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ANIMAL PRODUCTION |
* Departments of Animal and Poultry Sciences and
and
Biomedical Sciences and Pathobiology, Virginia Polytechnic Institute and State University, Blacksburg 24061
Abstract
This study was designed to evaluate breed differences in resistance to Haemonchus contortus in lambs. A total of 181 ewe lambs representing crossbred Dorsets (DO) and Dorpers (DP; out of 
-Dorset, 
-Rambouillet, -Finnsheep ewes) and straightbred Katahdins (KT) were evaluated over 3 yr. An additional 144 DO, DP, KT, and Barbados Blackbelly x St. Croix (HH) wethers were evaluated over 2 yr. Lambs were weaned at 60 to 90 d of age. After deworming at about 4 mo of age, ewe lambs received approximately 10,000 infective larvae and were evaluated for parasite resistance in drylot, whereas wethers were evaluated on pasture under conditions of natural infection. Each sex was analyzed separately. Egg counts per gram of feces (FEC), log-transformed FEC (LFEC), packed cell volumes (%), and body weights (kg) measured at 3, 4, 5, and 6 wk after deworming and reinfection were analyzed. Breed influenced all traits (P < 0.05) except BW in ewe lambs and PCV in wethers. Year and week influenced (P < 0.05) all traits. At most times, DP had the highest FEC, DO had the lowest PCV, and KT and HH had lower FEC and higher PCV than either DO or DP. Clearly, Dorper sheep were not more resistant to parasites than DO, but they were able to cope with infection better by maintaining similar or higher (P < 0.05) PCV and similar BW. Katahdin and HH were more resistant, with lower FEC (P < 0.05) than DO or DP. Breed differences were more apparent when infection levels were higher; DO and DP were less affected when infection levels were low and when animals were on a better plane of nutrition. Caribbean hair sheep originated in hot, humid regions of West Africa. They and the derivative Katahdin breed seem relatively resistant to parasitism. The Dorper, in contrast, was developed in more arid regions and exhibited little parasite resistance.
Key Words: Breeds • Disease • Health • Internal Parasites • Sheep
Introduction
Endoparasitic infections are a major concern for sheep producers. They increase costs of management and treatment, reduce production, and may cause deaths (Barger and Cox, 1984
; Larsen et al., 1995). Haemonchus contortus is prevalent in tropical, subtropical, and temperate regions, especially under warm and wet conditions, and is one of the more devastating endoparasites. This worm sucks blood, which may cause anemia, and is very prolific. Reports of anthelmintic resistance in H. contortus and related parasite species are now common (van Wyk and Malan, 1988; Overend et al., 1994; Gopal et al., 1999).
One option to control helminth infections is to breed sheep that are resistant to these parasites. Genetic variation in response to parasite infection has been documented within (Bishop et al., 1996; Morris et al., 1997; Woolaston and Windon, 2001) and among sheep breeds, mostly involving tropical and subtropical breeds, such as the Red Maasai, St. Croix, Florida Native, and Barbados Blackbelly (Gamble and Zajac, 1992; Baker et al., 1999; Notter et al., 2003). These breeds have consistently been more resistant to trichostrongyle infections than temperate breeds, such as Dorset and Rambouillet.
The Katahdin (KT) is a hair-type sheep developed in the United States from West African hair sheep and wooled British sheep (Parker et al., 1991). The Dorper was developed in South Africa from the Dorset Horn and Blackheaded Persian for use in arid regions under both extensive and intensive management (de Waal and Combrinck, 2000). Both breeds have good production capabilities and do not require shearing, but Dorpers in Kenya were more susceptible to parasitism than Red Maasai (Baker et al., 1999). The objective of this study was to evaluate differences among crossbred Dorset (DO), crossbred Dorper (DP), straightbred KT, and Caribbean hair sheep crosses (HH; St. Croix x Barbados Blackbelly) in response to both artificial and natural pasture challenge with H. contortus.
Materials and Methods
The experiment was conducted over 3 yr (Table 1). Responses of DO, DP, and KT ewe lambs to artificial challenge were monitored in 2000, 2001, and 2002. The DO, DP, KT, and HH wethers were evaluated with a natural pasture challenge in 2001 and 2002. Procedures for artificial challenge and evaluation of resistance were approved by the Virginia Polytechnic Institute and State University Animal Care Committee.
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The DO and DP crossbred lambs were produced at the Southwest Virginia Agricultural Research and Extension Center, Glade Spring, by mating Dorset and Dorper rams to ewes of 50% Dorset, 25% Rambouillet, and 25% Finnsheep breeding. In 2000, DP lambs were sired by four imported rams used by AI. In 2001 and 2002, two different DP sires were used in each year by natural service. These four rams came from two flocks and were offspring of four different sires. Thus, a total of eight DP rams by eight different sires were represented. Three DO rams were used in each year. Seven DO rams were produced in the Virginia Tech Dorset flock from five different sires and were used for only 1 yr each. An eighth DO ram was purchased and used for 2 yr.
Unregistered, commercial KT ewe lambs were purchased (Table 1
). Most were born in April, although six were born between March 20 and April 1. In each year, ewe lambs were purchased from four different flocks (two to six lambs per flock). In all, 10 flocks were represented, with 2 to 11 lambs per flock. In most cases, two sires were represented for each flock. The KT wether lambs were purchased in 2001 and 2002. In 2001, 15 lambs by two sires came from a single flock. In 2002, 15 wethers were sampled from four flocks (including a second sample from the flock sampled in 2001), with 3 to 5 wethers per flock. The HH wethers were produced at the Virginia Tech Sheep Center in Blacksburg by rotational crossing of St. Croix and Barbados Blackbelly ewes and rams. One St. Croix and one Barbados Blackbelly ram were represented in each year.
Experimental Procedures
The DO and DP crossbred ewe lambs were weaned in mid-June at about 60 d of age in 2000 and 2001, moved to drylot, and fed a complete pelleted diet containing primarily corn, soybean meal, and soybean hulls and with approximately 14.5% CP, 23% fiber, and 71% TDN (DM basis). In 2002, ewe lambs were weaned at about 90 d, after being creep-fed for 1 mo before weaning. The KT and HH lambs were weaned and delivered to the station at approximately 60 d of age, and all ewe lambs were maintained together in drylot after weaning. Wether lambs were maintained as contemporaries on pasture except in 2002, when purchased wethers were maintained in drylot until DO and DP wether lambs were weaned at about 90 d of age. In 2001, wethers were creep-fed the same diet provided to the ewe lambs after weaning at a level (0.9 ng/d) anticipated to maintain daily gains of approximately 200 g/d. However, realized gains in 2001 were less than anticipated (approximately 150 g/d), so the wether lambs were creep-fed ad libitum in 2002, with mean gains of 196 g/d.
At 4 to 5 mo of age, all lambs were dewormed with levamisole hydrochloride (Tramisol; Schering-Plough Animal Health, Union, NJ) at a dosage of 8 mg/kg BW. Ewe lambs were then dosed with approximately 10,000 infective larvae of H. contortus 2 to 4 d after deworming and subsequently remained in drylot. Wether lambs were returned to pasture after deworming and evaluated under natural infection. Jugular blood samples to estimate packed cell volume (PCV, %), rectal fecal samples to estimate fecal egg count (FEC, eggs per gram), and BW were obtained at 3, 4, 5, and 6 wk after infection. In 2001 and 2002, PCV and BW were also obtained at deworming. Initial BW (IBW) and PCV were not measured in 2000, but body weight 10 d postinfection was taken as the initial weight in 2001. In ewe lambs, BW were taken only at 4 and 6 wk postinfection in 2000; hence, only these weights were reported across years.
Animals were removed from the experiment and dewormed if their PCV fell below 18% and they lost weight. In 2001, 14 wethers (five DO, six DP, two KT, and one HH) with low PCV were dewormed after 5 wk postinfection and 1 DP wether died at 4 wk postinfection from causes apparently unrelated to parasitism. Also, in 2001, nine ewe lambs (five DO and four DP) were accidentally dewormed after sampling at 5 wk postinfection and were removed from the experiment. In 2002, one DP ewe lamb was removed from the study and dewormed after sampling at 5 wk postinfection because of low PCV; two wether lambs (one KT and one DP) died after 4 wk postinfection from causes apparently unrelated to parasitism. All lambs were dewormed immediately after the final sampling.
Fecal egg counts were estimated using the Modified McMaster’s method (Whitlock, 1948), with each egg counted representing 25 eggs per gram. Samples of less than 2 g were discarded. Jugular blood samples were used to determine PCV by the microhematocrit centrifuge method. Missing observations occasionally resulted when fecal samples of adequate size could not be obtained or from clotting of blood samples. The frequency of missing values was 11% for FEC and 6% for PCV in wethers and 10.5% for FEC and 3.6% for PCV in ewe lambs.
Statistical Analysis
Data from ewe lambs and wethers were analyzed separately. The distribution of FEC was not normal, so observed values were transformed as LFEC = ln(FEC + 100). One very high FEC observation of 30,725 eggs per gram for a 2002 DP ewe lamb was deleted as an outlier. For each sex, a repeated-measures analysis of variance was conducted using the mixed model procedure of the SAS software package (SAS Inst. Inc., Cary, NC) to describe weekly changes in BW, FEC, LFEC, and PCV. The model included fixed effects of year, breed, week (the repeated effect), and all interactions.
A set of summary traits was also defined for each animal and used to investigate interrelationships among variables. Summary traits included initial PCV (IPCV) and means for BW (MBW), PCV (MPCV), FEC (MFEC), and LFEC (MLFEC) during infection. In calculation of summary traits, missing values for FEC or PCV were replaced by predicted values derived from a nested analysis of variance (from the linear models procedure of SAS) that included fixed effects of year, breed, week, their two-way interactions, and a random effect of animal nested within breed and year. The mixed-model procedure did not provide predictions of missing values. Summary traits were then analyzed using multivariate analysis of variance with year, breed, their two-way interactions, and the continuous effect of deviations from the breed-year mean IBW in the model. Least squares means were thus adjusted to the breed-year mean IBW. Correlations among summary traits were obtained from these analyses, and regression analyses were conducted to quantify associations between MPCV and IPCV, MFEC and MPCV, MLFEC and MPCV, MFEC and MBW, and MLFEC and MBW. In particular, MPCV was adjusted, by linear regression, for differences in MFEC to evaluate breed differences in ability to cope with parasitism at same level of infection.
Results and Discussion
Ewe Lambs
In the repeated-measures analysis, year, week, and year x week interaction influenced all traits (P < 0.05). There was an effect of breed (P < 0.05) on all traits except BW (P = 0.29). There was breed x week interaction for FEC and LFEC (P < 0.05) and breed x year interaction for BW and PCV (P < 0.05). For summary traits, year and breed affected all traits (P < 0.01), and there was a breed x year interaction for MBW and MPCV (P < 0.05).
Ewe lambs generally continued to grow during the study period, but body weight differences were not consistent across years (Figure 1). In 2000, DP were heavier (P < 0.05) than both DO and KT. In 2001, DP ewe lambs did not differ in IBW from DO (P = 0.11) or KT (P = 0.16) but had lower MBW (P < 0.05) than either DO or KT. In 2002, DP were heavier (P < 0.05) than KT but were not different from DO (P = 0.11). Overall, there were no consistent differences in body weight among ewe lambs of these three breeds.
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). The DP consistently had higher FEC than KT (P < 0.01 to P < 0.05; Figure 2). Although there were no differences between DP and DO in 2000 (P = 0.30 to P = 0.60), DP had higher (P < 0.05) FEC than DO five times out of eight in 2001 and 2002 (Figure 2). For summary traits, FEC were lower (P < 0.05) in 2000 than in 2001 or 2002 (Figure 1). Dorper ewe lambs had higher mean FEC in 2001 and 2002 (P < 0.05) and, although differences among breeds were not significant in 2000 when FEC levels were relatively low, they were consistent, in terms of breed rankings, with the larger differences observed in 2001 and 2002.
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and then remained relatively stable through wk 6. For summary traits, MPCV was higher (P < 0.05) in 2000, when mean FEC were low, than in 2001 and 2002. Across years, KT had higher IPCV than DO (P < 0.05) although there were no significant differences between breeds for IPCV within each year (P = 0.08 to P = 0.36; Figure 1). The KT had significantly higher MPCV (P < 0.01) than DO in 2 of the 3 yr but differed from DP (P < 0.01) only in 2001. There were no differences between DO and DP in MPCV in any year (P = 0.08 to P = 0.53), despite the generally higher FEC observed in the DP. Breed differences in MPCV were not observed in 2002, despite the relatively high mean FEC and relatively low mean PCV (Figure 1). Ewe lambs in 2002 were weaned later and were about 3 wk older (but not heavier) when infected, which may have influenced their response to infection. Responses to infection were more variable in KT ewe lambs. Four KT in 2002 had high FEC and low PCV, and one KT did not show any effects of parasitism, with FEC of 25 eggs per gram at all measurement times. However, the greater variation observed in KT was not limited to 2002; coefficients of variation for LFEC over the 3 yr ranged from 15 to 19% for KT, compared with 7 to 12% for DO and 6 to 13% for DP. Greater variability in FEC responses may be associated with sampling from different KT flocks and with variation in parasite exposure during the preweaning period. However, lambs were maintained as contemporaries for 60 d before evaluation of parasite resistance. The observed variation may also be a characteristic of this relatively recently formed hair-wool composite breed.
Correlations among MBW, MPCV, MFEC, and MLFEC are shown in Table 2. Initial BW was not associated with any summary trait except MBW (P < 0.01). Both measures of FEC were negatively correlated with MBW and MPCV. Mean PCV and MBW were not correlated. An increase in MFEC of 100 eggs per gram led to a 0.07 ± 0.01% decrease (P < 0.05) in MPCV and a 1-unit increase in MLFEC led to a 1.85 ± 0.26% decrease (P < 0.05) in MPCV. After adjustment of MPCV for MFEC, DP had a higher (P < 0.05) MPCV than DO and were similar in MPCV to KT. Thus, DP appear to be more resilient than DO, with higher PCV at the same infection level. Regression of MBW on MFEC and MLFEC indicated that increased FEC measures adversely affected (P < 0.05) MBW; a 1-unit increase in MLFEC was associated with a 1.65 ± 0.65 kg decrease (P < 0.05) in MBW, and an increase of 1,000 eggs per gram of MFEC led to an decrease of 0.51 ± 0.23 kg (P < 0.05) in MBW.
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Wethers
For the repeated-measures analysis, there was an effect of year x breed x week interaction for FEC and LFEC (P < 0.05) and a year x breed interaction for BW and PCV (P < 0.05). Breed and breed x week interaction affected all traits (P < 0.05) except PCV (P = 0.08), and there was an effect of year, week, and year x week interaction on all traits (P < 0.05).
Overall, wethers continued to grow during the study, although an initial mean weight loss of 1.09 ± 0.92 kg and minimal overall growth were observed for HH wethers in 2001. In 2001, DO and DP were heavier (P < 0.05) than KT and HH (Figure 3). However, in 2002, KT had IBW similar to those of DO and DP and higher MBW (P < 0.05) than all other breed groups. The 2001 KT lambs came from a single source and were mainly offspring of 1-yr-old ewes. Thus, body weights of these lambs were likely less representative of the KT breed than those observed for ewe lambs and 2002 wethers.
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). Across years, fecal egg counts were highest in DO and DP, lowest in HH, and intermediate for KT (Figure 3). However, in 2002, KT and HH did not differ (P = 0.78) in FEC, and both had lower (P < 0.05) MFEC and MLFEC than DO and DP.
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). In both years, PCV increased by an average of 4.9% from the time of deworming until the third week postinfection in DO and DP, but changes in PCV of only 1.2 and 0.5% were observed over this period for KT and HH, respectively. In 2001, PCV then declined in a nearly linear manner between 3 and 6 wk postinfection but the overall magnitude of the decline was greater for DO (from 29.2 ± 0.6% at wk 3 to 20.5 ± 0.5% at wk 6), DP (from 30.5 ± 0.7% at wk 3 to 19.6 ± 0.5 at wk 6), and KT (31.1 ± 0.9% at wk 3 to 23.7 ± 0.6 at wk 6) than for HH (29.9 ± 0.9% at wk 3 to 25.5 ± 0.6% at wk 6). In contrast, changes in PCV after 3 wk postinfection were almost negligible in 2002 and no consistent time trend was observed. In terms of summary traits (Figure 3), large breed differences were observed in IPCV, with smaller, but corresponding, differences in MPCV. In 2002, there were no differences among breeds in IPCV, but DP had higher (P < 0.05) MPCV than HH and KT. Estimates for MFEC, MLFEC, and MPCV at 6 wk postinfection may have been influenced by estimation of missing values for the five DO and six DP that were removed from the study at the fifth week postinfection because of dangerously low PCV.
The initial increase in PCV observed in wethers after deworming presumably reflects recovery from effects of previous infection acquired on pasture. This result is in contrast to that observed in ewe lambs, which showed an initial decline in PCV after deworming and infection. The FEC in wethers was expected to increase throughout the period of study because they were being continually reinfected. This pattern was seen in 2001, but in 2002 FEC began to decline by the fifth week postinfection (Figure 4) and PCV remained stable throughout the period. Mean FEC in 2002 were lower than in 2001 (P < 0.01), suggesting lower levels of infection on pasture under the dry weather conditions experienced in 2002. Lambs on pasture were also fed ad libitum in 2002, which may have reduced effects of infection. Preston and Allonby (1978) and Kambara et al. (1993) reported that animals on a better plane of nutrition, and especially those receiving protein supplementation, appeared to be more resistant.
Correlations between summary traits in wethers are given in Table 2. Both MFEC and MLFEC were negatively correlated with MBW, MPCV, and IPCV. An increase in MFEC of 1,000 eggs per gram led to a decrease of 1.7 ± 0.3% in MPCV and a 1-unit increase in MLFEC led to a 2.6 ± 0.5% decrease (P < 0.05) in MPCV. Adjustment of MPCV for MFEC did not affect breed rankings, but, after adjustment of MPCV for MLFEC, DO had higher MPCV than HH or KT (P < 0.05). Regression of MBW on MFEC and MLFEC indicated that FEC measures were unfavorably associated with MBW (P < 0.05); an increase of 1,000 eggs per gram of MFEC led to a decrease of 1.57 ± 0.46 kg (P < 0.05) in MBW, and a 1-unit increase in MLFEC led to 2.86 ± 0.79 kg decrease (P < 0.05) in MBW. Mean PCV and IPCV were positively correlated with each other, and both were positively correlated with MBW. The MPCV increased by 0.28 ± 0.04% for each unit increase in IPCV (P < 0.05). Initial BW was positively associated with MBW, IPCV, and MPCV and negatively associated with MFEC and MLFEC.
General Discussion
Despite considerable variation in observed breed differences among years and sexes, this study provides a generally consistent picture of differences among these breeds in resistance to H. contortus. When ewe lambs were artificially infected and maintained in drylot without further infection, FEC were consistently and generally significantly higher for DP than for DO lambs. However, in wether lambs grazing infected pastures, mean FEC were lower than those expressed in artificially infected ewe lambs, and mean differences in FEC between DP and DO wethers were not observed.
Despite their generally higher FEC, DP lambs maintained PCV levels that were equal, or slightly superior, to those of DO lambs. Dorper lambs thus show no evidence of greater resistance to H. contortus compared to DO lambs when resistance is defined as the ability of the host to resist establishment of the parasite and modify its egg production (Gray, 1995). This result is perhaps not surprising, in that the Blackhead Persian, which is the fat-tailed hair sheep ancestor of the Dorper, evolved under arid, Middle Eastern conditions, where exposure to internal parasites was low. Further, the Dorper in South Africa is most common in arid regions and would not necessarily have developed resistance to internal parasites. However, in characterizing host responses to parasite infection, Gray (1995) also defined resilience to infection as the ability of the host to thrive in the presence of the parasite, and DP crosses thus appear to be relatively resilient.
Because DP lambs were of only 50% Dorper breeding and because both resistance and resilience are anticipated to increase with age (Courtney et al., 1985), purebred Dorper ewes may be able to tolerate a reasonable level of parasite challenge as adults in this environment. Burke and Miller (2002) reported that mature Dorper crossbred ewes were more resistant to parasitic infections than Hampshire ewes and that their resistance levels were comparable to those of St. Croix and Katahdin ewes in the southeastern United States. However, Dorper ewes in the subhumid coastal region of Kenya were more susceptible to H. contortus infection than indigenous, fat-tailed Red Maasai ewes (Baker et al., 1999). Increasing the proportion of Dorper genes in Dorper x Red Maasai crossbred lambs reduced both resistance (i.e., increased FEC) and resilience (i.e., decreased PCV) in Kenya (Baker et al., 2003). Use of a resilient, but not resistant, ewe breed in crossing with terminal sires of a susceptible breed is also a concern because resulting crossbred lambs would be exposed to high levels of pasture contamination.
The St. Croix x Barbados Blackbelly crossbred wethers had much lower FEC than either DP or DO crossbred wethers. These results support early reports of high levels of resistance to internal parasites in lambs of Caribbean hair breeds (Gamble and Zajac, 1992). Thus, under conditions of natural infection, breed differences in FEC between St. Croix and Dorset lambs were observed at 47 d after first exposure to H. contortus at 8 wk of age, and again at 24 d after deworming and reinfection (Gamble and Zajac, 1992). Saint Croix lambs had 99% fewer worms in the abomasum compared to Dorset lambs, were more refractory to reinfection, and had higher levels of acquired resistance in response to both natural and experimental infections. In other studies involving H. contortus, Zajac et al. (1990) found that 9- to 10-mo-old Dorset x Rambouillet lambs were less resistant to infection than age-matched St. Croix and Florida Native lambs; Courtney et al. (1985) reported that St. Croix lambs were more resistant to a secondary infection at 4 to 6 mo of age than domestic crossbred lambs (-Suffolk, -Dorset, -Finnsheep or -Dorset, -Finnsheep, and -Rambouillet); and Notter et al. (2003) reported that 4-mo-old St. Croix x Barbados Blackbelly lambs had lower FEC and higher PCV than lambs of 50% Dorset, 25% Rambouillet, and 25% Finnsheep breeding in response to an artificial infection.
Katahdin lambs consistently had lower log-transformed fecal egg counts than DO or DP lambs, but, because of greater variability in FEC in KT, differences in untransformed FEC were not always significant. Corresponding differences in PCV were not observed in wether lambs under the modest FEC observed on pasture but KT ewe lambs in drylot had higher PCV than DO or DP in 2 of 3 yr. Similarly, mature Katahdin and St. Croix ewes were found to have similar levels of resistance, in both natural and artificial nematode infections (Burke and Miller, 2002). The Katahdin breed was developed from crosses between Caribbean hair breeds and U.S. wool sheep, with some subsequent infusion of Wiltshire Horn breeding. Caribbean hair breeds were derived from small-tailed West African hair breeds (Bradford and Fitzhugh, 1983) and have a different origin and a greater history of exposure to internal parasites than either the Dorper or its Blackhead Persian ancestor. The level of resistance observed in KT lambs in this study is thus consistent with the proportion of Caribbean hair sheep genes represented in the various breed groups: 100% for HH, approximately 50% for KT, and 0% for DO and DP.
Implications
Breed differences in resistance to Haemonchus contortus in lambs were observed in response to both artificial infection and natural pasture challenge. Caribbean hair sheep and Katahdin lambs were more resistant to infection than Dorset and Dorper crossbred lambs. Dorper lambs were not resistant but seemed to cope better with infection than Dorsets. Differences among breeds in the ability to cope with infection were decreased when animals were maintained on a higher plane of nutrition, and were less apparent, especially between Dorset and Dorper, when infection levels were low.
Footnotes
1 This paper is dedicated to the memory of Mr. Allen Brock in recognition of his many years of skilled animal care at the Southwest Virginia Agric. Exp. Stn. The authors also wish to thank the American Dorper Sheep Association for donation of semen used to produce the 2000 Dorper lambs and R. Gamble and J. McCray of the USDA-ARS, Beltsville, MD, for cultures of H. contortus larvae. 
2 Correspondence—phone: 540-231-5135; fax: 540-231-3010; e-mail: drnotter{at}vt.edu.
Received for publication April 14, 2003. Accepted for publication October 20, 2003.
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