Saturday, January 23, 2010

Echinococcosis

Echinococcus granulosus is a tiny tapeworm with a global distribution, being found in South America (especially Argentina and Uruguay), Europe, North America, New Zealand, Australia and Europe and Middle East. It has a major focus in North Africa around the Mediterranean region, especially Tunisia, Morocco, Libya, and Algeria and, to some extent, Egypt. The other major African focus is in East Africa, an area that includes Kenya, Sudan, Ethiopia, Uganda and Tanzania. The distribution in the rest of the continent of Africa is patchy.
Global distribution of E.granulosus
The adult worm is 3 to 6 mm long and usually consists of 3 to 4 proglottids. The scolex is armed with suckers and a rostellum that is ringed by a row of small hooks. The worms occur in hundreds or thousands in the small intestines of carnivores. The larval form, known as the hydatid cyst, occurs in the tissues of ruminants, such as sheep, cattle, goats and camels. Among its carnivore definitive hosts, the dogs are the main source of infection to man.
E. granulosus is tiny tapeworm

Life cycle
Gravid segments release shelled eggs that are voided with canine faeces. When these are ingested by sheep, cattle, pigs or other ruminants, or by man, the onchospheres emerge from the shells in the small intestine, penetrate the host’s intestinal wall, and reach the circulatory system where they are carried to the liver (50% to 75% lodge here), lungs and other organs of the body. The larvae encyst in these organs while undergoing undergrowth, reaching 1 cm in diameter in about 5 months.
The life of E.granulosus. A, adult parasites in the intestine of a dog lays eggs which are ingested by man or domestic herbivore. B, hydatid cysts develop in the liver of man and herbivores. C, hydatid cyst removed from man or domestic herbivore. 


In the viscera, each young worm becomes vacuolated and slowly grows into a cystic structure called the hydatid cyst. The hydatid cyst consists of an outer part or pericyst which is a fibrous cellular envelope formed from the host tissues. Internal to this is another layer, the ectocyst. Inward to the ectocyst is syncytial layer known as the endocyst. The syncytial layer is the embryogenetic or germinal layer out of which brood capsules are budded. The brood capsules in turn give rise to numerous protoscoleces. A mature protoscolex has 4 suckers and a rostellum with hooklets and can be observed in the hydatid fluid.
Growth of the main larval cyst, or hydatid, is slow but the organism may reach a diameter of 20 cm in 10 to 12 years, depending on its location in the host. A mature cyst contains several litres of sterile fluid. The hydatid cyst may live in the intermediate host for up to 50 years.
When a carnivore ingests brood capsules, the protoscoleces are liberated in the stomach. After evagination, the protoscoleces become established in the small intestine, with the rostellum buried in a crypt of Lieberkuhn and for the first four to five days, they undergo little growth. By day 14, the scolex has grown in size, and the first segment has appeared. By the 35th day, the worm is fully developed with fully developed shelled embryos. However, in heavy infections, the worms take a considerably longer time to mature.
Pathogenesis
Hydatid cysts grow slowly at a rate of about 1 cm per year on average. Clinical symptoms usually develop when the cyst is about 10 cm in diameter. The severity of hydatidosis depends on the size, the location and the pressure exerted by the cyst in the host structures. Small cysts do not seem to do any appreciable damage to the host. On the other hand, large cysts exert considerable pressure on the surrounding organs. Rupture of the cyst may cause toxic/allergic reactions such as rash, wheal and redness of the skin. In some individuals, the symptoms may include fever, shortness of breath, cyanosis, vomiting and diarrhoea. Severe anaphylaxis resulting in a sudden death may follow a rupture of a large cyst and release of large quantities of fluid. Moreover, the rupture may spread the protoscoleces to other parts of the body where they will implant themselves and originate secondary hydatidosis.
Brood capsule containing scolices

One of the greatest dangers in hydatidosis is the involvement of the lungs where one side of the pulmonary cavity becomes blocked causing respiratory deficiency. Death usually occurs due to pulmonary oedema. Hydatid cysts in the brain are associated with increasing occurrence of intracranial tumours. Bone hydatidosis erodes the bone gradually until it fractures or crumbles.
Epidemiology
The incidence of echinococcosis in humans and in domestic ruminants is very much tied up with the keeping of dogs. Even in the Islamic countries of North Africa where the keeping of dogs is not encouraged, dogs still play a very important role in the transmission of infection. In East Africa, particularly among the pastoral groups, the prevalence of infection is quite high in the population, which is related to the large numbers of dogs that are usually kept ostensibly for herding, and protection against predators. The owners of these dogs do not look after them well and so the animals are always hungry. They will therefore feed on anything that resembles food, including offal and rotting carcasses and acquire infection.
      The dog plays a central role in the transmission of E.granulosus
The Turkana people of Kenya are pastoralists, living in the arid northwestern corner of the country. They keep large numbers of dogs, about 70% of them heavily infected with E. granulosus. The incidence of hydatidosis in the population ranges from 20 to 200 cases per 100000 people. Dogs carry heavy parasite loads that are as high as 3000 worms per dog. This is far in excess of the 30 worms per dog that seems to be the case in most endemic areas of the world.
The Turkana keep sheep, camels and goats, all of which are usually infected with hydatid cysts. In addition, many of the wild carnivores, such as the leopards, lions, hyenas, and jackals carry the infection. It is not surprising the incidence of human hydatidosis in that part of Kenya has been judged the highest in the world.
Diagnosis
Diagnosis is difficult because cysts grow deep in the internal organs. The liver is the most affected organ, recording 65 – 75 % of all the infections, the lungs, 10 – 20 %, and to a lesser extent the spleen, kidney, bones, orbit and brain. Among the Turkana people, a swollen belly is usually a sure diagnosis of E. granulosus. In mature hydatid infections, the size of the cyst may exceed 20 cm in diameter and contain many litres of fluid. Useful diagnostic tools include x-rays for pulmonary cysts, C T scan or echography and serological tests. Serology has less sensitivity in infections that do not involve the liver or lungs.
                                                 Doctor hold cysts removed from a Turkana woman
Control
1. Prevention of E. granulosus involves meat inspection and destruction of cysts by boiling or burying infected meat to prevent it from being eaten by dogs and other carnivores. The eggs of Echinococcus are very resistant to adverse conditions and survive for up to 2 years in water and cool temperatures, and more than a year on the ground under humid conditions. In pure formalin, the eggs remain viable for 2 weeks.
2. Animals should be slaughtered in designated abattoirs where the carcass can be inspected for cysts. Offal from infected animals should be buried or incinerated to prevent dogs from feeding on it.
3. Dogs should be treated regularly with anthelminthics and stray dogs should be eliminated.
4. Human treatment is based on surgical removal of the cyst and\ or treatment. The usual treatment involves albendazole 10 mg/kg body weight. However, success with albendazole is only reported to be about 30%. Administration of praziquantel by itself or in combination with albendazole is 100% effective against the protoscoleces.
The species of Echinococcus
Speciation in this genus is complex and confusing. Initially it was considered that there was only one species of Echinococcus granulosus, although different patterns of development of hydatid cysts were known by parasitologists. These were however thought to be due to development in different intermediate hosts. This view was later refuted and abandoned. Four types of hydatid cysts are now recognised.
Unilocular: characterised by single well-defined bladders, in which the laminated membrane continuously encloses the germinal membrane. Its geographical distribution is global. Example, E. granulosus.
Multivesicular: characterised by many adjoining and connected bladders, each having its own germinal membrane.
Alveolar: characterised by a malignant type of growth with jelly-filled proliferating vesicles embedded in a common dense stroma. The very thin laminated membrane does not restrict the germinal membrane that grows out into the host surrounding tissues. This parasite is restricted to the northern hemisphere.
Multilocular: characterised by many small bladders embedded in a common enclosing membrane. Has the same geographical distribution as E. granulosus.