| Echinococcosis | |
|---|---|
| Classification and external resources | |
Echinococcus life cycle (click to enlarge) |
|
| ICD-10 | B67 |
| ICD-9 | 122.4, 122 |
| DiseasesDB | 4048 |
| eMedicine | med/629 med/1046 |
| MeSH | D004443 |
Echinococcosis, which is often referred to as hydatid disease or echinococcal disease, is a parasitic disease that affects both humans and other mammals, such as sheep, dogs, rodents and horses.[1] There are three different forms of echinococcosis found in humans, each of which is caused by the larval stages of different species of the tapeworm of genus Echinococcus. The first of the three and also the most common form found in humans is cystic echinococcosis (also known as unilocular echinococcosis), which is caused by Echinococcus granulosus. The second is alveolar echinococcosis (also known as alveolar colloid of the liver, alveolar hydatid disease, alveolococcosis, multilocular echinococcosis, “small fox tapeworm”), which is caused by Echinococcus multilocularis and the third is polycystic echinococcosis (also known as human polycystic hydatid disease, neotropical echinococcosis), which is caused by Echinococcus vogeli and very rarely, Echinococcus oligarthus. Alveolar and polycystic echinococcosis are rarely diagnosed in humans and are not as widespread as cystic echinococcosis, but polycystic echinococcosis is relatively new on the medical scene and is often left out of conversations dealing with echinococcosis, and alveolar echinococcosis is a serious disease that not only has a significantly high fatality rate but also has the potential to become an emerging disease in many countries.
Echinococcosis is a disease that has been recognized by humans for centuries. There has been mention of it in the Talmud. It was also recognized by ancient scholars such as Hippocrates, Aretaeus, Galen and Rhazes. Although echinococcosis has been well known for the past two thousand years, it wasn’t until the past couple of hundred years that real progress was made in determining and describing its parasitic origin. The first step towards figuring out the cause of echinococcosis occurred during the 17th century when Francisco Redi illustrated that the hydatid cysts of echinococcosis were of “animal” origin. Then, in 1766, Pierre Simon Pallas predicted that these hydatid cysts found in infected humans were actually larval stages of tapeworms. A few decades afterwards, in 1782, Goeze accurately described the cysts and the tapeworm heads while in 1786, E. granulosus was accurately described by Batsch. Half a century later, during the 1850s, Carl von Siebold showed through a series of experiments that Echinoccocus cysts do cause adult tapeworms in dogs. Shortly after this, in 1863, E. multilocularis was identified by Rudolf Leuckhart. Then, during the early to mid 1900s, the more distinct features of E. granulosus and E. multilocularis, their life cycles and how they cause disease were more fully described as more and more people began researching and performing experiments and studies. While E. granulosus and E. multilocularis were both linked to human echinococcosis before or shortly after the 20th century, it wasn’t until the mid 1900s that E. oligarthus and E. vogeli were identified as and shown as being causes of human echinococcosis.[2][3][4]
E. granulosus is present virtually worldwide since there are very few countries that are considered to be completely free of E. granulosus.[5] An important fact to keep in mind is that the areas of the world where there is a high incidence of infection by E. granulosus often coincide with rural, grazing areas where dogs are able to ingest organs from infected animals.
E. multilocularis mainly occurs in the Northern hemisphere, including central Europe and the northern parts of Europe, Asia, and North America. However, its distribution was not always like this.[6] For instance, until the end of the 1980s, E. multilocularis endemic areas in Europe were known to exist only in France, Switzerland, Germany, and Austria. But during the 1990s and early 2000s, there was a shift in the distribution of E. multilocularis as the infection rate of foxes escalated in certain parts of France and Germany. As a result, several new endemic areas were found in Switzerland, Germany, and Austria and surrounding countries such as the Netherlands, Belgium, Luxembourg, Poland, the Czech Republic, the Slovak Republic, and Italy. While alveolar echinococcosis is not extremely common, it is believed that in the coming years, it will be an emerging or re-emerging disease in certain countries as a result of E. multilocularis’ ability to spread.[7]
Unlike the previous two species of Echinococcus, E. vogeli and E. oligarthus are limited to Central and South America. Furthermore, infections by E. vogeli and E. oligarthus (polycystic echinococcosis) are considered to be the rarest form of echinococcosis.[8]
Echinococcus eggs contain an embryo that is called an oncosphere or hexcanth. The name of this embryo stems from the fact that these embryos have six hooklets. The eggs are passed through the feces of the definitive host and it is the ingestion of these eggs that lead to infection in the intermediate host.[8]
From the embryo released from an egg develops a hydatid cyst, which grows to about 5–10 cm within the first year and is able to survive within organs for years.[9] Cysts sometimes grow to be so large that by the end of several years or even decades, they can contain several liters of fluid. Once a cyst has reached a diameter of 1 cm, its wall differentiates into a thick outer, non-cellular membrane, which covers the thin germinal epithelium. From this epithelium, cells begin to grow within the cyst. These cells then become vacuolated and are known as brood capsules, which are the parts of the parasite from which protoscolices bud. Often, daughter cysts will also form within cysts.[8]
Echinococcus adult worms develop from protoscolices and are typically 6mm or less in length and have a scolex, neck and typically three proglottids, one of which is immature, another of which is mature and the third of which is gravid (or containing eggs).[8] The scolex of the adult worm contains four suckers and a rostellum that has about 25-50 hooks.[10]
The major morphological difference among different species of Echinococcus is the length of the tapeworm. E. granulosus is approximately 2 to 7 mm while E. multilocuralis is often smaller and is 4 mm or less.[11] On the other hand, E. vogeli is found to be up to 5.6 mm long and E. oligarthus is found to be up to 2.9 mm long.[6] In addition to the difference in length, there are also differences in the hydatid cysts of the different species. For instance, in E. multilocularis, the cysts have an ultra thin limiting membrane and the germinal epithelium may bud externally. Furthermore, E. granulosus cysts are unilocular and full of fluid while E. multilocularis cysts contain little fluid and are multilocular. For E. vogeli, its hydatid cysts are large and are actually polycystic since the germinal membrane of the hydatid cyst actually proliferates both inward, to create septa that divide the hydatid into sections, and outward, to create new cysts. Like E. granulosus cysts, E. vogeli cysts are filled with fluid.[8]
As one can see from the life cycles illustrated above, all disease-causing species of Echinococcus are transmitted to intermediate hosts via the ingestion of eggs and are transmitted to definitive hosts by means of eating infected, cyst-containing organs. When thinking about transmission, it is important to remember that humans are accidental intermediate hosts that become infected by handling soil, dirt or animal hair that contains eggs.[11]
While there are no biological or mechanical vectors for the adult or larval form of any Echinococcus species, coprophagic flies, carrion birds and arthropods can act as mechanical vectors for the eggs.[12]
The incubation period for all species of Echinococcus can be months to years or even decades.[13] It largely depends on the location of the cyst in the body and how fast the cyst is growing.[11]
Like many other parasite infections, the course of Echinococcus infection is complex. The worm has a life cycle that requires definitive hosts and intermediate hosts. Definitive hosts are normally carnivores such as dogs, while intermediate hosts are usually herbivores such as sheep and cattle. Humans function as accidental hosts, because they are usually a 'dead end' for the parasitic infection cycle.
E. granulosus: dogs and other canidae
E. multilocularis: foxes, dogs, other canidae and cats
E. vogeli: bush dogs and dogs
E. oligarthus: wild felids[6][14]
E. granulosus: sheep, goats, swine and other wild herbivores
E. multilocularis: small rodents
E. vogeli: rodents
E. oligarthus: small rodents[6][14]
1) An adult worm resides in the small intestine of a definitive host. 2) Afterwards, gravid proglottids release eggs that are passed in the feces of the definitive host. The egg is then ingested by an intermediate host. 3) The egg then hatches in the small intestine of the intermediate host and releases an oncosphere that penetrates the intestinal wall and moves through the circulatory system into different organs, in particular the liver and lungs. Once it has invaded these organs, the oncosphere develops into a cyst. 4) The cyst then slowly enlarges, creating protoscolices and daughter cysts within the cyst. The definitive host then becomes infected after ingesting the cyst-containing organs of the infected intermediate host. 5) After ingestion, the protoscolices attach to the intestine. 6) They then develop into adult worms and the cycle starts all over again.[6]
In order to formally diagnose a patient with any type of echinococcosis, one must use a combination of tools that involve imaging techniques, histopathology and/or nucleic acid detection and serology. For cystic echinococcosis, imaging is the main method that is relied on for diagnosis while serology tests (such as indirect hemogglutination, ELISA (enzyme linked immunosorbent assay), immunoblots or latex agglutination) that use antigens specific for E. granulosus are used to verify the imaging results. The imaging technique of choice for cystic echinococcosis is ultrasonography since it is not only able to visualize the cysts in the body’s organs [15] but it is also inexpensive, non-invasive and gives instant results.[16] In addition to ultrasonography, both MRI and CT scans can and are often used although an MRI is often preferred to CT scans when diagnosing cystic echinococcosis since it gives better visualization of liquid areas within the tissue.[15]
As with cystic echinococcosis, ultrasonography is the imaging technique of choice for alveolar echinococcosis and is usually complemented by CT scans since CT scans are able to detect the largest number of lesions and calcifications that are characteristic of alveolar echinococcosis. MRIs are also used in combination with ultrasonography though CT scans are preferred. Like cystic echinococcosis, imaging is the major method used for the diagnosis of alveolar echinococcosis while the same types of serologic tests (except now specific for E. multilocularis antigens) are used to verify the imaging results. It is also important to note that serologic tests are more valuable for the diagnosis of alveolar echinococcosis than for cystic echinococcosis since they tend to be more reliable for alveolar echinococcosis since more antigens specific for E. multilocularis are available.[11] In addition to imaging and serology, identification of E. multilocularis infection via PCR or a histological examination of a tissue biopsy from the patient is another way to diagnose alveolar echinococcosis.[15]
Similar to the diagnosis of alveolar echinococcosis and cystic echinococcosis, the diagnosis of polycystic echinococcosis uses imaging techniques, in particular ultrasonography and CT scans, to detect polycystic structures within the patient’s body. However, imaging is not the preferred method of diagnosis since the method that is currently considered the standard is the isolation of protoscoleces during surgery or after the patient’s death and the identification of definitive features of E. oligarthus and E. vogeli in these isolated protoscoleces. Although this is the main way in which PE is diagnosed, there are current studies that have shown that PCR may be used to identify E. oligarthus and E. vogeli in patients’ tissues.[17] The only drawback of using PCR to diagnose polycystic echinococcosis is that there aren’t many genetic sequences that can be used for PCR that are specific only to E. oligarthus or E. vogeli.[11]
In the human manifestation of the disease, E. granulosus, E. multilocularis, E. oligarthus and E. vogeli are localized in the liver (in 75% of cases), the lungs (in 5-15% of cases) and other organs in the body such as the spleen, brain, heart and kidneys (in 10-20% of cases). In the patients who are infected with E. granulosus and therefore have cystic echinococcosis, the disease develops as a slow-growing mass in the body. These slow-growing masses, often called cysts, are also found in patients that are infected with alveolar and polycystic echinococcosis. The cysts found in those with cystic echinococcosis are usually filled with a clear fluid called hydatid fluid, are spherical and typically consist of one compartment and are usually only found in one area of the body. While the cysts found in those with alveolar and polycystic echinococcosis are similar to those found in those with cystic echinococcosis, alveolar and polycystic echinococcosis cysts usually have multiple compartments and have infiltrative as opposed to expansive growth.[2][18]
Depending on the location of the cyst in the body, the patient could be asymptomatic even though the cysts have grown to be very large or be symptomatic even if the cysts are absolutely tiny. If the patient is symptomatic, the symptoms that an infected patient exhibits will also depend largely on where the cysts are located. For instance, if the patient has cysts in the lungs and is symptomatic, they will have a cough, shortness of breath and/or pain in the chest. On the other hand, if the patient has cysts in the liver and is symptomatic, they will suffer from abdominal pain, abnormal abdominal tenderness, hepatomegaly with an abdominal mass, jaundice, fever and/or anaphylactic reaction. In addition, if the cysts were to rupture while in the body, whether during surgical extraction of the cysts or by some kind of trauma to the body, the patient would most likely go into anaphylactic shock and suffer from high fever, pruritus (itching), edema (swelling) of the lips and eyelids, dyspnea, stridor and rhinorrhea.[19]
Unlike intermediate hosts, definitive hosts are usually not hurt very much by the infection. Sometimes, a lack of certain vitamins and minerals can be caused in the host by the very high demand of the parasite.
For simple cases of cystic echinococcosis, the most common form of treatment is surgical removal of the cysts combined with chemotherapy using albendazole and/or mebendazole before and after surgery. However, if there are cysts in multiple organs or tissues, or the cysts are in risky locations, surgery becomes impractical. For inoperable cases such as these, chemotherapy and/or PAIR (puncture-aspiration-injection-reaspiration) become alternative options of treatment.[20] In the case of alternative treatment using just chemotherapy, albendazole is preferred with an adult dosage of 400 mg orally, twice a day for 1–5 months and a pediatric dosage of 15 mg/kg/day (max. of 800 mg) for 1–6 months.[21] An alternative to albendazole is mebendazole at a dosage of 40 to 50 mg/kg/day for at least 3 to 6 months. The other alternative to surgery is PAIR with chemotherapy. PAIR is a minimally invasive procedure that involves three steps: puncture and needle aspiration of the cyst, injection of a scolicidal solution for 20-30 min, and cyst-re-aspiration and final irrigation. Patients who undergo PAIR typically take albendazole or mebendazole from 7 days before the procedure until 28 days after the procedure. While surgery still remains as the standard for cystic echinococcosis treatment, there have been a number of studies that suggest that PAIR with chemotherapy is more effective than surgery in terms of disease recurrence, and morbidity and mortality.[22] In addition to the three abovementioned treatments, there is currently research and studies looking at new treatment involving percutaneous thermal ablation (PTA) of the germinal layer in the cyst by means of a radiofrequency ablation device. This form of treatment is still relatively new and requires much more testing before being widely used.[11] Another treatment that is also currently being looked into is laparoscropy hydatid liver surgery, whose efficacy is still controversial even though studies have suggested that it has clear benefits.[23]
For alveolar echinococcosis, surgical removal of cysts combined with chemotherapy (using albendazole and/or mebendazole) for up to two years after surgery is the only sure way to completely cure the disease.[24] However, in inoperable cases, chemotherapy by itself can also be used. In treatment using just chemotherapy, one could use either mebendazole at a dosage of 40 to 50 mg/kg/day in three doses or albendazole at a dosage of 10 to 15 mg/kg/day in two doses. Since chemotherapy on its own is not guaranteed to completely rid the patient of disease, patients are often kept on the drugs for extended periods of times (i.e. more than 6 months, years). In addition to surgery and chemotherapy, liver transplants are being looked into as a form of treatment for alveolar echinococcosis although it is seen as incredibly risky since it often leads to echinococcosis re-infection in the patient afterwards.[11]
Since polycystic echinococcosis is constrained to such a particular area of the world and is not well described or found in many people, treatment of polycystic echinococcosis is less defined than that of cystic and alveolar echinococcosis. While surgical removal of cysts was the treatment of choice for the previous two types of echinococcosis, chemotherapy is the recommended treatment approach for polycystic echinococcosis. While albendazole is the preferred drug, mebendazole can also be used if the treatment is to be for an extended period of time. Only if chemotherapy fails or if the lesions are very small is surgery advised.[11]
There are several different strategies that are currently being used to prevent and control cystic echinococcosis. Most of these various methods try to prevent and control cystic echinococcosis by targeting the major risk factors for the disease and the way it is transmitted. For instance, health education programs focused on cystic echinococcosis and its agents, and improved water sanitation attempt to target poor education and poor drinking water sources, which are both risk factors for contracting echinococcosis. Furthermore, since humans often come into contact with Echinococcus eggs via touching contaminated soil, animal feces and animal hair, another prevention strategy is improved hygiene. In addition to targeting risk factors and transmission, control and prevention strategies of cystic echinococcosis also aim at intervening at certain points of the parasite’s life cycle, in particular, the infection of hosts (i.e. especially dogs) that reside with or near humans. For example, many countries endemic to echinococcosis have implemented programs geared at de-worming dogs and vaccinating dogs and other livestock, such as sheep, that also act as hosts for E. granulosus.
There are also a number of strategies geared towards the prevention and control of alveolar echinococcosis, most of which are similar to those of cystic echinococcosis. For instance, health education programs, improved water sanitation, improved hygiene and de-worming of hosts, in particular, red foxes, are all ways in which a country can prevent and control the spread of alveolar echinococcosis. Unlike cystic echinococcosis, though, where there is a vaccine against E. granulosus, there is currently no canidae or livestock vaccine against E. multilocularis.[25]
While there are a number of control and prevention strategies dealing with cystic and alveolar echinococcosis, there aren’t many methods specified for the control and prevention of polycystic echinococcosis. This is probably due to the fact that polycystic echinococcosis is restricted to only Central and South America and that the way in which humans become accidental hosts of E. oligarthus and E. vogeli is still not completely understood.[11]
Currently there are no human vaccines against any form of echinococcosis. However, there are studies being conducted that are looking at possible vaccine candidates for an effective human vaccine against echinococcosis.[26]
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