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Resistance of exotic and domestic lambs to experimental infection with Haemonchus contortus
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RESISTANCE OF EXOTIC AND DOMESTIC LAMBS TO EXPERIMENTAL INFECTION WITH HAEMONCHUS CONTOR TUS C. H. COURTNEY,*$
C. F. PARKfiR,j-§
K. E. MCCLURE?
and R. P. HERD*
*Department of Veterinary Pathobiology, The Ohio State University, 1925 Coffey Road, Columbus, OH 43210, U.S.A. j-Department of Animal Science, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, OH 44691, U.S.A. (Received 2 March 1984; in revisedform 19 June 1984) Abstract-COURTNEY C. H., PARKERC. F., MCCLUREK. E. and HBRDR. P. 1985. Resistance of exotic and domestic lambs to experimental infection with Haemonchus contortus. International Journal for Purusifology 15: 101-109. Resistance to primary and secondary infections with H. contorfus was compared in St. Croix, 3/4 St. Croix, Florida Native, Barbados Blackbelly and domestic crossbred lambs. On the basis of worm counts, fecal egg counts and packed cell volumes, St. Croix and 314 St. Croix lambs were the most resistant to secondary infections, Florida Native and Barbados lambs were intermediate in resistance, and the domestic lambs were the least resistant. Breed differences were highly variable during primary infection. Breed differences were most pronounced prior to puberty, with domestic Iambs highly susceptible to secondary infections. After puberty, domestic lambs were better able to resist secondary infections. Age apparently had little or no effect on the parasite resistance of exotic breeds. Sex differences were significant only after puberty when ewe lambs were more resistant to secondary infection than ram lambs. In one of three experiments lambs of hemoglobin genotype AB were more resistant to secondary infection than lambs of genotype BB in terms of fecal egg counts, packed cell volumes and the proportion of ‘responder’ and ‘nonresponder’ lambs but not in terms of worm counts, although subtle differences may not have been detected because of small numbers of lambs.
INDEX KEY WORDS: Huemonchus contorfus; genetic resistance; hemoglobin genotype; Florida Native sheep; St. Croix sheep; Barbados Blackbelly sheep.
INTRODUCTION AN INCREASEDability to resist natural infections of H. contortus was demonstrated in certain individuals
than Dorset lambs (Altaif & Dargie, 1978b). Australian workers have shown that individual Merino lambs differed in their ability to respond to immunization with irradiated Lj of Trichostrongylus colubriformis and that these differences were heritable (Dineen, Gregg & Lascelles, 1978; Dineen & Windon, 1980a, b; Windon, Dineen & Kelly, 1980; Windon & Dineen, 1981). The successful demonstration of a repeatable model of acquired resistance to H. contortus in young lambs would be invaluable as an aid to the study of the immunology of age-related responsiveness in lambs as well as to vaccine development and selective breeding for increased parasite resistance. This paper reports a study of the influence of breed, age, sex and hemoglobin genotype on the ability of lambs to resist reinfection with H. contortus following primary sensitization with the same parasite. Emphasis is placed on three exotic breeds originating in the humid tropics and subtropics (Florida Native, Barbados Blackbelly and St. Croix) where natural selection would be expected to favor lambs that acquired resistance to gastrointestinal helminths at
within a flock of Dorset sheep and this increase was heritable (Whitlock, 1955). Other workers have found Florida Native sheep to be more resistant than conventional breeds (Suffolk or Rambouillet) in both natural and experimental infections with H. contortus (Loggins, Swanson & Koger, 1965; Jilek & Bradley, 1969; Radhakrishnan, Bradley & Loggins, 1972; Bradley, Rahhakrishnan, Patil-Kulkarni, & Loggins, 1973). After a primary infection with normal larvae of H. contortus, Florida Native lambs were more resistant to reinfection than Rambouillet lambs (Bradley et al., 1973). Likewise, Scottish Blackface lambs were found to be more resistant to reinfection *Present address for correspondence: Department of Preventive Medicine, College of Veterinary Medicine, Box J-136 JHMHC, University of Florida, Gainesville, FL 32610, U.S.A. #Present address: U.S. Sheep Experiment Station, Dubois, ID 83423, U.S.A. 101
102
C. H. COURTNEY, C. F. PARKER, K. E. McC~u~~and
an early age. A description in an Herd,
earlier 1984).
paper
of these breeds was given (Courtney, Parker, McClure &
MATERIALS
AND METHODS
Experimental design. Breed differences
in the susceptibility of exotic and domestic lambs to experimental infections with H. contortus were studied in three experiments involving a total of 144 lambs. In Expt. 1 breed differences in responses to both primary and secondary infections were compared. In Expt. 2 breed, sex and hemoglobin genotype differences in resistance to secondary infections were compared in lambs prior to puberty (4 months of age) whereas in Expt. 3 these variables were compared in lambs after sexual maturity (9 months of age). Experiment 1. Ten St. Croix, 10 Barbados Blackbelly, 10 Florida Native and 10 domestic crossbred 1980-born male lambs were used. The lambs, aged 6-16 weeks at the start of the experiment, were given a primary infection of 6000 L3 of H. contortus at week 0. At week 6, five lambs of each breed were necropsied and the primary infections were removed from the remaining lambs with a drench of 10 mg kg-l levamisole. At week 7 they were challenged with 12,000 L3 and necropsied at week 12. Blood and fecal samples were collected at weeks 0 and 3-12. Experiment 2. Eleven Barbados Blackbelly, nine St. Croix, nine three quarter St. Croix, 10 Florida Native and 14 domestic crossbred 1981-born lambs were used. The lambs were aged 14-18 weeks at the start of the experiment. Female lambs were used in all breed groups except Barbados Blackbelly. Lambs were given a primary infection of 6000 L3 of H. contortus at week 0, drenched with 10 mg kg-l levamisole at week 5, challenged at week 6 with 12,000 L3 and necropsied at week 11. Five of the domestic lambs, designated challenge controls, were not given the initial primary infection, but were treated with levamisole and given a primary infection of 12,000 L3 when the remaining lambs received their challenge infections. Blood and fecal samples were collected from all lambs at weeks 0 and 3-l 1. Experiment 3. Six Barbados Blackbelly, 10 St. Crotx, nine three quarter St. Croix, 10 Florida Native and 17 domestic crossbred 1981-born lambs of both sexes were used. Lambs were 36-40 weeks of age at the start of the experiment. Lambs were given a primary infection of 10,000 L3 at week 0, drenched with levamisole (10 mg kg-l) at week 5, challenged with 16,OtXl L3 at week 6 and necropsied at week 11. Five domestic lambs served as
challenge controls. Blood and fecal samples were collected from all lambs at weeks 0 and 3-11. Sheep and management. The domestic crossbred lambs were half Suffolk, quarter Finn, quarter Dorset (experiment 1) and half Suffolk or Dorset, quarter Finn, quarter Rambouillet (experiments 2 and 3). Three quarter St. Croix lambs were quarter Dorset or Rambouillet. All lambs were reared in confinement on expanded metal flooring free of parasites other than coccidia and Strongyloides papillosus. They were kept in the same trichostrongylid free environment throughout the study. Lambs were fed a standard diet of corn silage and 15% protein supplement except just prior to the start of Expt. 3 when the diet was changed to corn stover and the amount of protein supplement reduced since the lambs were becoming overfat. Due to miscalculation this changed diet was inadequate for maintenance of lambs and caused considerable loss of condition before the error was detected during week 5 and the diet corrected.
R. P. HERD
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Parasitological techniques. The local Wooster strain of H. contortus used in all experiments was isolated in 1979. Live, gravid female H. contortus, removed from a sheep at necropsy, were lightly ground in a Ten Broeck tissue grinder to release ova. Ova were cultured in nematode free sheep feces and the resulting 200 L3 were given to a worm free donor lamb. After patency, feces from the donor lamb were collected and incubated in shallow trays at a depth of l-2 cm for lo-14 days at 27°C. Cultures were stirred daily to reduce fungal overgrowth. Larvae were recovered from feces by Baermannization through a 150 pm screen and prior to use were stored for 2-8 weeks at 4°C in tissue culture flasks containing a 2-4 mm layer of water. This isolate was subsequently maintained in the laboratory by passage in worm free donor lambs. Experimental lambs were infected by oral drenching (primary infection of Expt. 1) or by peroral intubation of the rumen (remaining infections). Fecal egg counts were made by the modified McMaster technique (Whitlock, 1948). At necropsy worm counts were made from 10% aliquots of abomasal contents. The abomasal mucosa was scraped from the muscularis and digested in 200 ml of 1% HCI at 37°C for 3 h. After digestion was stopped by the addition of 10% formalin, 10% aliquots were examined for worms. Hematology. Packed cell volumes (PCVs) were determined by the microhematocrit technique (Schalm, Jain & Carroll, 1975). Hemoglobin genotypes were determined by electrophoresis of hemoglobin extracted from week 0 blood samples (Gebott &Peck, 1978). Statistical analysis. Data were analyzed by analysis of variance. Worm count data were transformed by log lo (X+ 1) and fecal egg count data by square roots to normalize data. Multiple comparisons were made by Duncan’s New Multiple Range Test. Lambs from Expt. 2 were declared “responders” if their adult worm burdens were less than the lower limit of the 99% confidence interval of the geometric mean adult worm burden of the challenge control lambs. If counts were greater than this limit, lambs were declared “nonresponders” (Dineen et a/., 1978). Proportions of responders and nonresponders were compared between breeds, sexes and hemoglobin genotypes using Fisher’s Exact Test. Differences were considered significant when p < 0.05.
RESULTS
Unless a statement is made to the contrary, any differences reported were significant at p < 0.05. In all experiments, both fecal egg counts and PCVs changed with time. As a general pattern fecal egg counts increased after primary infection, fell to zero after levamisole treatment and rose again after reinfection. PCVs generally fell after primary infection, rose after levamisole treatment and fell again after reinfection. There were, however, some interactions with the effects of breed, sex and hemoglobin genotype, so comparisons of these factors must be made at each sample date. In all experiments, very few larval worms were recovered at necropsy, so data on adult worms only are reported herein. After storage at 4”C, the Wooster strain of H. contortus does not undergo a significant degree of arrested development in experimental infections.
Resistance of exotic and domestic lambs to
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25 60 1 50 l
40 x w I
30
20
IO
WEEK FIG. 1. Effect of breed on egg count, Expt. 1. A-A,
Native; 0 -
mean PCV and square root fecal Barbados; A- - -A, Florida 0, St. Croix; 0 - - - l , domestic.
Expt. 1. Peak fecal egg counts during primary infection (week 4) were higher in Barbados than in St. Croix and Florida Native lambs (Fig. 1). Egg counts of domestic lambs were intermediate and not significantly different from any other breed. After reinfection fecal egg counts were greater in domestic lambs than in the exotic breeds at weeks 10, 11 and 12, and counts from St. Croix lambs were lower than Florida Native and Barbados lambs at weeks 11 and 12. There was no significant effect of hemoglobin genotypes AB and BB on fecal egg counts (Fig. 2). Hemoglobin genotype AA was excluded from analysis because only four genotype AA lambs were present and all were necropsied after primary infection. At week 4, St. Croix lambs had higher PCVs than the three other breeds, and at week 6 the domestic lambs had significantly lower PCVs than the exotic breeds (Fig. 1). At week 10 St. Croix and Florida Native lambs had higher PCVs than Barbados and domestic lambs. Hemoglobin genotypes AB and BB had no significant effect on PCV (Fig. 2). After primary infection St. Croix lambs had markedly fewer adult H. contortus than the other breeds but differences were not significant (Table 1). After secondary infection, St. Croix lambs had significantly fewer worms than all other breeds, and the domestic lambs had more worms than all other breeds. There was no effect of hemoglobin genotypes AB and BB on worm counts. Experiment 2. At week 3 the St. Croix lambs shed more ova than the other lambs, and at week 4 the St.
H. contortus
103
Croix and Barbados lambs shed more ova than the three quarter St. Croix lambs (Fig. 3). At weeks 9, 10 and 11 (3, 4 and 5 weeks after reinfection) the domestic and challenge control lambs shed more ova than the exotic lambs except that at week 11 their fecal egg counts were not significantly greater than the Barbados lambs. At week 11 the Barbados lambs had greater fecal egg counts than the St. Croix lambs. There were lower mean fecal egg counts at weeks 9, 10 and 11 in lambs of hemoglobin genotype AB than BB (Fig. 4). Genotype AA occurred only in Barbados and Florida Native sheep and was therefore not included in analysis. There were no significant differences in fecal egg counts associated with lamb sex (Fig. 5). No female Barbados lambs were available, so this breed was excluded from analysis of sex effects. The challenge control group was excluded from analysis of the effects of both hemoglobin genotype and sex. After reinfection, PCVs were higher in the exotic breeds than in the domestic lambs or challenge controls at weeks 9 and 10 (Fig. 3). At week 11 the PCVs of the exotic lambs were higher than the domestic lambs, but only the Barbados and St. Croix had higher PCVs than the challenge controls. Prior to infection, PCVs of lambs or hemoglobin genotype AB were higher than those of genotype BB (Fig. 4). Challenge control lambs and lambs of genotype AA were excluded from analysis. At weeks 6-l 1 genotype AB lambs again had higher
40
1
35> 2
30-
25 60
L 8 9
WEEK
IO II
12
FIG. 2. Effect of hemoglobin genotype on mean PCV and square root fecal egg count, Expt. 1. A-A, AA; 0 -0, AB; O---o, BB.
C. H. COURTNEY, C. F. PARKER, K. E. MCCLURE and R. P. HERD
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TABLE I-MEAN H. contortus RECOVERY BY BREED AND BY HEMOGLOBINGENOTYPE, EXPT. 1. DATA TRANSFORMED BY (x+ 1) FOR CALCoLATlON OF TRANSFORMEDMEANAND STANDARD DEVIATION. VALUES WITHIN A COLUMN WITH DIFFERENT
log
SUPERSCRIPTS
Primary
WERE
SIGNIFICANTLY
DIFFERENT
@
infection
< 0.05)
Challenge
infection
Group No. lambs
Arithmetic mean
Transformed mean
S.D.
No. lambs
Arithmetic mean
Transformed mean
S.D.
Breed St. Croix Florida Native Barbados Domestic
5 5 5 5
12 178 326 284
0.77 1.54 1.99 2.35
0.72 1.04 1.16 0.34
5 5 5 5
4 106 258 3694
0.3oa l.lsb 1.78b 3.46c
0.59 0.95 1.13 0.30
Hemoglobin AA AB BB
4 7 9
295 196 161
2.25 1.40 1.61
0.51 1.15 1.03
9 11
1461 651
1.79 1.56
1.53 1.33
Genotype
were found. There were no “responders” among the domestic lambs and no “nonresponders” among the St. Croix lambs, whereas Florida Native, Barbados, and three quarter St. Croix lambs were a mixture of “responders” and “nonresponders”. When analyzed by Fisher’s exact test, St. Croix and three quarter St. Croix lambs were found to have proportions of “responders” and “nonresponders” different from the domestic lambs. Hemoglobin genotype AB lambs had a greater proportion of “responders” than genotype BB lambs, but there were no significant differences in worm burdens. Genotype AA lambs occurred only in Florida Native and Barbados lambs
WEEK FIG. 3. Effect of breed on mean PCV and square root fecal egg count, Expt. 2. A- - -A, Barbados; A- - -A, Florida Native; 0 O, St. Croix; 0 0, 3/4 St. Croix; ‘O-O , domestic; n - - -II ) challenge control.
PCVs than genotype BB lambs. There were no significant sex differences in PCV; the apparent sex difference seen in Fig. 5 was caused by a difference in the distribution of sexes within breeds. St. Croix lambs had fewer worms than all other breeds and three quarter St. Croix lambs had fewer worms than all breeds except St. Croix (Table 2). No significant differences in worm burdens of Florida Native, Barbados, domestic, and challenge control lambs
WEEK TIC;. 4. Effect of hemoglobin genotype on mean PCV and square root fecal egg count, Expt. 2.6-A. AA; O--c1 , AB; 0 --, BB.
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Resistance of exotic and domestic lambs to H. contortus
40 1
25 90
3 4
5 6 7 8 9 IO II
WEEK FIG. 5.
Effect of sex on mean PCV and square root fecal egg count, Expt. 2. A-A, male; o -0 , female.
and were therefore
excluded from a general analysis. Within these two breeds, worm burdens in genotype AA lambs were not significantly different from those
of type AB or BB lambs. Neither worm burden nor proportion of “responders” to “nonresponders” was influenced by lamb sex. Experiment 3. During the primary infection, mean fecal egg counts at weeks 3,4 and 5 were higher in the Florida Native and domestic lambs than in the St. Croix and three quarter St. Croix lambs (Fig. 6).
WEEK
Rti. 6. Effect of breed on mean PCV and square root fecal egg count, Expt. 3. A-A, Barbados; A- - -A, Florida Native; 0 0, St. Croix; 0 - - - 0, 3/4 St. Croix; 0 0 , domestic; l - - - l , challenge control.
TABLE ~--RECOVERY OF H, contortus ADULTS BY BREED, SEX, AND HEMOGLOBIN GENOTYPE, EXPT. 2. DATA TRANSFORMED BY lOg(X+l)FOR TRANSFORMED MEAN AND STANDARD DEVIATION. !UiALYSIS OF HEMOGLOBIN GENOTYPEEXCLUDESCHALLENGECONTROLS;ANALYSISOFSE~EXCLUDESBARBADOSANDCHALLENGECONTROLS. ~~UESINACOLUMNWITHDIFFERENTSUPERSCRIPTSWERESIGNIFICANTLYDIFFERENTO] < 0.05)
Group
No. lambs
Breed Challenge control Domestic Barbados Florida Native 3/4 St. Croix St. Croix
5 8 11 10 9 9
Hemoglobin genotype AA AB BB Sex Male Female
ResponderUNonresponders
Arithmetic mean
Transformed mean
S.D.
web
3970 5238 1809 1625 1044 11
3.57a 3.71a 305a 3.01 a 2.16” 0.22c
0.18 0.12 0947 0~48 1.36 0.67
6 20 21
313 18/2a 8/13b
1808 590 3090
3.09 1.85 2.82
0.50 1.32 1.43
19 17
13/6 IO/7
1916 1838
2.33 2.17
1.51 1.56
0/8a 6/5a,b 713a.b 7/2b
C. H. COURTNEY, C. F. PARKER,K. E. MCCLURE and R. P.
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present and was excluded from analysis. Male lambs more ova than female lambs during weeks 3-5 and 9-11 (Fig. 7). PCVs decreased sharply in the challenge control group after week 4 (Fig. 6) even though they were not infected at week 0 and no ova were found in their feces until 3 weeks after challenge infections were given. At weeks 9, 10 and 11 St. Croix and three quarter St. Croix lambs had higher PCVs than the Florida Native, domestic and challenge control lambs. There was no significant effect of hemoglobin genotypes AB and BB on PCV, but PCVs were higher in female lambs than male lambs (Fig. 7). Breed and hemoglobin genotype had no effect upon worm burdens, but there were more worms in male than female lambs (Table 3). There was a trend towards the same pattern of breed differences in worm burdens seen in Expt. 2; however, larger variances in Expt. 3 prevented statistically significant differences given the present breed sample sizes. Likewise, the greater variance in the challenge control group precluded the designation of “responders” and “nanresponders” in this experiment. shed
25 60
a@-
P30w
20I o-
WEEK FIG. 7.
Effect of hemoglobin genotype and sex on mean PCV and square root fecal egg count, Expt. 3. A- - -A, AB; l - - -0, BB; A-A, male; 0 cl, female.
After reinfection, the challenge control, domestic, and Florida Native lambs shed more ova at weeks 9, 10 and 11 than the St. Croix and three quarter St. Croix lambs. There was no significant effect of hemoglobin genotypes AB or BB on fecal egg counts (Fig. 7), but only one animal of genotype AA was
DISCUSSION When 4-6-month-old lambs were infected with H. contortus, treated with levamisole after 5 or 6 weeks and reinfected, St. Croix lambs were highly resistant to reinfection, whereas domestic lambs were uniformly susceptible. Three quarter St. Croix, Florida Native, and Barbados Blackbelly lambs were intermediate in resistance. In Expt. 3, Florida Native lambs were apparently undernourished. They had lower PCVs and higher fecal egg counts than Barbados lambs but had fewer worms at necropsy. One can therefore rank the resistance of breeds in decreasing order as St. Croix, three quarter St. Croix,
TABLE 3--MEAN RECOYERY OF H. CO~iO~~~~ ADULTS BY BREED,SEX HEM~GLOBINGENOTYPE,EXPT.~.ANALYS~SOFHEMOOLOBINOENOTYPEAND SEXEXCLUDESCHALLENGE CONTROLS.VALUESINACOLUMNWlTHDIFFERENT SUPERSCRIPTSWERESIGNIFICANTLYDIFFERENT@< 0.05)
AND
No. lambs
Arithmetic mean
Transformed mean
S.D.
5 12 5 10 9 10
3870 2319 2960 1800 256 480
3.45 2.46 2.87 2.49 1.39 1.86
0.42 1.56 0.78 1.40 1.35 1.33
Hemoglobin genotype AA AB BB
1 25 20
5150 1514 1272
3.71 1.78 2.59
1.57 1.01
Sex Male Female
26 20
2296 438
2.96a l.lsb
0.8t 1.36
Group Breed Challenge controls Domestic Barbados Florida Native 3/4 St. Croix St. Croix
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Resistance of exotic and domestic lambs to H. contortus
Florida Native, Barbados, and domestic. After puberty it appeared that the female domestic lambs became nearly as resistant as the female exotic lambs, and male lambs were generally less resistant than female lambs. During primary infection, breed differences were more variable and not as pronounced as after reinfection. This suggests that breed differences after reinfection were due, at least in part, to differences in acquired immunity. Wakelin (1978) proposed that genetically determined resistance to H. contortus consisted of both immunological and non-immunological components and that a strong immunological component acted at the level of parasite establishment. Larvae of H. contortus apparently fail to reach their predilection site in the gastric pits of immune sheep, and over 90% are expelled within 48 h of infection (Miller, Jackson, Newlands, & Appleyard, 1983). In the present study, larger sample sizes and the use of a challenge control group for each breed would have given stronger evidence for or against acquired immunity as the mechanism involved in breed resistance, but sufficient lambs were not available. The experiments were further confounded by two other problems-low establishment of worms in the primary infection of Expt. 1 and a nutritional problem in Expt. 3. In Expt. 1 the 5% worm recovery seen in domestic lambs after primary infection was much less than the 25-40% recovery in later infections. This infection was given by oral drench rather than by a stomach tube, and larvae may have entered the abomasum directly via the esophageal groove reflex. Since H. contortus larvae normally exsheath in the rumen (Rogers, 1962) this bypass may have reduced their establishment rate. In spite of the low establishment rate, higher fecal egg counts and lower PCVs were seen in the exotic breeds given the less viable primary infection than in the subsequent secondary infection characterized by normal establishment rates in domestic lambs. Prior to the start of Expt. 3 the amount of protein supplement was reduced to keep the lambs from becoming over fat. At the same time Florida Native and challenge control lambs were repeatedly pushed away from the feed trough by the other sheep and their wool was plucked. Since this experiment was conducted in midwinter in an unheated barn, the affected sheep were under considerably more stress than their penmates. This probably explains the poor performance of the Florida Native lambs during Expt. 3 as well as the untimely fall in PCV in the uninfected challenge control lambs after week 4. Transmission of H. contortus in the pens was highly unlikely since it was midwinter and temperatures in the barn fell to as low as -20°C and were usually below 0°C. There has been only one previous study of the resistance of St. Croix sheep to helminth parasites, although helminths are reported to be the most
107
important cause of disease in their native habitat, the U.S. Virgin Islands (Mason, 1980). Loggins (personal communication) reported that St. Croix and St. Croix x Florida Native sheep performed as well as Florida Native sheep during the first year at pasture in subtropical Gainesville, Florida, U.S.A. However, in the second and third years of the study the St. Croix and crossbred sheep began to show signs of haemonchosis. During the third year they were treated with an anthelmintic and housed to prevent mortality. As a result, further study of this breed was discontinued at the University of Florida. The strong resistance to reinfection with H. contortus shown by St. Croix and three quarter St. Croix lambs in the present study is somewhat surprising in light of the Florida study. The two studies should be compared with caution since the Florida study involved a ewe flock kept continually at pasture whereas the present study was done with worm free lambs given experimental infections, Other workers have found Florida Native sheep to be more resistant than conventional breeds (Loggins et al., 1965; Jilek & Bradley, 1969; Radhakrishnan et al., 1972; Bradley el al., 1973), but evaluation of the Barbados Blackbelly breed for nematode resistance has given equivocal results (Yazwinski, Goode, Moncol, Morgan 8~ Linnerud 1979, 1980; Todd, Mansfield & Levine, 1978). Age appeared to have little or no effect upon the resistance of the exotic breeds. They may have acquired immune competence before the age at which the earliest experiments (1 and 2) were conducted. In contrast, it appeared that domestic lambs, especially females, were better able to resist challenge after puberty. Attempts to vaccinate conventional lambs against H. contortus have been disappointing even though young lambs can respond to bacterial and viral vaccines and humoral and cell-mediated responses are present in fetal lambs (Sterzl & Silverstein, 1976). It appears that lambs less than 8 months of age develop little or no immunity to primary infection or reinfection with H. contortus or to challenge with H. contortus after the administration of irradiated larval vaccines (Manton, Peacock, Poynter, Silverman, &Terry, 1962; Jarrett, Jennings, McIntyre, Mulligan & Sharp, 1961; Knight & Rodgers, 1974; Benitez-Usher, Armour, Duncan, Urquhart & Gettinby, 1977; Urquhart, Jarrett, Jennings, McIntyre, Mulligan & Sharp, 1966; Urquhart, Jarrett, Jennings, McIntyre & Mulligan, 1966; Duncan, Smith L Dargie, 1978; Smith & Angus, 1980). The exotic lambs are by contrast better able to acquire resistance at a young age than the domestic lambs. Sex differences were not present until after puberty (Expt. 3) when ewe lambs became more resistant than ram lambs. Thorson (1970) proposed that in the female, natural resistance to nematodes increases dramatically after puberty, whereas in the male it develops gradually from birth to adulthood. Windon
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C. H. COURTNEY, C. F. PARKER, K. E. MCCLURE and R. P. HERD
& Dineen (1981), however, found greater responses in female than in male lambs vaccinated with irradiated larvae of T. colubriformis before puberty. In Expts. 1 and 3 there was no significant effect of hemoglobin genotypes AB and BB on fecal egg counts, PCVs or worm burdens, but in Expt. 2 genotype AB lambs were found to be superior in terms of fecal egg counts, PCVs and the proportion of “responders” to “nonresponders”, but not in terms of worm counts. In all three experiments, genotype AA was excluded from general analysis since only a small number of lambs of this genotype were present and they were unevenly distributed among breeds. Likewise, after genotypes AB and BB were partitioned among the various breed and sex groups, few degrees of freedom remained and, as a result, may have been inadequate to detect subtle differences. Some workers have reported an association between hemoglobin genotype and resistance to parasites with genotype AA (Evans, Blunt & Southcott, 1963; Jilek & Bradley, 1969; Altaif & Dargie, 1976, 1978a, 1978b, 1978~; Allonby & Urquhart, 1976; Preston & Allonby, 1979; Luffau, Pery & Petit, 1981) or genotype AB (Radhakrishnan et al., 1972; Bradley et al., 1973) as most resistant. Other workers have found no relationship between hemoglobin genotype and parasite resistance in sheep (Yazwinski et al., 1979; Windon et al., 1980; Ross, 1970). Le Jambre (1978) concluded that sheep hemoglobin genotypes were not suitable as a marker for identifying parasite resistant sheep. The selection of sheep for resistance to H. contortus would be useful in humid tropical regions where high levels of pasture contamination with infective larvae are present throughout much of the year. It may be more rewarding to breed sheep for the ability to acquire immunity to nematodes than to select for resistance to primary infection. In the selection of animals for increased acquired resistance to parasites, young lambs (about 2-4 months old) could be vaccinated and challenged as in Expt. 2. Animals could then be selected for breeding stock on the basis of PCV and fecal egg counts 5 weeks after reinfection since PCVs and fecal egg counts 5 weeks after reinfection proved to be the most reliable antemortem indicator of worm burdens in the present study. Separate selections should be made for each sex, and older lambs should be excluded since agerelated changes in resistance may confound selection (Le Jambre, 1978). If more than one breed is being selected, separate challenge controls should be used for each breed. Since some authors suggest that pasture-reared lambs do not acquire resistance as readily as lambs reared worm free (Donald & Waller, 1982; Urquhart, 1980), it may be desirable to select resistant sheep by exposing lambs to natural infections rather than an experimental primary infection or vaccine.
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REFERENCES ALLONBY E. W. & URQUHART G. M. 1976. A possible relationship between haemonchosis and haemoglobin polymorphism in Merino sheep in Kenya. Research in
Veterinary Science 20: 212-214. ALTAIF K. I. & DARFIE J. D. 1976. Genetic resistance of sheep to Haemonchus contortus. In: Nuclear Techniques in Animal Production and Health, pp. 449-491. International Atomic Energy Agency, Vienna. ALTAIF K. I. & DARFIE J. D. 1978a. Genetic resistance to helminths. The influence of breed and haemoglobin type on the response of sheep to primary infections with
Haemonchus
contortus. Parasitology 77: 161-175.
ALTAIF K. I. & DARCIE J. D. 1978b. Genetic resistance to helminths. The influence of breed and haemoglobin type on the response of sheep to reinfection with Haemonchus contortus. Parasitology 77: 177-187. ALTAIF K. I. & DARGIE J. D. 1978~. Genetic resistance to helminths. Comparison of the development of Ostertagia circumcincra infections in Scottish Blackface sheep of different haemoglobin type. Research in Veterinary Science 24: 391-393. BENITEZ-USHER C., ARMOUR J., DUNCAN J. L., URQUHART Cl. M. & GETTINBY G. 1977. A study of some factors influencing the immunization of sheep against Haemonthus contortus using attenuated larvae. Veterinary
Parasitology 3: 327-342. BRADLEY R. E., RA~HAKRISHNAN C. V., PATIL-KULKARNI V. G. & LOGGINS P. E. 1973. Responses in Florida Native and Rambouillet lambs exposed to one and two oral doses of Haemonchus contortus. American Journal of
Veterinary Research 34: 729-735. COURTNEY C. H., PARKER C. F., MCCLURE K. E. and HERD R. P. 1984. A comparison of the periparturient rise in fecal egg counts of exotic and domestic ewes.
International Journalfor Parasitology 14: 377-381. DINEEN J. K., GREGG P. & LASCELLES A. K. 1978. The response of lambs to vaccination at weaning with irradiated Trichostrongylus colubriformis larvae: Segregation into “responders” and “nonresponders”. Inter-
national Journalfor
Parasitology 8: 59-63.
DINEEN J. K. & WINDON R. G. 1980a. The effect of sire selection on the response of lambs to vaccination with irradiated Trichostrongylus colubriformis larvae. International Journalfor Parasitology 10:189-196. DINEEN J. K. & WINDON R. G. 198Ob. The effect of acquired resistance on adult worms of Trichostrongylus colubriformis in lambs. International Journal for Parasitology 10: 249-252. DONALD A. D. & WALLER P. J. 1982. Problems and prospects in the control of helminthiasis in sheep. In: Biology and Control of Endoparasites (Edited by SYMONS L. E. A., DONALD A. D. and DIN&EN J. K.). McMaster Laboratory 50th Anniversary Symposium in Parasitology, University of Sydney, 5-6 November 1981. Academic Press, Sydney. DUNCAN J. L., SMITH W. D. & DARGIE J. D. 1978. Possible relationship of levels of mucosal IgA and serum IgG to immune unresponsiveness of lambs to Haemonchus
contortus. Veterinary Parasitology 4: 2 1-27. EVANS J. T., BLUNT M. H. & SOUTHCOTT W. H. 1963. The effects of infection with Haemonchus contortus on the sodium and potassium concentrations in erythro-
cytes
and
plasma,
in sheep
of different
haemoglobin
I.J.P. VOL. 15. 1985
Resistance
of exotic and domestic
Australian Journal of Agricultural Research 14: 549-558. GEBOTT M. D. & PECK J. M. 1978. Beckman Microzone Manual. Instruments, Electrophoresis Beckman types.
Fullerton,
California.
JARRETT W. F. H., JENNINGS F. W., MCINTYRE W. I. M., MULLIGAN W. & SHARP N. C. C. 1961. Studies on
immunity to Haemonchus contortus infection-double vaccination of sheep with irradiated larvae. American Journal of Veterinary Research 22: 186-l 88. JILEK A. F. & BRADLEY R. E. 1969. Hemoglobin resistance to Haemonchus contortus in sheep.
types and
American
KNIGHT R. A. & RODGERS D. 1974. Age resistance of lambs to single inoculation with Haemonchus contortus. Pro-
ceedings of the Helminthological Society of Washington in helminth
In: The Epidemiology and Control of Gastrointestinal Parasites of Sheep in Australia (Edited by
DONALD A. D., SOUTHCOTT W. H. and DINEEN J. K.). Commonwealth Scientific and Industrial Research Organization, Australia. LOGOINS P. E., SWANSON L. E. & KOGER M. 1965. Parasite levels in sheep as affected by heredity. Journal of Animal
Science 24: 286-287. and LUFFAU G., PBRY P. & PETIT A. 1981. Self-cure immunity following infection and reinfection in ovine
Veterinary Parasitology 9: 57-67.
MANTON V. J. A., PEACOCK R., POYNTER D., SILVERMAN P. & TERRY R. J. 1962. The influence of age on naturally
acquired
resistance
to Haemonchus contortus in lambs.
Research in Veterinary Science 3: 308-3 14. L. 1980. Conservation of Animal Genetic Resources: Prolific Tropical Sheep. Food and Agri-
MASON I.
cultural
Organization
of the United Nations,
Rome.
MILLER H. R. P., JACKSON F., NEWLANDS G. & APPLEYARD W. T. 1983. Immune exclusion, a mechanism of protection against the ovine nematode Haemonchus
contortus. Research in Veterinary Science 35: 357-363. PRESTON J. M. & ALLONBY E. W. 1979. The influence
hemoglobin
phenotype
on the susceptibility Haemonchus contortus infection in Kenya. Veterinary Science 26: 140-144.
of of sheep to
Research in
RADHAKRISHNAN C. V., BRADLEY R. E. & LOGGINSP. E. 1972. Host responses of worm-free Florida Native and
Rambouillet
lambs
experimentally
infected
with
Haemonchus contortus. American Journal of Veterinary Research 33: 817-823. ROGERSW. P. 1962. The Nature of Parasitism. Academic Press, New York and London. Ross J. G. 1970. Genetic differences in the susceptibility of sheep to infection with Trichostrongylus axei. A comparison of Scottish Blackface and Dorset breeds. Research in Veterinary Science 11: 465-468. SCHALM 0.
W.,
JAIN N.
Veterinary Hematology, Philadelphia.
C.
3rd
& CARROLL E.
ed.
D. & ANGUS K. W. 1980. Haemonchus contortus: attempts to immunise lambs with irradiated larvae. Research in Veterinary Science 29: 45-50.
STERZL J. & SILVERSTEIN A. M. 1967. aspects of immunity. Advances in
Developmental
Immunology
6:
337-459. THORSON R.
Immunity
E.
1970.
Direct
infection
to Parasitic Animals,
nematodes.
Vol.
2 (Edited
In:
by
JACKSON G. J., HERMAN R. and SINGER I.). AppletonCentury-Crofts, New York. TODD K. S., MANSFIELD M. E. & LEVINE N. D. 1978.
Haemonchus
contortus
infections Black-Belly cross
in Targhee and lambs. American
Journal of Veterinary Research 39: 865-866. URQUHART G. M. 1980. The application of immunity in the control of parasitic disease. Veterinary Parasitology
6: 217-239. factors
control.
haemonchosis.
109
SMITH W.
Targhee-Barbados
Journal of Veterinary Research 30: 1773-l 778.
41: 116. LE JAMBRE L. F. 1978. Host genetic
lambs to H. contortus
Lea
and
J.
1975.
Febiger,
URQUHART G. M., JARRETT W. F. H., JENNINGS F. W., MCINTYRE W. I. M., MULLIGAN W. & SHARP N. C. C. 1966. Immunity to Haemonchus contortus infection:
Failure
of X-irradiated
larvae to immunise
young
lambs.
American Journal of Veterinary Research 27: 1641-1643. URQUHART G. M., JARRETT W. F. H., JENNINGS F. W., MCINTYRE W. I. M. & MULLIGAN W. 1966. Immunity to Haemonchus contortus infection: Relationship
between age and successful vaccination with irradiated larvae. American Journal of Veterinary Research 27:
1645-1648. WAKELIN D. 1978. Genetic control of susceptibility and resistance to parasitic infection. Advances in Parasitology
16: 219-308. WHITLOCK H. V. 1948. Some modifications of the McMaster helminth egg-counting technique apparatus.
Journal of the Council for Scientsfic and Zndustrial Research 21: 177-180. WHITLOCK J. H. 1955. A study resistance to trichostrongylosis
of
the inheritance
in
sheep.
of
Cornell
Veterinarian 45: 422-439. WINDON R. G., DINEEN J. K. & KELLY J. D. 1980. The segregation of lambs into “responders” and “non-
responders”:
Response
to vaccination
with
irradiated
Trichostrongylus colubriformis larvae before weaning. International Journalfor Parasitology 10: 65-73. WINDON R. G. & DINEEN J. K. 1981. The effect of selection of both sire and dam on the response of F1 generation lambs to vaccination with irradiated Trichostrongylus
colubriformis larvae. sitology 11: 11-16.
International Journal for Para-
YAZWINSKI T. A., GOODE L., MONCOL D. J., MORGAN G. W. & LINNERUD A. C. 1979. Parasite resistance in straightbred and crossbred Barbados Blackbelly sheep.
Journal of Animal Science 49: 919-926. YAZWINSKI T. A., GOODE L., MONCOL D. J., G. W. & LINNERUD A. C. 1980. Haemonchus
resistance Blackbelly
279-284.
MORGAN
contortus
in straightbred and crossbred Barbados sheep. Journal of Animal Science 51:
‘G I985 Australian
$3.mI+0.00
Pergamon Press Ltd. Socrety for Parasifolo~y
RESISTANCE OF EXOTIC AND DOMESTIC LAMBS TO EXPERIMENTAL INFECTION WITH HAEMONCHUS CONTOR TUS C. H. COURTNEY,*$
C. F. PARKfiR,j-§
K. E. MCCLURE?
and R. P. HERD*
*Department of Veterinary Pathobiology, The Ohio State University, 1925 Coffey Road, Columbus, OH 43210, U.S.A. j-Department of Animal Science, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, OH 44691, U.S.A. (Received 2 March 1984; in revisedform 19 June 1984) Abstract-COURTNEY C. H., PARKERC. F., MCCLUREK. E. and HBRDR. P. 1985. Resistance of exotic and domestic lambs to experimental infection with Haemonchus contortus. International Journal for Purusifology 15: 101-109. Resistance to primary and secondary infections with H. contorfus was compared in St. Croix, 3/4 St. Croix, Florida Native, Barbados Blackbelly and domestic crossbred lambs. On the basis of worm counts, fecal egg counts and packed cell volumes, St. Croix and 314 St. Croix lambs were the most resistant to secondary infections, Florida Native and Barbados lambs were intermediate in resistance, and the domestic lambs were the least resistant. Breed differences were highly variable during primary infection. Breed differences were most pronounced prior to puberty, with domestic Iambs highly susceptible to secondary infections. After puberty, domestic lambs were better able to resist secondary infections. Age apparently had little or no effect on the parasite resistance of exotic breeds. Sex differences were significant only after puberty when ewe lambs were more resistant to secondary infection than ram lambs. In one of three experiments lambs of hemoglobin genotype AB were more resistant to secondary infection than lambs of genotype BB in terms of fecal egg counts, packed cell volumes and the proportion of ‘responder’ and ‘nonresponder’ lambs but not in terms of worm counts, although subtle differences may not have been detected because of small numbers of lambs.
INDEX KEY WORDS: Huemonchus contorfus; genetic resistance; hemoglobin genotype; Florida Native sheep; St. Croix sheep; Barbados Blackbelly sheep.
INTRODUCTION AN INCREASEDability to resist natural infections of H. contortus was demonstrated in certain individuals
than Dorset lambs (Altaif & Dargie, 1978b). Australian workers have shown that individual Merino lambs differed in their ability to respond to immunization with irradiated Lj of Trichostrongylus colubriformis and that these differences were heritable (Dineen, Gregg & Lascelles, 1978; Dineen & Windon, 1980a, b; Windon, Dineen & Kelly, 1980; Windon & Dineen, 1981). The successful demonstration of a repeatable model of acquired resistance to H. contortus in young lambs would be invaluable as an aid to the study of the immunology of age-related responsiveness in lambs as well as to vaccine development and selective breeding for increased parasite resistance. This paper reports a study of the influence of breed, age, sex and hemoglobin genotype on the ability of lambs to resist reinfection with H. contortus following primary sensitization with the same parasite. Emphasis is placed on three exotic breeds originating in the humid tropics and subtropics (Florida Native, Barbados Blackbelly and St. Croix) where natural selection would be expected to favor lambs that acquired resistance to gastrointestinal helminths at
within a flock of Dorset sheep and this increase was heritable (Whitlock, 1955). Other workers have found Florida Native sheep to be more resistant than conventional breeds (Suffolk or Rambouillet) in both natural and experimental infections with H. contortus (Loggins, Swanson & Koger, 1965; Jilek & Bradley, 1969; Radhakrishnan, Bradley & Loggins, 1972; Bradley, Rahhakrishnan, Patil-Kulkarni, & Loggins, 1973). After a primary infection with normal larvae of H. contortus, Florida Native lambs were more resistant to reinfection than Rambouillet lambs (Bradley et al., 1973). Likewise, Scottish Blackface lambs were found to be more resistant to reinfection *Present address for correspondence: Department of Preventive Medicine, College of Veterinary Medicine, Box J-136 JHMHC, University of Florida, Gainesville, FL 32610, U.S.A. #Present address: U.S. Sheep Experiment Station, Dubois, ID 83423, U.S.A. 101
102
C. H. COURTNEY, C. F. PARKER, K. E. McC~u~~and
an early age. A description in an Herd,
earlier 1984).
paper
of these breeds was given (Courtney, Parker, McClure &
MATERIALS
AND METHODS
Experimental design. Breed differences
in the susceptibility of exotic and domestic lambs to experimental infections with H. contortus were studied in three experiments involving a total of 144 lambs. In Expt. 1 breed differences in responses to both primary and secondary infections were compared. In Expt. 2 breed, sex and hemoglobin genotype differences in resistance to secondary infections were compared in lambs prior to puberty (4 months of age) whereas in Expt. 3 these variables were compared in lambs after sexual maturity (9 months of age). Experiment 1. Ten St. Croix, 10 Barbados Blackbelly, 10 Florida Native and 10 domestic crossbred 1980-born male lambs were used. The lambs, aged 6-16 weeks at the start of the experiment, were given a primary infection of 6000 L3 of H. contortus at week 0. At week 6, five lambs of each breed were necropsied and the primary infections were removed from the remaining lambs with a drench of 10 mg kg-l levamisole. At week 7 they were challenged with 12,000 L3 and necropsied at week 12. Blood and fecal samples were collected at weeks 0 and 3-12. Experiment 2. Eleven Barbados Blackbelly, nine St. Croix, nine three quarter St. Croix, 10 Florida Native and 14 domestic crossbred 1981-born lambs were used. The lambs were aged 14-18 weeks at the start of the experiment. Female lambs were used in all breed groups except Barbados Blackbelly. Lambs were given a primary infection of 6000 L3 of H. contortus at week 0, drenched with 10 mg kg-l levamisole at week 5, challenged at week 6 with 12,000 L3 and necropsied at week 11. Five of the domestic lambs, designated challenge controls, were not given the initial primary infection, but were treated with levamisole and given a primary infection of 12,000 L3 when the remaining lambs received their challenge infections. Blood and fecal samples were collected from all lambs at weeks 0 and 3-l 1. Experiment 3. Six Barbados Blackbelly, 10 St. Crotx, nine three quarter St. Croix, 10 Florida Native and 17 domestic crossbred 1981-born lambs of both sexes were used. Lambs were 36-40 weeks of age at the start of the experiment. Lambs were given a primary infection of 10,000 L3 at week 0, drenched with levamisole (10 mg kg-l) at week 5, challenged with 16,OtXl L3 at week 6 and necropsied at week 11. Five domestic lambs served as
challenge controls. Blood and fecal samples were collected from all lambs at weeks 0 and 3-11. Sheep and management. The domestic crossbred lambs were half Suffolk, quarter Finn, quarter Dorset (experiment 1) and half Suffolk or Dorset, quarter Finn, quarter Rambouillet (experiments 2 and 3). Three quarter St. Croix lambs were quarter Dorset or Rambouillet. All lambs were reared in confinement on expanded metal flooring free of parasites other than coccidia and Strongyloides papillosus. They were kept in the same trichostrongylid free environment throughout the study. Lambs were fed a standard diet of corn silage and 15% protein supplement except just prior to the start of Expt. 3 when the diet was changed to corn stover and the amount of protein supplement reduced since the lambs were becoming overfat. Due to miscalculation this changed diet was inadequate for maintenance of lambs and caused considerable loss of condition before the error was detected during week 5 and the diet corrected.
R. P. HERD
I.I.P.
VOL.
15. 1985
Parasitological techniques. The local Wooster strain of H. contortus used in all experiments was isolated in 1979. Live, gravid female H. contortus, removed from a sheep at necropsy, were lightly ground in a Ten Broeck tissue grinder to release ova. Ova were cultured in nematode free sheep feces and the resulting 200 L3 were given to a worm free donor lamb. After patency, feces from the donor lamb were collected and incubated in shallow trays at a depth of l-2 cm for lo-14 days at 27°C. Cultures were stirred daily to reduce fungal overgrowth. Larvae were recovered from feces by Baermannization through a 150 pm screen and prior to use were stored for 2-8 weeks at 4°C in tissue culture flasks containing a 2-4 mm layer of water. This isolate was subsequently maintained in the laboratory by passage in worm free donor lambs. Experimental lambs were infected by oral drenching (primary infection of Expt. 1) or by peroral intubation of the rumen (remaining infections). Fecal egg counts were made by the modified McMaster technique (Whitlock, 1948). At necropsy worm counts were made from 10% aliquots of abomasal contents. The abomasal mucosa was scraped from the muscularis and digested in 200 ml of 1% HCI at 37°C for 3 h. After digestion was stopped by the addition of 10% formalin, 10% aliquots were examined for worms. Hematology. Packed cell volumes (PCVs) were determined by the microhematocrit technique (Schalm, Jain & Carroll, 1975). Hemoglobin genotypes were determined by electrophoresis of hemoglobin extracted from week 0 blood samples (Gebott &Peck, 1978). Statistical analysis. Data were analyzed by analysis of variance. Worm count data were transformed by log lo (X+ 1) and fecal egg count data by square roots to normalize data. Multiple comparisons were made by Duncan’s New Multiple Range Test. Lambs from Expt. 2 were declared “responders” if their adult worm burdens were less than the lower limit of the 99% confidence interval of the geometric mean adult worm burden of the challenge control lambs. If counts were greater than this limit, lambs were declared “nonresponders” (Dineen et a/., 1978). Proportions of responders and nonresponders were compared between breeds, sexes and hemoglobin genotypes using Fisher’s Exact Test. Differences were considered significant when p < 0.05.
RESULTS
Unless a statement is made to the contrary, any differences reported were significant at p < 0.05. In all experiments, both fecal egg counts and PCVs changed with time. As a general pattern fecal egg counts increased after primary infection, fell to zero after levamisole treatment and rose again after reinfection. PCVs generally fell after primary infection, rose after levamisole treatment and fell again after reinfection. There were, however, some interactions with the effects of breed, sex and hemoglobin genotype, so comparisons of these factors must be made at each sample date. In all experiments, very few larval worms were recovered at necropsy, so data on adult worms only are reported herein. After storage at 4”C, the Wooster strain of H. contortus does not undergo a significant degree of arrested development in experimental infections.
Resistance of exotic and domestic lambs to
I.J.P. VOL. 15. 1985
25 60 1 50 l
40 x w I
30
20
IO
WEEK FIG. 1. Effect of breed on egg count, Expt. 1. A-A,
Native; 0 -
mean PCV and square root fecal Barbados; A- - -A, Florida 0, St. Croix; 0 - - - l , domestic.
Expt. 1. Peak fecal egg counts during primary infection (week 4) were higher in Barbados than in St. Croix and Florida Native lambs (Fig. 1). Egg counts of domestic lambs were intermediate and not significantly different from any other breed. After reinfection fecal egg counts were greater in domestic lambs than in the exotic breeds at weeks 10, 11 and 12, and counts from St. Croix lambs were lower than Florida Native and Barbados lambs at weeks 11 and 12. There was no significant effect of hemoglobin genotypes AB and BB on fecal egg counts (Fig. 2). Hemoglobin genotype AA was excluded from analysis because only four genotype AA lambs were present and all were necropsied after primary infection. At week 4, St. Croix lambs had higher PCVs than the three other breeds, and at week 6 the domestic lambs had significantly lower PCVs than the exotic breeds (Fig. 1). At week 10 St. Croix and Florida Native lambs had higher PCVs than Barbados and domestic lambs. Hemoglobin genotypes AB and BB had no significant effect on PCV (Fig. 2). After primary infection St. Croix lambs had markedly fewer adult H. contortus than the other breeds but differences were not significant (Table 1). After secondary infection, St. Croix lambs had significantly fewer worms than all other breeds, and the domestic lambs had more worms than all other breeds. There was no effect of hemoglobin genotypes AB and BB on worm counts. Experiment 2. At week 3 the St. Croix lambs shed more ova than the other lambs, and at week 4 the St.
H. contortus
103
Croix and Barbados lambs shed more ova than the three quarter St. Croix lambs (Fig. 3). At weeks 9, 10 and 11 (3, 4 and 5 weeks after reinfection) the domestic and challenge control lambs shed more ova than the exotic lambs except that at week 11 their fecal egg counts were not significantly greater than the Barbados lambs. At week 11 the Barbados lambs had greater fecal egg counts than the St. Croix lambs. There were lower mean fecal egg counts at weeks 9, 10 and 11 in lambs of hemoglobin genotype AB than BB (Fig. 4). Genotype AA occurred only in Barbados and Florida Native sheep and was therefore not included in analysis. There were no significant differences in fecal egg counts associated with lamb sex (Fig. 5). No female Barbados lambs were available, so this breed was excluded from analysis of sex effects. The challenge control group was excluded from analysis of the effects of both hemoglobin genotype and sex. After reinfection, PCVs were higher in the exotic breeds than in the domestic lambs or challenge controls at weeks 9 and 10 (Fig. 3). At week 11 the PCVs of the exotic lambs were higher than the domestic lambs, but only the Barbados and St. Croix had higher PCVs than the challenge controls. Prior to infection, PCVs of lambs or hemoglobin genotype AB were higher than those of genotype BB (Fig. 4). Challenge control lambs and lambs of genotype AA were excluded from analysis. At weeks 6-l 1 genotype AB lambs again had higher
40
1
35> 2
30-
25 60
L 8 9
WEEK
IO II
12
FIG. 2. Effect of hemoglobin genotype on mean PCV and square root fecal egg count, Expt. 1. A-A, AA; 0 -0, AB; O---o, BB.
C. H. COURTNEY, C. F. PARKER, K. E. MCCLURE and R. P. HERD
104
I.J.P. VOL. 15. 1985
TABLE I-MEAN H. contortus RECOVERY BY BREED AND BY HEMOGLOBINGENOTYPE, EXPT. 1. DATA TRANSFORMED BY (x+ 1) FOR CALCoLATlON OF TRANSFORMEDMEANAND STANDARD DEVIATION. VALUES WITHIN A COLUMN WITH DIFFERENT
log
SUPERSCRIPTS
Primary
WERE
SIGNIFICANTLY
DIFFERENT
@
infection
< 0.05)
Challenge
infection
Group No. lambs
Arithmetic mean
Transformed mean
S.D.
No. lambs
Arithmetic mean
Transformed mean
S.D.
Breed St. Croix Florida Native Barbados Domestic
5 5 5 5
12 178 326 284
0.77 1.54 1.99 2.35
0.72 1.04 1.16 0.34
5 5 5 5
4 106 258 3694
0.3oa l.lsb 1.78b 3.46c
0.59 0.95 1.13 0.30
Hemoglobin AA AB BB
4 7 9
295 196 161
2.25 1.40 1.61
0.51 1.15 1.03
9 11
1461 651
1.79 1.56
1.53 1.33
Genotype
were found. There were no “responders” among the domestic lambs and no “nonresponders” among the St. Croix lambs, whereas Florida Native, Barbados, and three quarter St. Croix lambs were a mixture of “responders” and “nonresponders”. When analyzed by Fisher’s exact test, St. Croix and three quarter St. Croix lambs were found to have proportions of “responders” and “nonresponders” different from the domestic lambs. Hemoglobin genotype AB lambs had a greater proportion of “responders” than genotype BB lambs, but there were no significant differences in worm burdens. Genotype AA lambs occurred only in Florida Native and Barbados lambs
WEEK FIG. 3. Effect of breed on mean PCV and square root fecal egg count, Expt. 2. A- - -A, Barbados; A- - -A, Florida Native; 0 O, St. Croix; 0 0, 3/4 St. Croix; ‘O-O , domestic; n - - -II ) challenge control.
PCVs than genotype BB lambs. There were no significant sex differences in PCV; the apparent sex difference seen in Fig. 5 was caused by a difference in the distribution of sexes within breeds. St. Croix lambs had fewer worms than all other breeds and three quarter St. Croix lambs had fewer worms than all breeds except St. Croix (Table 2). No significant differences in worm burdens of Florida Native, Barbados, domestic, and challenge control lambs
WEEK TIC;. 4. Effect of hemoglobin genotype on mean PCV and square root fecal egg count, Expt. 2.6-A. AA; O--c1 , AB; 0 --, BB.
I.J.P. VOL.15. 1985
Resistance of exotic and domestic lambs to H. contortus
40 1
25 90
3 4
5 6 7 8 9 IO II
WEEK FIG. 5.
Effect of sex on mean PCV and square root fecal egg count, Expt. 2. A-A, male; o -0 , female.
and were therefore
excluded from a general analysis. Within these two breeds, worm burdens in genotype AA lambs were not significantly different from those
of type AB or BB lambs. Neither worm burden nor proportion of “responders” to “nonresponders” was influenced by lamb sex. Experiment 3. During the primary infection, mean fecal egg counts at weeks 3,4 and 5 were higher in the Florida Native and domestic lambs than in the St. Croix and three quarter St. Croix lambs (Fig. 6).
WEEK
Rti. 6. Effect of breed on mean PCV and square root fecal egg count, Expt. 3. A-A, Barbados; A- - -A, Florida Native; 0 0, St. Croix; 0 - - - 0, 3/4 St. Croix; 0 0 , domestic; l - - - l , challenge control.
TABLE ~--RECOVERY OF H, contortus ADULTS BY BREED, SEX, AND HEMOGLOBIN GENOTYPE, EXPT. 2. DATA TRANSFORMED BY lOg(X+l)FOR TRANSFORMED MEAN AND STANDARD DEVIATION. !UiALYSIS OF HEMOGLOBIN GENOTYPEEXCLUDESCHALLENGECONTROLS;ANALYSISOFSE~EXCLUDESBARBADOSANDCHALLENGECONTROLS. ~~UESINACOLUMNWITHDIFFERENTSUPERSCRIPTSWERESIGNIFICANTLYDIFFERENTO] < 0.05)
Group
No. lambs
Breed Challenge control Domestic Barbados Florida Native 3/4 St. Croix St. Croix
5 8 11 10 9 9
Hemoglobin genotype AA AB BB Sex Male Female
ResponderUNonresponders
Arithmetic mean
Transformed mean
S.D.
web
3970 5238 1809 1625 1044 11
3.57a 3.71a 305a 3.01 a 2.16” 0.22c
0.18 0.12 0947 0~48 1.36 0.67
6 20 21
313 18/2a 8/13b
1808 590 3090
3.09 1.85 2.82
0.50 1.32 1.43
19 17
13/6 IO/7
1916 1838
2.33 2.17
1.51 1.56
0/8a 6/5a,b 713a.b 7/2b
C. H. COURTNEY, C. F. PARKER,K. E. MCCLURE and R. P.
I.J.P.VOL.15. 1985
HERD
present and was excluded from analysis. Male lambs more ova than female lambs during weeks 3-5 and 9-11 (Fig. 7). PCVs decreased sharply in the challenge control group after week 4 (Fig. 6) even though they were not infected at week 0 and no ova were found in their feces until 3 weeks after challenge infections were given. At weeks 9, 10 and 11 St. Croix and three quarter St. Croix lambs had higher PCVs than the Florida Native, domestic and challenge control lambs. There was no significant effect of hemoglobin genotypes AB and BB on PCV, but PCVs were higher in female lambs than male lambs (Fig. 7). Breed and hemoglobin genotype had no effect upon worm burdens, but there were more worms in male than female lambs (Table 3). There was a trend towards the same pattern of breed differences in worm burdens seen in Expt. 2; however, larger variances in Expt. 3 prevented statistically significant differences given the present breed sample sizes. Likewise, the greater variance in the challenge control group precluded the designation of “responders” and “nanresponders” in this experiment. shed
25 60
a@-
P30w
20I o-
WEEK FIG. 7.
Effect of hemoglobin genotype and sex on mean PCV and square root fecal egg count, Expt. 3. A- - -A, AB; l - - -0, BB; A-A, male; 0 cl, female.
After reinfection, the challenge control, domestic, and Florida Native lambs shed more ova at weeks 9, 10 and 11 than the St. Croix and three quarter St. Croix lambs. There was no significant effect of hemoglobin genotypes AB or BB on fecal egg counts (Fig. 7), but only one animal of genotype AA was
DISCUSSION When 4-6-month-old lambs were infected with H. contortus, treated with levamisole after 5 or 6 weeks and reinfected, St. Croix lambs were highly resistant to reinfection, whereas domestic lambs were uniformly susceptible. Three quarter St. Croix, Florida Native, and Barbados Blackbelly lambs were intermediate in resistance. In Expt. 3, Florida Native lambs were apparently undernourished. They had lower PCVs and higher fecal egg counts than Barbados lambs but had fewer worms at necropsy. One can therefore rank the resistance of breeds in decreasing order as St. Croix, three quarter St. Croix,
TABLE 3--MEAN RECOYERY OF H. CO~iO~~~~ ADULTS BY BREED,SEX HEM~GLOBINGENOTYPE,EXPT.~.ANALYS~SOFHEMOOLOBINOENOTYPEAND SEXEXCLUDESCHALLENGE CONTROLS.VALUESINACOLUMNWlTHDIFFERENT SUPERSCRIPTSWERESIGNIFICANTLYDIFFERENT@< 0.05)
AND
No. lambs
Arithmetic mean
Transformed mean
S.D.
5 12 5 10 9 10
3870 2319 2960 1800 256 480
3.45 2.46 2.87 2.49 1.39 1.86
0.42 1.56 0.78 1.40 1.35 1.33
Hemoglobin genotype AA AB BB
1 25 20
5150 1514 1272
3.71 1.78 2.59
1.57 1.01
Sex Male Female
26 20
2296 438
2.96a l.lsb
0.8t 1.36
Group Breed Challenge controls Domestic Barbados Florida Native 3/4 St. Croix St. Croix
I.J.P.VOL.
15. 1985
Resistance of exotic and domestic lambs to H. contortus
Florida Native, Barbados, and domestic. After puberty it appeared that the female domestic lambs became nearly as resistant as the female exotic lambs, and male lambs were generally less resistant than female lambs. During primary infection, breed differences were more variable and not as pronounced as after reinfection. This suggests that breed differences after reinfection were due, at least in part, to differences in acquired immunity. Wakelin (1978) proposed that genetically determined resistance to H. contortus consisted of both immunological and non-immunological components and that a strong immunological component acted at the level of parasite establishment. Larvae of H. contortus apparently fail to reach their predilection site in the gastric pits of immune sheep, and over 90% are expelled within 48 h of infection (Miller, Jackson, Newlands, & Appleyard, 1983). In the present study, larger sample sizes and the use of a challenge control group for each breed would have given stronger evidence for or against acquired immunity as the mechanism involved in breed resistance, but sufficient lambs were not available. The experiments were further confounded by two other problems-low establishment of worms in the primary infection of Expt. 1 and a nutritional problem in Expt. 3. In Expt. 1 the 5% worm recovery seen in domestic lambs after primary infection was much less than the 25-40% recovery in later infections. This infection was given by oral drench rather than by a stomach tube, and larvae may have entered the abomasum directly via the esophageal groove reflex. Since H. contortus larvae normally exsheath in the rumen (Rogers, 1962) this bypass may have reduced their establishment rate. In spite of the low establishment rate, higher fecal egg counts and lower PCVs were seen in the exotic breeds given the less viable primary infection than in the subsequent secondary infection characterized by normal establishment rates in domestic lambs. Prior to the start of Expt. 3 the amount of protein supplement was reduced to keep the lambs from becoming over fat. At the same time Florida Native and challenge control lambs were repeatedly pushed away from the feed trough by the other sheep and their wool was plucked. Since this experiment was conducted in midwinter in an unheated barn, the affected sheep were under considerably more stress than their penmates. This probably explains the poor performance of the Florida Native lambs during Expt. 3 as well as the untimely fall in PCV in the uninfected challenge control lambs after week 4. Transmission of H. contortus in the pens was highly unlikely since it was midwinter and temperatures in the barn fell to as low as -20°C and were usually below 0°C. There has been only one previous study of the resistance of St. Croix sheep to helminth parasites, although helminths are reported to be the most
107
important cause of disease in their native habitat, the U.S. Virgin Islands (Mason, 1980). Loggins (personal communication) reported that St. Croix and St. Croix x Florida Native sheep performed as well as Florida Native sheep during the first year at pasture in subtropical Gainesville, Florida, U.S.A. However, in the second and third years of the study the St. Croix and crossbred sheep began to show signs of haemonchosis. During the third year they were treated with an anthelmintic and housed to prevent mortality. As a result, further study of this breed was discontinued at the University of Florida. The strong resistance to reinfection with H. contortus shown by St. Croix and three quarter St. Croix lambs in the present study is somewhat surprising in light of the Florida study. The two studies should be compared with caution since the Florida study involved a ewe flock kept continually at pasture whereas the present study was done with worm free lambs given experimental infections, Other workers have found Florida Native sheep to be more resistant than conventional breeds (Loggins et al., 1965; Jilek & Bradley, 1969; Radhakrishnan et al., 1972; Bradley el al., 1973), but evaluation of the Barbados Blackbelly breed for nematode resistance has given equivocal results (Yazwinski, Goode, Moncol, Morgan 8~ Linnerud 1979, 1980; Todd, Mansfield & Levine, 1978). Age appeared to have little or no effect upon the resistance of the exotic breeds. They may have acquired immune competence before the age at which the earliest experiments (1 and 2) were conducted. In contrast, it appeared that domestic lambs, especially females, were better able to resist challenge after puberty. Attempts to vaccinate conventional lambs against H. contortus have been disappointing even though young lambs can respond to bacterial and viral vaccines and humoral and cell-mediated responses are present in fetal lambs (Sterzl & Silverstein, 1976). It appears that lambs less than 8 months of age develop little or no immunity to primary infection or reinfection with H. contortus or to challenge with H. contortus after the administration of irradiated larval vaccines (Manton, Peacock, Poynter, Silverman, &Terry, 1962; Jarrett, Jennings, McIntyre, Mulligan & Sharp, 1961; Knight & Rodgers, 1974; Benitez-Usher, Armour, Duncan, Urquhart & Gettinby, 1977; Urquhart, Jarrett, Jennings, McIntyre, Mulligan & Sharp, 1966; Urquhart, Jarrett, Jennings, McIntyre & Mulligan, 1966; Duncan, Smith L Dargie, 1978; Smith & Angus, 1980). The exotic lambs are by contrast better able to acquire resistance at a young age than the domestic lambs. Sex differences were not present until after puberty (Expt. 3) when ewe lambs became more resistant than ram lambs. Thorson (1970) proposed that in the female, natural resistance to nematodes increases dramatically after puberty, whereas in the male it develops gradually from birth to adulthood. Windon
108
C. H. COURTNEY, C. F. PARKER, K. E. MCCLURE and R. P. HERD
& Dineen (1981), however, found greater responses in female than in male lambs vaccinated with irradiated larvae of T. colubriformis before puberty. In Expts. 1 and 3 there was no significant effect of hemoglobin genotypes AB and BB on fecal egg counts, PCVs or worm burdens, but in Expt. 2 genotype AB lambs were found to be superior in terms of fecal egg counts, PCVs and the proportion of “responders” to “nonresponders”, but not in terms of worm counts. In all three experiments, genotype AA was excluded from general analysis since only a small number of lambs of this genotype were present and they were unevenly distributed among breeds. Likewise, after genotypes AB and BB were partitioned among the various breed and sex groups, few degrees of freedom remained and, as a result, may have been inadequate to detect subtle differences. Some workers have reported an association between hemoglobin genotype and resistance to parasites with genotype AA (Evans, Blunt & Southcott, 1963; Jilek & Bradley, 1969; Altaif & Dargie, 1976, 1978a, 1978b, 1978~; Allonby & Urquhart, 1976; Preston & Allonby, 1979; Luffau, Pery & Petit, 1981) or genotype AB (Radhakrishnan et al., 1972; Bradley et al., 1973) as most resistant. Other workers have found no relationship between hemoglobin genotype and parasite resistance in sheep (Yazwinski et al., 1979; Windon et al., 1980; Ross, 1970). Le Jambre (1978) concluded that sheep hemoglobin genotypes were not suitable as a marker for identifying parasite resistant sheep. The selection of sheep for resistance to H. contortus would be useful in humid tropical regions where high levels of pasture contamination with infective larvae are present throughout much of the year. It may be more rewarding to breed sheep for the ability to acquire immunity to nematodes than to select for resistance to primary infection. In the selection of animals for increased acquired resistance to parasites, young lambs (about 2-4 months old) could be vaccinated and challenged as in Expt. 2. Animals could then be selected for breeding stock on the basis of PCV and fecal egg counts 5 weeks after reinfection since PCVs and fecal egg counts 5 weeks after reinfection proved to be the most reliable antemortem indicator of worm burdens in the present study. Separate selections should be made for each sex, and older lambs should be excluded since agerelated changes in resistance may confound selection (Le Jambre, 1978). If more than one breed is being selected, separate challenge controls should be used for each breed. Since some authors suggest that pasture-reared lambs do not acquire resistance as readily as lambs reared worm free (Donald & Waller, 1982; Urquhart, 1980), it may be desirable to select resistant sheep by exposing lambs to natural infections rather than an experimental primary infection or vaccine.
I.J.P. VOL. 15. 1985
REFERENCES ALLONBY E. W. & URQUHART G. M. 1976. A possible relationship between haemonchosis and haemoglobin polymorphism in Merino sheep in Kenya. Research in
Veterinary Science 20: 212-214. ALTAIF K. I. & DARFIE J. D. 1976. Genetic resistance of sheep to Haemonchus contortus. In: Nuclear Techniques in Animal Production and Health, pp. 449-491. International Atomic Energy Agency, Vienna. ALTAIF K. I. & DARFIE J. D. 1978a. Genetic resistance to helminths. The influence of breed and haemoglobin type on the response of sheep to primary infections with
Haemonchus
contortus. Parasitology 77: 161-175.
ALTAIF K. I. & DARCIE J. D. 1978b. Genetic resistance to helminths. The influence of breed and haemoglobin type on the response of sheep to reinfection with Haemonchus contortus. Parasitology 77: 177-187. ALTAIF K. I. & DARGIE J. D. 1978~. Genetic resistance to helminths. Comparison of the development of Ostertagia circumcincra infections in Scottish Blackface sheep of different haemoglobin type. Research in Veterinary Science 24: 391-393. BENITEZ-USHER C., ARMOUR J., DUNCAN J. L., URQUHART Cl. M. & GETTINBY G. 1977. A study of some factors influencing the immunization of sheep against Haemonthus contortus using attenuated larvae. Veterinary
Parasitology 3: 327-342. BRADLEY R. E., RA~HAKRISHNAN C. V., PATIL-KULKARNI V. G. & LOGGINS P. E. 1973. Responses in Florida Native and Rambouillet lambs exposed to one and two oral doses of Haemonchus contortus. American Journal of
Veterinary Research 34: 729-735. COURTNEY C. H., PARKER C. F., MCCLURE K. E. and HERD R. P. 1984. A comparison of the periparturient rise in fecal egg counts of exotic and domestic ewes.
International Journalfor Parasitology 14: 377-381. DINEEN J. K., GREGG P. & LASCELLES A. K. 1978. The response of lambs to vaccination at weaning with irradiated Trichostrongylus colubriformis larvae: Segregation into “responders” and “nonresponders”. Inter-
national Journalfor
Parasitology 8: 59-63.
DINEEN J. K. & WINDON R. G. 1980a. The effect of sire selection on the response of lambs to vaccination with irradiated Trichostrongylus colubriformis larvae. International Journalfor Parasitology 10:189-196. DINEEN J. K. & WINDON R. G. 198Ob. The effect of acquired resistance on adult worms of Trichostrongylus colubriformis in lambs. International Journal for Parasitology 10: 249-252. DONALD A. D. & WALLER P. J. 1982. Problems and prospects in the control of helminthiasis in sheep. In: Biology and Control of Endoparasites (Edited by SYMONS L. E. A., DONALD A. D. and DIN&EN J. K.). McMaster Laboratory 50th Anniversary Symposium in Parasitology, University of Sydney, 5-6 November 1981. Academic Press, Sydney. DUNCAN J. L., SMITH W. D. & DARGIE J. D. 1978. Possible relationship of levels of mucosal IgA and serum IgG to immune unresponsiveness of lambs to Haemonchus
contortus. Veterinary Parasitology 4: 2 1-27. EVANS J. T., BLUNT M. H. & SOUTHCOTT W. H. 1963. The effects of infection with Haemonchus contortus on the sodium and potassium concentrations in erythro-
cytes
and
plasma,
in sheep
of different
haemoglobin
I.J.P. VOL. 15. 1985
Resistance
of exotic and domestic
Australian Journal of Agricultural Research 14: 549-558. GEBOTT M. D. & PECK J. M. 1978. Beckman Microzone Manual. Instruments, Electrophoresis Beckman types.
Fullerton,
California.
JARRETT W. F. H., JENNINGS F. W., MCINTYRE W. I. M., MULLIGAN W. & SHARP N. C. C. 1961. Studies on
immunity to Haemonchus contortus infection-double vaccination of sheep with irradiated larvae. American Journal of Veterinary Research 22: 186-l 88. JILEK A. F. & BRADLEY R. E. 1969. Hemoglobin resistance to Haemonchus contortus in sheep.
types and
American
KNIGHT R. A. & RODGERS D. 1974. Age resistance of lambs to single inoculation with Haemonchus contortus. Pro-
ceedings of the Helminthological Society of Washington in helminth
In: The Epidemiology and Control of Gastrointestinal Parasites of Sheep in Australia (Edited by
DONALD A. D., SOUTHCOTT W. H. and DINEEN J. K.). Commonwealth Scientific and Industrial Research Organization, Australia. LOGOINS P. E., SWANSON L. E. & KOGER M. 1965. Parasite levels in sheep as affected by heredity. Journal of Animal
Science 24: 286-287. and LUFFAU G., PBRY P. & PETIT A. 1981. Self-cure immunity following infection and reinfection in ovine
Veterinary Parasitology 9: 57-67.
MANTON V. J. A., PEACOCK R., POYNTER D., SILVERMAN P. & TERRY R. J. 1962. The influence of age on naturally
acquired
resistance
to Haemonchus contortus in lambs.
Research in Veterinary Science 3: 308-3 14. L. 1980. Conservation of Animal Genetic Resources: Prolific Tropical Sheep. Food and Agri-
MASON I.
cultural
Organization
of the United Nations,
Rome.
MILLER H. R. P., JACKSON F., NEWLANDS G. & APPLEYARD W. T. 1983. Immune exclusion, a mechanism of protection against the ovine nematode Haemonchus
contortus. Research in Veterinary Science 35: 357-363. PRESTON J. M. & ALLONBY E. W. 1979. The influence
hemoglobin
phenotype
on the susceptibility Haemonchus contortus infection in Kenya. Veterinary Science 26: 140-144.
of of sheep to
Research in
RADHAKRISHNAN C. V., BRADLEY R. E. & LOGGINSP. E. 1972. Host responses of worm-free Florida Native and
Rambouillet
lambs
experimentally
infected
with
Haemonchus contortus. American Journal of Veterinary Research 33: 817-823. ROGERSW. P. 1962. The Nature of Parasitism. Academic Press, New York and London. Ross J. G. 1970. Genetic differences in the susceptibility of sheep to infection with Trichostrongylus axei. A comparison of Scottish Blackface and Dorset breeds. Research in Veterinary Science 11: 465-468. SCHALM 0.
W.,
JAIN N.
Veterinary Hematology, Philadelphia.
C.
3rd
& CARROLL E.
ed.
D. & ANGUS K. W. 1980. Haemonchus contortus: attempts to immunise lambs with irradiated larvae. Research in Veterinary Science 29: 45-50.
STERZL J. & SILVERSTEIN A. M. 1967. aspects of immunity. Advances in
Developmental
Immunology
6:
337-459. THORSON R.
Immunity
E.
1970.
Direct
infection
to Parasitic Animals,
nematodes.
Vol.
2 (Edited
In:
by
JACKSON G. J., HERMAN R. and SINGER I.). AppletonCentury-Crofts, New York. TODD K. S., MANSFIELD M. E. & LEVINE N. D. 1978.
Haemonchus
contortus
infections Black-Belly cross
in Targhee and lambs. American
Journal of Veterinary Research 39: 865-866. URQUHART G. M. 1980. The application of immunity in the control of parasitic disease. Veterinary Parasitology
6: 217-239. factors
control.
haemonchosis.
109
SMITH W.
Targhee-Barbados
Journal of Veterinary Research 30: 1773-l 778.
41: 116. LE JAMBRE L. F. 1978. Host genetic
lambs to H. contortus
Lea
and
J.
1975.
Febiger,
URQUHART G. M., JARRETT W. F. H., JENNINGS F. W., MCINTYRE W. I. M., MULLIGAN W. & SHARP N. C. C. 1966. Immunity to Haemonchus contortus infection:
Failure
of X-irradiated
larvae to immunise
young
lambs.
American Journal of Veterinary Research 27: 1641-1643. URQUHART G. M., JARRETT W. F. H., JENNINGS F. W., MCINTYRE W. I. M. & MULLIGAN W. 1966. Immunity to Haemonchus contortus infection: Relationship
between age and successful vaccination with irradiated larvae. American Journal of Veterinary Research 27:
1645-1648. WAKELIN D. 1978. Genetic control of susceptibility and resistance to parasitic infection. Advances in Parasitology
16: 219-308. WHITLOCK H. V. 1948. Some modifications of the McMaster helminth egg-counting technique apparatus.
Journal of the Council for Scientsfic and Zndustrial Research 21: 177-180. WHITLOCK J. H. 1955. A study resistance to trichostrongylosis
of
the inheritance
in
sheep.
of
Cornell
Veterinarian 45: 422-439. WINDON R. G., DINEEN J. K. & KELLY J. D. 1980. The segregation of lambs into “responders” and “non-
responders”:
Response
to vaccination
with
irradiated
Trichostrongylus colubriformis larvae before weaning. International Journalfor Parasitology 10: 65-73. WINDON R. G. & DINEEN J. K. 1981. The effect of selection of both sire and dam on the response of F1 generation lambs to vaccination with irradiated Trichostrongylus
colubriformis larvae. sitology 11: 11-16.
International Journal for Para-
YAZWINSKI T. A., GOODE L., MONCOL D. J., MORGAN G. W. & LINNERUD A. C. 1979. Parasite resistance in straightbred and crossbred Barbados Blackbelly sheep.
Journal of Animal Science 49: 919-926. YAZWINSKI T. A., GOODE L., MONCOL D. J., G. W. & LINNERUD A. C. 1980. Haemonchus
resistance Blackbelly
279-284.
MORGAN
contortus
in straightbred and crossbred Barbados sheep. Journal of Animal Science 51: