Echinococcus granulosus

Echinococcus granulosus

1. ECHINOCOCCUS Santosh Kumar Yadav M.Sc. Clinical Microbiology IOM, TUTH

2. Introduction to cestodes Adult worm is usually found in the intestine of men and animals and are long, segmented, flattened dorsoventrally , tape like worms hence also called as tapeworms. Size : vary from few mm to several meter. Hymenolepis nana is the smallest tapeworm (1–4 cm) where as Diphyllobothrium is the longest Cestode measuring 10 meters or more. Adult worm consists of three parts: Head or scolex Neck Strobila (body or trunk) : immature, mature and gravid.

3. No body cavity Body Wall (or Tegument) : It is made up of three layers—outer microvillus like structure called as microthrix , middle basal plasma membrane and inner muscular layer (outer circular and inner longitudinal muscle coats). Female Reproductive Organs : Present on the ventral side and consists of: A bilobed ovary, Oviduct, Ootype , Vagina, Uterus, Vitelline gland and Mehlis ’ gland. Male Genital Organs : testes, vas deferens, seminal vesicle and cirrus (equivalent of penis). Nervous System : It is rudimentary, consists of brain like structure (central ganglion, lateral and rostellar ganglia connected by central nerve ring) present in the scolex from which the longitudinal nerve trucks arise and pass through all the segments and joined by transverse nerves in each segment.

4. No circulatory system Excretory System : It is also rudimentary and present in each segment. It consists of two lateral canals (dorsal and ventral) connected by transverse canals in each segment. The excretory canals are built up of flame cells (terminal cells) and canal cells. Eggs : Eggs are released into the uterus of parasites following fertilization and fill the gravid proglottids .

5. Larva : Embryonated eggs undergo further development to form larva Pseudophyllidean cestodes : Larva is solid without any sac. They are: Coracidium : First stage larva of Diphyllobothrium Procercoid : Second stage larva of Diphyllobothrium Plerocercoid : Third stage larva of Diphyllobothrium Sparganum : Larval stage of Spirometra Cyclophyllidean Cestodes : Larvae contain bladder like sacs. They are: Cysticercus : Larval stage of Taenia Hydatid cyst: larval stage of Echinococcus Coenurus : Larval stage of Multiceps Cysticercoid : Larval stage of Hymenolepis .

6. Life cycle : Cestodes complete their life cycle in two hosts (definitive host and intermediate host) except: Hymenolepis (requires only one host—man) Diphyllobothrium requires three hosts (one definitive host—man and two intermediate hosts– cyclops and fish).

7. Echinococcus spp Echinococcus causes hydatid disease. There are four species of Echinococcus known to infect humans. E. granulosus : Causes cystic hydatid disease E. multilocularis : Causes alveolar hydatid disease E. vogeli and E. oligarthrus : Cause polycystic hydatid disease.

8. Echinococcus granulossus Echinococcus granulosus , also called dog tape worm or hydatid worm belongs to class Cestoda . E. granulosus causes cystic echinococcosis or hydatid disease in livestock and humans beings which acts as intermediate hosts and parasitize the small intestines of adult canids i.e. definitive hosts. It is a zoonotic disease.

9. Geographical Distribution The disease is prevalent in most parts of the world, though it is most extensive in the sheep and cattle raising areas of Australia, Africa, and South America. It is also common in Europe, China, the Middle East and India. It is seen more often in temperate than in tropical regions.

10. Habitat The adult worm lives in the jejunum and duodenum of dogs and other canine carnivora (wolf and fox). The larval stage ( hydatid cyst) is found in liver and other visceral organs of humans and herbivorous animals (sheep, goat, cattle and horse)

11. Morphology Adult worm : The adult tapeworm ranges in length from 3 mm to 6 mm. It comprises of a scolex (head) , neck and strobilla or proglottids consisting of 3 or 4 segments. The 1 st segment is immature , 2 nd is mature and the last one or two is gravid. The terminal segment is by far the biggest (2-3mm x 0.6mm) The scolex bears four suckers and also has a rostellum with two rows of hooks. The neck is short and thick. Its body shape is ribbon-like.

12. Echinococcus is triploblastic , anus is absent and it has no digestive system. Its body is covered by tegument and the worm is divided into a scolex , a short neck, and three to six proglottids . The adult worm lives for 6–30 months Egg : ovoid in shape and resembles other taenia . Measures 32-36 μ m x 25-32 μ m and contains a hexacanth embryo with 3 pairs of hooks. The egg is infective to man, cattle, sheep and other herbivorous animals.

13. Larval Form Found within the hydatid cyst developing inside various organs of the intermediate host. Represents the structure of the scolex of adult worm and remains invaginated within a vesicular body. After entering the definitive host, the scolex with suckers and rostellar hooklets , becomes exvaginated and develops into adult worm.

14. Life Cycle Host: E. granulosus life cycle passes through two hosts: 1. Definitive host: Dogs and other canine animals 2. Intermediate host: Sheep and other herbivores. Man acts as an accidental intermediate host (dead end). Mode of transmission: Men (and other intermediate hosts) acquire the infection by ingestion of food contaminated with dog’s feces containing E. granulosus eggs. Rarely flies serve as a mechanical vector of the eggs.

15. Development in man Eggs transform to larva ( hydatid cyst): In duodenum of man or sheep and cattle , the oncosphere is released by the rupture of ova. Onchosphere hatches and penetrates into the intestinal wall, enters the portal circulation and carries to the liver (60–70% of cases) or lungs or rarely to other organs. Host immune response may destroy many oncospheres , but few may escape destruction and develop into hydatid cyst. The oncospheres are encysted by the fibrous tissue (produced by fibroblasts) and transform into fluid filled bladder like cyst called as hydatid cysts The hydatid cyst undergoes maturation increases in size at a rate of 1 cm/month.

16. Full development takes 10–18 months in sheep. This stage is infective to dog and other definitive hosts Man is a dead end (as dogs don’t feed on human viscera) Development in Dog Dog and other canine animals acquire infection by consumption of the contaminated viscera of intermediate hosts (sheep and cattles ) containing mature hydatid cysts. The hydatid cyst (larva) transforms into adult worm in dog’s intestine. The adult worm becomes sexually mature, self fertilizes to produce eggs which are passed in feces and are infective to man and other intermediate host .

17. Life Cycle

18. Pathogenicity Pathogenicity is related to the deposition of the hydatid cysts (larval form of the parasite) in various organs. Hydatid cyst : Fully developed hydatid cyst of E. granu losus is unilocular , subspherical in shape and size varies from few milimeters to more than 30 cm (usual size 5–8 cm). It appears as fluid filled bladder like cyst.

19. Cyst wall consists of three layers: Pericyst (outer layer, host derived): Consists of fibrous tissue and blood vessels produced by the host cellular reaction Ectocyst (middle layer, parasite derived): It is a tough elastic, glycan rich acellular hyaline layer of variable thickness (1 mm). It resembles the white of a hardboiled egg Endocyst ( inner layer, parasite derived): Germinal layer, 22–25 μm thickness. It consists of number of nuclei embedded in protoplasmic mass. Its function is to form the ectocyst outside and on the inner side it forms brood capsule and secretes the hydatid fluid

20. Hydatid fluid : It is clear, colorless to pale yellow It has a pH of 6.7 and specific gravity of 1.005 to 1.010 Chemical composition : sodium chloride, sodium sulfate, sodium phosphate and succinates It is antigenic, toxic and anaphylactic Brood capsules : arise from the inner side of the endocyst and contains number of protoscolices (future head) Hydatid sand: Some of the brood capsules and protoscolices break off and gets deposited at the bottom as granular deposit to form the hydatid sand

21. Variety of hydatid cyst: Primary cyst: Formed directly from the oncosphere released from the eggs ingested Secondary cysts: Formed due to the breakage of the primary cyst by trauma. The secondary cysts are carried in the circulations to various organs Acephalocyst : Cysts without brood capsules and protoscolices Endogenous daughter cysts: Formed by the breakage of the brood capsule into the hydatid fluid; surrounded by ectocyst and endocyst Fate of the hydatid cyst: Spontaneous resolution may happen to few cysts Rupture of the cyst may either lead to: − Formation of secondary cysts − Anaphylactic reaction to the hydatid fluid antigens.

22. Clinical Features Infection usually occurs in childhood but gets manifested in adult life. Site: Most common site of location of the cyst is liver (60–70%, right lobe) or lung (20–30%) but may be found in any organs like spleen and kidney (3–5%), brain and heart (1–1.5%) and rarely bones They grow upto 5–10 cm in size within the first year and can survive for years or even decades Asymptomatic: Many cases are asymptomatic and infection is detected only incidentally by imaging studies

23. Symptoms occur due to: Pressure effect of the enlarging cyst : Leads to palpable abdominal mass, hepatomegaly , abdominal tenderness, portal hypertension and ascites Obstruction: Daughter cyst may erode into the biliary tree or a bronchus and enter into the lumen to cause cholestasis and dyspnea Secondary bacterial infection can cause pyogenic abscess formation in the hydatid cysts Anaphylactic reactions: Cyst leakage or rupture may be associated with a severe allergic reaction to hydatid fluid antigens; leading to hypotension, syncope and fever Outcome of the disease: It depends on the cyst size and location Younger children are more associated with extrahepatic cysts in lungs, brain and orbital sites In 20–40% of cases, multiple cysts or multiple organ involvement have been reported.

24. Skin test ( Casoni test) : It is an immediate hypersensitivity reaction to hydatid fluid antigens. Developed by Casoni in 1911 Antigen used: Sterile hydatid fluid derived from unilocular cysts from dog or man (sterilized by filtration) Procedure: 0.2 mL of the antigen is injected in one arm; sterile saline is injected to the other arm as control Interpretation: Sensitive patients develop large wheal measuring 5 cm or more with formation of pseudopodia within 30 minutes with no reaction in the control arm Disadvantage : It has low sensitivity (60-80%) and gives false positive results in cross reactive cestode infections It is obsolete now days and replaced largely by the serological tests. Laboratory Diagnosis

25. Antibody detection Screening tests: Various antibody detection methods are evaluated using crude E. granulosus cyst fluid antigen. They show variable results (60–90% sensitivity) .These tests are: Indirect hemagglutination (IHA) Latex agglutination test (LAT) Indirect fluorescent antibody tests (IFA) ELISA Confirmatory tests: a. Immunodiffusion and electro immunodiffusion b. Western blot: Detecting antibody against antigen B fragment . This test is 92% sensitive and 100% specific. Antigen B fragment binds specifically to IgG4 antibodies. Patients with liver cysts show better results than extrahepatic cysts.

26. Antigen detection : ELISA, CIEP (countercurrent immunoelectrophoresis ) and LAT are available to detect specific antigens in serum and urine. Imaging methods : Imaging methods play an important role as they are noninvasive methods, which can detect the cysts incidentally in asymptomatic individuals and in seronegative cases. a. Xrays : to detect hepatomegaly and calcified cysts and cysts in lungs. b. Ultrasound (USG): detects both single and multiple cystic lesions c. Computed tomography (CT scan): It can detect 90–100% of cases. It detects more accurately the number, location of the cyst and the complications. d. MRI : It can be used as an alternate to CT scan.

27. Hydatid fluid microscopy : exploratory cyst puncture ( not advised) Aspirated hydatid fluid is examined for brood capsules and protoscolices by direct microscopy or staining with acid fast stain. Drop of centrifuged fluid is placed between two slides and the slides are rubbed over the fl uid . Hydatid sand is felt as grating of the sand grains in between the slides. Purulent material can be examined after treating with hydrochloric acid. Histological examination : Surgically removed cysts can be subjected to histopathological stains like Giemsa , hematoxylin and eosin (H & E) and Periodic acid Schiff (PAS) stain to demonstrate the three layers of the cyst wall and attached brood capsules.

28. Molecular Methods : PCR targeting mitochondrial DNA has been developed. PCRRFLP can be used to detect genotypes of E. granulosus.10 genotypes have been identifi ed from G1 to G10; each has a particular host specificity and geographical distribution. G1 is the most common genotype in India. Other tests Eosinophilia is present in 20–25% cases Hypergammaglobulinemia .

29. Echinococcus eggs in feaces of dog

30. Treatment Surgical Removal of Hydatid Cyst : 90% effective but can be risky depending on location, size, and advancement of cyst It may need chemotherapy to prevent recurrance Chemotherapy: Albendazole is preferred treatment because it penetrates into hyatid cysts.  Dosage: 10mg/kg body weight or 400mg 2x daily for 4 weeks Mebendazole Dosage: 40mg/kg body weight 3x daily for 3-6 months Dogs are effectively treated with Praziquental PAIR Treatment : Puncture, aspiration, injection and re-aspiration: It is an alternate method recommended instead of surgery. It involves four basic steps: Percutaneous puncture of the cyst Aspiration of 10–15 mL of cyst fl uid Infusion of scolicidal agents like hypertonic saline, cetrimide , or ethanol Re-aspiration of the fluid after 5 minutes

31. Prevention In order to prevent transmission to dogs from intermediate hosts, dogs can be given anthelminthic vaccinations  Clean slaughter and high surveillance of potential intermediate host during slaughter is key in preventing the spread this cestode to its definitive host Proper disposal of carcass and offal after slaughter to prevent dogs access to offal from livestock Boiling livers and lungs which contain hydatid cysts for 30 minutes has been proposed as a simple, efficient and saving way to kill the infectious larvae

32. Echinococcus multilocularis Most of the E. multiculoris are said to be sterile because they don’t have protoscolices in their parent cyst. In general the rupture of hydatid cyst release the hydatid sand. Life Cycle : Life cycle is similar to that of E. granulosus . Only the hosts are different. Definitive host: Foxes and wolves (and also dogs and cats) Intermediate hosts: Small wild rodents like squirrels, voles, mice, etc. Man is an accidental intermediate host.

33. Clinical Features E. multilocularis is the causative agent of alveolar (or multilocular ) hydatid disease. So named because the cysts have multiple locules or cavities with no fluid or no free brood capsule/ scolices . Liver is the most common organ aff ected (98% of cases)

34. (Intermediate)

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