Echinococcosis is a parasitic disease caused by tapeworms of the Echinococcus type. The two main types of the disease are cystic echinococcosis and alveolar echinococcosis. Less common forms include polycystic echinococcosis and unicystic echinococcosis.
The disease often starts without symptoms and this may last for years. The symptoms and signs that occur depend on the cyst's location and size. Alveolar disease usually begins in the liver but can spread to other parts of the body, such as the lungs or brain. When the liver is affected, the patient may experience abdominal pain, weight loss, along with yellow-toned skin discoloration from developed jaundice. Lung disease may cause pain in the chest, shortness of breath, and coughing.
The infection is spread when food or water that contains the eggs of the parasite is ingested or by close contact with an infected animal. The eggs are released in the stool of meat-eating animals that are infected by the parasite. Commonly infected animals include dogs, foxes, and wolves. For these animals to become infected they must eat the organs of an animal that contains the cysts such as sheep or rodents. The type of disease that occurs in human patients depends on the type of Echinococcus causing the infection. Diagnosis is usually by ultrasound though computer tomography (CT) or magnetic resonance imaging (MRI) may also be used. Blood tests looking for antibodies against the parasite may be helpful as may biopsy.
Prevention of cystic disease is by treating dogs that may carry the disease and vaccination of sheep. Treatment is often difficult. The cystic disease may be drained through the skin, followed by medication. Sometimes this type of disease is just watched. The alveolar form often requires surgical intervention, followed by medications. The medication used is albendazole, which may be needed for years. The alveolar disease may result in death.
The disease occurs in most areas of the world and currently affects about one million people. In some areas of South America, Africa, and Asia, up to 10% of certain populations are affected. In 2015, the cystic form caused about 1,200 deaths; down from 2,000 in 1990. The economic cost of the disease is estimated to be around US$3 billion a year. It is classified as a neglected tropical disease (NTD) and belongs to the group of diseases known as helminthiases (worm infections). It can affect other animals such as pigs, cows and horses.
Terminology used in this field is crucial since echinococcosis requires the involvement of specialists from nearly all disciplines. In 2020, an international effort of scientists, from 16 countries, led to a detailed consensus on terms to be used or rejected for the genetics, epidemiology, biology, immunology, and clinical aspects of echinococcosis.
Signs and symptoms
In the human manifestation of the disease, E. granulosus, E. multilocularis, E. oligarthrus and E. vogeli is 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 people 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 people who 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, 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, the alveolar and polycystic echinococcosis cysts usually have multiple compartments and have infiltrative as opposed to expansive growth.
Depending on the location of the cyst in the body, the person could be asymptomatic even though the cysts have grown to be very large or be symptomatic even if the cysts are tiny. If the person is symptomatic, the symptoms will depend largely on where the cysts are located. For instance, if the person 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 person has cysts in the liver and is symptomatic, they will experience 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 trauma to the body, the person would most likely go into anaphylactic shock and have high fever, pruritus (itching), edema (swelling) of the lips and eyelids, dyspnea, stridor, and rhinorrhea.
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.
The incubation period for all species of Echinococcus can be months to years or even decades. It largely depends on the location of the cyst in the body and how fast the cyst is growing.
Cause 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, unless eaten by dogs or wolves after death.
Hosts
Organism Definitive Hosts Intermediate HostsE. granulosus dogs and other canidae sheep, goats, cattle, camel, buffalo, swine, kangaroos, and other wild herbivoresE. multilocularis foxes, dogs, other canidae and cats small rodentsE. vogeli bush dogs and dogs rodentsE. oligarthruswild felids small rodents
Life cycle An adult worm resides in the small intestine of a definitive host. A single gravid proglottid releases eggs that are passed in the feces of the definitive host. The egg is then ingested by an intermediate host. 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. The cyst then slowly enlarges, creating protoscolices (juvenile scolices), and daughter cysts within the cyst. The definitive host then becomes infected after ingesting the cyst-containing organs of the infected intermediate host. After ingestion, the protoscolices attach to the intestine. They then develop into adult worms and the cycle starts all over again.
Eggs
Echinococcus eggs contain an embryo that is called an oncosphere or hexacanth. 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 leads to infection in the intermediate host.
Larval/hydatid cyst stage From the embryo released from an egg develops a hydatid cyst, which grows to about 5–10 cm within the first year and can survive within organs for years. 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 also form within cysts.
Adult worm Echinococcus adult worms develop from protoscolices and are typically 6 mm 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). The scolex of the adult worm contains four suckers and a rostellum that has about 25–50 hooks.
Morphological differences 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. multilocularis is often smaller and is 4 mm or less. On the other hand, E. vogeli is found to be up to 5.6 mm long and E. oligarthrus is found to be up to 2.9 mm long. 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 polycystic since the germinal membrane of the hydatid cyst 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.
Transmission 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 eating infected, cyst-containing organs. Humans are accidental intermediate hosts that become infected by handling soil, dirt, or animal hair that contains eggs.
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.
Aberrant cases There are a few aberrant cases in which carnivores play the role of the intermediate hosts. Examples are domestic cats with hydatid cysts of E. granulosus.
Diagnosis
Classification The most common form found in humans is cystic echinococcosis (also known as unilocular echinococcosis), which is caused by Echinococcus granulosus sensu lato. The second most common form 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 oligarthrus. 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 has a significantly high fatality rate, and may have the potential to become an emerging disease in many countries.
Cystic