Studies in the epidemiology and control of echinococcus granulosus in Kenya

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 hydatidosis 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. 9ranulosus strains in Kenya, the infection pressure of the Parasite to dogs and hence to humans, the number, survival an- 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. granulosus infections in dogs, were also evaluated. In an attempt to characterize E. granulosus 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 isoenzyme 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 faster in Turkana dogs than,..in Nairobi dogs, ue 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 found 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

<3ifferen"t Turkana and Nairobi environments was also studied. It was found that the average number of eggs in E. granulosus 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. granulosus 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. granulosus 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. granulosus, will be of value to the education programme as it can serve as an indicator of ^°9s having been fed on raw offal. In addition the large Sl2e of the T. hydatigena parasite would help the people to

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 d°gs and the basic elements of hygiene that will reduce the currently exceptionally high level of man-dog contact.

The field application of the controlled release glass ncapsulated praziquantel would be of great value to the control programme. Further studies are needed in tJ>e development of the capsules of varied digestibilities in dogs which would allow for a prolonged dosing interval.