	35.3	395	2.16	84.5
Residual SD		0.42	1.9	4.4	439	0.51	31.6
CV (%)		17.2	21.0	12.4	111	23.5	37.4

ns = not significant;
* P<0.05;
** P<0.01;
*** P<0.001.

Table 3. Least squares means by breed, lambing group and breed x lambing group interaction for birth weight (BWT), weaning weight (WWT), packed cell volume (PCV), anti-log of logarithm transformed faecal egg count (ALFEC) and lamb survival from birth to weaning (SURV).

Effect		No. of lambs born	BWT (kg)	WWT (kg)	PCV (%)	ALFEC (epg)	SURV (%)
Breed
	Menz	573	2.16	8.4	35.7	153	85.8
	Horro	381	2.50	8,9	33.3	183	73.1
Significance			***	***	***	*	***
Lambing group (LG)
	1	283	2.07	8.7	33.4	137	84.9
	2	340	2.75	9.7	34.9	502	94.1
	3	331	2.18	7.4	35.2	69	59.3
Significance			***	***	***	***	***
Breed x LG
	Menz 1	149	1.90	8.7	35.6	214	92.3
	2	228	2.54	9.0	35.8	353	96.8
	3	196	2.02	7.4	35.7	48	68.3
	Horro 1	134	2.23	8.8	31.3	87	77.5
	2	112	2.92	10.4	33.9	708	91.4
	3	135	2.33	7.5	34.6	98	50.3
Significance			ns	***	***	***	*

ns = not significant;
* P<0.05;
** P<0.01;
*** P<0.001.

There was a significant (P) interaction of breed by lambing group for all traits except BWT (Table 3). For lamb survival, WWT and PCV the interaction was manifested by a change in the magnitude of differences between the breeds in different lambing groups rather than any change in breed ranking. For example, the considerably higher PCV of the Menz compared to the Horro lambs in the first lambing group (4.3%) declined to 1.9% and 1.1% in the second and third lambing groups, respectively. For LFEC the breed ranking changed in different lambing groups, with the Menz having higher FEC than the Horro lambs in the first lambing group and the reverse being the case in the other two lambing groups.

Analysis of faecal samples of the lambs born in the dry season (groups 1 and 3) at day 60 showed a low egg count whereas packed cell volume was within the normal range. Faecal culture from lambs born in the wet season (group 2) showed that the predominant nematode species included Ostertagia trifurcata (54%), Trichostrongylus colubriformis (28%) and Haemonchus contortus (18%).

Heritability estimates for BOOT, WWT, PCV and LFEC are presented in Table 4. For both BWT and WWT there was significant additive genetic maternal variance.

Table 4. Heritabilities^a for birth weight (BWT), weaning weight (WWT), packed cell volume (PCV) and logarithm transformed faecal egg count (LFEC).

Trait	Model 1	Model 3
	h2 ± SE	h2A ± SE	h2M ± SE
BWT	0.66±.25	0.07±.06	0.34±.06
WWT	0.10±.09	0.04±.05	0.43±.07
PCV	0.34±.14	0.29±.13	0.08±.09
LFEC	0.32±.13	0.30±.13	0.04±.09

^a Heritabilities estimated by DFREML (Meyer 1991).

Model 1: h² = total heritability (ignoring maternal additive genetic variance)

Model 3: h²_A = heritability of direct (additive genetic) component of performance.
h²_M = heritability of maternal (additive genetic) component of performance.

Discussion

Breed differences in traits analysed were significant but, in general, not of great biological importance except for the difference in survival. Estimated breed differences in birth weight, weaning weight and survival in the present study are consistent with some previous results from this location (Gautsch 1992).

The general levels of nematode parasite infection was too low for this to be a major cause of sheep mortality in the study. Indeed, post-mortem results showed that respiratory diseases were the primary causes of death. Specifically, the low mean FEC (even for the group 2 lambs) suggests that this level of infection was unlikely to result in major pathogenic effects. Optimum conditions for transmission of Ostertagia, Trichostrongylus and Haemonchus are present from July to November (total monthly rainfall of 50 mm or more with mean temperatures between 11°C and 13°C). There is therefore an overlap of the three nematode species. However, because of a temperate climate in the highlands, Ostertagia and Trichostrongylus thrive better than Haemonchus which generally predominates in relatively warm climates. Thus, although the Horro breed had, on average, lower PCV and higher LFEC values than their Menz contemporaries, there was no direct evidence from these data that the breed difference in lamb survival was due to differential resistance to endoparasites. Additionally, breed differences in LFEC were not consistent across lambing groups.

The low survival of the Horro in this study may be due to lack of adaptation of the breed to this environment. Additionally, the lower PCV of the breed compared to the mean for the Menz may be due to the fact that the Horro is less adapted to this (higher) altitude, although both are highland breeds. However, hard data is not yet available to substantiate this speculation.

The heritability estimates for indicators of resistance to parasites were not different from values reported elsewhere. The average heritability for single FEC measurements from estimates reviewed by Baker et al (1992) was 0.32, while the average estimate for PCV was 0.35. Heritability of the mean of multiple (2 to 3) egg counts recorded in different infections increased to about 0.5-0.6. On the basis of available data, there seems to be substantial within-breed variation in resistance to endoparasites (i.e. PCV and LFEC) in these lambs at a young age. There may be opportunity for within-breed improvement of this trait through selection on either PCV or LFEC.

With only one and two lamb crops in wet and dry seasons, respectively, the results on lambing group differences are too preliminary for any definite conclusions to be made on seasonal effects. The wet season was, as would be expected, associated with the highest parasite challenge, as reflected in faecal egg counts. However, this is also the season with the most abundant grazing, best growth performance, and highest lamb survival. The estimates of group means for the various traits in this study emphasise the point already made, that the level of nematode infections encountered to date at Debre Birhan is not too debilitating.

Except for faecal egg counts, the effect of breed x lambing group interactions were of no major practical significance. However, no final conclusions can be drawn until more data and additional traits (post-weaning growth, PCV, FEC as well as reproduction) are analysed.

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