- Email: [email protected]
The value of circulating eosinophil count as a selection criterion for resistance of sheep to trichostrongyle parasites
Intematimd
JournalforParasitology,
Pergamon
0020-7519(95)00105-0
RESEARCH
Vol. 26. No. 1, pp. 123126, 1996 Australian Society for Parasitology Ekvier Science LKI Printed in Great Britain 002&7519/96 S15.00 + 0.00
NOTE
The Value of Circulating Eosinophil Count as a Selection Criterion for Resistance of Sheep to Trichostrongyle Parasites R. R. WOOLASTON,*t P. MANUELI,$ S. J. EADY,? I. A. BARGER,§ L. F. LE JAMBRE,§ D. J. D. BANKS7 and R. G. WINDONS TCSIRO Division of Animal Production, PMB Armidale, NSW 2350, Australia $Ministry of Primary Industries, Box 358, Suva, Fgi $CSIRO Division of Animal Health, Ph4B Armidale, NSW 2350, Australia IDept of Primary Industries & Energy, GPO Box 858 Canberra, ACT 2601, Australia (Received IO January 1995; accepted 4 September 1995)
Abstract-Woolaston R. R., Manueli P., Eady S. J., Barger I. A., Le Jambre L. F., Banks D. J. D. & Windon R. G. 1996. The value of circulating eosinopbil count as a selection criterion for resistance of sheep to trichostrongyle parasites. Znternutionaf Joumdfor Purasitology 26: 123426. In Merino sheep bred for either increased or decreased resistance to Hoemonchus coorortus, faecal worm egg counts @EC) were lower in the resistant line (6,831 vs 17,645 epg, PcO.Ol), and circulating eosinopkils (EOS) were higher, but not signithntly so (3.40 x 10’ III-’ vs 1.40 x 104 ml-‘, P = 0.11). Another Bock was artitlcially infected with Trichostrongylus cokbrifomis and significant genetic variation was found in both FEC (heritabiity 0.40 + 0.11) and EOS (0.19 + 0.08). In a tkird flock comprising tropical sheep breeds, a natural challenge with T. colubrijhwtis resulted in significant sire effects on FEC (heritability 0.20 kO.10) but not EOS (heritability inestimable). We conclude that EOS offers no advantage over FEC as a selection criterion for resistance.
Key words: genetic resistance; sheep; immunity; eosinophils.
Long-term experiments have shown that faecal worm egg counts (FEC) can be used as a selection criterion to breed sheep for improved resistance to homologous trichostrongyle infections (Windon, Dineen Kc Wagland, 1987; Woolaston, Barger & Piper, 1990) and to field infections with mixed species (Baker et al., 1991; Vlassoff et al., 1991; Cummins et al., 1991). The development of acquired immunity to Trichostrongylus colubriformisis accompanied by a rise in the number of circulating eosinophils (EOS), with a more pronounced eosinophilia occurring in animals
*To whom correspondence should be addressed E-mail: [email protected]; Fax: 067 761 371.
bred for increased resistance (Dawkins, Windon & Eagleson, 1989; Buddle et al., 1992; Rothwell et al., 1993). Rothwell et al. (1993) and others (Hohenhaus, Outteridge Jr Payne, 1994) also found a negative phenotypic association between EOS and FEC in unselected sheep and suggested that EOS may be a useful trait for identifying resistant sheep. In this paper, we report studies of the value of circulating eosinophils as an indicator of resistance to Huemonchus contortus in 1 flock and T. colubriformisin 2 other flocks. The CSIRO Haemonchw lines have been selected since 1977 for different levels of resistance to H. contortus,using artificial challenge (flock and procedures described by Woolaston et al., 1990). Ewe
123
124
R.
R. Woolaston et al.
lambs for this experiment were chosen at random from progeny born in 1989, 20 representing the 5 sire groups of the increased resistance line (IRH) and 20 representing the 5 sire groups of the decreased resistance line (DRH). Five weeks after artificial challenge with 10,000 L3 H. contortus per head, FEC were determinedusinga modified McMaster methodwith a lower limit of detection of 100epg and EOS were countedin blood samplesusingthe methoddescribed by Dawkins et al. (1989). Transformation was necessaryto remove skewnessand to homogenise residualvariances(cube-root for FEC and logarithmic for EOS). Although untransformed meansare reported throughout, tests of significancerefer to transformeddata. The averageageof the lambsat samplingwas 166 days. The overall mean (+S.D.) FEC was 12,238&-13,820 epg and the mean EOS was 2.40+4.40 x IO4 ml-i of blood. IRH lambs had lower FEC than DRH lambs(6831vs 17,645epg, ttest P~0.01) but the differencein EOS, although of similarmagnitude,was low and variable and did not reachsignificance(3.40x lo4 ml-’ vs 1.40x lo4 ml-‘, P = 0.11). The pooled within-line correlation between transformed FEC and EOS was -0.37, significantly lessthan zero (P-C 0.05). Thus EOS had a negativephenotypic relationshipwith FEC, but the lack of a consistentbetween-linedifference suggests that EOS would be of limited value as a selection criterion to reduceFEC during H. contortus infection. The second flock comprised 1081 mixed sex Merinos (ewesand wethers) sampledfrom 9 fine and 2 medium wool studs. Animals were born and raisedtogether in a common environment at Armidale on the Northern Tablelands of New South Wales, Australia. Faecal samplestaken prior to testing showedthat the flock had been exposedto natural infections with T. colubriformis at pasture.
When testedat about 13months of age, the animals had been allocated at random into 5 separate managementgroups. One week after anthelmintic treatment with ivermectin at the recommendedrate (Ivomec, MSD AGVET), all animalswere given an oral infection of 20,000 L3 T. colubriformis per head, then left to graze relatively uncontaminatedpastures until faecaland blood sampleswerecollected4 weeks post-infection for determination of FEC and EOS. Laboratory methodswere similar to thosedescribed above. Analysis of variance was performed on transformeddata using the method of least-squares (Harvey, 1990),which accountsfor imbalanceamong the various sources of variation. Effects fitted included stud, sire, maternal handicap (twin or single,maidenor mature dam), managementgroup, age (in days, fitted as a covariate) and first order interactions.Any effectsfound to be not significant (defined by P>O.O5) were omitted from the final model. He&abilities and genetic correlations were estimated using restricted maximum likelihood as programmed by Meyer (1989), fitting the effects found to be significantin the analysesof variance. A total of 1072animalswere measuredfor FEC and 1056for EOS, while 1047animalshad data available for both traits. The means (&-SD.) for FEC and EOS were 311+581 epg and 21.5&-19.0~10~ ml-’ of blood, respectively,and the traits had a phenotypic correlation of -0.26. Maternal handicap and interactions had no significanteffect on either trait, but all other effects had a highly significant effect on FEC (PcO.01, Table 1). Wethers had higher FEC than ewes(355 vs 228 epg) and younger sheephad higher FEC than older sheep, decreasingat the rate of 4.3k2.2 epg per day of age. Stud, sireand management group all affectedEOS (P
Table l-Mean squares from analysis of variance showing genetic and significant non-genetic sources of variation for faecal egg counts (cube-root transformed) and circulating eosinophils ( x 104, log,, transformed) in sheep from 2 flocks infected with T. colubrifomis Flock
Source
of variation
Faecal d.f.
Fine and medium
Merinos
Fijian synthetic and Barbados Blackbelly
*P
l *P
Flock Sire Management Sex Age (days) Error
group
Sire Sex/paddock Age (days) Error ***P
ns-not
significant.
egg count Mean square
Eosinophil d.f. 10 63 4
count Mean square
10 63 4 1 1 992
296.99*** 33.00*** 426.58*** 121.24** 93.48** 12.96
1.0274*** 0.2563**+ 2.5160***
978
0.1458
91 1 1 671
9.55* 115.36*** 436.56+++ 7.06
91 1 1 671
0.182P 13.2842*** 5.9738*** 0.2123
125
Research note FEC (and standard error) was 0.40 (0.11) and of EOS was 0.19 (0.08), with a genetic correlation of -0.62 (0.20). These parameters suggest that if the aim of a breeding program is to reduce FEC, then the efficiency of indirect selection on the basis of EOS will be 0.62 x (0.19/0.40)“.5 = 43% relative to direct selection on FEC (Falconer, 1981). The third flock was tested on the Fijian island of Makogai and comprised 671 lambs of a synthetic breed of tropical sheep based on a Wiltshire Barbados Blackbelly cross and 94 pure-bred Barbados Blackbclly lambs. From birth, all sheep were exposed to pastures contaminated with T. colubriformis and H. contortus larvae, with ewes and entire ram lambs running in separate but similar paddocks. Shortly after weaning, all sheep were treated with levamisole (Nilverm, ICI Australia) and 4 weeks later were also treated with closantel (Seponver, SmithKline Animal Health Products) to prevent development of patent H. contortus infections. Both anthelmintic treatments were delivered at the recommended rates. Nine weeks after levamisole treatment, when the sheep were an average age of 144 days (range 80-186 days), faecal samples and blood were taken from all animals for FEC and EOS determination, respectively. Laboratory and statistical methods were as described above, except that effects fitted included breed, sire, sex (confounded with paddock), birth type (single or twin), first order interactions and the linear effect of age (in days). Means (kS.D.) for FEC and EOS were 517+521 epg and 6.14 + 9.62 x lo4 ml-’ of blood, respectively. EOS increased significantly with age (P
mental infections, perhaps due to synchronization of the immune response with stage of infection. During field infections a number of environmental and genetic factors may interact to weaken the association. Given that EOS appears to be less heritable than FEC, is only informative under certain conditions (see also Rothwell et al., 1993 and Hohenhaus et al., 1994) and is no simpler to measure, we conclude that EOS offers no advantage over FEC as a selection criterion for resistance. AcknowledgementsWork with Merinos was supported by Australian woolgrowers through the International Wool Secretariat and studies in Fiji were supported by the Australian Centre for International Agricultural Research. Mr R. L. Elwin, MS J. L. Ward, Mr P. N. TyIer and staff of the Makogai Laboratory
Research are thanked
Station and Koronivia Veterinary for their technical help.
REFERENCES Baker R. L., Watson R. L., Bisset S. A., Vlassoff A. & Douch P. G. C. 1991. Breeding sheep in New Zealand for resistance to internal parasites: research results and commercial application. In: Breeding for Disease Resistance in Sheep (Edited by Gray G. D. & Woolaston R. R.), pp. 19-32. Australian Wool Corporation, Melbourne. Buddle B. M., Jowett G., Green R. S., Douch P. G. C. & Ridson P. L. 1992. Association of blood eosinophilia with the expression of resistance in Romney lambs to nematodes. International Journal for Parasitology 22: 955-960. Cummins L. J., Thompson R. L., Yong W. K. RitIkin G. G., Goddard M. E., Callinan A. P. L. & Saunders M. J. 1991. Genetics of Gstertagia selection lines. In: Breeding for Disease Resistance in Sheep (Edited by Gray G. D. & Woolaston R. R.), pp. 11-18. Australian Wool Corporation, Melbourne. Dawkins H. J. S., Windon R. G. & Eagleson G. K. 1989. Eosinophil responses in sheep selected for high and low responsiveness to Trichostrongylus colubrtformis. International Journal for Parasitology 19: 199-205. Falconer D. S. 1981. Introduction to Quantitative Genetics, p.320. 2nd Edition, Longman, Edinburgh. Harvey W. R. 1990. User’s Guide for LSMLMW and MIXMDL: PC-2 version. Mixed Model Least-squares and Maximum Likelihood Computer Program. Hohenhaus M. A., Outteridge P. M. & Payne G. 1994. An investigation of immunogenetic tests for internal parasite resistance in sheep and their association with production characters. Proceedings of the Australian Society of Animal Production 20: 386. Meyer K. 1989. Restricted maximum likelihood to estimate variance components for animal models with several random effects using a derivative-free algorithm. Genetics, Selection, Evolution 21: 317-340. Rothwell T. L. W., Windon R. G., Horsburgh B. A. & Anderson B. H. 1993. Relationship between eosinophilia and responsiveness to infection with Trichostrongylus
126 colubriformis in sitology 23: 203-2
R. R. Woolaston et al.
sheep. International Journal for Para11. Windon R. G. Dineen J. K. & Wagland B. M. 1987. Genetic control of immunological responsiveness against the intestinal nematode Trichostrongylus colubriformis in lambs. In: Merino Improvement Programs in Australia
(Edited by McGuirk B. J.), pp. 371-375. Australian Wool Corporation, Melbourne. Woolaston R. R., Barger I. A. & Piper L. R. 1990. Response to helminth infection of sheep selected for resistance to Haemonchus sitology 20:
contortus.
1015-1018.
International
Journal
for
Para-
JournalforParasitology,
Pergamon
0020-7519(95)00105-0
RESEARCH
Vol. 26. No. 1, pp. 123126, 1996 Australian Society for Parasitology Ekvier Science LKI Printed in Great Britain 002&7519/96 S15.00 + 0.00
NOTE
The Value of Circulating Eosinophil Count as a Selection Criterion for Resistance of Sheep to Trichostrongyle Parasites R. R. WOOLASTON,*t P. MANUELI,$ S. J. EADY,? I. A. BARGER,§ L. F. LE JAMBRE,§ D. J. D. BANKS7 and R. G. WINDONS TCSIRO Division of Animal Production, PMB Armidale, NSW 2350, Australia $Ministry of Primary Industries, Box 358, Suva, Fgi $CSIRO Division of Animal Health, Ph4B Armidale, NSW 2350, Australia IDept of Primary Industries & Energy, GPO Box 858 Canberra, ACT 2601, Australia (Received IO January 1995; accepted 4 September 1995)
Abstract-Woolaston R. R., Manueli P., Eady S. J., Barger I. A., Le Jambre L. F., Banks D. J. D. & Windon R. G. 1996. The value of circulating eosinopbil count as a selection criterion for resistance of sheep to trichostrongyle parasites. Znternutionaf Joumdfor Purasitology 26: 123426. In Merino sheep bred for either increased or decreased resistance to Hoemonchus coorortus, faecal worm egg counts @EC) were lower in the resistant line (6,831 vs 17,645 epg, PcO.Ol), and circulating eosinopkils (EOS) were higher, but not signithntly so (3.40 x 10’ III-’ vs 1.40 x 104 ml-‘, P = 0.11). Another Bock was artitlcially infected with Trichostrongylus cokbrifomis and significant genetic variation was found in both FEC (heritabiity 0.40 + 0.11) and EOS (0.19 + 0.08). In a tkird flock comprising tropical sheep breeds, a natural challenge with T. colubrijhwtis resulted in significant sire effects on FEC (heritability 0.20 kO.10) but not EOS (heritability inestimable). We conclude that EOS offers no advantage over FEC as a selection criterion for resistance.
Key words: genetic resistance; sheep; immunity; eosinophils.
Long-term experiments have shown that faecal worm egg counts (FEC) can be used as a selection criterion to breed sheep for improved resistance to homologous trichostrongyle infections (Windon, Dineen Kc Wagland, 1987; Woolaston, Barger & Piper, 1990) and to field infections with mixed species (Baker et al., 1991; Vlassoff et al., 1991; Cummins et al., 1991). The development of acquired immunity to Trichostrongylus colubriformisis accompanied by a rise in the number of circulating eosinophils (EOS), with a more pronounced eosinophilia occurring in animals
*To whom correspondence should be addressed E-mail: [email protected]; Fax: 067 761 371.
bred for increased resistance (Dawkins, Windon & Eagleson, 1989; Buddle et al., 1992; Rothwell et al., 1993). Rothwell et al. (1993) and others (Hohenhaus, Outteridge Jr Payne, 1994) also found a negative phenotypic association between EOS and FEC in unselected sheep and suggested that EOS may be a useful trait for identifying resistant sheep. In this paper, we report studies of the value of circulating eosinophils as an indicator of resistance to Huemonchus contortus in 1 flock and T. colubriformisin 2 other flocks. The CSIRO Haemonchw lines have been selected since 1977 for different levels of resistance to H. contortus,using artificial challenge (flock and procedures described by Woolaston et al., 1990). Ewe
123
124
R.
R. Woolaston et al.
lambs for this experiment were chosen at random from progeny born in 1989, 20 representing the 5 sire groups of the increased resistance line (IRH) and 20 representing the 5 sire groups of the decreased resistance line (DRH). Five weeks after artificial challenge with 10,000 L3 H. contortus per head, FEC were determinedusinga modified McMaster methodwith a lower limit of detection of 100epg and EOS were countedin blood samplesusingthe methoddescribed by Dawkins et al. (1989). Transformation was necessaryto remove skewnessand to homogenise residualvariances(cube-root for FEC and logarithmic for EOS). Although untransformed meansare reported throughout, tests of significancerefer to transformeddata. The averageageof the lambsat samplingwas 166 days. The overall mean (+S.D.) FEC was 12,238&-13,820 epg and the mean EOS was 2.40+4.40 x IO4 ml-i of blood. IRH lambs had lower FEC than DRH lambs(6831vs 17,645epg, ttest P~0.01) but the differencein EOS, although of similarmagnitude,was low and variable and did not reachsignificance(3.40x lo4 ml-’ vs 1.40x lo4 ml-‘, P = 0.11). The pooled within-line correlation between transformed FEC and EOS was -0.37, significantly lessthan zero (P-C 0.05). Thus EOS had a negativephenotypic relationshipwith FEC, but the lack of a consistentbetween-linedifference suggests that EOS would be of limited value as a selection criterion to reduceFEC during H. contortus infection. The second flock comprised 1081 mixed sex Merinos (ewesand wethers) sampledfrom 9 fine and 2 medium wool studs. Animals were born and raisedtogether in a common environment at Armidale on the Northern Tablelands of New South Wales, Australia. Faecal samplestaken prior to testing showedthat the flock had been exposedto natural infections with T. colubriformis at pasture.
When testedat about 13months of age, the animals had been allocated at random into 5 separate managementgroups. One week after anthelmintic treatment with ivermectin at the recommendedrate (Ivomec, MSD AGVET), all animalswere given an oral infection of 20,000 L3 T. colubriformis per head, then left to graze relatively uncontaminatedpastures until faecaland blood sampleswerecollected4 weeks post-infection for determination of FEC and EOS. Laboratory methodswere similar to thosedescribed above. Analysis of variance was performed on transformeddata using the method of least-squares (Harvey, 1990),which accountsfor imbalanceamong the various sources of variation. Effects fitted included stud, sire, maternal handicap (twin or single,maidenor mature dam), managementgroup, age (in days, fitted as a covariate) and first order interactions.Any effectsfound to be not significant (defined by P>O.O5) were omitted from the final model. He&abilities and genetic correlations were estimated using restricted maximum likelihood as programmed by Meyer (1989), fitting the effects found to be significantin the analysesof variance. A total of 1072animalswere measuredfor FEC and 1056for EOS, while 1047animalshad data available for both traits. The means (&-SD.) for FEC and EOS were 311+581 epg and 21.5&-19.0~10~ ml-’ of blood, respectively,and the traits had a phenotypic correlation of -0.26. Maternal handicap and interactions had no significanteffect on either trait, but all other effects had a highly significant effect on FEC (PcO.01, Table 1). Wethers had higher FEC than ewes(355 vs 228 epg) and younger sheephad higher FEC than older sheep, decreasingat the rate of 4.3k2.2 epg per day of age. Stud, sireand management group all affectedEOS (P
Table l-Mean squares from analysis of variance showing genetic and significant non-genetic sources of variation for faecal egg counts (cube-root transformed) and circulating eosinophils ( x 104, log,, transformed) in sheep from 2 flocks infected with T. colubrifomis Flock
Source
of variation
Faecal d.f.
Fine and medium
Merinos
Fijian synthetic and Barbados Blackbelly
*P
l *P
Flock Sire Management Sex Age (days) Error
group
Sire Sex/paddock Age (days) Error ***P
ns-not
significant.
egg count Mean square
Eosinophil d.f. 10 63 4
count Mean square
10 63 4 1 1 992
296.99*** 33.00*** 426.58*** 121.24** 93.48** 12.96
1.0274*** 0.2563**+ 2.5160***
978
0.1458
91 1 1 671
9.55* 115.36*** 436.56+++ 7.06
91 1 1 671
0.182P 13.2842*** 5.9738*** 0.2123
125
Research note FEC (and standard error) was 0.40 (0.11) and of EOS was 0.19 (0.08), with a genetic correlation of -0.62 (0.20). These parameters suggest that if the aim of a breeding program is to reduce FEC, then the efficiency of indirect selection on the basis of EOS will be 0.62 x (0.19/0.40)“.5 = 43% relative to direct selection on FEC (Falconer, 1981). The third flock was tested on the Fijian island of Makogai and comprised 671 lambs of a synthetic breed of tropical sheep based on a Wiltshire Barbados Blackbelly cross and 94 pure-bred Barbados Blackbclly lambs. From birth, all sheep were exposed to pastures contaminated with T. colubriformis and H. contortus larvae, with ewes and entire ram lambs running in separate but similar paddocks. Shortly after weaning, all sheep were treated with levamisole (Nilverm, ICI Australia) and 4 weeks later were also treated with closantel (Seponver, SmithKline Animal Health Products) to prevent development of patent H. contortus infections. Both anthelmintic treatments were delivered at the recommended rates. Nine weeks after levamisole treatment, when the sheep were an average age of 144 days (range 80-186 days), faecal samples and blood were taken from all animals for FEC and EOS determination, respectively. Laboratory and statistical methods were as described above, except that effects fitted included breed, sire, sex (confounded with paddock), birth type (single or twin), first order interactions and the linear effect of age (in days). Means (kS.D.) for FEC and EOS were 517+521 epg and 6.14 + 9.62 x lo4 ml-’ of blood, respectively. EOS increased significantly with age (P
mental infections, perhaps due to synchronization of the immune response with stage of infection. During field infections a number of environmental and genetic factors may interact to weaken the association. Given that EOS appears to be less heritable than FEC, is only informative under certain conditions (see also Rothwell et al., 1993 and Hohenhaus et al., 1994) and is no simpler to measure, we conclude that EOS offers no advantage over FEC as a selection criterion for resistance. AcknowledgementsWork with Merinos was supported by Australian woolgrowers through the International Wool Secretariat and studies in Fiji were supported by the Australian Centre for International Agricultural Research. Mr R. L. Elwin, MS J. L. Ward, Mr P. N. TyIer and staff of the Makogai Laboratory
Research are thanked
Station and Koronivia Veterinary for their technical help.
REFERENCES Baker R. L., Watson R. L., Bisset S. A., Vlassoff A. & Douch P. G. C. 1991. Breeding sheep in New Zealand for resistance to internal parasites: research results and commercial application. In: Breeding for Disease Resistance in Sheep (Edited by Gray G. D. & Woolaston R. R.), pp. 19-32. Australian Wool Corporation, Melbourne. Buddle B. M., Jowett G., Green R. S., Douch P. G. C. & Ridson P. L. 1992. Association of blood eosinophilia with the expression of resistance in Romney lambs to nematodes. International Journal for Parasitology 22: 955-960. Cummins L. J., Thompson R. L., Yong W. K. RitIkin G. G., Goddard M. E., Callinan A. P. L. & Saunders M. J. 1991. Genetics of Gstertagia selection lines. In: Breeding for Disease Resistance in Sheep (Edited by Gray G. D. & Woolaston R. R.), pp. 11-18. Australian Wool Corporation, Melbourne. Dawkins H. J. S., Windon R. G. & Eagleson G. K. 1989. Eosinophil responses in sheep selected for high and low responsiveness to Trichostrongylus colubrtformis. International Journal for Parasitology 19: 199-205. Falconer D. S. 1981. Introduction to Quantitative Genetics, p.320. 2nd Edition, Longman, Edinburgh. Harvey W. R. 1990. User’s Guide for LSMLMW and MIXMDL: PC-2 version. Mixed Model Least-squares and Maximum Likelihood Computer Program. Hohenhaus M. A., Outteridge P. M. & Payne G. 1994. An investigation of immunogenetic tests for internal parasite resistance in sheep and their association with production characters. Proceedings of the Australian Society of Animal Production 20: 386. Meyer K. 1989. Restricted maximum likelihood to estimate variance components for animal models with several random effects using a derivative-free algorithm. Genetics, Selection, Evolution 21: 317-340. Rothwell T. L. W., Windon R. G., Horsburgh B. A. & Anderson B. H. 1993. Relationship between eosinophilia and responsiveness to infection with Trichostrongylus
126 colubriformis in sitology 23: 203-2
R. R. Woolaston et al.
sheep. International Journal for Para11. Windon R. G. Dineen J. K. & Wagland B. M. 1987. Genetic control of immunological responsiveness against the intestinal nematode Trichostrongylus colubriformis in lambs. In: Merino Improvement Programs in Australia
(Edited by McGuirk B. J.), pp. 371-375. Australian Wool Corporation, Melbourne. Woolaston R. R., Barger I. A. & Piper L. R. 1990. Response to helminth infection of sheep selected for resistance to Haemonchus sitology 20:
contortus.
1015-1018.
International
Journal
for
Para-