| dc.description.abstract | Hydatid disease (hydatidosis, echinococcosis) caused by the
larval stage of the taeniid, Echinococcus granulosus (Batsch,
1786), is a cyclozoonosis of major medical and veterinary
importance in most countries of the world. In nature, the
disease is primarily transmitted between wild carnivores and
herbivores, but of importance to man is the domestic cycle
which involves dogs and domestic livestock.
Control of the disease has been instituted in more than
twelve countries. In Kenya, a pilot control programme began
in 1983 in the northwestern part of Turkana district, a
region with the highest known incidence of human hydat~dosis
in the world. However, a number of areas in the
epidemiology, biology, transmission and stability of the
parasite in Kenya were still unclear and yet were important
for effective control of the disease. The present study was
therefore, initiated in an effort to bridge the gaps in the
epidemiology of E. granulosus in Kenya, leading to a more
strategic and effective control programme against the
parasite in the country.
Included in the objectives of the study, were investigations
in the prepatent period and the existence and extent of E.
granulosus strains in Kenya, the infection pressure of the
parasite to dogs and hence to humans, the number, survival
·and release of E. granulosus eggs in the environment. The
application of regulated release encapsulated praziquantel in
the mass dog dosing programme, in the control of the disease
in Kenya and the effect of drought on the prevalence and
intensity of E. granu~osus infections in dogs, were also
evaluated.
In an attempt to characterize E. granu~osus in Kenya, the
rate of development of the parasite from human and domestic
hosts in experimentally infected dogs, was studied at days
14, 17, 21, 26, 35 and 42 post infection. Worms of different
intermediate host origin harvested at day 35 post infection,
were also compared morphologically among themselves and with
worms of similar infection age from Australia, Britain,
Canada, Switzerland and South Africa. In addition, Sau 96 I
digest fragments of DNA material, extracted from
protoscoleces obtained from hydatid cysts of goat, sheep,
cattle, human and camel origin were compared. The DNA
fragments were separated on horizontal gel electrophoresis.
Two strains of the Kenyan parasite were identified, one
infecting cattle, sheep, man and goat, while the other which
occurred less frequently, infecting goat and camel. The
strains were differentiated by electrophoretic separation of
DNA material after a complete restriction endonuclease
digestion. The results thus obtained were similar to those
reported from isoenzyroe profiles of parasite material of
similar origin (Macpherson and McManus, 1982). The strains
were found to be developmentally and morphologically similar.
However, the rate of development of the parasite was shown to
be faster in Turkana dogs than in Nairobi dogs.
The Kenyan parasite was found to closely resemble E. g.
granulosus (Verster, 1965) from South Africa, both in
morphology and rate of development, but significantly
different from the widespread domestic sheep/dog strain
(Thompson and Lymbery, 1988). E. granulosus in Turkana dogs
was shown to have short prepatent period comparable to E.
granulosus of cattle/dog cycle both from South Africa and
Switzerland.
The incidence of echinococcosis in praziquantel treated and
untreated dogs in the hydatid pilot control area, was
investigated over a period of four years (1984 - 1988) using
autopsy and arecoline hydrobromide purge methods. Purging of
dogs with arecoline hydrobromide as a method of diagnosis of
tapeworm infections, was foun~ to be ten times less sensitive
compared to autopsy. Of 58 undosed dogs killed and
autopsied, 63.8% were found infected with E. granulosus, an
infection level similar to that recorded in the same area, in
the previous four years. However, significant increase in E.
granulosus infection intensity in dogs was shown to have
occurred at the end of 1978/82 drought in Turkana. The
natural infection rate in dosed dogs was found to increase
with time and reverted to pre-control level by six months.
Taenia hydatigena was shown to be the only other commonly
found dog taeniid in Turkana.
The number of E. granulosus eggs in undetached and detached
segments in infected dogs was counted and compared in heavy,
medium and light infections. Survival of the eggs in
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different Turkana and Nairobi environments was also studied.
It was found that the average number of eggs in E. granu~osus
gravid segment was 825; this was not dependent on the
intensity of infection. The majority of the eggs (70%) were
released in the host gut, and contrary to earlier reports,
proglottid movements outside the definitive host were found
to have no significant role in egg dispersal under the
Turkana environment. E. granu~osus eggs lost viability in
less than three hours in the open ground heat in Turkana, but
survived for three days in the shade and for more than two
weeks in water. In contrast the taeniid eggs survived for
more than four weeks in the shade in Nairobi.
In an effort to find an effective treatment that would
provide prolonged protection against E. granu~osus
reinfection in dogs, an experiment was carried out to test
the application of controlled release glass encapsulated
praziquantel in dogs infected with Taenia hydatigena. Four
weeks following capsule implantations, seven of eight
experimental dogs were found on autopsy to have been cleared
of the tapeworm infections, while control dogs were still
infected. No side effects were observed.
The results of the present study had a number of implications
on the hydatid control programme in Kenya. T. hydatigena,
being the only other common dog taeniid in Turkana and having
a similar life cycle to E. granu~osus, will be of value to
the education programme as it can serve as an indicator of
dogs having been fed on raw offal. In addition the large
size of the T. hydatigena parasite would help the people to
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appreciate it better than the small E. granulosus tapeworm.
Surveillance using arecoline hydrobromide is not a reliable
technique, as often dogs fail to purge and when they do,
occasionally the worms are not expelled and young or light
infections can be missed. It is also a cumbersome method and
presents a greater risk of infection to personnel. Although
autopsy was found to be a highly sensitive method of
diagnosis of E. granulosus in dogs, it has limited use as a
surveillance technique in a hydatid control programme.
Development of a safe and appropriate diagnostic method,
therefore, requires further research.
From studies of the prepatent period of the parasite, it was
shown that gravid segments were shed by dogs less than six
weeks following infection. In addition the infection
pressure to dogs was shown to be high. Therefore, to reduce
transmission as much as possible between dogs and the
intermediate hosts, the dosing interval ought to be reduced
to every five weeks. The need for this was found to be more __
critical towards the end of a drought when infection pressure
is thought to be highest. With about 200 dogs spread over
9,000 sq. km., this would be an expensive exercise and the
long term answer, therefore, lies in the intensification of
the education programme, teaching people not to feed cysts to
dogs and the basic eiements of hygiene that will reduce the (
currently exceptionally high level of man-dog contact. I
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The field application of the controlled release glass
encapsulated praziquantel would be of great value to the
control programme. Further studies are needed in the
development of the capsules of varied digestibilities in dogs
which would allow for a prolonged dosing interval. | en |
