Hydatid disease, also known as echinococcosis, or cystic hydatid disease, is caused by an infection with the larvae of the tapeworm Echinococcus granulosus. This parasite causes slow-growing cystic formation and associated symptoms that depend on cyst location.
Hydatid disease occurs in Central and South America, Africa, and Asia, but rarely in North America.
What is the Echinococcus granulosus tapeworm?
The adult tapeworm typically measures between 3 and 6 mm in length and 0.5 mm wide. The head features four prominent suckers as well as a double row of hooklets on the rostellum. They reside in the intestine parasitically and are most commonly found in the jejunum, the middle area of the small intestine. After attaching itself to the mucous membrane, the worm takes approximately eight weeks to fully mature and is thought to have a lifespan five months. Male and female sex organs develop in the proglottids distal to the scolex and can contain several thousand eggs. Definitive hosts are carnivores such as dogs, with intermediate hosts being pigs, cows, and sheep. Dogs would become infected after eating intestines from intermediate hosts, whereas humans are accidental hosts.
3d illustration of an Echinococcus granulosus - Image Credit: Sebastian Kaulitzki / Shutterstock
Transmission of the Disease
During maturation of the distal proglottid, separation from the main body of the tapeworm occurs and subsequently disintegrates releasing ova which can then be passed into faeces. Any ova released may contaminate grass, vegetables or the fur of animals which excreted the ova. If animals then feed on the grass containing the ova, they then can become infected and become an intermediate host for the parasite. Definitive hosts are those that consume infected intestines from the intermediate hosts.
The disease can then be passed onto humans through the ingestion of contaminated water or vegetables, or by close contact with dogs who are infected (dog licking a person’s face or a person eating food contaminated with dog faeces). Following ingestion, the outer membrane of the ova is digested by either the intestinal or gastric juices. The ovum is then free to penetrate the mucus of the villi located in the intestine, followed by the capillaries and by traveling in the bloodstream, enter the liver. The sinusoids of the liver act as a filter, resulting in the retention of the ovum in this area. However, some may still escape and go onto to enter the lungs. In some cases, further transportation is seen whereby the ovum passes through the lungs and into the arterial blood resulting in the possibility of lodging in a number of organs in the body.
In the majority of human cases, approximately 15% of the ova reside in the lung, 70% in the liver, and the remaining ova are found in several other locations including but not exclusive to the brain, bone, kidney and spleen. Fewer incidents have been reported where the ova has lodged itself into the heart.
Any ovum retained in the organs can multiply to develop into a spherical mass which is surrounded by a homogenous layer of endocyst. A serous vesicle then forms at the centre of the mass. The whole vesicle enlarges resulting in the cells becoming arranged around the germinal layer. Neighbouring tissues of the host react to the foreign body by producing a membrane around the cystic structure. This membrane is often referred to as the ectocyst or adventitia and can vary depending on which organ the cyst is inhabiting.
What are the symptoms of hydatid disease?
Cysts that develop in cases of hydatid disease are often slow-growing which means that the infection may not produce noticeable symptoms until a few years following the initial ingestion.
In cases where symptoms do develop, they are dependent on where the cyst is located. For example, if lodged in the liver, nausea, vomiting and abdominal pain may be experienced. If found in the lung, individuals may develop chest pain, shortness of breath and a chronic cough. Other, non-specific signs may develop including weakness, weight loss and anorexia.
Research has found that the only effective method of treating cases of hydatid disease is through surgical removal of the cysts followed by a course of chemotherapy.
Currently, there aren’t any preventative methods such as vaccines to protect humans against developing the disease. However, certain measures such as not feeding dogs uncooked food, washing hands thoroughly after being in contact with dogs, washing fruit and vegetables before cooking and eating them can be taken to reduce the likelihood of contracting the disease.