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Resistance to Internal Parasites in Lambs of Hair Sheep Composite Breeds

Livestock Update, May 2003

Scott Greiner, Extension Animal Scientist, Beef, VA Tech

Summary of Results

A comparison of resistance to infection by barber-pole worms (Haemonchus contortus) in straightbred Katahdin, Dorper crossbred, Dorset crossbred, and Caribbean hair sheep crossbred lambs revealed that resistance levels were consistently higher for Katahdins than for Dorper crosses or Dorset crosses. In Katahdin ewe lambs exposed to artificial parasite infection in drylot (Table 1), average numbers of parasite eggs in the feces were 45% less than those observed in Dorset crosses and 62% less than those observed in Dorper crosses. Dorper crossbred ewe lambs had higher fecal egg counts than Dorset crosses in all 3 years. In wether lambs exposed to natural infection by grazing of contaminated pastures (Table 2), severity of infection was less than in artificially infected ewe lambs. However, fecal egg counts in Katahdin wethers on pasture were still 45% lower than those observed in Dorper and Dorset crosses. Differences in fecal egg counts between Dorper and Dorset crosses were not observed in grazing wethers. Caribbean hair sheep crosses (St. Croix x Barbados Blackbelly) were evaluated only on pasture with natural infection. Comparisons of fecal egg counts between Katahdins and Caribbean hair sheep were not consistent across years. Caribbean hair sheep had lower egg counts than Katahdins in 2001 but not in 2002.

Effects of parasitism on animal health are generally monitored by measurement of packed cell volume, which quantifies the percentage of red blood cells in a blood sample. The barber-pole worm affects its host by attaching to the gut wall and sucking blood. Animals become anemic with losses in production and, potentially, death resulting from blood loss in the gut. Low values for packed cell volume are thus indicative of anemia and are commonly associated with high fecal egg counts. Measures of packed cell volume in Katahdin lambs were generally equal to, or higher than, those of other breeds, confirming a level of resistance to parasitism. Interestingly, Dorper crosses exposed to either natural or artificial infection consistently had slightly, though not significantly, higher packed cell volumes than Dorset crosses, despite their generally higher fecal egg counts. Dorper crossbred wethers also had higher packed cell volume than Katahdin wethers under conditions of low parasite challenge in 2002.

This situation, in which an animal becomes infected by parasites but is still able to maintain reasonable health status, is sometimes referred to as resilience to infection. Dorper crosses thus appeared to be somewhat less resistant to internal parasite than Dorset crosses, at least under the more challenging environment provided by the artificial infection, but their somewhat greater resilience to infection allowed them to maintain similar packed cell volume. In contrast, Katahdin and Caribbean hair sheep crosses were clearly more resistant to parasitism, and also tended to become less anemic.

Although the Dorper and the Katahdin are both derived from hair sheep crosses, differences in parasite resistance between the two breeds are not surprising. The Dorper and the Katahdin were derived from very different types of hair sheep. The Dorper originated in South Africa from crosses between the Dorset and the Blackhead Persian. The Blackhead Persian is a fat-rumped hair breed from the arid lands of the Middle East. In South Africa, the Dorper is likewise most commonly found in arid and semi-arid regions where parasite challenge is often low. There is thus nothing in the evolutionary history of the Dorper breed to suggest that these animals would have developed resistance to internal parasites. In contrast, the Katahdin was developed from the thin-tailed Caribbean hair breeds. These breeds originally came from the hot, humid, high-rainfall regions of West Africa, where parasite challenge is extremely high and where development of parasite resistance would have been advantageous. The results observed in the current study are thus consistent with the evolutionary history of the breeds involved.

The Details

This experiment was conducted over 3 years (2000-2002) at the Southwest Virginia Agricultural Research and Extension Center in Glade Spring. Dorset and Dorper crosses were produced by mating rams of these breeds to whitefaced crossbred ewes (50% Dorset, 25% Rambouillet, and 25% Finnsheep). Four imported Dorper rams were used by AI in 2000; two different natural service rams were used in each of the next 2 years. A total of eight Dorset rams were represented. Katahdin lambs were purchased at weaning (approximately 60 days) from a total of 10 different flocks, and St. Croix x Barbados Blackbelly wethers were introduced at weaning from the Virginia Tech Sheep Center, Blacksburg. Wethers were evaluated only in 2001 and 2002, and no Caribbean hair sheep ewe lambs were tested.

Ewe lambs were maintained in drylot after weaning and at approximately 120 days of age were challenged with an oral drench of approximately 10,000 infective larvae of barber-pole worms. Ewe lambs remained in drylot after infection. Fecal samples were collected for determination of fecal egg counts and samples of blood were taken to determine packed cell volume at 3, 4, 5, and 6 weeks after infection in order to monitor the course of infection.

Wether lambs remained on pasture after weaning at about 90 days of age. They were provided with supplemental grain and dewormed as needed. At about 120 days of age, wethers were dewormed and returned to infected pastures. Fecal egg counts and packed cell volume were measured at 3, 4, 5, and 6 weeks after deworming.

Means for body weights, fecal egg counts, and packed cell volumes over the measurement period are shown for ewe lambs in each year in Table 1. Consistent breed differences in body weight were not observed. Dorper crossbred lambs sired by imported rams in 2000 were significantly heavier than lambs of other breeds, but this advantage in body weight was not observed for Dorper crosses in 2001 or 2002. Breed differences in fecal egg counts were quite consistent across years, even though considerably reduced under the low mean fecal egg counts observed in 2000. Breed differences in packed cell volume were likewise consistent in 2000 and 2001 but much-reduced in 2002.

Results for wether lambs grazing infected pastures in 2001 and 2002 are shown in Table 2. Katahdin wether lambs in 2001 were notably smaller than in 2002. The 2001 Katahdin wethers came from only one flock, and a high proportion were out of yearling ewes, so the Katahdin breed is probably better represented by the 2002 wethers and by the ewe lambs, where four flocks were sampled in each year. Despite the lower body weights of Katahdin wethers in 2001, breed rankings for fecal egg counts were consistent across years in wethers, indicating higher levels of worm resistance in breeds with Caribbean hair sheep ancestry. Breed differences in packed cell volume were consistent with those in fecal egg counts in 2001. In 2002, low rainfall in late summer reduced the level of worm challenge and required an increase in level of supplemental feeding. Under these conditions, breed differences in packed cell volume were reduced and the apparently high baseline level for packed cell volume in the Dorper was evident.

These results confirm high levels of parasite resistance in Caribbean hair sheep and a moderate level of resistance in Katahdins. Dorper crossbred lambs were not more resistant that Dorset crosses, but the Dorper appears to express a degree of resilience to infection that may reduce symptoms of parasitism in moderately infected animals.

This report was taken from the M.S. thesis of Hima Bindu Vanimisetti at Virginia Tech ( For more information, contact Dr. David Notter ( or Dr. Scott Greiner ( We would like to thank the Katahdin breeders who produced lambs for the study and to thank the American Dorper Sheep Breeders' Society for donation of the semen used to produce the 2000 Dorper crossbred lambs.

Table 1. Means and standard errors for body weights (lb), fecal egg counts (FEC; eggs/gram of feces) and packed cell volume (PCV; %) during infection for ewe lambs in drylot following artificial infection with barber-pole worm (Haemonchus contortus) over 3 years
Measurement Breed groupa 2000 2001 2002 Average
Mean body wt DO 91.8 1.1 90.7 0.8 86.8 1.5 89.8 0.7
DP 103.3 1.6 84.9 1.0 89.7 1.1 92.6 0.7
KT 90.7 1.1 90.5 1.2 85.2 1.2 88.8 0.7
Mean FEC DO 897 122 2835 303 2490 468 2074 190
DP 1064 219 4064 541 3866 564 2998 271
KT 539 79 1188 188 1720 265 1149 114
Mean PCV DO 27.9 .5 25.0 .4 26.1 .7 26.3 .3
DP 29.5 .8 26.2 .5 25.5 .6 27.1 .4
KT 30.8 .6 28.9 .6 26.2 .6 28.6 .3
aDO = Dorset crossbred, DP = Dorper crossbred, and KT = Katahdin.

Table 2. Means and standard errors for body weights (lb), fecal egg counts (FEC; eggs/gram of feces) and packed cell volume (PCV; %) during infection for wether lambs following deworming and return to contaminated pastures over 2 years
Measurement Breed groupa 2001 2002 Average
Mean body wt DO 69.6 1.0 83.2 1.3 76.4 0.8
DP 71.6 1.1 81.1 1.2 76.4 0.8
KT 50.3 1.4 90.8 1.6 70.6 1.1
HH 53.5 1.4 65.5 1.4 59.5 1.0
Mean FEC DO 1556 151 953 123 1255 97
DP 1556 160 944 108 1250 97
KT 1012 135 351 55 682 73
HH 437 60 284 39 83 36
Mean PCV DO 24.4 0.6 33.9 0.8 29.2 0.5
DP 25.9 0.6 35.5 0.7 30.7 0.5
KT 26.1 0.8 32.8 1.0 29.5 0.6
HH 27.4 0.9 32.6 0.9 30.0 0.6
aDO = Dorset crossbred, DP = Dorper crossbred, KT = Katahdin, and HH = St. Croix x Barbados crossbred.

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