Department of Animal Science Florida Agricultural
Mimeograph Series No. AN 66-2 Experiment Station
August, 1965 Gainesville, Florida
GENETIC INFLUENCE ON THE PARASITIC INFECTION LEVELS OF SHEEP1/
P. E. Loggins,.L. E. Swanson and M. Koger-
Previous work by Whitlock et al. (1955) showed an inherited resistance
to internal parasites in sheep and suggested that rams could be selected to
produce sheep with less susceptibility to parasite infections. Gibson et al.
(1953) showed an acquired resistance in sheep to infections of Trichostrongy-
lus axei. Unpublished observations by Florida Sheep Breeders suggest that
death losses due to parasitism may be due to a genetic difference that exists
in the Florida Native flocks. Preliminary data collected (1953-55) on 11
Southdown ewes maintained for teaching purposes showed that the internal
parasite infection levels were excessively high in this breed under Florida
These data were collected over a nine year period from sheep located at
the Florida Agricultural Experiment Station, Gainesville, Florida.
A flock of sheep was selected for a spring lamb production study in
1955. The flock consisted of three breed groups which originated from the
following area: Hampshire from Tennessee; Rambouillet from Texas and Florida
Native from within the state. Recommended management practices and anthel-
mentics were used to control internal parasites with all breed groups receiv-
ing equal treatment. The ewe flock was maintained as one group from 1956-62
except for a 60 to 90 day breeding season. The experimental flock was ex-
posed to the same pastures of equal infestation rate though not measured in
this study. In 1962 the breed grouDs were separated but maintained on similar
pastures. During the 1955-64 period various drenching programs were used and
the interval between treatments varied from 21 days, 28 days, 56 days, 90 days
and once a year. The ewe and lamb groups were randomly assigned to the dif-
ferent treatment groups. Control groups of ewes and lambs were maintained
for the anthelmintic experimental studies. Data from the negative control
and similar treated anthelmintic groups were used to evaluate the genetic
difference in the sheep flock.
The criteria used to measure the parasitic infection level were hemo-
globin levels, fecal ova counts per gram feces, necropsy worm counts and
death losses due to parasitic infections. Haemonchus contortus (large stomach
worms) infection levels were principally used in analyzing for the genetic
1/ This work was accomplished through a cooperative project with the
Veterinary Science Department.
2/ Loggins, Assistant Animal Husbandman; Koger, Animal Geneticist; Swanson,
Parasitologist, Veterinary Science Department. The assistance of J. U.
Stokes and L. C. Lewis, is gratefully acknowledged.
differences. The fecal ova counts and the necropsy worm counts were de-
termined by routine parasitology techniques. Hemoglobin levels were de-
termined on the ewe flock every 28 days to establish a pattern to aid in
diagnosis of an internal parasite infection. The data were analyzed sta-
tistically by methods as described by Snedecor (1956).
Results and Discussion
The genetic breed difference of the experimental sheep flock was first
statistically shown from data collected for hemoglobin level determinations
by these workers (Loggins et al., 1960). Hemoglobin level readings were
determined in the ewe flocks every 28-days over a 42-month period. A sum-
mary of these data are reported in Figure 1.
The hemoglobin levels varied significantly between years with the
Florida Natives reaching the highest average level of 10.5 in November,
1957. Holding weight constant by using covariance technique, it was found
that year, season within year and breed significantly influenced the hemoglo-
bin levels. The lighter weight Florida Native ewes had a higher hemoglobin
level than Hampshire than Rambouillet ewes.
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April Aug. Dec. Mar. July Nov.
1957 1957 1957 1958 1958 1958
Figure 1. Hemoglobin levels of Native, Hampshire, and
Rambouillet breeding ewes. 1956-1959.
Results obtained on fecal ova counts, necropsy worm counts and death
losses are reported in tables 1-4. These data were analyzed to measure breed
differences in the internal parasitic infection levels of sheep. Necropsy
counts on 19 lambs to determine the H. contortus levels are shown in table 1.
The 19 lambs were a negative control group for an anthelmintic study and were
slaughtered on the fifth day following the treatment period.
TABLE 1. AVERAGE NUMBER OF H. CONTORTUS AT NECROPSY
ON NEGATIVE CONTROL LAMBS (1959)
Number of lambs
Av. Hb level
Av. number of H. C.
' P <.01
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The data in table 1 shows a highly significant breed difference in the
level of H. contortus counts. Florida Native lamb hemoglobin levels were
significantly higher than the other breed groups. These data follow the
same pattern as reported earlier in this paper for the ewe groups.
The low production level of the Hampshire flock plus a high per cent
death loss, heavy internal parasite worm counts, resulted in their removal
from the study in 1959 after four years. All later comparisons were made
on the Rambouillet versus the Florida Native breed groups.
Table 2 shows
of feces from ewes
into drylot versus
the average number of H. contortus ova counts per gram
for the 1960-62 period. The ewes were randomly divided
pasture groups for this period.
TABLE 2. AVERAGE H. CONTORTUS OVA COUNTS PER GRAM FECES
t Fla. Natives
Ova counts 11-14-61
Ova counts 3-14-62
Ova counts 11-21-62
Ova counts 2-23-63
*: P <.01
The Florida Native ewes both in drylot and on pasture had
lower H. contortus fecal ova counts than the Rambouillet ewes.
overall significant difference between drylot and pasture.
There was no
Fecal ova counts on the lamb crops raised in a drylot versus pasture
study (1960-62) are shown in table 3. There was a significantly lower H.
contortus ova count for lambs raised in drylot compared to those on past7xre.
The genetic difference was more pronounced in the pasture raised Florida
Native lambs which were significantly lower in H. contortus fecal ova
TABLE 3. AVERAGE H. CONTORTUS OVA COUNTS PER GRAM FECES (LAMBS)
Rambouillet Fla. Natives Rambouillet Fla. Natives
Number lambs 17 20 23 22
Ova counts 3-14-62 51 39 2557 1356**
Ova counts 5-16-62 192 273 2618 578**
Number of lambs 23 26 19 28
Ova counts 2-23-63 330** 36 294 108**
Ova counts 5-17-63 14 27 2862 771*'
*A P <.01
Twenty lambs were slaughtered from the 1962 lamb crop and stomach worm
counts were determined. These results are reported in table 4.
TABLE 4. AVERAGE NUMBER OF STOMACH WORMS AT NECROPSY ON LAMBS (1962)
Rambouillet Fla. Natives Rambouillet Fla. Natives
Number of lambs 4 5 6 5
Av. number H. contortus 42 33 1272** 2073
Av. number 0. circumcinta 0 0 67* 160
Av. number T. axei 0 7 178 220
The actual worm counts in the drylot-raised lambs were significantly
lower than for the pasture-raised lambs. The 1962 Florida Native pasture-
raised lambs were significantly higher in H. contortus worm counts than the
Rambouillet. These are the only data (table 4) during the nine year period
where sheep of Florida Native breeding showed higher infection levels than
the Rambouillet. It is not known why this occurred.
Rambouillet and Florida Native ewes and lambs were separated and main-
tained as two separate flocks in April, 1962. These data collected on the
ewes and lambs to show a genetic breed difference are reported in tables 5
through 8. The use of anthelmintics was reduced to a minimum with the 1954
ewe flock. They received only one series of two phenothiazine drenches (21
day interval) 30 days prior to the lambing date. Lambs-were not drenched
until after weighing and grading on May 20 of each year. The Rambouillet
and Florida Native ewe and lamb groups received the same treatment and were
grazed on the same types of forages at all times.
H. contortus fecal ova counts (table 5) were determined in November of
each year and following weaning of the lambs (70 days of age) in February
and March. Ova counts were significantly lower in the Florida Native ewes.
TABLE 5. AVERAGE H. CONTORTUS OVA COUNTS PER GRAM FECES
Rambouillet Florida Natives
Number ewes 51 51
Ova counts 11-13-62 12 1**
Ova counts 3-15-63 7220 904**
Number ewes 49 52
Ova counts 10-15-63 868 76**
Ova counts 11-11-53 12 2**
Ova counts 2-27-64 3936 1624'
** P <.01
The data shown in table 6 were obtained from lambs that had not been
treated from birth to control internal parasites. A significantly lower
H. contortus counts were found in the Florida Native lambs.
TABLE 6. AVERAGE H. CONTORTUS OVA
FECES (LAMBS 1963-64)
Ova counts 3-15-63
Ova counts 5-20-63
COUNTS PER GRAM
Ova counts 2-27-64
Ova counts 5-20-64
** P <.01
Necropsy worm counts were determined on 56 lambs from the 1963-64
study. The lambs that were slaughtered in May of each year as shown in
table 7 had not been treated to control internal parasites. The lambs
slaughtered in September, 1964 received one treatment of thiabendazole
prior to the 90-day drylot summer feeding trial. Stomach worm counts
show a significantly lower count in the Florida Native lambs in all
three groups of lambs in table 7.
TABLE 7. AVERAGE NUMBER OF STOMACH
WORMS AT NECROPSY ON
Number Number Number Number
lambs H. contortus 0. circumcinta T. axei
Rambouillet 6 4,800 700 167
Florida Natives 6 1,367"* 300** 33**
Rambouillet 8 5,125 150 412
Florida Natives 8 2,762** 125 125**
Rambouillet 11 909 0 73
Florida Natives 17 194'* 0 6**
** P <.01
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During the production, of the 1965 lamb crop the reduced use of
anthelmintics and yearly climatic differences increased death losses due
to parasitism. The per cent death loss was high in the Rambouillet ewe and
lamb crop, due primarily to parasite infestation as shown in table 8.
TABLE 8. DEATH LOSSES IN THE 1965 LAMB CROP PRODUCTION
Rambouillet Florida Native
Ewes Lambs Ewes Lambs
Number ewes & lambs 49 55 51 65
Period of death losses
September December 31, 1964 1 0 0 0
January March 1, 1965 11 0 1 2
March May 31, 1965 2 11 0 0
June September 1, 1965 1 0 0 0
Total death losses 15 11 1 2
% death loss 31 20 2 3
Nine years of data were summarized to study the effect of breed difference.
on the hemoglobin levels and internal parasitism in sheep. The number of
sheep included approximately 100 ewes and their lamb production (90 larbs)
for each year of the study. Following is a summary of the results obtained:
1. Significantly higher hemoglobin levels were obtained in the Florida
2. H. contortus ova counts were significantly lower in the Florida
Native ewes and lambs.
3. Necropsy counts for H. contortus infection levels were signifi-
cantly lower for the Florida Native lambs.
4. Death losses were excessively high in the Rambouillet e!'es and
lambs during the production of the 1965 lamb crop between September
1, 1964, to September 1, 1965. These losses can be attributed to
out reduction in anthelmintic treatments during this phase of the
5. The lower infection level, resistance or tolerance to H. contortus
in sheep is best explained by the genetic differences between the
breeds studied. This difference in the Florida Native may be an
6. Further work is needed to determine the mechanism of this genetic
difference; also, if this phenomenon can be utilized in developing
a parasite control program for other species of animals.
1. Gibson, T. E. 1953. Great Britain. The development of acquired
resistance by sheep to infestation with Trichostrongylus axei. J.
2. Loggins, P. E., J. T. McCall and J. R. Crockett. 1960. Relationship
of hemoglobin levels and body weight of Hampshire, Rarbouillet and
Florida Native ewes at different seasons. J. Animal Sci. 19:653.
3. Loggins, P. E., L. E. Swanson and M. Koger. 1965. Parasite levels
in sheep as affected by heredity. J. Animal Sci. 24:287.
4. Snedecor, G. W. 1956. Statistical methods. 5th ed. The Iowa State
College Press, Ames, Iowa.
5. Unpublished observations by Florida Sheep Breeders.
6. Whitlock, J. W. 1955. A study of the inheritance of resistance to
Trichostronglidosis in sheep. Cornell Vet. 45:422-439.