| dc.description.abstract | Despite significant advances in the development of
cestocidal agents and improved understanding of the
epidemiological factors affecting the transmission of
tapeworms, there has been in fact an increase in human
taeniasis and bovine cysticercosis due to Taenia saginata
during the last two decades.
The control measures whicl) have been tried have largely
failed to achieve the expected results. In theory, this
cyclozoonosis can easily be controlled. In order to break the
cycle of man-cattie-man transmission, two approaches, or a
combination of the two, are evident. Firstly, the eradication
of human taeniasiz through appropriate treatment, thus
eliminating the source of infection for cattle. This approach
has largely failed because of the logistics involved, in spite
of the availability of efficient and non-toxic drugs. The
second logical approach would be to prevent infected meat from
r£aching the market. This approach is presently in operation
in the form of meat inspection legislation and procedures,
which inter alia are aimed at the detection of ~ saginata
metacestodes to prevent marketing of infective beef.
Unfor.tunately. experience and careful studies have shown that
the meat inspection procedures are unable to detect light
infection, resulting in the marketing of considerable
quantities of infective beef.
In view of the present unsatisfactory results of attempts
to control human taeniasis and bovine cysticercosis, it could
be surmised -that a serological diagnostic method might be of
value both in the ante-mortem as well as post-mortem diagnosis
of bovine cysticercosis. It is envisaged that a reliable
ante-mortem diagnostic method would provide the livestock
farmer with means of identifying infected animals. Such
animals may be disposed of immediately to prevent the losses
incurred in feeding of animals which may be condemned or
downgraded at the time of slaughter. A serological test may
also assist in identifying infected animals at the time of
slaughter, despite of negative findings during meat inspection.
In the future, cattle herds could be screened using a
serological method. Those giving positive reactions may be
considered for treatment when an efficient drug becomes
available. So far, however, drug treatment of bovine
cysticercosis has not been attempted except on an experimental
basis. Finally, it can be envisaged that the present meat
inspection procedure can be simplified through the introduction
of a serological method for bovine cysticercosis, in a way
similar to what has been done for porcine trichinellosis.
The fundamental aim of the present study was to examine
the antigenic components of T.saginata metacestodes in an
attempt to select antigen(s) suitable for the use in
serological tests. This required investigations of the
complexity of antigenic determinants shared with other common
paLasites of cattle, the selection of antigen(s) possessing
adequate specificity and the establishment of sensitive
seLological methods using these antigens.
In addition to the potential pLactical implications of
the development of such irr~unonssnys, these investigations
might contLibute to the undeLstanding of funda~ental asp~cts of
the inunun~r-esponae s to par-as i t i,c infections in gener-a l.
Chaptet' I describes the establishment of a reference
system for the antigens of 1saginata metacestodes. Fifteen
antigens wet'e defined by the Laurell crossed
immunoelectrophoresis technique. Antigens were given abitLary
numbers starting with the most anodic components. Antigens
1,3,4,5,7,8,9,10,11, and 13 were found to be shaLed betwecn
T. saginata cysts and the adult wor"" while Antigens
1,2,4,6,7,9,10,12 and 13 were common among cestodes. Antigens
10,14 and 15 were shared with con~only occurLing nematodes and
trematodes of domestic animals.
Antigens 4, 8 and 11 were considered potentially useful
and were selected for furthet' study. Antigen 4 was found to be
common among all cestodes examined, while Antigens 8 and 11 had
a more restricted di st rIbuti.on . Antigen 8 was only shared \-lith
1. saginata and its metacestode. Specific antisera to these
antigens wer-e pr-epar-ed in rabbi t s using iImnunoprecipitates
obtained in i~~unodiffusion tests using rabbit antiserum to
1saginata metacestodes, and metacestode extrect as untigen.
Chapter II describes the isolation and purification
procedures of these antigens. All the three antigens were
isolated using an affinity chromatography technique. The
antigens were analysed by isoelectric focusing and
polyacrylamide gel electropporesis in which sodium dodecyl
sulphate ~as incorporated (SDS-PAGE). Antigens 4,8 and 11 were
found to have molecular weights of 63 kd, 260 kd, and 68 kd and
save pI values of 5.20, 6.95 and 5.50, respectively.
Antigen 8 was found to consist of three components of
molecular weights 110 kd, 72 kd and 61 kd. All these
components showed reactions of identity in.immunodiffusion
tests, indicating that they shared the same epitopes.
Chapter III describes an attempt to determine whether the
three antigens could be used for the serodiagnosis of bovine
cysticercosis. The "sandwich"" enzymeimmunoassay and a
radioimmunoassay were first used to evaluate these antigens
using sera from experimentally infected animals. since
antibodies to all three antigens could be detected in
experimentally infected animals, the applicability of these
antigens in detecting antibodies in naturally infected animals
was evaluated.
The "sandwich" enzyme immunoassay using Antigen 8 gave a
sensitivity of 66~, a specificity of 40~and a predictive value
of 56~. Antigen 11 gave a sensitivity of 55~, a specificity of
48~, and a predictive value of 52~. Antigen 4 gave a
sensitivity of 45~, a specificity of 45~ and a predictive value
of 46~.
(vii)
There was no statistically significant difference between
the sensitivities obtained with Antigen 8 and 11 in the enzyme
irrmunoassay (t = 1.925t P> 0.05). There was a significant
difference in the sensitivities obtained when Antigen 8 was
compared with Antigen 4 (t = 2.528~ P<0.05) Antigen 8 was more
sensitive than f~tigen 4. A comparison between Antigen 11 and
Antigen 4 also gave a significant difference in sensitivity (t
= 5.885~ ~0.05) with Antigen 11 giving a higher sensitivity
than Antigen 4. There was no statistical difference between
the specificities obtained when Antigen 8 was compared with
Antigen 11 in enzymeiwmunoassay (t = 0.980; P)0.05), while
there was a statistically significant difference in
specificities shown by Antigen 8 and Antigen 4 (t 3.261; P<
0.05), with Antigen 8 giving a higher specificity than Antigen
4. Antigen 11 gave a higher specificily than Antigen 4 (t =
2.297; P<"0.05).
In the radioimmunoassay, using labelled antigens, a
sensitivity of 82%, a specificicity of 25% and a predictive
value of 58% were obtained with Antigen 8. A sensitivity value
of 76%, a specificity of 30% and a predictive value of 58% were
obtained with Antigen 11. Antigen 4 was not evaluated in this
assay.
There was no statistical difference when the
sensitivities obtained with P~tigen 8 were compared with
Antigen 11 (t = 0.712: P> 0.05). Neither was there any
difference in the specificities obtained with these antigens in
radioimmunoassays (t = 1.05/,; P.) 0.05) .
There was a statistical difference in sensitivity when
Antigen 8 was used in radioinununoassay, as compared to its use
in enzymeirr~unoassay (t = 3.803~ P~.05) with radioimmunoassays
using Antigen 8 giving a higher sensitivity. There was also a
difference in specificities obtained in radioimmunoassay as
compared to enzymeimmunoassay (t = 2.042; P <.0.05) with
enzymeinununoassay using Antigen 8 giving a higher specificity.
A significant difference in sensitivity was obtained when
Antigen 11 was used in a radioimmunoassay as compared to
enzymeimmunoassay (t = 2.617; P< 0.05) with enzyme i~~unoassay
using Antigen 11 being more specific.
This difference in sensitivities and specificities could
be attributed to the use in the radioimmunoassay of labelled
antigens, while in the "sandwich" enzyme immunoassay an
anti-bovine I~G conjugate was used.
The high percentage of apparent false positive reactions
obtained with the immunoassays may in fact be true positive
reactions because of the poor reliability of meat inspection
procedures which were used as the definitive parameter for the
diagnosis. On the other hand, false positive reactions may
have been due to abortive infections as the results of the
ingestion of senescent eggs. Such eggs may fail to develop
into matur.e cysts but nevertheless tr.igger an irr~uneresponse
and give rise to detectable antibody levels.
There are severaI ways in which the i.mmunoassays
describ8d in this study may be applied. For example, the
enzymeimmunoassay using Antigen 8 and 11 may be used for the
selection of animals which are heavily infected. Such animals
could be separated for treatment or slaughtered at an early
age, thereby saving the farmer from keeping animals which can
be downgraded or condemned at the time of slaughter. The same
method could be.used in the selection of animals which are to
be taken to a feedlot for fattening.
These assays could also be used for the detection of
lightly infected animals as an adjunct to the current meat
inspection procedure. This could justify the cold storage of
all carcasses from animals which gave a positive reaction in
the immunoassay. On the other hand, animals \ihich gave
negative reactions in the serological tests could be subjected
to a less vigorous inspection.
The presence of antibodies to T. saginata metacestodes in
cattle from a certain farm or area may also be used as indirect
way of identifying persons with taenia£is. The treatment of
persons identified in this way should be an intergral part of
control measures for bovine cysticercosis.
In this study it has been sho~m that the selection of
certain antigens of T. saginata metacestodes can easily be made
from complex mixture of antigens by the use of a reference
pattern based on the Laurell crossed immunoelectrophoresis
technique. The same r-ef er-enc e sys tem can be used to elucidate
the relationship of these antigens with those of other
parasites.
The selection of three antigens which were consistently
present in T. saginata metacestodes and absent in most other
prasites was considered the most appropriate choice of antigens
for use in specific serodiagnostic tests for bovine
cysticercosis. Encoura~ing results were obtained with Antigens
8 and 11 and these antigens could be used as an adjunct to
current meat inspection procedures. | en |
