| dc.description.abstract | The gastric nematodes, Haemonchus contortus and Ostertagia
(TeladrosagiaJ circumcincta, are the most pathogenic and economically
important parasites of sheep in tropical and temperate areas,
respectively. There are no vaccines available and the control methods
rely entirely on grazing management and anthelmintic treatment.
However, there are increasing constraints on the continued use of
these conventional methods and alternative approaches for controlling
gastric nematodes are under investigation. One of these, namely,
genetic resistance to helminth infection is considered in this thesis.
Chapter 1 is a general introduction and review of gastric
nematodes with respect to production losses, pathogenesis, limitations
of the conventional methods of control, and evidence for genetic
variation in resistance. The association between eosinophilia and
parasite infection, including, the role of Tvlymphocytes, chemotactic
factors 'and possible effector mechanisms, are considered.
Chapter 2 is a general description of the materials and methods
and includes, the type of sheep, their management, laboratory
techniques and the methods of statistical analysis used to test for
differences between groups or individual animals, including other
interactive factors such as sex, time, drug treatment.
Chapter 3 reports the results of an investigation on the
eosinophil response of lambs to natural mixed nematode infection,
predominantly O.circumcincta. Faecal egg counts and peripheral blood
eosinophil counts were found to be overdispersed, with wide variations
among the lambs. The correlation coefficient between faecal egg count
and peripheral blood eosinophilia was negative and significant
suggesting that peripheral blood eosinophilia might be an indication of
resistance to gastric nematode infection.
The repeatability of faecal egg counts taken three times at
monthly intervals was not significant from zero (repeatability was
0.06), while that of peripheral blood eosinophil counts was 0.25 and
was significant.
Chapter 4 describes the results of an investigation on the
existence of diurnal variation of peripheral blood eosinophil counts in
sheep infected with 50,000 L3 O.circumcincta. Diurnal variation was
observed during low to moderate levels of peripheral eosinophilia with
the highest counts around 0900 hrs and the lowest around midnight.
However, at time of peak peripheral blood eosinophilia following
infection, diurnal rhythm was not observed. Thus, the time of
sampling peripheral blood eosinophilia is an important factor to
consider during low to moderate levels of infection, but not at peak
infection period. The factors that might influence diurnal variation,
including cortisol levels in blood are considered.
Chapter 5 presents the results of an investigation on eosinophil
responses of 20 Scottish Blackface lambs following three experimental
infections with 10,000 L3 H.contortus. The greatest decrease in
growth rate of infected lambs was observed during the first infection.
Following the second infection, there was a slight increase in growth
rates, while followingthe third infection, growth further increased.
There were marked differences in the degree of resistance to
infection with H.contortus. Four lambs died before the end of the
experiment and were classified as susceptible. The 16 lambs that
survived, classified as resistant, showed marked differences in weight
gains, in faecal egg counts, in the ability to maintain stable packed red
cell volumes, in peripheral blood and abomasal tissue eosinophilia, and
in worm burdens.
There was a significant negative correlation between faecal egg
output and body weight gains during the second and third infections
and, between PCV and' body weight gains, there was a significant
positive correlation during the third infection. In all three infections,
the correlation between faecal egg output and PCV was significant and
positive.
With respect to peripheral blood eosinophilia, it was
significantly and positively correlated with body weight gains during
the first and second infections, significantly and negatively correlated
with faecal egg counts during the second and third infections, and
significantly and positively correlated with PCV values in the second
and third infections. At the same time, peripheral blood eosinophilia
and abomasal tissue eosinophilia were significantly and positively
correlated; both were significantly and negatively correlated with
worm burdens.
Thus, peripheral blood eosinophilia was significantly correlated
to paramet=rs that reflect resilience (body weight) and parameters that
reflect resistance (worm burden, faecal egg output and PCY) and might
have a potential as a marker for selection.
Chapter 6 is a general discussion with proposals for further
investigations on eosinophils, including, heritability estimates of
eosinophilia, association of eosinophilia with major histocompatibility
complex, and eosinophil chemotaxis and adherence. | en |
