| dc.description.abstract | The objective of these investigations was
to study the rabbit reactions to the repeated
infestation by each of the life cycle stages of
Rhipicephalus appendicu1atus and to try and
e1ucidatefue possible mechanisms on which the
tick resistance phenomenon, a characteristic
of this particular host-parasite system, is based.
Eighteen mature intact male rabbits
(Orycto10gus Cunicu1us) were divided into three
groups of six rabbits each, each group being set
aside for the infestation by one of the life
cyc1e.stages according to methods of Bailey, (1960).
Each group was itself subdivided into three subgroups
of two rabbits.,each. One subgroup had three
infestations, another, two infestations and the
third subgroup had only one infestation. The
infe~tations were done at two weeks interva1s.
Rabbit reactions to the ticks were studied during
each infestation. Two weeks after the last
respective nymphal or larval infestation, adult
feeding readiness and the associated rabbit
reactions were examined. Similar investigations were made
'in the case of nymphs and for larvae feeding on
the adult infested group of rabbits.
To find out if histamine release during
the infestations played a role in the resistance
mechanism, one of the rabbits which had three
previous infestations of its respective developmental
stage ~f the tick was treated with antihista
mine (Promethazine HeL) at a doss rate of Smg/kg
for three consecutive days after applying the
corresponding life cycle stage of the ticks.
The feeding readiness of the ticks was assessed.
To demonstrate the involvement of humoral
factors in the resistance mechanism(s), sera from
tick resistant rabbits, was transferred into six!
rabbits with no previous tick exposure. Feeding
readiness of each life cycle stage and the
accompanying host reactions were examined.
Six other rabbits divided into three
groups of two rabbits each were subcutaneously
inoculated with O.Sml of 1:40 unheated whole
larval saline extract conjugated in 1:1 ratio
with Freund's Complete Adjuvant. Twenty days
after the last sensitizing infestation, each group
of two rabbits was artificially infested with
one of the developmental stages of the tick to
assess the respective feeding readiness and the
attendant .host reactions.
feeding readiness, an index of resistance
to the ticks, was assessed according to the
criteria of length of engorgement period, weight
and colour of engorged ticks and percentage of
successful engorgement.
The gross, haematological, immunological
and histopathological reactions accompanying any
level of resistance attained during any infestation
were studied.
It has been observed that the feeding
readiness of each developmental stage of appendiculatus
declined with successive infestations;
prior infestation by one of the life cycle stages
reduced the feeding readiness of another life
cycle stage latter applied on the same rabbits;
prior sensitization decreased the feeding readiness .
of the larvae. Treatment of the resistant rabbits
with an established antihistamine, improved the
feeding readiness of larvae only.
It has also been observed that tick
infestation and the resultant tick resistance was
accompanied by the rabbit physiopathological
"reactions .of epidermal hyperplasia, lengthening
of hair/follicles, epidermal tongue like projections
into the dermis and inflammatory cell infiltration
of the feeding lesion. Immunological reactions
manifested as the appearance of both homocytotropic
and precipitating antibodies in sera and the
occurrence of blood and tissue eosinophilia
also accompanied the subsequent infestations.
It is concluded that the time relationship
between the occurrenceof the various rabbit
reactions and the development of resistance suggest
that tick resistance in rabbits is acquired and its
mechanisms are multicomponent in origin and based on
mainly Arthus type of Immediate hypersensitivity
which is superimposed on and which enhance the
non-specific and probably innate physiopathological
reactions of the rabbits. These tick resistance
mechanisms are non absolute in effect but rather
are geared to offsetting the high reproductive
potential of the tick by rendering the attachment
and feeding site unsuitable for the feeding of the
tick. | |
