Also known as hydatid worm, hyper tapeworm or dog tapeworm, Echinococcus granulosus is primarily found in areas where sheep or other herbivores are raised and are in close contact with dogs or wild canines. Other criteria for at-risk areas include those where there is close contact between canines and humans. These areas include Britain, parts of South America, Australia, parts of Africa, Asia, and China, and selected portions of the Middle East. Several cases have been reported in the U.S., particularly in Alaska, as well as in the West and Southwest.
Morphological characteristics of Echinococcus granulosus
Echinococcus granulosus Egg
- The eggs of Echinococcus are indistinguishable from those of Taenia species.
- It is ovoid in shape and brown in color
- It contains an embryo with 3 pairs of hooklets.
Fortunately, Echinococcus granulosus ‘ diagnostic stage is that of the Hydatid Cysts larval stage described later.
Hydatid Cysts larval stage
Echinococcus granulosus hydatid cyst larval stage found in human tissue, consists of several structures. These structures somewhat overlap in their definitions, resulting in terms being confused and making a clear and concise description of them a challenge. The description below should be clear.
The whole structure is called a hydatid cyst. Miniaturization of the whole hydatid cyst can occur within the cyst; these are called daughter cysts. A protective cyst wall and laminated layers of germinal tissue surround both types of cysts. Furthermore, from the inner germinal layer, brood capsules, which lack a protective cyst wall, form. These structures include the development of scolices. When present in the definitive host, each scolex, once fully developed, has the ability to develop into an adult worm.
The hydatid sand, which is defined as component in an earlier Echinococcos granulosus cyst, can evolve, as is shown in some of the hydatid cysts, which typically involve the daughter of cysts, free scolices, hooklets and diverse unspecified material. In this case, certain hydatid cysts may develop such structures, but sterilization or death, followed by calcification, may occur as a result of secondary bacterial infections.
Adults Echinococcus granulosus worm
The average adult Echinococcus granulosus is only 4.5 mm in length, relatively small.The worm is made up of a scolex, a small neck, and three proglottids, one at each stage of development — immature, mature, and gravid.The scolex has four suckers and about 36 hooks.Typically, this form is not seen in humans, but is commonly found in canines that serve as definitive hosts.
Echinococcus granulosus Life Cycle
Human beings serve Echinococcus granulosus as accidental intermediate hosts.The typical intermediate host is sheep, although other herbivores may also serve in this role.Human infection begins after echinococcus eggs have been ingested by contact with contaminated dog feces.Larvae from eggs enter the intestine and migrate to a number of tissue sites, especially the lungs and liver, through the bloodstream.In the infected tissue, a hydatid cyst develops.Humans are considered dead end hosts because the life cycle of Echinococcus ends in human tissue.
The life cycle can be completed when the sheep act as the intermediate host.The hydatid cyst forms in the sheep’s viscera (soft parts and internal organs of the body’s major cavities).Infected sheep viscera are consumed by the definitive host, the dog or the wild Canine.Each scolex becomes an adult worm when ingested with the cyst.The adult worms are in the intestine of the host.Eggs are produced and passed through the feces into the environment where they can initiate a new cycle.
Pathogenesis of echinococcus granulosus
Evolution of Hydatid Cyst
The embryo slowly develops at the deposition site into a hollow bladder or cyst filled with fluid. This becomes the hydatid cyst (meaning “a drop of water “ in Greek hydatis).It enlarges slowly and in about 6 months it reaches a diameter of 0.5–1 cm. The growing cyst evokes the reaction of host tissue leading to fibrous capsule deposition around it.The embryo’s cyst wall consists of three indistinguishable layers :
- Pericyst is the outer host inflammatory reaction of fibroblastic proliferation, mononuclear cells, eosinophils, and giant cells that eventually develop into dense fibrous capsules which may even calcify.
- Ectocyst is the intermediate layer consisting of characteristic acellular, chitinous, laminated hyaline material. It has the appearance of a hard – boiled egg white.
- Endocyst is the cellular inner germ layer comprising a number of nuclei embedded in a protoplasmic mass and extremely thin (22–25 mm).The germinal layer is the cyst’s vital layer and is the site of asexual reproduction that produces brood capsules with scolices
It is a clear colorless or pale yellow fluid called hydatid fluid fills the inside of the cyst. Approximately 6.7 (acidic) fluid pH.
- It contains salts (0.5 percent sodium chloride, sodium sulphate, sodium phosphate, succinic acid salts) and proteins.
- It is antigenic and highly toxic so that its release into circulation results in severe eosinophilia or even anaphylaxis.
- The fluid was used as the antigen for Casoni’s intradermal test.
At the bottom of the cyst is a granular deposit or hydatid sand made up of free brood capsules and protoscolices and loose hooklets.
Small knob – like excrescences or gemmules protrude into the cyst’s lumen from the germinal layer. These will be enlarged, vacuolated, and filled with fluid. These are called Brood capsules
- Initially they are attached by a stalk to the germinal layer, but later they escape free into the cyst cavity filled with fluid.
- Protoscolices (new larvae) develop from the brood capsules ‘ inner wall, representing the head of the potential worm, along with invaginated scolex, bearing suckers and hooklets.
- Several thousand protoscolices develop into a mature hydatid cyst, thus representing a high magnitude asexual reproduction.
- Further generation of cysts, daughter cysts and grand – daughter cysts may develop within mature hydatid cysts. The cyst often grows slowly, taking 20 years or more to get big enough to cause clinical disease, and is therefore especially seen in humans.
Some cysts are sterile and may never produce brood capsules, while some brood capsules may not produce scolices. These are known as acephalocysts.
Fate of hydatid cysts
After an inflammatory reaction, the cyst may be calcified or evacuated spontaneously. Hydatid liver cyst may break into the lung or other cavity of the body resulting in disseminated hydatid lesions.
Clinical Presentation of Echinococcus granulosus infection
Infection is mostly asymptomatic and discovered accidentally.Clinical disease only develops when the hydatid cyst has grown large enough to cause obstructive symptoms. Disease mainly results from pressure effects caused by the enlarged cysts.The primary hydatid cyst occurs in the liver (63 %) in about half of the cases, mostly in the right lobe. The usual manifestations are hepatomegaly, pain, and obstructive jaundice.The next common site is the lung (25 %) (the lower lobe of the right lung is the most common).
The clinical picture consists of cough, hemoptysis, chest pain, pneumothorax, and dyspnea.Hydatid cyst in the kidney (2 %) causes pain and hematuria. Other affected sites include spleen (1 %), brain (1 %), pelvic organs, orbit, and bones (3 %).Cerebral hydatid cysts may present as focal epilepsy.
When hydatid cyst is formed within the bones, due to containment by dense osseous tissues, the laminated layer is not well developed. The parasite migrates as naked excrescences along the bony canals that erode the tissue of the bone. This is called hydatid cyst osseous. Bone erosion can result in pathological fractures. Other pathogenic mechanism in hydatid disease, apart from pressure effects, is hypersensitivity to the echinococcal antigen. By minute amounts of hydatid fluid flowing through the capsule, the host is sensitized to the antigen. Hypersensitivity can cause hives. But if a hydatid cyst breaks spontaneously or during surgical interference, severe, even fatal anaphylaxis can be caused by massive release of hydatid fluid.
Echinococcus granulosus Laboratory diagnosis
There are several ways to diagnose echinococcus granulosus. The presence of scolices, daughter cysts, brood capsules, or hydatid sand may be examined on biopsy samples. Care must be taken when choosing this diagnostic method because anaphylaxis may occur in infected patients if fluid leaks from the hydatid cyst during collection of specimens. There are available serological tests such as ELISA, indirect hemagglutination, and Western blot test. Radiography, computed tomography (CT), or ultrasound scanning techniques can be used to detect the hydatid cyst.
Echinococcus granulosus treatment
Historically, surgical removal of the hydatid cyst has been considered the treatment of choice for Echinococcus if it is located in an appropriate area for surgery. However, if appropriate, the advent of antiparasitic drugs has offered an alternative to surgery. In situations where the hydatid cyst was inoperable, the medications mebendazole, albendazole, and praziquantel were particularly useful.
Prevention and Control of Echinococcus granulosus
To break the life cycle of Echinococcus granulosus and then stop the spread of human disease, it is essential to implement several measures. These include implementing appropriate personal hygiene practices to prevent egg intake, discontinuing the practice of feeding canines with potentially contaminated viscera, promptly treating infected canines and humans, and setting up a comprehensive parasite transmission education program for those in high-risk areas.