Echinococcus: Introduction, Morphology, Life Cycle, Clinical Features, Pathogenecity, Lab Diagnosis And Treatment

Echinococcus

 Introduction of Echinococcus 

Echinococcus granulosus, also called the hydatid worm or dog tapeworm is a cestode. It causes cystic echinococcosis, also known as hydatid disease. Infection through contaminated water is not common. The eggs being a heavier sink to the bottom. Infecting agent: eggs in dog feces. Portal of entry: Alimentary tract. Site of localization: Viscera ( liver, lungs, and other organs) Infection is acquired by the ingestion of eggs in the dog’s feces. This occurs in the following ways.

i) by direct contact (handling) with infected dogs

ii) by allowing the dog to feed on the same dish

iii) by taking vegetables contaminated with infected dog feces

Geographical Distribution of Echinococcus

In sheep and cattle raising areas like Australia, Africa, South America, Europe, China, India, etc.

Habitat

The adult worm is present in the intestine of Dog, fox, jackal whereas, larval form in men, cattle, sheep, pigs.

Scientific classification

Kingdom: Animalia

Phylum: Platyhelminthes

Class: Cestoda

Order: Cyclophyllidea

Family: Taeniidae

Genus: Echinococcus

Species: E. granulosus

Binomial name

Echinococcus granulosus

Morphology of Echinococcus

Adult worm

It is a small tapeworm, measuring 3-6 mm in length. It comprises Scolex (head), neck, and Strobila consisting of 3 segments. 1st: Immature 2nd: Mature 3rd: Gravid. The terminal segment is the biggest, measuring 2-3 mm in length and 0.6 mm in width. The Scolex bears four suckers and a protrusible rostellum.

Egg

It is spherical in shape and measures 32- 36 µmx 25-32 µm. It contains a hexacanth embryo with 3 pairs of hooks. Eggs are indistinguishable from those of other Taenia species.

Larval form

This is found within the hydatid cyst and represents the scolex of the future adult worm. It remains invaginated within a vesicular body. When it enters the definitive host, the scolex with four suckers and hooklets becomes evaginated and develops into an adult worm.

Life cycle  of Echinococcus

The worm passes its life cycle in two hosts.

Definitive host: Dog, wolf, fox, and jackal.

Intermediate host: sheep, goat, man.

The eggs are discharged with the feces of the definitive host (dogs and allied animals). These eggs are swallowed by the intermediate host sheep and other domestic animals while grazing in the field and also by man.

Pathogenicity

The mode of infection is ingestion of eggs ( in the stools of dogs) including contaminated edibles.

Hydatid cyst

The cyst wall consists of two layers: The outer layer (ectocyst). Inner or germinal layer ( endocyst). It is a clear colorless fluid (maybe pale yellow in color). It is highly toxic when absorbed gives rise to anaphylactic shock.

Clinical features

The chief clinical manifestation is dependent upon local signs and if the cyst is situated superficially it may cause visible swelling. The disease remains symptom-less for many years and its presence is only detected at autopsy.

Laboratory diagnosis of Echinococcus

Parasitological: Detection of scolices in cyst fluid or centrifuged deposit (hydatid cyst) is 100% confirmatory.
Casoni test: It is an immediate hypersensitivity (type-I) skin test introduced by Casoni in 1911. Intradermal injection of 0.2 ml of fresh sterile hydatid fluid into the volar space of the forearm. The reaction is seen after 30 minutes, a large wheel (5 cm in diameter)  multiple pseudopodia is seen.
Detection of antibodies: Enzyme-Linked Immunosorbent Assay (ELISA), Complement Fixation Test (CFT), Indirect hemagglutination Assay (IHA).

Detection of Antigen: ELISA can be used for the detection of antigen in the serum.

Blood Examination: Eosinophilia (20-25)% may be present.
Radiological: This is often helpful in the diagnosis of hydatid cysts of the lungs and liver. USG of the whole abdomen is useful in locating the site of the hydatid cyst of the abdominal organ. CT scan is more helpful. Histological examination: Surgically, removed. The cyst may be examined to reveal different layers of the hydatid cyst i.e. ectocyst and endocyst.

Treatment of  echinococcosis

Surgical removal of the hydatid cyst can be performed but there may be recurrences in 2-25% of cases of surgery. Hence postoperative chemotherapy may be given for at least two years after surgery. Praziquantel and albendazole chemotherapeutic agent used for the treatment of hydatid cyst.

Prophylaxis

Personal hygiene including avoidance of close contact with dogs.

Control in the movement of dogs.

General health education.

Not allowing the dogs to eat carcasses of slaughtered animals in endemic areas.

Further Readings

  1. Molecular Medical Parasitology. Editors: J. Joseph Marr, Timothy W. Nilsen, and Richard W. Komuniecki, Publisher Academic Press, an imprint of Elsevier Science.
  2. Medical Parasitology by Abhay R. Satoskar, Gary L. Simon, Peter J. Hotez and Moriya Tsuji
  3. Atlas of Medical Helminthology and protozoology -4th edn  -P.L.  Chiodini, A.H. Moody, D.W. Manser
  4. Merkel and Voge’s medical parasitology
    9th edition.
  5. Parasitology: 12th edition
    By K. D. Chatterjee
  6. District laboratory practice in Tropical countries –Part-I.
    By Monica Chesbrough.
  7. Isenberg clinical microbiology procedures Handbook
    2nd edition. Vol. 2
  8. Atlas of Human Parasitology, Lawrence R Ash, Thomas C. Orihel, 3 rd ed, Publisher ASCP Press, Chicago.
  9. Topley & Wilson’s Principle of parasitology. Editors: M.T. Parker & amp; L.H. Collier, 8 th ed 1990, Publisher Edward Arnold publication, London.
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