Research Results - (PDF, 101 mb) - USAID

The second approach was to continue
screening kids raised in confinement by challenge
with 10,000 H.contortus larvae. Two
groups (42 and 27 kids) have been screened.
Two bucks and three does have been identified
that had fewer than 500 EPG in challenge infections.
These results are interesting because
other kids had greater than 10,000 EPG. We
plan to screen two additional groups of kids
before January 1991 using the same design. In
addition, we will expose kids with less than 500
EPG (in primary infections) to a secondary
challenge for confirmation. Kids experiencing
low EPG identified in these two approaches will
be included in breeding challenge experiments
planned for 1991.
Project Achievements
Goats have been identified that resist H.
contortus larval challenge.
Changes in Workplan
We initially planned to compare two
groups of at least 10 goats each-one group with
low egg counts averaging less than 300 eggs per
gram of feces (EPG), and the other group averaging
more than 1000 EPG. Because of irsufficient
kids available for the low EPG group, we
have focused our efforts on obtaining additional
kids that resist H. contortusinfections. We have
used two approaches to identify these kids. In
the first, four does from a low EPG group were
bred to one buck from the same group. This
produced five kids that were raised in confinement
and challenged with 10,000 H. contortus.
One doe had undetectable EPG on both the
primary and secondary challenge infections.
Additional kids will be available from this
breeding stock for testing in the coming year.
Activity No. 2. Identification of Antigens
for the Diagnosis and Prevention of
Heartwater in Goats.
Objective
Identify and evaluate Cowdria
ruminantiumsurface proteins.
Problem Statement and Approach
Heartwater is caused by the rickettsia C.
ruminantiumand infects goats, sheep, and cattle.
In goats, the disease causes a very high mortality
and can be devastating in susceptible populations.
Ti;.nsmission is by ticks, primarily
Amblyomma variegatum,which is widely distributed
in Kenya. The approach is to identify C.
ruminantiumsurface proteins in expression
libraries made from genomic DNA.
Justification
The identification of recombinant surface
proteins of the organism is the first step toward
development of a subunit vaccine for the disease.
Such a vaccine could be used to prevent
the disease and eventually become part of a
multivalent recombinant vaccine that would
induce protection against the major goat diseases.
Project Progress
During the evaluation of the genomic
DNA library for expression of surface proteins it
became obvious that one or more of the cloned
genes could be used to develop a diagnostic
DNA probe. The DNA probe could be used to
identify infected goats and ticks. This identification
could be very important foy epidemiology
studies to define areas where the disease occurs
and to define conditions where goats are at risk
of infection.
Two DNA clones were identified: pCR9,
which is derived from the Kiswani, Kenya,
isolate of C. ruminantiumand pCS20, which is
derived from the Crystal Springs, Zimbabwe,
isolate. Initial specificity was determined by
hybridization with labeled DNA from infected
culture cells, but not to DNA from uninfected
cells. Infurther studies, it was found that the
labeled DNA insert from pCR9 bound to large
amounts of some bacterial DNAs causing
specificity problems. In contrast, the insert from
pCS20 was more specific and did not bind to .
ng of DNA isolated from several different
rickettsia, bacteria and protozoa. Neither probe
bound to DNA extracted from A.variegatum
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