Echinococcosis

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University of Oklahoma Study Abroad Microbiology in Arezzo, Italy[1]

Description

Echinococcosis is a disease caused by the ingestion of the eggs of tapeworm genus Echinococcus through contaminated food, water, or through direct animal contact. Among the four forms of the disease, this page will discuss Cystic echinococcosis because it is the most significant form of the disease in regards to human health. Cystic echinococcosis, also known as hydatid disease, is caused by the parasite Echinococcus granulosus [2][3].

Epidemiology

More than 1 million people are experiencing echinococcosis at any one time [3]. Cystic Echinococcosis occurs in Africa, Europe, Asia, the Middle East, and Central and South America, with higher rates of infection among populations that raise and slaughter livestock [3]. In areas where Echinococcosis reaches endemic levels, prevalence rates reach 5%-10% among humans, and 20%-95% among livestock. Endemic areas for human cystic echinococcosis include Argentina, Central Asia, Chile, China, East Africa, Japan, Russia, and Uruguay [5].

Transmission

Transmission depends on a definitive primary host, typically a dog or wolf, to pass the Echinococcus eggs through its fecal matter. From there, the eggs may be transmitted directly to the human host via fecal-oral transmission. Alternatively, the eggs can be passed to an intermediate host, typically some form of livestock, before being transmitted to the human host via fecal-oral transmission.

After ingesting the Echinococcus eggs, the human host's digestive proteolytic enzymes begin to break down the egg membranes, effectively "hatching" the eggs and releasing the larvae into the small intestine.

After they hatch, the larvae are absorbed into the intestinal mucosal wall, and are carried through blood vessels to various organs throughout the body. The most common organs for the larvae to attach include the liver, lungs, brain, and heart.

Once the larvae attach, they begin forming cysts, called hydatids, that slowly grow and fill with clear fluid. The presence of these cysts can go undetected for months or even years, until they increase in size enough to cause noticeable effects in the surrounding tissues.

Pathogenesis

Echinococcus' main method of harming the host comes from the cysts formed by the larvae form, not by the pathogen itself. The cysts, if developed enough, will swell and put pressure on the surrounding tissues, causing eventual blood vessel obstruction and necrosis of the tissue. Additional harm may be inflicted upon rupture of the cysts can cause an allergic reaction and possible anaphylactic shock if the fluid from the cysts spreads via the blood stream to other parts of the body. These ruptures are normally caused by trauma or medical treatment [5].

Virulence Factors

The chief virulence factor of Echinococcus granulosus is its ability to synthesize and secrete hydatid fluid (HF), which contains various antigens, including antigen 5 and antigen B, which are considered to be responsible for the survival of E. granulosus within a human host. Antigen B has been found to interfere with dendrictic cell function by suppressing differentiation into immature dendritic cells, which hinders their ability to mature once they encounter the pathogen's PAMP's on its cell surface.
Antigen B has also been found to alter dendritic cell maturation, skewing DC development and creating DC's that signal to CD4 t-cells to differentiate into T-helper II cells. Additionally, one study [4] found that antigen B also alters a DC's cytokine production, decreasing the synthesis of IL-12 and TNF-α, which blocks T-helper I differentiation in favor of T-helper II. This is thought to aid the pathogen's survival by hindering part of the host's innate immune response, perhaps because removing the presence of THI cells severely limits the ability of macrophages to immediately and effectively respond to the parasite by phagocytosing it and signaling inflammation via the release of cytokines.
Looking forward, scientists are now researching possible uses of parasites like E. granulosus in creating helpful anti-inflammatory immunotherapy agents [5].

Clinical Features

Symptoms

After infection, an individual can be asymptomatic for several years until the hydatid cysts become large enough to trigger symptoms. Echinococcosis symptoms can include (depending on the location of the hydatids): weight loss, weakness, abdominal pain, nausea, vomiting, chronic cough, chest pain, and difficulty breathing [5].

Diagnosis

Diagnosis is usually based on results from ultrasonography imaging, validated by computed tornography or magnetic resonance imaging (MRI) scans. These imaging techniques can detect the presence of cysts, which can be further confirmed by serological tests that indicate antibodies specific for the cysts. Biopsies can provide differential diagnosis of cysts from tumors [3].

Treatment

Treatment choices are largely dependent on the human resources available, the cost associated with the option, and the stage of the cysts, determined by ultrasound imaging. In many patients, who aren't diagnosed until a very advanced stage of cyst formation and development, treatment options are very limited.

Today, three treatments options exist:
1. Surgical removal, whose effectiveness is limited by the stage of cyst development. In cases where surgery has been used as the treatment option, it has been about 90% effective [1].
2. PAIR technique, which stands for Puncture, Aspiration, Injection, and Re-Aspiration [7].
3.Chemotherapy, with Albendazole as the preferred treatment because of its ability to penetrate into the harmful cysts [5].

Prevention

Periodic deworming of dogs, improved hygiene in livestock slaughtering, and public education campaigns about human hygiene have been found to greatly reduce transmission and infection rates. The World Health Organization has focused on the eradication of the cystic form of Echinococcosis in recent years, and claims: "A programme combining vaccination of lambs, deworming of dogs and culling of older sheep could lead to elimination of cystic echinococcosis disease in humans in less than 10 years" [6].

Damage Response Framework

Because the hydatid cysts may go undetected for many months or years after initial infection, and because the main harm done to the host occurs upon growth or rupture of the cysts and not initial infection or action by the parasite, it is more difficult to draw a connection between the host immune response, the pathogen, and the overall damage done to the host. E. granulosus's production of antigen B serves to weaken the host innate immune response by limiting the differentiation of THI cells, which has been shown to hinder anti-inflammatory response by the host innate immune system [4]. If this is considered to create a weak host immune response, then the damage response framework of E.granulosus would involve high levels of damage at a weaker host immune response, as the pathogen is able to survive better when the host immune response does not signal inflammation.
Additionally, greater levels of damage are caused by a very strong host immune response, if the host recognizes the presence of the hydatid cysts in the tissues and attempts to destroy them, thus leading to cyst rupture and possible allergic reaction and anaphylaxis [5].

References

1. Parasites- Echinococcosis: Biology, 2012
http://www.cdc.gov/parasites/echinococcosis/biology.html

2. Parasites-Echinococcosis: General Information: Cystic Echinococcosis FAQ’s, 2012
http://www.cdc.gov/parasites/echinococcosis/gen_info/ce-faqs.html

3. Echinococcosis: Fact sheet, 2015
http://www.who.int/mediacentre/factsheets/fs377/en/

4. Infection and Immunity: Echinococcus granulosus Antigen B, 2007
http://iai.asm.org/content/75/4/1667.short

5. Parasites: Echinococcus, 2006
http://web.stanford.edu/group/parasites/ParaSites2006/Echinococcus/main.html

6. The Imaging of Tropical Diseases: Epidemiology and Pathology: Echinococcus granulosus
http://tmcr.usuhs.mil/tmcr/chapter3/epidemiology.htm

7. PAIR: Puncture, Aspiration, Injection, Re-Aspiration: An option for the treatment of Cystic Echinococcosis

http://whqlibdoc.who.int/hq/2001/who_cds_csr_aph_2001.6.pdf