| dc.description.abstract | The lack of basic information in the field of tick endocrinology
led to this investigation on the effects of an insect moulting
hormone, beta-ecdysone, and a phytoecdysone, ponasterone A, on
adult and nymphal O. p. porcinus ticks.
Ticks were bred at 2SoC with a relative humidity of 75 to SO% on
four laboratory hosts : rabbit, chicken, rat and guinea pig. In
vitro breeding techniques were also used to study bovine and
porcine defibrinated blood as possible substrates for the mass
rearing of ticks. The suitability of a host or substrate was
assessed by the feeding performance of the ticks, their reproductive
efficiency, the percent hatch of eggs and subsequent
moulting success and rate of adult maturation. Observations
indicated that rabbit- and chicken-fed ticks laid more eggs per
unit weight of bloodmeal and t~e resulting eggs had better
hatching rates than rat- and guinea pig-fed ticks. The moulting
success and rates of development and adult maturation were also
better among rabbit- and chicken-fed ticks than rat- or guinea
pig-fed ticks. Porcine blood wa3 an excellent substrate for the
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promotion of high levels of eggs output while bovine blood gave
poor results. In some respects, rabbit blood was a superior
substrate in comparision to porcin~. blood but rabbit blood
haemolyses easily and it is not suitable because of resulting
tick deaths. Porcine blood was greatly superior to bovine
blood in most parameters of tick feeding and development.
Topical application of beta-ecdysone (1 to 5~g/tick) to adult
O. p. porcinus gave higher mortality among males than females,
while both males and females supermoulted. Topical application
of beta-ecdysone and ponasterone A to 2nd through 5th nymphal
instar ticks, the same day or 4 days post-feeding, produced
higher mortality among treated nymphs but the premoulting period
was also shortened among nymphs treated 4 days post-feeding.
There were differences in the rates of moult acceleration among
the surviving nymphs with different ecdysteroid doses and in some
cases the dose responses were linear.
Ticks fed on b Lcodme aI wi th added ecdys teroids at 1 to 8f..lg/molf
blood showed low (5.2%) and high (41.3%)mo rtali. ty responses
among female and male ticks, respectively. Among ponasterone
A treated ticks, a high frequency of mortality ranging from
3.2 to 90%, was observed among both male and female ticks.
Supermoulting among beta-ecdysone fed ticks occurred at higher
(74%) and lower (19.3%) rates among females and males,respectively.
Ponasterone A fed ticks also showed higher (55.8%)
rates among females than among males (21.6%). Some betaecdysone
fed females underwent a second supermoult on the ecdysone
from the first bloodmeal and the dose response curve for
this response was linear. Among nymphal ins tars, feeding on
ecdysteroids caused moult acceleration,the rate of which depended
on the nymphal stage. Older (and larger) nymphal stages
experienced better moult acceleration than the younger (and
smaller) ones. Age did not affect supermoulting potential ~n
o. p. porcinus. Both A. persicus and O. tholozani showeda very
slight supermoulting response when fed on bloodmeal with added
e cdys te ro i ds .
Moulting hormone equi,v'a.lent titre changes (determined by MUsca
bioassay) in the haemolymph o.f normally-fed 5th instar nymphs
occurred in a pattern with 2 'peaks; the lower 1st peak appeared
on day 4 and the higher 2nd peak "appeared on day 9, jus t
before moulting. Ecdysone-fed 5th ins tar nymphs showed an early
titre peak on day one followed by a normal pattern with the
2 basic peaks. Normally-fed and mated females showed a low titre
peak on the 7th day post-feeding which coincided with the onset
of vitellogenesis. In ecdysone-fed unmated females, the
2 basic peaks of the nymphal moulting pattern were observed; an
early lower moulting peak appeared on day 5 and a 2nd higher
peak appeared on day la, preceding supermoulting. In the ecdysone-
fed mated females, the 2 basic titre peaks of the nymphal
moulting pattern·also occurred. Comparisons between the equivalent
moulting hormone titre changes in normally-fed 5th ins tar
nymphs and ecdysone-fed fern~les were essentially the same,
both in terms of timing and in the magni tude of the changes.
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The biology of the supermoulted ticks revealed that while the
supermoulted ticks grew larger with subsequent ecdysone-meals,
their conversion of normal bloodmeal into eggs was less
effective compared to that of normal females. Their total
oxygen consumption was also comparable to that of normal females
during the period of post-bloodmeal digestion and oocyte
maturation. Anatomical and histological comparison~revealed
significant differences between the adjusted mean size indices
(in terms of unfed weight) of normal and ecdysteroid treated
females. Thus supermoulted females weigh less than would be
expected for their size. These supermoulted ticks also had
enlarged salivary glands and. the salivary glands of twice supermoulted
females were proportionally larger than those of normal
or once supermoulted females. Cuticular patterns of normal and
supermoulted adult females also showed differences; the number
of mammillae per optic field became fewer as they g rew larger
and became more prominent wi th each subsequent moul t . Supermoulted
females also showed accumulation of salt crystals around
their mouth parts which may imply that faulty water balance
mechanisms contribute to their greater mortality in comparison
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to normal females.
From the foregoing observations it was concluded that :
1. For ~n vivo mass rearing, rabbits and chickens are better
laboratory hosts than are rats and guinea pigs. The female
ticks fed on rabbits and chickens can be expected to have
higher levels of egg production. Nymphal ticks fed on
rabbit and chicken hosts also show low levels of
mortality coupled wi th rapid moulting and adul t development.
2. Defibrinated porcine blood is an excellent substrate for
the promotion of good in vitro feeding and high levels of
egg production. Nymphs fed in vitro on this substrate had
good develQP~ental rates with early adult maturation. These
positive effects of defibrinated pig blood make the in iri tvo
breeding technique the method of choice for colony rear1ng
and maintenance.
3. Adult male O. p. porc1-nus show significantly higher mortality
responses to topically applied bet a+e cdys one (at any
particular dose level) than do adult females. Both sexes
show low supermoulting responses to topical, ecdysteroids with
males responding at a slightly higher rate.
4. Topical application of ecdysteroids to nymphs will produce
high mortality responses with the sensitivity of nymphs
increasing significantly from day I to day 4 post-feeding.
Since mortality was always less than 80%, other ecdysteroids
should be tested at a range of practical dose levels. As
a potentially successful growth regulator, any ecdysteroid
must cause more than 90% mortality before it could be tested
for potential in the field.
5. The moulting acceleration which was observed among the surviving
nymphs is undesirable in terms of tick control.
6. At lower doses(lto4~g/ml), ingestion of ecdysteroids will
cause supermoulting and low mortality among beta-ecdys~ne
and po nas terone A fed ticks. Higher do seslf to l~g/ml) cause
mortality of up to 90% among both males and females. Ecdy-
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sene-fed ticks may sllpermoult as many as 3 times.
Ornithodoros p. porcinus'adults do not lose their supermou
lti.ng potential with Lnc reasi ng age and they can also respond
to many other ecdysteroids.
7. Ingestion of beta-ecdysone or ponasterone A causes moderate
to high mortality among nymphal instars. Because of the
absencq of a generally linear dose-response curve it is not
now feasible to test either of these ecdysteroids as a
systemic growth regulator for ticks. Ingestion of these
ecdys teroi.d s al so causes nymphal moul t acce lera tion among
the survivors.
8. <There are 2 basic pe~~s of moulting hormone activity 1n the
haemolymph of fed nymphs and supermoulting adults. The two
peaks appear to be related to the moulting process. In
normally-fed and mated females a peak of moulting hormone
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activity appears on day 7 and this seems to be related to
egg maturation. Therefore ticks show titre changes in
association with both moulting and reproductive cycles and
may have endocrine patterns and mechanisms similar to those
found in insects.
9. Supermoulted female O. p. porci.nus take larger bloodmeals
and lay more eggs than normal females. On a comparative basis,
however, their conversion of bloodmeal into eggs is less
efficient and they have shorter adult lives. The shortening
of adult life seems to be related to physiological defects
1n their water balance mechanisms. Accordingly, the total
egg production of supermoulted females might not be greater
than that of the longer-lived and more efficient normal
females.
10. There may be some potential for the development of an analogue
to the natural tick ecdysone(s) which could be used
for tick control by systemic administration through livestock
hosts. Any such analogue would have to persist at'
relatively high levels in the host circulation and should not
contribute to th ec.s upe rmou Lti.ng of adults or moult acceleration
of nymphs. | en |
