Echinococcus

This article is about the organism. For the infection, see Echinococcosis.

Echinococcus
Necropsy of a cotton rat infected with Echinococcus multilocularis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Platyhelminthes
Class:	Cestoda
Order:	Cyclophyllidea
Family:	Taeniidae
Genus:	Echinococcus Rudolphi, 1801

schematic representation of the life cycle of Echinococcus

General description of the egg and oncosphere of Echinococcus spp.

Echinococcus is a genus within Cestoda, a parasitic class of the platyhelminthes phylum (colloquially known as flatworms). Human echinococcosis is an infectious disease caused by the following species: E. granulosus, E. multilocularis, E. vogeli or E. oligarthrus.

Echinococcus is triploblastic – it has three layers – outermost ectoderm, middle mesoderm, and inner endoderm. An anus is absent, and it has no digestive system. Its body is covered by tegument and the worm is divided into a scolex, a short neck, and three to six proglottids. Its body shape is ribbon-like.

In humans, Echinococcus spp. cause a disease called echinococcosis. The three types of echinococcosis are cystic echinococcosis caused by E. granulosus, alveolar echinococcosis caused by E. multilocularis, and polycystic echinococcosis caused by E. vogeli or E. oligarthrus. A worm's incubation period is usually long and can be up to 50 years. Cystic echinococcosis is mostly found in South and Central America, Africa, the Middle East, China, Italy, Spain, Greece, Russia, and the western United States (Arizona, New Mexico, and California).

Echinococcosis is a zoonosis. The definitive hosts are carnivorous predators – dogs, wolves, foxes, and lions. The adult tapeworm lives in their small intestines and delivers eggs to be excreted with the stool. The intermediate hosts are infected by ingesting eggs. Sheep, goats, cattle, camels, pigs, wild herbivores, and rodents are the usual intermediate hosts, but humans can also be infected. Humans are dead-end hosts, since their corpses are nowadays seldom eaten by carnivorous predators.

The egg hatches in the digestive system of the intermediate host, producing a planula larva. It penetrates the intestinal wall and is carried by bloodstream to liver, lung, brain, or another organ. It settles there and turns into a bladder-like structure called hydatid cyst. From the inner lining of its wall, protoscoleces (i.e. scoleces with invaginated tissue layers) bud and protrude into the fluid filling the cyst.

After the death of the normal intermediate host, its body can be eaten by carnivores suitable as definitive hosts. In their small intestines, protoscoleces turn inside out, attach, and give rise to adult tapeworms, completing the lifecycle. In humans, the cysts persist and grow for years. They are regularly found in the liver (and every possible organ: spleen, kidney, bone, brain, tongue and skin) and are asymptomatic until their growing size produces symptoms or are accidentally discovered. Disruption of the cysts (spontaneous or iatrogenic e.g. liver biopsy) can be life-threatening due to anaphylactic shock.

Cysts are detected with ultrasound, X-ray computed tomography, or other imaging techniques. Antiechinococcus antibodies can be detected with serodiagnostic tests – indirect fluorescent antibody, complement fixation, ELISA, Western blot, and other methods.

Taxonomy

A phylogenetic tree has been created for several species in this genus – Echinococcus oligarthrus, Echinococcus vogeli, Echinococcus multilocularis, Echinococcus shiquicus, Echinococcus equinus, Echinococcus ortleppi, and Echinococcus granulosus. The first diverging species are the neotropical endemic species E. oligarthrus and E. vogeli. E. ortleppi and E. canadensis are sister species, as are E. multilocularis and E. shiquicus. E. canadensis is related to E. granulosus.

The origin of these parasites based on host-parasite co-evolution comparisons was North America or Asia, depending on whether the ancestral definitive hosts were canids or felids.

Echinococcus oligarthrus and Echinococcus vogeli are basal in this genus. The genus is a sister to the genus Taenia from which it diverged more than 10 million years ago. The genus Echinococcus evolved in North America in canids and began to diversify 5.8 million years ago.

In 2020, an international effort of scientists from 16 countries lead to a detailed consensus on terminology, i.e. the terms to be used or rejected for the genetics, epidemiology, biology, immunology and clinical aspects linked with Echinococcus species.

Prevention

There is no vaccine against Echinococcus multilocularis. However, it is possible to protect humans from the fox tapeworm by deworming the main hosts.

Prevention of Echinococcosis (Hydatid Disease) involves a comprehensive, multi-sectoral approach that integrates veterinary care, public health, and environmental management. These strategies aim to interrupt the transmission of Echinococcus parasites, which involve definitive hosts (e.g., carnivores such as dogs and foxes) and intermediate hosts (e.g., herbivores and humans). Effective prevention requires coordinated efforts at the animal, environmental, and human levels.

1. Control in Animals: Since dogs and other carnivores are the definitive hosts of Echinococcus, managing their populations and preventing them from shedding Echinococcus eggs is essential in controlling the disease.

• Deworming Programs: Regular deworming of domestic dogs and other carnivores is a fundamental strategy in preventing Echinococcosis. Anthelmintic drugs such as albendazole or praziquantel are commonly used to eliminate adult tapeworms in the intestines of infected dogs. Deworming programs are typically conducted at least twice a year in endemic areas, but the frequency may be higher depending on local risk.

• Stray Dog Control: The presence of stray dogs contributes to the spread of Echinococcus. Effective control measures include vaccination, sterilization, and in some cases, culling of stray dog populations.

• Animal Husbandry Practices: In livestock farming, preventing dogs and wild carnivores from accessing animal carcasses or offal (internal organs) can significantly reduce the risk of transmission. Proper disposal of offal and carcasses by slaughterhouses is also crucial in limiting contamination.

2. Control in Livestock: As intermediate hosts, livestock become infected with Echinococcus larvae. Prevention strategies for livestock include:

• Vaccination: While not universally available, vaccination of livestock has shown promise in reducing infection rates. For example, the EG95 vaccine for sheep and cattle has proven effective in decreasing the prevalence of Echinococcus larvae in these animals. Expanding the use of vaccines in endemic regions could significantly reduce the burden of infection.

• Control of Access to Contaminated Water and Pastures: Livestock should be prevented from grazing in areas where they might ingest Echinococcus eggs. This includes areas that may be contaminated by dog feces or poorly managed waste. Ensuring proper sanitation of grazing areas and drinking water sources is key to reducing infection.

• Hygiene and Sanitation: Ensuring proper hygiene in farming operations is essential. This includes the safe disposal of animal waste, regular cleaning of feeding and watering systems, and maintaining clean living conditions for animals.

3. Human Health Strategies: Humans become infected with Echinococcus by ingesting the eggs of the parasite, typically through contaminated food, water, or contact with infected animals. Preventive measures for humans include:

• Education and Public Awareness: Public health campaigns and education programs are essential to raise awareness about the risks of Echinococcosis and the importance of hygiene. Educating communities in endemic regions about the risks of handling dogs, eating undercooked meat, and consuming contaminated water is a crucial step in prevention.

• Proper Handling of Meat: One way humans can contract Echinococcosis is through the consumption of contaminated meat, particularly organ meats. Ensuring proper cooking of meat (especially offal) can kill the parasite and reduce infection risk. Additionally, meat inspection and safe meat handling practices can help prevent the sale of infected carcasses.

• Safe Water and Sanitation: Drinking untreated or contaminated water can lead to the ingestion of Echinococcus eggs. In endemic areas, communities should be encouraged to use clean, treated water sources. Proper water treatment and improved sanitation facilities, especially in rural areas, can reduce the likelihood of infection.

• Personal Protective Measures: In regions of high risk, individuals working with livestock or in environments where Echinococcus is common should take precautions, such as wearing gloves, face masks, and other protective clothing when handling animals, particularly during slaughter or carcass disposal.

4. Environmental Management: Environmental control plays a key role in preventing the spread of Echinococcus:

• Environmental Sanitation: Proper disposal of dog feces and livestock waste is crucial in preventing contamination of water sources and grazing lands. Community-based waste management efforts are essential for reducing the risk of environmental contamination.

• Controlling Wild Carnivore Populations: In some areas, wild carnivores such as foxes and wolves act as definitive hosts. Managing these populations through controlled hunting or vaccination programs may be necessary to reduce the spread of the parasite. However, these efforts should be balanced with local wildlife conservation priorities.

• Restoration of Ecosystems: The overlap between wildlife, domestic animals, and livestock in certain ecosystems can contribute to the persistence of Echinococcus. Restoration of ecosystems that minimize this overlap, such as through improved livestock management or habitat protection, can help curb the transmission of the parasite.

5. Integrated One Health Approach: The One Health approach, which emphasizes the interconnection between human, animal, and environmental health, is central to the prevention and control of Echinococcosis. This framework advocates for coordinated efforts among veterinarians, public health professionals, environmental scientists, and local communities.

• Surveillance Systems: Integrated surveillance systems that track Echinococcus infections in humans, animals, and the environment are essential for early detection and targeted interventions. Such systems help identify areas at high risk and monitor the success of control efforts.

• Cross-Sector Collaboration: Effective control requires collaboration between veterinary, health, and environmental agencies. Governments, international organizations, and local communities must work together to implement strategies such as deworming programs, vaccination campaigns, and educational outreach.

6. Global and Regional Efforts: Several international organizations and countries are engaged in efforts to control Echinococcosis:

• World Health Organization (WHO): The WHO has issued guidelines for the prevention and control of Echinococcosis, including recommendations for surveillance, control of definitive host populations, and community education.

• Regional Programs: Countries with high rates of Echinococcus infection, such as New Zealand, Turkey, and China, have implemented national and regional strategies that combine deworming, public education, vaccination, and environmental sanitation. These programs aim to reduce both human and animal infection rates.

• Collaborative Research: Ongoing research into vaccines for both humans and animals, improved diagnostic tools, and novel treatment options will further enhance global efforts to combat Echinococcosis.