''Echinococcus granulosus'', also called the hydatid worm or dog tapeworm, is a
cyclophyllid cestode
Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, commonly known as tapeworms. Their bodies co ...
that dwells in the
small intestine
The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...
of
canids as an adult, but which has important
intermediate host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
s such as
livestock
Livestock are the Domestication, domesticated animals that are raised in an Agriculture, agricultural setting to provide labour and produce diversified products for consumption such as meat, Egg as food, eggs, milk, fur, leather, and wool. The t ...
and humans, where it causes
cystic echinococcosis, also known as hydatid disease. The adult
tapeworm ranges in length from 3 mm to 6 mm and has three
proglottid
Cestoda is a Class (biology), class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, commonly known as tapeworms. T ...
s ("segments") when intact—an immature proglottid, mature proglottid and a gravid proglottid. The average number of eggs per gravid proglottid is 823. Like all cyclophyllideans, ''E. granulosus'' has four suckers on its
scolex ("head"), and ''E. granulosus'' also has a
rostellum
The rostellum is a projecting part of the column in Orchidaceae flowers, and separates the male androecium from the female gynoecium, commonly preventing self-fertilisation. In many orchids, such as '' Orchis mascula'', the pollinia or pollen m ...
with hooks. Several strains of ''E. granulosus'' have been identified, and all but two are noted to be infective in humans.
The lifecycle of ''E. granulosus'' involves dogs and wild carnivores as a definitive
host
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
* Host Island, in the Wilhelm Archipelago, Antarctica
People
* ...
for the adult tapeworm.
[ Definitive hosts are where parasites reach maturity and reproduce. Wild or domesticated ]ungulate
Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...
s, such as sheep, serve as an intermediate host.[ Transitions between life stages occur in intermediate hosts. The larval stage results in the formation of echinococcal ]cyst
A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubb ...
s in intermediate hosts.[ Echinococcal cysts are slow growing,][ but can cause clinical symptoms in humans and be life-threatening.] Cysts may not initially cause symptoms, in some cases for many years.[ Symptoms developed depend on location of the cyst, but most occur in the liver, lungs, or both.][
''Echinococcus granulosus'' was first documented in Alaska but is distributed worldwide. It is especially prevalent in parts of Eurasia, north and east Africa, Australia, and South America.][ Communities that practice sheep farming experience the highest risk to humans,][ but wild animals can also serve as an avenue for transmission. For example, dingoes serve as a definitive host before larvae infect sheep in the mainland of Australia.][ Sled dogs may expose moose or reindeer to ''E. granulosus'' in parts of North America and Eurasia.][
]
Transmission
''E. granulosus'' requires two host types, a definitive host and an intermediate host. The definitive host of this parasite are dogs and the intermediate host are most commonly sheep, however, cattle, horses, pigs, goats, and camels are also potential intermediate hosts. Humans can also be an intermediate host for ''E. granulosus'', however this is uncommon and therefore humans are considered an aberrant intermediate host.[
]
''Echinococcus granulosus'' is ingested and attaches to the mucosa of the intestines in the definitive host and there the parasite will grow into the adult stages. Adult ''E. granulosus'' release eggs within the intestine which will be transported out of the body via feces.[ When contaminated waste is excreted into the environment, intermediate host has the potential to contract the parasite by grazing in contaminated pasture, perpetuating the cycle.][
''Echinococcus granulosus'' is transmitted from the intermediate host (sheep) to the definitive host (dogs) by frequent feeding of ]offal
Offal (), also called variety meats, pluck or organ meats, is the internal organ (anatomy), organs of a butchered animal. Offal may also refer to the by-products of Milling (grinding), milled grains, such as corn or wheat.
Some cultures strong ...
, also referred to as "variety meat" or "organ meat". Consuming offal containing ''E. granulosus'' can lead to infection; however, infection is dependent on many factors.[
While adult ''E. granulosus'' is harmless in dogs, the larval form can be a huge problem in humans. Although rare, the parasite can form a cyst causing cystic echinococcus also known as hydatid disease. The cyst can cause pressure on surrounding tissue which may lead to abnormal organ function, spontaneous fracture of bones, and other neurological effects.
The frequency of offal feedings, the prevalence of the parasites within the offal, and the age of the intermediate host are factors that affect infection pressure within the definitive host.][ The immunity of both the definitive and intermediate host plays a large role in the transmission of the parasite, as well as the contact rate between the intermediate and the definitive host (such as herding dogs and pasture animals being kept in close proximity where dogs can contaminate grazing areas with fecal matter).][
The life expectancy of the parasite, coupled with the frequency of anthelminthic treatments, will also play a role in the rate of infection within a host. The temperature and humidity of the environment can affect the survival of ''E. granulosus''.][
Once sheep are infected, the infection typically remains within the sheep for life. However, in other hosts, such as dogs, treatment for annihilating the parasite is possible. However, the intermediate host is assumed to retain a greater life expectancy than the definitive host.]
Diagnosis
Diagnosis in the definitive host, the dog, may be done by ''post mortem'' examination of the small intestine, or with some difficulty ''ante mortem'' by purging with arecoline hydrobromate. Detection of antigen
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.
...
s in feces by ELISA
The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay is a solid-phase type of enzyme immunoassay (EIA) to detect the presence of ...
is currently the best available technique. The prevalence of ''Echinococcus granulosus'' was found to be 4.35% in a 2008 study in Bangalore
Bengaluru, also known as Bangalore (List of renamed places in India#Karnataka, its official name until 1 November 2014), is the Capital city, capital and largest city of the southern States and union territories of India, Indian state of Kar ...
, India employing this coproantigen detection technique. Polymerase chain reaction
The polymerase chain reaction (PCR) is a method widely used to make millions to billions of copies of a specific DNA sample rapidly, allowing scientists to amplify a very small sample of DNA (or a part of it) sufficiently to enable detailed st ...
(PCR) is also used to identify the parasite from DNA isolated from eggs or feces. However, it is difficult to determine the eggs in feces because it is indistinguishable from other taeniid eggs.
Diagnosis in humans can be done by x-rays, CAT scans, and ultrasounds.
Treatment
If a human becomes infected there are a variety of methods for treatment. The most common treatment in the past years has been surgical removal of the hydatid cysts.[ The fluid in the cysts contain antigens that can immunologically sensitize the host, so cyst manipulation should be performed with caution, as spilling of cyst contents can cause anaphylactic shock. However, in recent years, less invasive treatments have been developed such as cyst puncture, aspiration of the liquids, the injection of chemicals, and then re-aspiration.][ Benzimidazole-based chemotherapy is also a new treatment option for humans.][
]
Prevention
In order to prevent transmission to dogs from intermediate hosts, dogs can be given anthelminthic vaccinations.[ In the case of intermediate hosts, especially sheep, these anthelminthic vaccinations do cause an antigenic response—meaning the body produces specific antibody—however it does not prevent infection in the host.][
Clean slaughter and high surveillance of potential intermediate host during slaughter is key in preventing the spread this cestode to its definitive host. It is vital to keep dogs and potential intermediate host as separated as possible to avoid perpetuating infection.]
According to mathematical modeling, vaccination of intermediate hosts, coupled with dosing definitive hosts with anthelminths is the most effect method for intervening with infection rates.[
Proper disposal of carcasses and offal after home slaughter is difficult in poor and remote communities and therefore dogs readily have access to offal from livestock, thus completing the parasite cycle of ''Echinococcus granulosus'' and putting communities at risk of cystic ]echinococcosis
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'' an ...
. Boiling livers and lungs which contain hydatid cysts for 30 minutes has been proposed as a simple, efficient and energy- and time-saving way to kill the infectious larvae.
See also
* ''Echinococcus multilocularis
''Echinococcus multilocularis'', the fox tapeworm, is a small Cyclophyllidea, cyclophyllid Cestoda, tapeworm found extensively in the northern hemisphere. ''E. multilocularis,'' along with other members of the ''Echinococcus'' genus (especially ' ...
''
* ''Fasciola hepatica
''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitism, parasitic trematode (fluke (flatworm), fluke or flatworm, a type of helminth) of the class (biology), class Trematoda, phylum Platyhelminthes. It ...
''
* ''Taenia solium
''Taenia solium'', the pork tapeworm, belongs to the cyclophyllid cestode family Taeniidae. It is found throughout the world and is most common in countries where pork is eaten. It is a tapeworm that uses humans (''Homo sapiens'') as its definit ...
''
References
Further reading
*
External links
Echinococcus
from the US CDC
''Echinococcus granulosus''
in the NCBI Taxonomy Browser
{{Authority control
Cestoda
Animals described in 1786
Sheep and goat diseases
Parasites of dogs
Veterinary helminthology