Echinococcus granulosus: Etiological agent of echinococcosis or cystic hydatidosis - IgG antibodies ; Molecular diagnosis (PCR).


Information 26-05-2018.


Echinococcosis or human hydatidosis is a zoonotic disease caused by cestodes of the genus Echinococcus. There are different types of hydatidosis, depending on the species that causes the disease. Among them, the echinococcosis or cystic hydatidosis, caused by Echinococcus granulosus, is a chronic disease characterized by the development of one or more hydatid cysts located more frequently in the liver and lungs, and less frequently in the bones, kidneys, spleen, muscles, central nervous system and eyes. This zoonosis has a cosmopolitan distribution, although it is especially prevalent in sheep-producing countries in rural areas. South America, the Middle East, Australia, Russia and China are the regions where a greater number of cases are reported. Cystic echinococcosis has a considerable impact on both human and animal health, and important economic consequences derived from the costs of medical treatment and morbidity for human cases and losses in animal productivity.

Currently, the existence of nine species within the genus Echinococcus is accepted: E. multilocularis, E. vogeli, E. oligarthra, E. shiquicus, E. granulosus, E. equinus, E. ortleppi, E. canadensis, and E. felidis. Species of this genus differ in morphology, development, host specificity, infectivity and pathogenicity in humans, and other aspects. Among them, four species are considered more important from the point of view of public health and their distribution and economic impact worldwide. In the first place, E. granulosus, which causes cystic echinococcosis, and E. multilocularis, the etiological agent of alveolar echinococcosis, are the most frequent. On the other hand, E. vogeli, the cause of polycystic echinococcosis, and E. oligarthrus, which gives rise to unicystic echinococcosis, have been reported much less frequently in humans. However, the main responsible for hydatidosis in humans is E. granulosus.

E. granulosus is known as the dog tapeworm because the adult form of the parasite is found exclusively in dogs and other wild canids such as the wolf and the jackal, where it does not usually present pathology. This helminth is transmitted cyclically between canines and numerous herbivorous animals of cattle, which can serve as intermediate hosts. The human is an accidental host, affecting especially people who have a relationship with livestock and the owners of contaminated dogs.

The adult cestode, located in the small intestine of the definitive hosts, the canids, measures around 2 to 6 mm in length, usually consists of 3 proglottids (immature, mature and gravid), and a scolex with four suckers and a double crown of hooks. The eggs of these cestodes (30 - 40 μm) are eliminated in feces of these carnivorous animals and are the infective form for intermediate hosts, herbivorous or omnivorous mammals (mainly sheep, pigs, cattle, goats, horses), and the human, as an accidental host. The elimination of eggs by each parasite is the cause of contamination of the soil, crops, water, and the environment in general where dogs travel, and where they can survive for several months. If the man or some intermediate host ingests these eggs in the vegetables or the contaminated grass, the resulting larvae will form cysts in various tissues. Meanwhile, if the dog or some canid ingests the meat or the viscera of the parasitized animal, the wall of the cyst will disintegrate in the intestine and a new adult helminth will develop. The larvae or hidatides that develop and encyst in the internal organs and tissues of the herbivores and omnivores, cause the deterioration in the health of the animal and diminution of the productivity. The natural death of a sick animal in the field or its sacrifice to obtain meat for consumption, produce the availability of viscera infected with hydatid cysts that can close the carnivore-omnivore circuit. The dog-sheep, dog-bovine, dog-pig, dog-camel, dog-kangaroo or dog-pig cycle is important in most areas where E. granulosus is endemic and is the form that parasiticism has to perpetuate in a region. In particular, cystic echinococcosis is mainly maintained in a dog-sheep-dog cycle.

Infection in humans occurs after ingestion of eggs through contaminated food, water or soil, or by direct and close contact with parasitized dogs. It has been observed that the eggs adhere to the coat of the canids, mainly around the anus, snout, thighs and legs. Infection occurs most often in childhood due to close contact with pets. When eggs of E.granulosus reach the stomach, the chitin layer of the egg is destroyed by the hydrochloric acid of the gastric juice and the hexacant embryos that cross the gastric and intestinal mucosa are released and carried by the portal circulation, reaching the liver. Many of these embryos are phagocytosed and destroyed by the mononuclear phagocytic system, although some evolve the larvae state and encyst in the liver, and others, in small quantity, embolize in pulmonary capillaries where they follow a similar evolution, encyst in the lung or pass into the systemic circulation and spread through the rest of the body. Once they reach their final destination, the metacestodes implant in these tissues, and form the hydatid cysts. Of the hydatid cysts, it is estimated that 65-75% is located in the liver, more frequently in the right lobe, between 25-30% in the lung, and around 20% in other locations. There are reports of bone, kidney, muscle, spleen, central nervous system and eye parasitosis.

The hydatid cyst or hydátide is a sphere or vesicle of variable size (1 to 20 cm), filled with a transparent and colorless liquid, defined as "rock water". Hydatid fluid is a complex mixture of glycolipoproteins, carbohydrates, amino acids and salts, and products of metabolism of the metacestode in a 98% water base. Some of its components also come from the host, mainly albumin and immunoglobulins. The vesicular metacestode has two layers, one internal, the germinative, where the daughter vesicles are produced, and one external, acellular, known in its entirety as an endocyst. The germinal layer gives rise to vesicles in which protoscolices develop, with 4 suckers and crown of hooks. These vesicles may be attached to the wall or free, in the fluid, forming child cysts. The set of membrane and protoscolic remains constitute the so-called hydatid sand. All this structure is covered by the pericyst, which is formed by the inflammatory tissue resulting from the response of the host to the parasite. The induction of a granulomatous reaction in the host leads to the formation of a third layer, of connective tissue, the adventitia, pericystic or ectocic layer, which on the one hand isolates the parasite and on the other serves as a mechanical support. The parasite in turn begins to form around it a spherical seal of inert chitinous material with central vacuolization and thus establishes the first phase of a viable cyst.

Infected subjects may be asymptomatic for months, years or permanently. The manifestations depend on the affected organ, the number of cysts, their size, development and the pressure exerted on adjacent tissues or organs. It should be noted that cysts located in the brain or at the ocular level can lead to early clinical manifestations. The main pathogenic mechanism of this structure is mechanical. The development of hydatid cysts is slow, but over time it can lead to an occupying mass that can cause very important tissue displacements. In its development it can compress adjacent structures, fissure, become infected and more rarely break. It is also possible the dissemination of protoscolices, which will lead to a secondary hydatidosis.

When clinical manifestations are absent or non-specific, diagnosis can be difficult. The most frequent nonspecific clinical manifestations are: hepatomegaly, pain in the right hypochondrium, epigastric, nausea, vomiting, urticaria, abdominal distension, cholestasis, portal hypertension, biliary cirrhosis, ascites and other signs and symptoms associated with the occupational mass, such as pulmonary compromise. When the cyst ruptures, a picture of acute abdominal pain accompanied by fever, pruritus and the appearance of an urticarial rash or an anaphylactic reaction may occur.

The diagnosis of cystic echinococcosis is based on epidemiological antecedents, origin of endemic area, contact with dogs, clinical findings, imaging techniques, serology and molecular techniques. Serological techniques have limited utility, because, regardless of their location, intact cysts cause a minimal immune response, and only cysts with cracks or ruptured are associated with a strong response. Molecular methods such as the polymerase chain reaction (PCR) with superior sensitivity and specificity and with the ability to provide a definitive parasitic diagnosis, have an advantage over conventional serological tests and are very useful for the detection of the parasite in feces of dog, or liver biopsies or of another organ where the cysts are located in intermediate and human hosts.

Tests carried out in IVAMI:

  • Molecular diagnosis of Echinococcus granulosus (PCR).
  • IgG antibodies against Echinococcus granulosus (ELISA).

Recommended sample:

  • For the detection of IgG antibodies to the parasite, samples of plasma or serum (1 mL) separated from blood extracted with EDTA are accepted. Alternatively, whole blood (5 mL) extracted with EDTA will be accepted for the subsequent separation of the serum in the laboratory.
  • For the detection of E. granulosus DNA, samples of biopsies, aspirate of hydatid fluid or wall of extracted hydatid cysts will be accepted. Serum or plasma samples will be accepted only in case of suspected rupture of the hydatid cyst.
  • For the detection of E. granulosus DNA in definitive hosts (canids - dogs, etc.), stool samples are accepted.
  • For the detection of E. granulosus DNA in intermediate hosts, samples from the viscera contaminated with hydatid cysts will be accepted.

Conservation and shipment of the sample:

  • Refrigerated (preferred) for less than 2 days, or frozen if the storage period is longer.
  • In the case of sending whole blood extracted with EDTA, the sample must be kept refrigerated, not frozen, and arrive at our laboratory in a period not exceeding 48 hours.

Delivery of results:

  • Molecular detection of Echinococcus granulosus DNA (PCR): 24 to 48 hours.
  • Detection of IgG antibodies against Echinococcus granulosus (ELISA): 4 days.

Cost of the test:

  • Detection of IgG antibodies against Echinococcus granulosus: Consult