|E. granulosus scolex|
Echinococcus granulosus, also called the Hydatid worm or Hyper Tape-worm or Dog Tapeworm, is a cyclophyllid cestode that parasitizes the small intestine of canids as an adult, but which has important intermediate hosts such as livestock and humans, where it causes cystic echinococcosis, also known as hydatid disease. The adult tapeworm ranges in length from 2 mm to 7 mm and has three proglottids ("segments") when intact — an immature proglottid, mature proglottid and a gravid proglottid. Like all cyclophyllideans, E. granulosus has four suckers on its scolex ("head"), and E. granulosus also has a rostellum with hooks. Several strains of E. granulosus have been identified, and all but two are noted to be infective in humans (Eckert & Deplazes 2004).
The lifecycle of E. granulosus involves dogs and wild carnivores as a definitive host for the adult tapeworm (Moro & Shantz 2008). Definitive hosts are where parasites reach maturity and reproduce. Wild or domesticated ungulates, such as sheep, serve as an intermediate host (Moro & Shantz 2008). Transitions between life stages occur in intermediate hosts. The larval stage results in the formation of echinococcal cysts in intermediate hosts (Moro & Shantz 2008). Echinococcal cysts are slow growing (Moro & Shantz 2008), but can cause clinical symptoms in humans and be life-threatening (McManus et al. 2003). Cysts may not initially cause symptoms, in some cases for many years (Moro & Shantz 2008). Symptoms developed depend on location of the cyst, but most occur in the liver, lungs, or both (McManus et al. 2003).
E. granulosus was first document in Alaska but is distributed world-wide. It is especially prevalent in parts of Eurasia, north and east Africa, Australia, and South America (McManus et al. 2003). Communities that practice sheep farming experience the highest risk to humans (McManus et al. 2003), 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 (McManus et al. 2003). Sled dogs may expose moose or reindeer to E. granulosus in parts of North America and Eurasia (McManus et al. 2003).
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 (Torgerson & Heath 2003). Humans can also be an intermediate host for E. granulosus, however this is uncommon and therefore humans are considered an aberrant intermediate host (Torgerson & Heath 2003).
E. 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 (Bazalar et al. 1997). Adult E. granulosus release eggs within the intestine which will be transported out of the body via feces (Bazalar et al. 1997). 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 (Torgerson & Heath 2003, Lamar et al. 2004).
E. granulosus is transmitted from the intermediate host (sheep) to the definitive host (dogs) by frequent feeding of offal, 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 (McManus et al. 2003).
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 (Bazalar et al. 1997). 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) (McManus et al. 2003).
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 (McManus et al. 2003).
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 (McManus et al. 2003 and Lahmar et al. 2004).
Diagnosis in the definitive host, the dog, is difficult by ordinary microscopy as it cannot differentiate between Taenia and Echinococcus eggs. Detection of antigens in feces by ELISA is currently the best available technique. The prevalence of Echinococcus granulosus was found to be 4.5% in Bangalore, India by a study conducted by Centre of Advanced Studies in Veterinary Parasitology, Veterinary College, Hebbal, Bangalore employing this coproantigen detection technique. Newer techniques like PCR is also used to identify the parasite from DNA isolated from eggs or feces.
Risk in Humans
Humans should avoid handling fecal matter of canines and avoid consuming infected animals and home slaughtering animals. If a human, becomes infected, there are a variety of methods for treatment (Eckert and Deplazes 2004, Craig et al. 2007). The most common treatment in the past years has been surgical removal of the hydatid cysts (Craig et al. 2007). 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 (Eckert and Deplazes 2004). Benzimidazole-based chemotherapy is also a new treatment option for humans (Eckert and Deplazes 2004).
In order to prevent transmission to dogs from intermediate hosts, dogs can be given anthelminthic vaccinations (Moro & Schantz 2009, Craig et al. 2007). In the case of intermediate hosts, especially sheep, these anthelminthic vaccinations do cause an antigenic response—meaning the body produces antibodies in response—however it does not prevent infection in the host(Moro & Schantz 2009 and Craig et al. 2007).
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 (Moro & Schantz 2009).
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 (Moro & Schantz 2009).
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