Veterinary Epidemiology and Zoonosis

HYDATIDOSIS

Synonyms

• Echinococcosis, Hydatidosis, Hydatid disease, ‘Swollen-belly disease’ of man

Type of zoonosis

• Either Obligatory or Non-obligatory cyclozoonosis (life cycle may be completed even without involvement of man)

Definition

• Hydatidosis is an either obligatory or non-obligatory cyclozoonotic disease, caused by Echinococcus granulosus, E.multilocularis and E.vogeli. Development of hydatid cyst in various organs such as lung and liver, and also spleen, kidney, bones, orbit, brain and peritoneal cavity leads to functional disorders of respective organ due to pressure atrophy, eventually may lead to death. Symptoms include abdmonal pain, bulging of abdomen, jaundice, hepatomegally, weight loss, anaemia, fever and palpable abdominal masses.

Etiology

• The larval stage of following cestodes
  • Echinococcus granulosus - causes "cystic" disease, called unilocular hydatid cyst.
  • Echinococcus multilocularis - causes "alveolar" disease, called multilocular hydatid cyst.
  • Echinococcus vogeli - causes "polycystic" disease, called non-invasive hydatid cyst.
• Definitive hosts: Dogs (stray / domestic / wild), lions, cats, wolf and fox.
• Intermediate hosts: Man, sheep, cattle, goats, pigs, horses, donkeys and buffaloes.
• Intermediate stage: Hydatid cyst (65-75% in liver and 10-20% in lung), is a fluid-filled cyst with brood capsules.

Reservoir and incidence

• Man is an accidental host.
• Prevalence of hydatidosis in man is high in countries with livestock, especially sheep raising, in rural areas, and among people of limited economic and cultural means.
• Disease in animals is asymptomatic throughout the life.
• Definitive hosts (carnivores) harbour the adult tapeworms in the small intestine.
• The life cycle for E.multilocularis involves foxes as definitive host and microtine (voles and meadow mice) and rodents as intermediate host. Domestic dogs and cats can also become infected with the adult tapeworm when they eat infected wild rodents.
• The principle definitive host for E.vogeli is the bush dog, the main intermediate hosts are the paca and spiny rat. Domestic hunting dogs are also definitive hosts, and serve as an important source of human infection.
• Only one gravid segment is eliminated in faeces every two weeks by the definitive host.
• Eggs are resistant to desiccation and extreme temperatures.
• Dispersion of eggs is of great epidemiological interest.
• Radial dispersion of eggs from faeces may be about 80 m after 10 days.
• Mechanical dispersion of eggs by vectors, especially carrion birds and coprophagic flies.

Transmission and life cycle

• Ingestion is the chief route of transmission.
• There are different pattern in completion of life cycle, which basically depends on type of association between definitive host and intermediate host. Examples: Dog-sheep-dog cycle or dog-man-dog cycle.
• Human infection occurs when eggs passed in dog faeces (or also faeces of other definitive hosts) are accidentally swallowed.
• Adult cestode lives attached deep inside the mucosal crypts of the definitive host's small intestine.
• The adult parasite has three proglottids, of which only the last is gravid (contains several hundreds of eggs), which detaches from the strobila, is expelled with the faeces and disintegrates in the environment.
• Each egg contain an hexacanth embryo (oncosphere), and this must be ingested by the intermediate hosts to complete the life cycle.
• After ingestion of egg by the intermediate host, oncosphere is released in the small intestine and passes through the intestinal wall, and is carried by the blood circulation to various organs. It develops in to larval stage, called "hydatid".
• After 3 weeks, the hydatid cyst measures about 250 micrometer in diameter, with central cavity.
• After 5 months, the hydatid cyst measures about 1 cm, with two layers (external, cuticular or laminar layers and internal, germinative or proligerous cellular syncytium).
• The larval form of E.granulosus, typically consists of a single cavity, is unilocular, which is filled with fluid, is hydatid fluid. Brood capsules bud off from the germinative layer and invaginated protoscolices, which constitute the infective agent of the parasite. These capsules either adhere or float freely in the hydatid fluid. The capsules and the protoscolices that float freely in the hydatid fluid are known as "hydatid sand". Some hydatid do not form capsules, and sometime the capsule do not form protoscolices, is called "sterile larvae" or "sterile cyst". As the larva develops and the tissues of the host are compressed.
• The cycle will be completed when definitive host ingests the viscera of an intermediate host in which there are fertile hydatid cysts.
• Each scolex develops in to an adult cestode in the definitive host's small intestine and produces infective eggs after 45 to 60 days of ingestion (a single cyst may produce thousands of adult cestode).
• Gravid proglottids are passed in the faeces of definitive host.

Disease in animals

• Usually no clinical signs. Disease in animals is asymptomatic throughout the life, except for enteritis in heavy infestations.
• Economic loss due to condemnation of meat and loss of production, especially in sheep.

Disease in man

• Symptoms include abdmonal pain, bulging of abdomen, jaundice, hepatomegally, weight loss, anaemia, fever and palpable abdominal masses.
• Cystic hydatid disease (Echinococcus granulosus)
  • Cyst in liver is common (~70%), which may be remain silent for 10 to 20 years.
  • The symptoms are due to pressure effects or rupture and leakage of hydatid cyst.
  • Signs of abdominal pain, nausea and vomiting may be noticed.
  • Rupture of cyst results in anaphylaxis and death.
  • Mild and slow leakage of hydatid particles may cause allergic reactions.
  • Dissemination of germinal elements may be followed by the development of multiple secondary cysts.
  • Brain cysts may cause seizures.
  • Cysts in the bone marrow may cause fracture of bone.
  • Multiple cysts may be in 20% cases, and death may occur in 15% of untreated patients.
• Alveolar disease (Echinococcus multilocularis)
  • Alveolar cysts primary localyze in the liver, where they may extend locally or metastasize to other tissues.
  • Poorly defined borders of alveolar cyst mass, which behaves like a neoplasm.
  • High mortality in untreated cases (90%) within 10 years.
• Polycystic form of human hydatid disease (Echinococcus vogeli)
  • Cyst size and location determines the clinical pictures.
  • Numerous microcysts are produced by the brood capsules.

Diagnosis

• Based on history and clinical signs (swollen-bellied condition), and epidemiological circumstances.
• Ultrasonography, radiography and scanning.
• Immunological assays, such as:
  • Intradermal skin test (Cassoni’s skin test)
  • Agar gel immunodiffusion test
  • Latex agglutination test
  • Indirect haemagglutination test
  • ELISA, which replaced all the immunological test, because it is ~95 % sensitive, but gives cross reaction with taeniosis and ascariosis.
  • Detection of coproantigen in the faeces by using ELISA and PCR.

Treatment

• Surgical removal of cysts. Sudden removal of cyst may lead to shock. So, removal of hydatid fluid should be done over a period of time.
• Chemotherapy with albendazole (10 mg / kg body weight) or albendazole + praziquantel combination or mebendazole can be prescribed.

Prevention and control

• Strict meat inspection should be practiced.
• Safe disposal of infested carcasses so as to avoid eating by dogs.
• In endemic areas, prophylactic treatment should be followed in pet dogs with praziquantel (100% effective against adult parasite) to prevent this problem.
• Administration of arecoline hyrochloride in dogs to interrupt the life cycle of dog-man-dog pattern.
• "Albendazole village" has been constructed for the purpose of elimination of this problem in human beings in several African countries, especially in Turkana.
• Close contact with dogs should be avoided.
• Personal hygiene should be followed, like hand washing before eating food.
• Ingestion of raw vegetables and water should be avoided, because that may have been contaminated with the faeces of infected dogs.
• Socio-economic and cultural conditions of the people should be upgraded through public awareness and health education programmes.