2025.10.14
Introduction to hydatid cyst disease
Hydatid cyst disease is a parasitic disease caused by the larval stage of tapeworms of the Echinococcus genus (Echinococcus spp.). The word "hydatid" is derived from the Greek word hydatis or hydor, meaning "a water-filled sac." This disease affects not only domestic animals especially livestock such as cows and sheep but also humans, and thus is classified as a zoonotic disease (transmissible between animals and humans). It is also known as human echinococcosis and generally manifests in humans in two main forms: cystic echinococcosis, also known as hydatid cyst, which is the most common form of the disease and is caused by Echinococcus granulosus and alveolar echinococcosis, a rarer form, caused by Echinococcus multilocularis.
History
The history of hydatid cyst disease dates back to ancient times. Around 400 BC, the Greek physician Hippocrates mentioned masses in the abdominal and liver regions in his writings, which were likely hydatid cysts. In the early centuries AD, these cysts were described by prominent physicians such as Aretaeus and Galen. Moreover, in Islamic civilization, the renowned Persian physician Muhammad ibn Zakariya al-Razi in the 9th century also referred to diseases with characteristics similar to hydatid cyst disease.
In the 17th century, the Italian biologist Francesco Redi was the first to identify the animal origin of hydatid cysts and recognized that these cysts were caused by a parasite. In 1801, the German scientist Karl Rudolphi introduced the term Echinococcus, derived from the Greek words echinos (spiny) and kokkos (berry), referring to the microscopic spiny and round appearance of the parasite. Later, in the mid-19th century, the two main forms of hydatid disease cystic and alveolar echinococcosis were distinguished. In the 19th and 20th centuries, significant advancements in biological sciences and the use of microscopes led to a precise understanding of the life cycle of the hydatid cyst parasite.
In the 21st century, international research expanded, and organizations such as the World Health Organization (WHO) and the World Organisation for Animal Health (OIE) worked to assess the prevalence, mortality, and other impacts of hydatid disease across various regions. These organizations have identified the disease as one of the most important zoonotic infections.
Life cycle of the hydatid cyst parasite
The hydatid cyst parasite requires two types of hosts intermediate and definitive to complete its life cycle, as each stage of its development occurs in a specific host. It is an obligate parasite, meaning it cannot survive or complete its life cycle outside a host’s body. The cystic stage develops in intermediate hosts, and the adult, egg-producing stage occurs in the intestines of definitive hosts. Intermediate hosts include herbivorous animals such as sheep, cattle, goats, etc., which become infected by ingesting food or water contaminated with parasite eggs. The definitive hosts are typically dogs and other carnivores, in whose intestines the adult worms reside and lay eggs. The hydatid cyst parasite is an internal parasite because all its developmental stages occur inside the host’s body.
If a dog is infected with the hydatid cyst parasite, microscopic eggs (not visible to the naked eye) are excreted in its feces and contaminate the environment soil, plants, or water. This initial contamination usually occurs in pastures, farms, or near livestock. When animals like sheep or cattle ingest contaminated plants or water, the eggs enter their bodies. In the small intestine, the eggs hatch, releasing embryos that penetrate the intestinal wall, enter blood capillaries, and are transported to organs such as the liver or lungs. There, they gradually develop into fluid-filled cysts hydatid cysts. Inside the cysts, immature parasite heads (protoscolices) form over time. If a dog or other carnivore eats infected organs (e.g., liver) from these animals, the immature parasite heads develop into adult worms in the dog’s intestines. These adult worms lay eggs that are again excreted in feces, contaminating the environment. These eggs can then infect humans in several ways.
Common routes of transmission to humans
Some of the main routes of transmission of hydatid cyst eggs to humans include:
- If a person unknowingly consumes contaminated water or raw vegetables, the eggs may enter the body and lead to infection.
- Contact with soil contaminated with eggs can also transmit the parasite.
- Direct contact with infected definitive hosts, such as dogs (e.g., petting), may also result in transmission. The parasite eggs excreted in feces can remain around the anus or on the fur, paws or skin of the animal. As a result, individuals who do not observe proper personal hygiene may unintentionally ingest the parasite's eggs through contaminated hands and become infected with the disease.
Signs and symptoms of hydatid cyst disease in humans
Hydatid cyst disease primarily affects the liver and lungs, however, in some cases, other organs such as the kidneys, spleen, muscles, and brain may also be affected. The disease usually has a latent (asymptomatic) period i.e., a delay between the parasite's entry and symptom onset. In the case of hydatid cysts, this period can last for years, during which the individual may show no symptoms. Once in the body, the parasite eggs hatch in the small intestine, and the embryos penetrate the intestinal wall, enter the bloodstream, and travel to various organs. In most cases, the liver is the first organ affected, where the embryos gradually grow into hydatid cysts. These cysts expand over months or years, eventually occupying space and exerting pressure on surrounding tissues, impairing organ function (e.g., the liver’s detoxification capacity).
Liver involvement may present as abdominal pain, nausea, vomiting, etc. Lung involvement can cause chronic cough, chest pain, shortness of breath, etc. Other symptoms include appetite loss, weight loss, general weakness, etc. Additionally, the disease can lead to severe complications and be life-threatening in advanced cases. Anaphylaxis a severe allergic reaction is the most common mechanism of death, which can occur if a hydatid cyst leaks or ruptures within the body.
Prevalence of hydatid cyst disease
The hydatid cyst parasite has a wide geographical distribution and can survive in various environments, including temperate, polar, tropical, semi-tropical, and arid regions. According to the World Health Organization, in endemic areas (regions with high parasite prevalence) such as the Mediterranean, South and Central Russia, Central Asia and China, Australia, South America, North and East Africa, etc., over 1 million people may be affected simultaneously. Furthermore, in these regions, the incidence can exceed 50 cases per 100,000 people annually.
Residents of areas with a high rate of dog ownership and individuals with limited access to healthcare services (such as the absence of regular programs for dog control and vaccination), which are key factors in sustaining the parasite's life cycle are at greater risk of contracting hydatid cyst disease. In addition, poor sanitary conditions, including lack of personal hygiene, can further increase the risk of infection. Certain occupational groups are also at higher risk due to the nature of their work and frequent exposure to potentially contaminated sources. For example, vegetable vendors are more likely to be exposed to this parasitic infection, as they handle raw vegetables on a daily basis, which may have been irrigated or washed with water contaminated with hydatid cyst eggs.
Preventive measures for hydatid cyst disease
To prevent hydatid cyst disease, the following precautions should be observed:
1- Wash vegetables and other agricultural products especially those in direct contact with soil thoroughly using proper methods before consumption. This significantly reduces the risk of transmission of disease-causing agents, including hydatid cyst eggs.
2- Regularly wash hands with soap and water, particularly after contact with dogs, as their fur may be contaminated with eggs. Educating children on personal hygiene especially handwashing before meals and after playing with animals is essential.
3- Ensure the safety and cleanliness of drinking water, as contaminated water may carry parasite eggs. In areas where water quality is uncertain, it is recommended to boil the water for at least 10 minutes before use to generate sufficient heat to penetrate the resistant egg shell and destroy the infectious larva inside.
4- Use gloves when in contact with soil (e.g., during gardening), since the soil may be contaminated with hydatid cyst eggs, and direct contact increases the risk of infection.